Tag Archives: saturn

Magnetic field readings point to the structure of Saturn’s interior

Researchers at the Johns Hopkins University have completed a new model of Saturn’s interior, which hints at a thick layer of helium rain that modulates the gas giant’s magnetic field.

Saturn’s interior with stably stratified Helium Insoluble Layer (HIL). Image credits Yi Zheng (HEMI/MICA Extreme Arts Program) / Johns Hopkins University.

The so-called ‘gas giants’ are notoriously hard to peer into, and they remain some of the most mysterious planets out there. Given the extreme environments they represent, it’s likely going to be a while before this changes, and an even longer while before any astronauts can actually go see for themselves.

That doesn’t mean we can’t draw some conclusions based on what we do know, however. And a team from Johns Hopkins University did just that, creating a new digital model looking into Saturn’s interior. This model hints at a temperature difference in the helium rain layer between the planet’s equator (where it is hotter) and the poles (where it gets colder).

Hot waist

“By studying how Saturn formed and how it evolved over time, we can learn a lot about the formation of other planets similar to Saturn within our own solar system, as well as beyond it,” said co-author Sabine Stanley, a Johns Hopkins planetary physicist.

“One thing we discovered was how sensitive the model was to very specific things like temperature,” she adds. “And that means we have a really interesting probe of Saturn’s deep interior as far as 20,000 kilometers down. It’s a kind of X-ray vision.”

Saturn is unique among the other gas giants in that its magnetic field is almost perfectly symmetrical around its axis. Since magnetic fields are generated by structures inside a planet’s body, this tidbit could help us glean some information about Saturn’s interior layout.

Using data recorded by NASA’s Cassini mission, researchers at Johns Hopkins University created detailed computer simulations using software typically employed for weather and climate simulations. The models indicate that there is a heat gradient in Saturn’s interior, with higher temperatures towards the equator. Overall, this could point to the existence of a layer of liquid helium around the planet’s core.

The magnetic field of Saturn seen at the surface. Image credits Ankit Barik / Johns Hopkins University.

This structure creates a dynamo-like mechanism, which goes on to produce the striking magnetic field recorded around Saturn. On Earth, the planet’s iron core and molten metal mantle play the role of dynamo. It was expected that gas giants rely on a different structure to create their magnetic field, given their different chemical composition and extreme mass, but this is the first study to actually pinpoint one candidate structure for this role in gas giants.

Apart from this, the simulations also suggest that a certain level of non-axisymmetry could be present near Saturn’s north and south poles.

“Even though the observations we have from Saturn look perfectly symmetrical, in our computer simulations we can fully interrogate the field,” said Stanley.

Naturally, until we can put a person on Saturn to check, we can’t confirm these findings. Until then, models will have to suffice.

The paper “Recipe for a Saturn‐Like Dynamo” has been published in the journal AGU Advances.

Saturn is tilted. The fault lies with its moons

Even the greatest of us have to take those around us into account. It’s true on a social level and, as new research shows, it’s true on an astronomical level as well.

Image via Pixabay.

Planets and the moons that orbit them form interdependent systems in which both influence one another. We see this with our Moon being tidally locked to our planet, while it, in turn, exerts its gravitational influence on Earth’s oceans.

These are undeniably massive effects that the two bodies exert on one another. But they’re not the most dramatic ones we’ve found so far. A new paper reports that Saturn, one of the titans of our Solar system, has a tilted rotation axis — and, according to the team, this is the doing of its moons.

Bowing to influence

The team, with members from CNRS (France’s national research center), Sorbonne University, and the University of Pisa working with the Paris Observatory report that Saturn’s satellites can explain the mystery of its tilted axis. They also predict that the planet will keep tilting in the future for a few billion years.

This tilt is caused by the gravitational pull of Saturn’s moons as they migrate away from their host planet. Titan, Saturn’s largest natural satellite, bears the lion’s share of the blame, they add.

Saturn’s moons are gradually wrestling free of the giant’s gravitational influence and are slowly inching away from it. While we were aware this was happening, the study showcases that the process is unfolding much faster than previously estimated. By using the new migration rate into our models and calculations, the team concluded that it is, in fact, tied to the planet’s tilt. Furthermore, as Saturn’s moons get further away from the planet, its tilt will keep increasing.

However, a decisive event (in regards to Saturn’s tilt) likely occurred recently in cosmological terms, the team adds. For around three billion years after its formation, Saturn’s axis was only slightly tilted; however, around one billion years ago, the tilting process took root.

At that time, the team explains, the movement of Saturn’s moons triggered a “resonance phenomenon” that continues to this day. We’re seeing the middle stages of this phenomenon currently. While this was already known as well, it was assumed that it started four billion years ago due to a change in Neptune’s orbit and that Saturn’s orbit today is stable — now we know neither are true. Over the next billion years or so, Saturn’s inclination relative to its axis could more than double.

Jupiter, the team adds in a different paper, is likely to undergo a similar tilting with the migration of its own moons due to the influence of Uranus. This process will likely take place over five billion years, says the team, and could take it from its current inclination of 3° to more than 30°.

The first paper “The large obliquity of Saturn explained by the fast migration of Titan” has been published in the journal Nature Astronomy.

The second paper “The future large obliquity of Jupiter” has been published in the journal Astronomy & Astrophysics.

Titan’s largest methane sea is over 1000 feet deep, says a new paper

Titan’s seas should be deep enough for a robotic submarine to wade through, a new paper explains. This should help pave the way towards our exploration of Titan’s depths.

Radar map of the polar region of Saturn’s moon Titan. Image credits NASA / JPL-Caltech.

Fancy a dip? Who doesn’t. But if you ever find yourself on Titan, Saturn’s biggest moon, you should stay away from swimming areas. A new paper reports that the Kraken Mare, the largest body of liquid methane on the moon’s surface is at least 1,000 feet deep near its center, making it both very deep and very cold.

While that may not be very welcoming to humans, such findings help increase our confidence in plans of exploring the moon’s oceans using autonomous submarines. It was previously unknown if Titan’s methane seas were deep enough to allow such a craft to move through.

Faraway seas

“The depth and composition of each of Titan’s seas had already been measured, except for Titan’s largest sea, Kraken Mare—which not only has a great name, but also contains about 80% of the moon’s surface liquids,” said lead author Valerio Poggiali, a research associate at the Cornell Center for Astrophysics and Planetary Science (CCAPS).

Titan is a frozen moon that shines with a golden haze as sunlight glints on its nitrogen-rich atmosphere. Beyond that, however, it looks surprisingly Earth-like with liquid rivers, lakes, and seas sprawling along its surface. But these are not made of water — they’re filled with ultra-cold liquid methane.

The findings are based on data from one of the last Titan flybys made during the Cassini mission (on Aug. 21, 2014). During this flyby, the probe’s radar was aimed at Ligeia Mare, a smaller sea towards the moon’s northern pole. Its goal was to understand the mysterious “Magic Island” that keeps disappearing and then popping back up again.

Its radar altimeter measured the liquid depth at Kraken Mare and Moray Sinus (an estuary on the sea’s northern shore). The authors of the paper, made up of members from both NASA’s Jet Propulsion Laboratory and Cornell University, used this data to map the bathymetry (depth) of the sea. They did this by tracking the return time on the radar’s signal for the liquid’s surface and the sea bottom while taking into account the methane’s effect on the signal (it absorbs some of the energy from the radio wave as it passes through, in essence dampening it to an extent).

Colorized mosaic of Titan’s Kraken Mare. Liquids are blue and black, land areas appear yellow to white. The surface was mapped using radar data from NASA’s Cassini. Image credits NASA / JPL-Caltech / Agenzia Spaziale Italiana / USGS via Wikimedia.

According to them, the Moray Sinus is about 280 feet deep, and the Kraken Mare gets progressively deeper towards its center. Here, the sea is too deep for the radar signal to pierce through, so we don’t know its maximum depth. The data also allowed us some insight into the chemical composition of the sea: a mix of ethane and methane, dominated by the latter. This is similar to the chemical composition of Ligeia Mare, Titan’s second-largest sea, the team explains. It might seem inconsequential, but it’s actually a very important piece of information: it suggests that Titan has an Earth-like hydrologic system.

Kraken Mare (‘mare’ is Latin for ‘sea’) is our prime choice for a Titan-scouting submarine due to its size — it is around as large as all five of America’s Great Lakes put together. We also have no idea why this sea doesn’t just evaporate. Sunlight is about 100 times less intense on Titan than Earth, but it’s still enough to make the methane evaporate. According to our calculations, this process should have completely depleted the seas in around 10 million years, but evidently, that didn’t happen. This is yet another mystery our space-faring submarine will try to answer.

“Thanks to our measurements,” he said, “scientists can now infer the density of the liquid with higher precision, and consequently better calibrate the sonar aboard the vessel and understand the sea’s directional flows.”

The paper “The Bathymetry of Moray Sinus at Titan’s Kraken Mare” has been published in the journal Journal of Geophysical Research: Planets.

Jovian planets — the giants of solar systems

The gas planets, the giants of the solar system, the jovian planets — call them what you will, these planets have fascinated mankind for centuries, and they’re still one of the more intriguing astronomical bodies out there.

Jovian literally means “Jupiter-like”, from “Jove” — another name for the Roman god Jupiter (called Zeus by the Greeks). They are primarily composed of gas or ice and are much larger than the Earth. They’re also much easier to detect than other planets — largely because they’re so big.

The name-giver of all Jovian planets: Jupiter. Jupiter is 318 times as massive as Earth, and it is 2.5 times larger than all the planets in the solar system combined. Image credits: NASA.

You can’t walk on a jovian planet

Jovian planets are comprised of fluid (gases or ices) rather than rock or other solid matter. Although giant rocky planets can exist, these are thought to be much rarer than gas or ice giants.

Jupiter is made up almost entirely of hydrogen and helium — the same elements found in the Sun, though at different temperatures and pressures. Here on Earth, hydrogen and helium are gases, but under the huge temperatures and pressures of Jupiter, hydrogen can be a liquid or even a kind of metal. We’re not entirely sure what lies at the center of Jupiter, but researchers believe that most likely, the core is similar to a thick, boiling-hot soup with a temperature of about 55,000 Fahrenheit (30,000 Celsius).

Saturn has a similar structure, with layers of metallic hydrogen, liquid hydrogen, and gaseous hydrogen, covered by a layer of visible clouds. Unlike Jupiter and Saturn, Uranus and Neptune have cores of rock and metal and different chemical compositions.

A potential internal structure of the jovian planets. It’s not clear if the core consists of rock, but it must be something very dense and hot. Image credits: University of Virginia.

We’re not sure exactly what the surface of jovian planets is like, but based on all we know, it’s not something you can walk on. Jovian planets tend to have very thick clouds (the clouds on Jupiter, for instance, are 30 miles or 50 km thick). After that, there’s gaseous hydrogen and helium, then more and more condensed gas, until you ultimately end up in liquid, metallic hydrogen. Saturn has a similar structure, though it is far less massive than Jupiter.

You might even have trouble realizing where the atmosphere ends and where the “planet” begins.

Methane clouds on Neptune. Image credits: NASA / JPL.

Uranus and Neptune, much smaller than both Jupiter and Saturn, have gaseous hydrogen surrounding a mantle of ice and a rocky core.

Jovian planets also have atmospheres with bands of circulating material. These bands typically encircle the planet parallel to the equator, with lighter bands lying at higher altitudes and being areas of higher pressure, and darker bands being lower in the atmosphere as low-pressure regions. This atmospheric circulation is similar only in principle to that on Earth, it has a very different structure.

Atmospheric bands on Jupiter. Image credits: NASA/JPL.

There are also other, smaller visible structures. The most famous of these is Jupiter’s Great Red Spot, which is essentially a giant storm that has been active for centuries. Saturn’s hexagon is another very well-known feature — both of these are much larger than the Earth itself.

These spinning balls of gas and liquids are truly impressive, and we’re still learning new things about them.

Jovian planets in our solar system

While still far smaller than the sun, jovian planets are by far the largest planets in our solar system. Image credits: NASA / University of Virginia.

Not all gas planets are alike. In fact, the reason why some astronomers prefer the term jovian planets to gas giants is that not all jovian planets are made of gas.

For instance, just Jupiter and Saturn are true gas giants, whereas Uranus and Neptune are ice giants. However, even this is a bit misleading: at the temperature and pressures on these planets, distinct gas and liquid phases cease to exist. Even so, the chemistry of the two groups is different: hydrogen and helium dominate Jupiter and Saturn, whereas, in the case of Uranus and Neptune, it’s water, methane, and ammonia. The two latter planets are thought to have a slushie-like mantle that spans over half of the planet diameter.

Image credits: NASA.

All four of these planets have large systems of satellites, and these satellites can be very interesting in their own right (we’ll get to that in a minute — there’s a good chance that life may be hiding in the jovian satellites). Saturn, for instance, has 82 designated satellites, and countless undesignated moonlets. Despite being larger, Jupiter “only” has 79 known satellites. Uranus has 27 and Neptune has 14.

All these four planets also have rings, though Saturn’s are by far the most pronounced.

Artist’s impression of the Voyager probe with the Jovian planets and some of their satellites. If you look closely, you can see Neptune’s rings. Image credits: Don Davis.

Much of what we know about gas and ice giants in general, we extrapolate from what we see in our own solar system. This being said, astronomers are aware that jovian planets can be very different and have a much greater variety than we see in our solar system.

Extrasolar Jovian planets

Roughly speaking, jovian planets can be split into 4 categories:

  • gas giants (like Jupiter and Saturn) — mostly consisting of hydrogen and helium, and only 3-13% heavier elements;
  • ice giants (like Neptune and Uranus) — a hydrogen-rich atmosphere covering an icy layer of water;
  • massive solid planets (somewhat similar to the Earth, but huge) — tangible evidence for this type of planet only emerged in 2014, and these planets are still poorly understood. Astronomers suspect that solid planets up to thousands of Earth masses may be able to form, but only around massive stars;
  • super puffs — planets comparable in size with Jupiter, but in mass with the Earth. These planets are super rarefied, and were only discovered in the past decade; the most extreme examples known are the three planets around Kepler-51.
Artistic rendering of Gliese 3470 b — a rare “superpuff”. The two are believed to be comparable in mass. Image credits: NASA.

Based on what we’ve seen so far, jovian planets seem pretty common across our galaxy. However, we’ve only started discovering exoplanets very recently, and it’s hard to say whether the planets we’ve found so far are representative of the larger picture.

However, based on the fact that researchers have discovered far more Neptune-sized planets than Jupiter-sized planets (although the latter are easier to discover), it’s pretty safe to say that it’s the Neptune-sized planets that are more common.

Image credits: NASA.

A particularly interesting class of jovian planets is the so-called Hot Jupiters.

Hot Jupiters are the easiest planets to detect. As the name implies, they are Jupiter-sized planets, but they lie very close to their stars and have a rapid orbital period that produces effects that are more easily detected. For instance, one such planet revolved around its star in only 18 hours, making for one very short year. Another freakish example of a Hot Jupiter is believed to have surface temperatures of 4,300°C (7,800°F) — which is hotter than some stars we know.

Extrasolar planets, and Hot Jupiters in particular, can shed a lot of light on the evolution of solar systems. It is believed that these planets form in the outer parts of solar systems (like Jupiter), but they slowly migrate towards the star, drawn by gravitational attraction. As they do so, they could wreak havoc on the entire solar system, much like a big billiards ball.

Artist’s impression of a Hot Jupiter. Image credits: ESO/L. Calçada.

Some jovian planets get so large that they blur the line between a planet and a brown dwarf. Brown dwarfs are neither truly stars nor planets. As a rule of thumb, jovian planets are only “planets” until they are 15 times the mass of Jupiter — after that, they “become” brown dwarfs.

Because of the limited techniques currently available to detect and study exoplanets, there are still many things we don’t know about exoplanets, even those as big as Jupiter. We tend to associate these planets by size with the ones in our own solar system, piecing together other available information (which is scarce). As our telescopes, equipment, and theoretical models become better, we will no doubt better our understanding of jovian planets, and exoplanets in general.

This image shows an artist’s impression of the ten hot Jupiter exoplanets studied by David Sing and his colleagues. From top left to to lower left these planets are WASP-12b, WASP-6b, WASP-31b, WASP-39b, HD 189733b, HAT-P-12b, WASP-17b, WASP-19b, HAT-P-1b and HD 209458b.

Life around Jovian planets

Jovian planets are not exactly life-friendly — at least not directly. A giant, spinning, mass of fluid you can’t even stand on, either very hot or very cold, doesn’t sound very attractive to life forms. But jovian satellites are a different story. In fact, astronomers are starting to believe that the satellites of Jupiter and Saturn may be the best places to look for extraterrestrial life in our solar system.

Both Jupiter and Saturn lie rather far from the Sun. They are cold, frigid places, as are their satellites — at least on the surface.

Jupiter’s icy moon, Europa. Image credits: NASA.

Researchers now believe that some of the icy satellites of Jupiter and Saturn (especially Europa and Enceladus) could host life under their frozen surfaces.

Although the surface temperatures are extremely low on these satellites, astronomers have some clues that both satellites may harbor oceans of liquid water beneath the frozen surface. Basically, the huge gravitational effect from their host planets causes friction and shear in the ice, which produces sufficient heat to melt the ice. This is called tidal heating. Geothermal and geological activity may also contribute to this effect, creating a liquid, salty water beneath the ice — and while this has not yet been confirmed, these could be suitable conditions for life to emerge.

Enceladus is believed to harbor a liquid ocean in its subsurface. Image credits: NASA / JPL.

In addition to Europa and Enceladus, several other jovian satellites could harbor life (with various degrees of likelihood): Callisto, Ganymede, Io, Triton, Dione, and even Pluto’s moon Charon could all have a liquid ocean compatible with life. NASA’s Clipper mission is set for launch in 2024, with the goal of exploring Europa’s potential habitability. Some scientists believe there are as many habitable exomoons as there are habitable exoplanets.

Jovian planets seem to play a key role in the structure of solar systems. Whether their satellites can hold life or not, they are an extremely important puzzle piece in our understanding of how solar systems form, evolve, and how the Earth fits in this grand cosmic puzzle.

Titan’s sand dunes could be caused by cosmic rays

Titan’s sand dunes cover over 10 million square kilometres of the moon’s surface, an area about the size of the US, including Alaska.

Saturn’s largest moon has long intrigued scientists as its chemical composition is believed to mirror that of our own primordial planet. Now, thanks to new data obtained by researchers by the University of Hawaii (UH) at Manoa, we might be able to provide some answers to key question’s about Titan’s surface.

The team, led by physical chemist Ralf I. Kaiser, examined remote sensing data from NASA’s Cassini-Huygens mission to Titan to study its huge swathes of desert which are covered in sand dunes. These dunes stretch across the moon’s equatorial region in a space over 10 million kilometers (6,213,712 miles) and reach heights of up to about 100 meters in some places — think Egyptian pyramids tall.

The UH Manoa team exposed acetylene ice — a chemical that is used on Earth in welding torches and exists at Titan’s equatorial regions — at low temperatures to proxies of high-energy galactic cosmic rays.

“Titan’s dunes represent the dominating surface sink of carbon in Titan’s organic chemistry,” said Matthew Abplanalp, former chemistry graduate student at UH’s W.M. Keck Research Laboratory in Astrochemistry, and current researcher at the Naval Air Warfare Center Weapons Division at China Lake. “Therefore, unraveling the origin and chemical pathways to form this organic dune material is vital not only to understand Titan’s chemical evolution, but also to grasp how alike the chemistries on Titan and on Earth might have been like before life emerged on Earth 3.5 million years ago.”

The UH researchers exposed a rapid cosmic-ray-driven chemistry which converts simple molecules like acetylene to more complex organic molecules like benzene and naphthalene, a compound which is found in mothballs, but hopefully without the scent.

These findings will have unprecedented implications for the next space mission to Titan. In 2034, the three‐meters‐long Dragonfly rotorcraft will land in the ‘Shangri‐La’ dune‐fields near the moon’s equator. From there, it will use 1its eight rotors to traverse dozens of sites across Titan’s surface, taking samples and performing analysis. The purposes of the mission is to search for alien life and its molecular precursors.

“Overall, this study advances our understanding of the complex organics and fundamental chemical processing of simple molecules in deep space and provides a scientifically sound and proven mechanism of formation of aromatic structures in extreme environments in low temperature ices,” Kaiser concluded. “Since Titan is nitrogen-rich, the incorporation of nitrogen in these PAHs may also lead to carbon-nitrogen moieties (parts of a molecule) prevailing in contemporary biochemistry such as in DNA and RNA-based nitrogen-bases.”

Discovered in 1655 by the Dutchman Christiaan Huygens, Titan is located approximately 760,000 miles (1,223,101 kilometers) from Saturn. Cassini showed us that Titan’s surface has lakes, rivers, and even seas of liquid ethane and methane (the main component of natural gas), as well as vast expanses of sand dunes. Its climate is such that the methane can form clouds and even rain, as water does here on Earth. The moon’s atmosphere is four times denser than ours and its gravity is approximately 1/7th of Earth’s. Because it is so far from the Sun, Titan’s surface temperature hovers around a chilly ‐290 degrees Fahrenheit (‐179 degrees Celsius).

Saturn is now the planet with the most moons

A team of researchers from Carnegie just discovered 20 new moons orbiting around Saturn, raising the total count to 82 — barely surpassing Jupiter’s 79.

Saturn is the king of moons — at least for now.

Depending on when you finished school, you might have learned that Jupiter or Saturn has the most moons. Kids finishing school this year will probably learn that Saturn has the most moons, as 20 new moons measuring around 5 km (3 miles) in diameter have been discovered around Saturn.

Three of these moons orbit Saturn “normally”, rotating in the same direction the planet does. The rest of 17, however, have a so-called retrograde rotation, meaning they rotate in the opposite direction.

“It’s exciting to find them,” said Scott Sheppard, an astronomer who led the work at the Carnegie Institution for Science in Washington DC. “These moons are very far away from the planet.” Each is about three miles across.

The newly discovered moons and their orbits. Image credits: Carnegie Institution for Science.

However, it’s not just a quirky new find or something topic students will have to learn — learning more about these moons could teach us something new about the evolution of their host planets, and the solar system as a whole.

“Studying the orbits of these moons can reveal their origins, as well as information about the conditions surrounding Saturn at the time of its formation,” said Dr Scott Sheppard, from the Carnegie Institution for Science in Washington DC, who led the team that discovered the moons.

Nowadays, if an asteroid zooms by a planet, the planet wouldn’t be able to capture it, because there’s nothing to slow it down and dissipate its energy. But in the solar system’s infancy, things were quite different.

In the early days of the solar system, when Saturn was still just forming, a cloud of dust and gas surrounded the planet. During those days, if an asteroid flew by Saturn, the cloud dissipated the asteroid’s energy, allowing the planet to capture the asteroid.

These moons have been alongside Saturn for a long time, it just took astronomers a long time to find them because they are so small.

These moons are at the very edge of what current satellites can discover — particularly, researchers used the Subaru telescope on Hawaii’s Mauna Kea volcano for their detection. From 2004 to 2007, Sheppard and colleagues used Subaru to sweep Saturn’s area looking for undiscovered moons. They did find some intriguing points of light, but it took a while to prove that they were in fact moons.

That’s why this discovery was more than a decade in the making. From 2004 to 2007, Sheppard and his colleagues used Subaru to comb the area around Saturn to search for undiscovered moons. While they did see some intriguing points of light, they struggled to prove that those pinpricks were, in fact, orbiting Saturn. It was improved computer algorithms that allowed astronomers to make the confirmation.

“We thought they were moons of Saturn, but we weren’t able to get full orbits to determine this,” said Dr Sheppard. “By using this new computer power, I was able to link these 20 objects that we thought were moons to officially find orbits for them.”

None of the new moons have names yet. Sheppard and colleagues have invited the public to offer suggestions for a period of 2 months, until December 6th.

Even without names, Saturn is, at least temporarily, the king of moons — overthrowing Jupiter from that position. However, that might not last for long. New, more powerful telescopes are currently being built, and there is a good chance that they will be able to discover even moons that are currently undetectable. Our current best capabilities allow astronomers to detect moons around 3 miles across around Saturn, and 1 mile across around Jupiter.

Saturn Illustration.

Perturbations in Saturn’s rings reveal how long a day is on the gas giant

Saturn’s days are 10 hours, 33 minutes, and 38 seconds long — and we know this by looking at wave patterns in its rings.

Saturn Illustration.

Illustration showing NASA’s Cassini spacecraft in orbit around Saturn.
Image credits NASA / JPL-Caltech.

New observations from NASA’s Cassini spacecraft allowed researchers at the University of California Santa Cruz to calculate Saturn’s rate of rotation. This measurement — the most precise determination of its rotation rate — was based on observations of wave patterns created within the planet’s rings.

Timekeeping rings

“Particles in the rings feel this oscillation in the gravitational field. At places where this oscillation resonates with ring orbits, energy builds up and gets carried away as a wave,” explained Christopher Mankovich, a graduate student in astronomy and astrophysics at UC Santa Cruz, and lead author of the study.

Just like our own planet, Saturn vibrates in response to perturbations (large-scale movement of matter). Unlike our planet, these perturbations come not from the movement of tectonic plates, but likely from heat-driven convection in the planet’s gassy bulk. Such internal oscillations move about massive quantities of gas, which has a noticeable impact on local densities within Saturn’s atmosphere. Such changes, in turn, cause noticeable changes in the planet’s localized gravitational pull. And, even better, the frequency of oscillation within Saturn carries over to the gravitational effects — in short, they share the same ‘fingerprint’, so these internal events can be linked to their external, gravitational effects.

Saturn rings.

Image of Saturn’s rings taken by NASA’s Cassini spacecraft on Sept. 13, 2017.
Image credits NASA / JPL-Caltech / Space Science Institute.

Naturally, we’d need satellites or other sorts of equipment in orbit across the planet to pick up on such gravitational fluctuations. Which we haven’t really brought over yet. Rather conveniently, however, Saturn has a sprawling ring system surrounding it. They do react to the planet’s gravitational pull, its fluctuations causing certain wave patterns to form inside the rings. Not all patterns seen inside the rings are caused by gravitational effects — but most are.

In effect, this makes the rings act similarly to seismographs, devices that we use to measure earthquakes.

“Some of the features in the rings are due to the oscillations of the planet itself, and we can use those to understand the planet’s internal oscillations and internal structure,” says Jonathan Fortney, professor of astronomy and astrophysics at UC Santa Cruz and paper coauthor.

NASA’s Cassini spacecraft allowed researchers to observe Saturn’s rings in unprecedented detail. Mankovich’s team developed a series of models of the planet’s internal structure and used them to predict the frequency spectrum of Saturn’s internal vibrations. Then they compared their predictions to waves observed by Cassini in Saturn’s C ring.

One of the main results of this study is an estimation of Saturn’s speed of rotation — which has been notoriously difficult to accurately pin down. Saturn is basically a huge clump of gas and, as such, its surface doesn’t have any fixed, distinctive features we could track as it rotates. The planet is also unusual in that its magnetic poles are nearly perfectly aligned to its axis of rotation — so we can’t track those either. On Earth, for example, the magnetic poles aren’t aligned with this axis.

Mankovich’s team determined that a day on Saturn lasts for 10 hours, 33 minutes, and 38 seconds — several minutes shorter than previous estimates (which were based on radiometry readings from the Voyager and Cassini spacecraft).

“We now have the length of Saturn’s day, when we thought we wouldn’t be able to find it,” said Cassini Project Scientist Linda Spilker.

“They used the rings to peer into Saturn’s interior, and out popped this long-sought, fundamental quality of the planet. And it’s a really solid result. The rings held the answer.”

The paper “Measurement and implications of Saturn’s gravity field and ring mass” has been published in the journal Science.

Cassini’s view of Saturn durings its final flybies of the gas giant. Credit: NASA/JPL-Caltech/Space Science Institute.

Scientists come up with most accurate age of Saturn’s rings yet

Cassini’s view of Saturn durings its final flybies of the gas giant. Credit: NASA/JPL-Caltech/Space Science Institute.

Cassini’s view of Saturn during its final flyby of the gas giant. Credit: NASA/JPL-Caltech/Space Science Institute.

Saturn’s rings have fascinated astronomers ever since Galileo Galilei first discovered them with his 20-power telescope. At the time (in 1610), he thought the rings were large moons on either side of the planet, writing “I have observed the highest planet [Saturn] to be tripled-bodied. This is to say that to my very great amazement Saturn was seen to me to be not a single star, but three together, which almost touch each other.”

Decades later, Christaan Huygens used a much more powerful telescope than Galileo, finding that Saturn was in fact surrounded by “a thin, flat ring, nowhere touching, and inclined to the ecliptic.”

Slowly, year by year, astronomers have come up with progressively better observations of Saturn’s rings. So much so that four centuries after Galileo’s discovery, humans have managed the incredible feat of sending a spacecraft to the gas giant’s orbit.

For 13 years, the Cassini spacecraft has looped around the ringed planet. In September 2017, it finally ran out of fuel and NASA sent it on a controlled path towards the planet. 

Although the leap destroyed Cassini, it managed to beam back important data about Saturn’s elusive rings as it passed through them. A new study analyzed some of that precious data, revealing new interesting insights about the physics that glue the debris rings together as well as how much mass they trap.

During its final maneuver, Cassini looped in and out of Saturn’s rings. Previously, it had only observed them from outside their range. By calculating the gravity of the bands which jostled the spacecraft between them and Saturn, scientists were able to come up with the most precise estimate of the mass of the rings to date. Combined, all that rock and ice that form Saturn’s bands has a mass about 2,000 times smaller than the moon. That may be surprising to hear considering how large they seem with a telescope, but the particles that comprise them are mostly tiny, like grains of sand (although they can be the size of boulders or even small mountains) and are widely spaced apart in certain areas.

According to the authors at NASA’s Jet Propulsion Laboratory, the estimate has an error margin of about 25%. That’s quite a lot, but even so, it’s the most accurate figure that we currently have.

This monochrome view is the last image taken by the imaging cameras on NASA's Cassini spacecraft. It looks toward the planet's night side, lit by reflected light from the rings, and shows the location at which the spacecraft would enter the planet's atmosphere hours later. Credit: NASA/JPL-Caltech/Space Science Institute.

This monochrome view is the last image taken by the imaging cameras on NASA’s Cassini spacecraft. It looks toward the planet’s night side, lit by reflected light from the rings, and shows the location at which the spacecraft would enter the planet’s atmosphere hours later. Credit: NASA/JPL-Caltech/Space Science Institute.

This new estimate published in the journal Science also helps answer a long-standing puzzle: how old are the rings? The most massive the rings, the older they should be. Not too long ago, researchers believed that Saturn’s band formed when the planet itself coalesced into its current shape, about 4.6 billion years ago. The new study suggests that the rings are much younger than that — somewhere between 10 million and 100 million years old. This means that if humans were alive during the age of the dinosaurs, our instruments would have seen a lonely Saturn, without its signature bands.

All of this leads us to another mystery: who put a ring on that planet? The lucky fellow may have been anything from a moon, comet, asteroid, or multiple things at once which strayed too close to the planet and was shattered to smithereens by the massive gas giant’s tugging forces. To really know the answer to this question, Cassini would have had to collect a sample from Saturn’s bands and analyze them. Perhaps a mission in the future might actually do this — but it might take several decades.

One thing to note is that Saturn will one day lose its rings. According to a study published in the journal Icarus, the ice particles that make up the rings are being pulled into the planet at a rate that could fill an Olympic-sized swimming pool every half hour. The rings could be gone as soon as 100 million years from now.

An artist's impression of how Saturn may look in the next hundred million years. Credit: NASA/Cassini/James O'Donoghue.

An artist’s impression of how Saturn may look in the next hundred million years. Credit: NASA/Cassini/James O’Donoghue.

Saturn rings.

Saturn’s rings are raining down — in about 100 million years, they’ll be gone

New research from NASA found that Saturn, the ring planet, is losing its rings.

Saturn and rings.

Image NASA / Cassini Imaging Team via Wikimedia.

Observations made decades ago by Voyager 1 and Voyager 2 show that Saturn is devouring its own rings, NASA reports. The particles making up these striking structures are falling onto the planet as a rain of dust and ice, propelled by Saturn’s gravity and magnetic field.

One ring to bind them

“We estimate that this ‘ring rain’ drains an amount of water products that could fill an Olympic-sized swimming pool from Saturn’s rings in half an hour,” said James O’Donoghue of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, the study’s lead author.

“[…] The rings have less than 100 million years to live. This is relatively short, compared to Saturn’s age of over 4 billion years.”

The research actually began with scientists trying to figure out if Saturn formed with its rings or acquired them later. The second scenario seems to be the more likely, the team reports. In fact, they estimate that the rings are no older than 100 million years. The team based this age on how much it would take for the C-ring to form from a (hypothetical) original B-ring-like structure. Here’s a chart for your convenience:

Saturn rings.

Saturn rings and with their major subdivisions.
Image credits NASA / JPL / Space Science Institute via Wikimedia.

There are quite a number of theories in regards to how Saturn got its rings (the most prevalent of which we’ve talked about here). If they’re younger than the planet itself, the rings could be the product of collisions between Saturn and small, icy moons. Such a mechanism would be supported by the rings’ present makeup — chunks of water ice ranging from several yards across to microscopic sizes.

Still, the finding that Saturn acquired its rings later in life is, perhaps, overshadowed by the realization that it’s eventually going to lose them. O’Donoghue says, we’re “lucky to be around” while Saturn still has rings. They’re probably around the middle of their lifetime, he adds. The other side of the coin is that we’ve perhaps missed out on seeing similarly lush ring systems around Jupiter, Uranus, and Neptune. While these gas giants do have ring systems today, they’re thin, wispy things.

Black belt giant

The first hints that Saturn’s rings were raining down on the planet came from Voyager readings on (seemingly) unrelated phenomena: variations in Saturn’s ionosphere (electrically-charged upper atmosphere), density variations in its rings, and the planet’s three dark bands These bands encircle the planet at high altitudes (stratosphere) at northern mid-latitudes, and were first spotted by the Voyager 2 mission in 1981.

Later, a NASA Goddard researcher named Jack Connerney linked (paper here) these bands to the planet’s massive magnetic field. Connerney’s hypothesis was that the bands form as electrically-charged ice particles from Saturn’s rings flowed down magnetic field lines. Tiny particles can get electrically charged by ultraviolet light from the Sun or by plasma clouds emanating from micrometeoroids impacting the rings.

Essentially, water pouring into the planet’s upper atmosphere was what formed these bands. The water would literally wash away haze in Saturn’s stratosphere, making them less reflective of light — so the bands appear darker.

So what actually causes the rings to rain down? Well, they’re generally kept in orbit by an interplay between the planet’s gravitational field (which pulls them down) and the centrifugal force generated by the rings’ rotation (which pushes them outwards, or ‘up’).

Things become more complicated when Saturn’s magnetic field gets involved, however. Those electrically-charged particles we talked about earlier also start feeling the pull of the planet’s magnetic field, which curves towards Saturn at its rings. In some parts of the rings, this magnetic pull is enough to dramatically shift the balance of forces on particles — it neutralizes, to an extent, the centrifugal force. Gravity takes hold, pulling the particles down on the planet.

These infalling bits of water chemically react in Saturn’s ionosphere, generating H3+ ions. O’Donoghue picked up on these ions using the Keck telescope in Mauna Kea, Hawaii, as H3+ ions glow in infrared light. The team saw glowing infrared bands in Saturn’s northern and southern hemispheres where magnetic field lines enter the planet. By analyzing the infrared light output, the team calculated the quantity of infalling ring matter (i.e. of how fast they are degrading).

The highest influx of infalling ice, the paper adds, is found in an area in southern Saturn. Some of the matter spewed by Enceladus’ ice geysers also finds its way down to the gas giant, which Connerney says isn’t “a complete surprise.”

So far, the results are pretty solid. However, the team says observing Saturn as it goes around the sun (on a 29.4-year orbit) would conclusively prove or disprove the findings. On its trek, Saturn’s rings will be exposed to various degrees of ultraviolet light — which charges ice particles in the rings. If researchers find that different levels of exposure to sunlight change the quantity of ‘rain’ on Saturn, the study’s conclusions would be confirmed.

The paper “Observations of the chemical and thermal response of ‘ring rain’ on Saturn’s ionosphere” has been published in the journal Icarus.

Saturn.

Planetary rings are surprisingly chemically-rich, paper reports

Saturn’s rings are very chemically complex, new research shows, and actively change the makeup of the planet’s atmosphere.

Saturn.

Titan in front of Saturn and its rings.
Image credits NASA / JPL-Caltech / Space Science Institute.

Data beamed back from the Cassini spacecraft during its final descent into the depths of Saturn shows that the giant’s rings are more chemically complex than we’ve believed.

If you like it, study the rings on it

“This is a new element of how our solar system works,” said Thomas Cravens, professor of physics & astronomy at the University of Kansas and a co-author of the new paper.

Cravens is a member of Cassini’s Ion and Neutral Mass Spectrometer (INMS) team. Back in 2017, as Cassini plunged into Saturn’s upper atmosphere, it sampled the chemical makeup of points at various altitudes between Saturn’s rings and atmosphere using its onboard mass spectrometer.

The paper reports finding a surprising chemical complexity in the planet’s rings. This challenges the current view, based on past observations, that the rings “would be almost entirely water”, Cravens explains.

“Two things surprised me. One is the chemical complexity of what was coming off the rings — we thought it would be almost entirely water based on what we saw in the past. The second thing is the sheer quantity of it — a lot more than we originally expected.”

“the mass spectrometer saw methane — no one expected that. Also, it saw some carbon dioxide, which was unexpected,” Cravens explains. “The rings were thought to be entirely water. But the innermost rings are fairly contaminated, as it turns out, with organic material caught up in ice.”

The INMS-readings were performed in the gap between the inner ring and upper atmosphere. They uncovered the presence of water, methane, ammonia, carbon monoxide, molecular nitrogen, and carbon dioxide in the rings.

Dust grains from Saturn’s D (innermost) ring constantly rain down into the planet’s upper atmosphere, carrying a coating of this ‘chemical cocktail’. This process takes place at an extraordinary rate, the team adds — 10 times faster than previously estimated. This process is powered by the different spin rates of the planet and its rings (the rings spin faster than the planet’s atmosphere). Over time, this process likely changed the carbon and oxygen content of Saturn’s atmosphere.

“We saw it was happening even though it’s not fully understood,” Cravens adds. “What we saw is this material, including some benzine, was altering the uppermost atmosphere of Saturn in the equatorial region. There were both grains and dust that were contaminated.”

The findings not only shed light on the chemical complexity of planetary rings, but also raise important questions pertaining to their formation, lifespan, and interaction with the host planet.

For example, given the very high rate of material transfer to the atmosphere, it may be safe to assume that planetary rings are much more short-lived than previously estimated. In the absence of a source of fresh material to make up for this particle flow, rings may simply drain away into nothingness. One possibility that derives from these findings is that Jupiter likely also had its own set of fully-fleshed out rings, which gradually drained into the wispy trail that surrounds the gas giant today.

The origin of these complex materials is also of interest to astronomers; “[is material in the rings] left over from the formation of our solar system? Does it date back to proto pre-solar nebula, the nebula that collapsed out of interstellar media that formed the sun and planets?”

Finally, the team reports that this influx of matter also impacts the planet’s ionosphere by converting hydrogen ions and triatomic hydrogen ions into heavier molecular ions — thereby depleting the ionosphere of charged particles.

But Cravens’ main contribution involved interpreting that data with a focus on how materials from the rings are altering Saturn’s ionosphere.

“My interest was in the ionosphere, the charged-particle environment, and that’s what I focused on,” Cravens said. “This gunk coming in chews up a lot of the ionosphere, affects its composition and causes observable effects — that’s what we’re trying to understand now. The data are clear, but explanations are still being modeled and that will take a while.”

The paper has been published in the journal Science.

NASA reveals new, crystal-clear images of Saturn’s moon Titan

NASA is spoiling us once again — this time with some stunning photos of Titan.

Image credits: NASA / JPL.

The six images above represent some of the clearest and sharpest images we have ever captured of the icy moon. They were taken by the Visual and Infrared Mapping Spectrometer (VIMS) instrument on board NASA’s Cassini spacecraft, whose mission to explore Saturn and its moons ended on September 15, 2017. Although Cassini’s mission ended, scientists are still analyzing data and images the shuttle sent our way.

The new images are the result of a multitude of different observations made under a wide variety of lighting and viewing conditions. Making mosaics from VIMS images of Titan is particularly challenging because the viewing angle and atmospheric conditions varied so greatly. As a result, most images had visible seams which overlapped on the image, but that was removed here through painstaking data analysis and hand processing.

With the seams now gone, this is quite possibly the best collection of Titan photos available, and the images can be used to study Titan’s features in unprecedented detail. NASA has also released a bigger, black & white map of Titan with labeled features — you can check that out here.

This map of Titan shows the names of many (but not all) features on the Saturnian moon that have been approved by the International Astronomical Union. Image credits: NASA / JPL.

From these images, it becomes apparent that Titan’s surface is anything but uniform. Even an untrained eye can quickly distinguish myriad different geological features, although what those features are remains a much more difficult question. For example, you can notice some equatorial dune fields, which appear a consistent brown color here. The bluish and purplish areas have a clearly different composition and possibly contain water ice.

It’s important to note that the image is not in the visual spectrum — observing the surface of Titan in the visible region of the spectrum is difficult, due to the globe enshrouding haze that envelops the moon. The aerosols in Titan’s atmosphere strongly scatter visible light, but leave a few infrared “windows” open — parts of the infrared spectrum where scattering and absorption are much weaker. This is where VIMS excels, and that’s how it was able to snap the good photos NASA worked on to develop the mosaic.

Titan is the largest moon of Saturn. It is the only moon known to have a dense atmosphere and the only object other than Earth where clear evidence of surface liquid has been found. Titan is primarily composed of water ice and rocky material, but, unlike Earth, which is largely covered by bodies of water, Titan features hydrocarbon lakes and seas. Titan’s methane cycle is analogous to Earth’s water cycle but at a much lower temperature (−179.2 °C; −290.5 °F).

Enceladus interior.

Enceladus “the only body besides Earth to satisfy all of the basic requirements for life,” Cassini reveals

Data beamed back by the Cassini spacecraft reveals that Enceladus, Saturn’s sixth-largest moon, isn’t shy about blasting large organic molecules into space.

Enceladus interior.

Hydrothermal processes in the moon’s rocky core could synthesize organics from inorganic precursors. Alternatively, these processes could be transforming preexisting organics by heating, or they could even generate geochemical conditions in the subsurface ocean of Enceladus that would allow possible forms of alien life to synthesize biological molecules.
Image credits NASA/JPL-Caltech/Space Science Institute/LPG-CNRS/Nantes-Angers/ESA

Mass spectrometry readings beamed back by NASA’s Cassini craft show that Enceladus is bursting with organic molecules. The moon’s icy surface is pockmarked with deep cracks that spew complex, carbon-rich compounds into space. Scientists at the Southwest Research Institute (SwRI) say these compounds are likely the result of interactions between the moon’s rocky core and warm waters from its subsurface ocean.

Why so organic?

“We are, yet again, blown away by Enceladus,” said SwRI’s Dr. Christopher Glein, co-author of a paper describin the discovery.

“Now we’ve found organic molecules with masses above 200 atomic mass units. That’s over ten times heavier than methane. With complex organic molecules emanating from its liquid water ocean, this moon is the only body besides Earth known to simultaneously satisfy all of the basic requirements for life as we know it.”

The Cassini mission, a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency, is widely-held to be one of the most ambitious space exploration missions we’ve ever embarked upon. Launched on October 15, 1997, Cassini spent some 13 years studying the gas giant and its moons. The craft also flew by Venus (April 1998 and July 1999), Earth (August 1999), the asteroid 2685 Masursky, and Jupiter (December 2000), before settling in on Saturn’s orbit on July 1st, 2001.

Enceladus. Image credits: NASA/JPL.

On September 15, 2017, NASA de-commissioned the aging craft with a bang: they deorbited Cassini, letting it fall towards Saturn’s core and burn up in its atmosphere.

However, the wealth of information this tiny craft beamed back from its travels is still giving astronomers a lot to work on. Before its fiery demise, Cassini sampled the plume material ejected from the subsurface of Enceladus. Using its Cosmic Dust Analyzer (CDA) and the SwRI-led Ion and Neutral Mass Spectrometer (INMS) instruments, the craft analyzed both the plume itself and Saturn’s E-ring — which is formed by ice grains from the plumes trapped in Saturn’s gravity well.

Chemicals Enceladus.

Synthesis path of different aromatic cations identified in Enceladus’ plume.
Image credits F. Postberg et al., 2018, Nature.

During one of Cassini’s particularly close flybys of Enceladus (Oct. 28, 2015), the INMS detected molecular hydrogen in the moon’s plume ejections. Previous flybys also revealed the presence of a global subsurface ocean and a rocky core. This was the first indication that the moon can boast active geochemical below the surface, most likely between water and rocks in hydrothermal vents.

The presence of hydrogen was also grounds for great enthusiasm at NASA — the element is a known source of chemical energy for microbes living in hydrothermal vents here on good ol’ Earth.

“Once you have identified a potential food source for microbes, the next question to ask is ‘what is the nature of the complex organics in the ocean?'” says SwRI’s Dr. Hunter Waite, INMS principal investigator and paper coauthor. “This paper represents the first step in that understanding — complexity in the organic chemistry beyond our expectations!”

The findings are significant enough to influence further exploration, Glen believes. Any spacecraft that flies towards Enceladus in the future should make a point of going through its plume to analyze these complex organic molecules with a high-resolution mass spectrometer to “help us determine how they were made.”

“We must be cautious, but it is exciting to ponder that this finding indicates that the biological synthesis of organic molecules on Enceladus is possible.”

The paper “Macromolecular organic compounds from the depths of Enceladus” has been published in the journal Nature.

Saturn’s tiny moons? It’s all ravioli and collisions

When Martin Rubin, an astrophysicist at the University of Bern, saw images of Saturn’s small inner moons, it reminded him of something rather odd: food — variations of pasta, to be more exact. That inspired him to further study these moons, and now, he has a good idea of how they formed.

The top row shows 3 small moons of Saturn imaged by the Cassini spacecraft. Shown at the bottom are computer model outcomes. Image credits: NASA/JPL-Caltech/Space Science Institute / University of Bern.

The photos Rubin saw were from the Cassini mission, one of the most ambitious efforts in planetary space exploration, which sent a probe to study the planet Saturn and its system, including its rings and natural satellites. Among Cassini’s 2017 images, something caught Rubin’s eye: something which NASA called “flying-saucers with diameters of about 30 km” — extremely small moons, which do look a bit like ravioli or spaetzle.

Saturn’s moons are numerous and diverse, ranging from tiny moonlets, less than 1 kilometer across, to the massive Titan, which is bigger than the planet Mercury. Saturn has a total of 62 moons with regular orbits, 24 of them being regular satellites. The extremely small, inner moons, however, are believed to originate from Saturn’s rings, a thin disk of ice and rock located around the planet’s equatorial plane.

Rubin asked his colleague Martin Jutzi whether they could be the outcome of collisions, instead of being formed through gradual accretion of material around a single core. Together, they started developing models of how collision-formed moons would look, and found that the models closely replicated the real thing.

Saturn isn’t really a perfect sphere, but rather oblate. Combining this shape with its huge mass makes it very difficult for any satellites to escape the gravitational attraction around Saturn’s equatorial plane.

Although they weren’t looking for it, researchers also found a possible explanation to the mystery of Saturn’s third-largest moon, Iapetus, explaining its unusual shape.

Iapetus might also have formed through a head-on collision of two bodies. Image credits: Adrien Leleu, Martin Jutzi and Martin Rubin/ University of Bern.

“Our modelling results suggest that these features may be a result of a merger of similar-sized moons taking place with a close to head-on impact angle, similar to the smaller moons,” the researchers summarize.

Lastly, this study also points out that Saturn could serve as a sandbox system, to enable us to understand this type of collision in other environments.

 “A significant fraction of such merging collisions take place either at the first encounter or after 1-2 hit-and-run events,” the authors summarize in their paper published today in Nature Astronomy. “In this respect, Saturn is almost a toy system to study these processes,” says Martin Rubin.

Journal Reference: A. Leleu, M. Jutzi, M. Rubin: “The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets”, Nature Astronomy. doi: 10.1038/s41550-018-0471-7.

Cassini's last full-view picture of Saturn. Credit: NASA/JPL-Caltech/Space Science Institute.

Cassini’s stunning farewell picture of Saturn

On September 15, 2017, the daredevil Cassini spacecraft ended its 20-year voyage through space. It did so in style, plunging into Saturn’s atmosphere — the planet it had most closely studied in the past 13 years. Two days before spiraling into oblivion, the spacecraft fired the shutters of its wide-angle camera one last time. The result is this stunning full-body view of Saturn and its dazzling rings.

Cassini's last full-view picture of Saturn. Credit: NASA/JPL-Caltech/Space Science Institute.

Cassini’s last full-view picture of Saturn. Credit: NASA/JPL-Caltech/Space Science Institute.

A total of 80 wide-angle images were acquired over two hours. This mosaic view was pieced together from 42 of these shots, which were taken using red, green, and blue spectral filters, which when combined produce a natural-color picture.

“It was hard to say goodbye, but how lucky we were to be able to see it all through Cassini’s eyes!”

Though rather faint, you can spot six of Saturn’s 53 moons in the image — Enceladus, Epimetheus, Janus, Mimas, Pandora, and Prometheus.

“For 37 years, Voyager 1’s last view of Saturn has been, for me, one of the most evocative images ever taken in the exploration of the solar system,” said Carolyn Porco, Cassini imaging team leader.

“In a similar vein, this ‘Farewell to Saturn’ will forevermore serve as a reminder of the dramatic conclusion to that wondrous time humankind spent in intimate study of our sun’s most iconic planetary system,” she added.

Annotated version. Credit: NASA/JPL-Caltech/Space Science Institute.

Annotated version. Credit: NASA/JPL-Caltech/Space Science Institute.

When it took the shot, Cassini was approximately 698,000 miles (1.1 million kilometers) from Saturn, facing the gas giant’s sunlit side about 15 degrees above the ring plane.

NASA’s Cassini-Huygens mission launched in 1997 and took seven years to reach its destination around Saturn. Over the course of its long mission, Cassini’s achievements were legion. Besides landing a freaking probe on Titan, a methane-filled world similar to the early days of Earth before life evolved, Cassini made the most planetary flybys of any spacecraft — over 100. Before arriving in Saturn’s orbit, Cassini circled Earth, Venus, and Jupiter.

“It was all too easy to get used to receiving new images from the Saturn system on a daily basis, seeing new sights, watching things change,” said Elizabeth Turtle, an imaging team associate at the Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland. “It was hard to say goodbye, but how lucky we were to be able to see it all through Cassini’s eyes!”

 

Artist impression of plumes gushing out of Enceladus' south pole. Credit: NASA.

Enceladus’ hidden ocean is kept warm by porous core

Saturn’s icy moon Enceladus is one of the most promising places in the solar system for extraterrestrial life. Buried under miles and miles of ice lies a warm ocean that stretches across the whole body. Recent observations, like those performed by Cassini before it perished, suggest that geysers emanate from hotspots, capable of warming the ocean long enough for some form of life to appear. Now, in a new study, scientists have come just a tad closer to understanding Enceladus’ dynamics after they found evidence that suggests the moon’s core isn’t rocky but rather porous.

Artist impression of plumes gushing out of Enceladus' south pole. Credit: NASA.

Artist impression of plumes gushing out of Enceladus’ south pole. Credit: NASA.

The resourceful Cassini spacecraft explored Saturn and its moon Enceladus for 13 years. A few months back its mission came to an end and NASA engineers instructed the craft to make a suicide jump into Saturn’s atmosphere. NASA thought it’s best to destroy the craft in a controlled fashion then risk having Cassini crash into Enceladus, contaminating the moon in the process.

When Cassini first arrived in Saturn’s system in 2004, NASA scientists marveled when they learned tall geysers were ejecting material hundreds of miles into space from the south pole. Eventually, scientists learned that there is a huge liquid ocean on the little moon and that the tall plumes are made of water-ice mixed with traces of carbon dioxide, ammonia, methane and other hydrocarbons. We also know that the ocean is convecting, meaning it’s active.

Heated from the core

The geysers erupt from cracks present on the moon’s southern polar region. These cracks are known as “tiger stripes” — parallel depressions that are 100km long and 500m deep. According to temperature readings made by Cassini, the tiger stripes are hotter than the rest of the icy crust. So, what’s the heat source?

Scientists are aware that tidal heating can explain some of the heat on the small moon, which is only 241 km (150 miles) in diameter. However, NASA has calculated that the power required to keep the geysers active is in the order of 5GW — enough to power the city of Chicago — and tidal heating can account for just a fraction of that.

In the new study, researchers at the Université de Nantes, France, have accounted for the missing heat. According to their study published in  Nature Astronomy, Enceladus’ tiny core is not solid but rather porous.

The mushy core takes in water from the ocean, which the French researchers calculated it comprises 20% of the core’s mass. The tidal forces associated with the pore water are now more than sufficient to explain how Enceladus’ heat is generated. The researchers are careful to note that the porous core is not really like a sponge but rather more like sand or gravel.

The team led by  Gael Choblet found heat dissipation from the core is not homogeneous, but rather appears as a series of interlinked, narrow upwellings with temperatures in excess of 363K (85°C). The computer model suggests that the hotspots are mainly concentrated at the south pole, in agreement with actual observations. Since the heat is concentrated on just one side of the moon, it’s natural to have enhanced hydrothermal activity which explains the hydrogen in the plumes.

One interesting finding is that the internal tide produces enough heat to warm Enceladus’ ocean for billions of years to come, with important consequences for the prospect of finding extraterrestrial life there. The moon itself is only a hundred million years old. By the most recent estimates, life here on Earth took about 640 million years to appear so if we’re to take this at reference, Enceladus still has a long way to go. Just as well, life might already be presented since the conditions there could be far more hospitable than the hell early Earth must have looked like.

Saturn rings

Cassini is no more, but it left us one of the most memorable photos of Saturn

After twenty years in space, thirteen of which orbiting Saturn and its moons, the enduring Cassini spacecraft finally met its end on September 15, 2017. The trove of images and scientific data it beamed back, however, will keep scientists busy and the general public entertaining for many more years to come. For instance, NASA spoils us with this amazing shot of Saturn and its icy rings — one of the most spectacular we’ve seen so far.

Saturn rings

Credit: NASA/JPL-Caltech/Space Science Institute.

 

The photo was taken just a month before the spacecraft made a suicide plunge into Saturn’s atmosphere. At the time, Cassini was a staggering 600,000 km away from the planet, which, even so, still looks enormous.

Saturn has four main groups of rings and three fainter, narrower ring groups, separated by gaps called divisions. Each ring group is made up of thousands of smaller rings made of ice and debris, in some places no thicker than a few meters. In between Saturn’s cloud tops and the innermost D ring, the spacecraft passed through 22 times before it ended its mission. Before Cassini’s plunges were routed, scientists spent countless hours analyzing such images in search for debris that might prove hazardous to the spacecraft.

Remarkably, besides Saturn and some of its ring groups, the image also features an outlier: Saturn’s moon Pandora. Discovered in October 1980 by Voyager 1, the potato-shaped moon has 25.3 miles (40.7 km) mean radius. In this particular image, seen in the top-right corner, it barely measures more than a single pixel.

The image was taken in  with the spacecraft’s wide-angle camera. To increase visibility, Pandora was brightened by a factor of 2.

Cassini's best close-up view of Saturn's F ring shepherd moon, Pandora, taken in December 2016. Credit: NASA.

Cassini’s best close-up view of Saturn’s F ring shepherd moon, Pandora, taken in December 2016. Credit: NASA.

Over the course of its long mission, Cassini’s achievements were legion. Besides landing a probe on Titan, a methane-filled world similar to the early days of Earth before life evolved, Cassini made the most planetary flybys of any spacecraft — over 100. Before arriving in Saturn’s orbit, Cassini circled Earth, Venus, and Jupiter.

goodbye-cassini

Cassini’s farewell photo of Saturn’s dark side

goodbye-cassini

Credit: NASA/JPL-Caltech/Space Science Institute

Last month, the formidable Cassini spacecraft ended its 20-year-mission with a nose dive into Saturn’s atmosphere. NASA’s Cassini-Huygens mission launched in 1997 and took seven years to reach its destination around Saturn and its 53 moons. Over the course of its long mission, Cassini’s achievements were legion. Besides landing a freaking probe on Titan, a methane-filled world similar to the early days of Earth before life evolved, Cassini made the most planetary flybys any man-made craft ever has — over 100. Before arriving in Saturn’s orbit, Cassini circled Earth, Venus, and Jupiter.

It was over the course of these flybys that Cassini took some of the most breath-taking pictures of Saturn. Now, NASA has another one worthy of Cassini’s ‘best of’ collection — a unique view of Saturn’s dark side.

Because Earth orbits much closer to the sun than Saturn, ground-based telescopes were never able to observe the gas giant’s dark side. As such, this image would have never been possible were it not for the Cassini spacecraft.

The picture was taken on 7 June 2017, with the spacecraft’s onboard wide-angle camera. At the time, Cassini was about 1.21 million kilometers from Saturn, facing the sun-lit side of the rings seven degrees above the plane of rings.

Though the spacecraft itself is toast, the mission will live on for many years. During its 20-year trip, Cassini has beamed back a trove of data and images which will keep scientists busy for a long time. Perhaps, the mission’s most important contributions to science are still a work in progress.

cassini-spacecraft

Cassini spacecraft ends 20-year-old voyage in style, crashes into Saturn

Earlier this month, NASA’s Cassini probe embarked on a death spiral around Saturn’s orbit. Now, on Friday morning, September 15, scientists reported that they have received the last-ever message relayed by the probe, shortly before in plunged into the gas giant’s atmosphere.

cassini-spacecraft

Illustration of Cassini spacecraft prepping to dive into Saturn’s atmosphere. Credit: NASA.

This was the climax of a 20-year mission which was the $4-billion probe travel over two billion miles to Saturn. The collaborative mission between NASA, the European Space Agency and the Italian Space Agency aimed to study Saturn and its moons from close up and in the process learn more about the solar system and how it was formed.

Cassini-Huygens launched in 1997 and took seven years to reach its destination around Saturn and its 53 moonsThe Huygens probe detached from the Cassini spacecraft in 2005 when it landed on Titan, Saturn’s largest moon. “This was humanity’s first successful attempt to land a probe on another world in the outer solar system,” the ESA says.

Over the course of its long mission, Cassini’s achievements were legion. Besides landing a freaking probe on Titan, a methane-filled world similar to the early days of Earth before life evolved, Cassini made the most planetary flybys any man-made craft ever has — over 100. Before arriving in Saturn’s orbit, Cassini circled Earth, Venus, and Jupiter. It was during these flybys that the most detailed true color photos of the gas giant ever recorded were beamed back to Earth.

Amazing real-life photo of Jupiter captured by Cassini in 2000. Credit: NASA.

Amazing real-life photo of Jupiter captured by Cassini in 2000. Credit: NASA.

Like a true explorer, Cassini has found hidden ‘lands and treatures’. No fewer than seven moons orbiting Saturn were identified by the spacecraft.  These include Methone, Pallene, Polydeuces, Daphnis, Anthe and Aegaeon.

The most important findings, however, were those considering Saturn’s icy moon Enceladus. During the spacecraft’s frequent flybys of the icy moon thought to host a hidden ocean of liquid water beneath hundreds of miles of ice, scientists found hints of conditions favorable for microbial life. NASA claims Enceladus has “water, organic carbon, nitrogen [in the form of ammonia], and an energy source,” which no other environment besides Earth can boast.

“As we continue to learn more about Enceladus, and compare data from different instruments, we are finding more and more evidence for a habitable ocean world,” Linda Spilker, Cassini Project Scientist, told NASA. “If life is eventually discovered in Enceladus’ ocean by a mission after Cassini, then our Enceladus discoveries will have been among the top discoveries for all planetary missions.”

After two decades of interplanetary travel, however, Cassini finally ran out of fuel. It still had enough left to power its boosters for another few years but NASA mission engineers didn’t want to take the risk of an uncontrolled landing on Titan or Enceladus. Instead, NASA seized the opportunity to plunge the spacecraft into Saturn’s atmosphere. For one last time, Cassini spread its wings and used its instruments to sample Saturn’s atmosphere. This information will teach us new things about a totally alien environment. A few minutes later, Cassini vaporized bellowing a ‘scream’ that took 83 minutes to reach Earth. It was then that the mission control announced ‘the end of the mission’ in a burst of applause, commemorating one of the most successful space missions in history.

We’re not done yet

Despite 13 years in Saturn’s orbit, there are still many unanswered questions. It’s still not clear how long a Saturnian day lasts and the planet’s magnetic field seems to behave capriciously. And though Cassini made extremely exciting discoveries that signal potentially habitable conditions on Enceladus, we still need more data before we can come to any sensible conclusions.

“We’ve left the world informed, but still wondering,” Cassini-Huygens program manager Earl Maize told reporters at a press conference days before Cassini’s suicidal death plunge. “As a scientist, I couldn’t ask for more.”

After all this mission, there’s a single conclusion we can draw: we have to come back. One thing’s for sure, Cassini didn’t perish in vain.

Yup, that's Earth as seen by Cassini from 1.44 billion kilometres away. Is this worth your tax dollar? YES!

Yup, that’s Earth as seen by Cassini from 1.44 billion kilometres away. Is this worth your tax dollar? YES! Credit: NASA/JPL.

This photo illustration shows selected moons of our solar system at their correct relative sizes to each other and to Earth. Pictured are Earth's Moon; Jupiter's Callisto, Ganymede, Io and Europa; Saturn's Iapetus, Enceladus, Titan, Rhea, Mimas, Dione and Tethys; Neptune's Triton; Uranus' Miranda, Titania and Oberon and Pluto's Charon. Credit: NASA.

What are the moons of the solar system and how many are there

This photo illustration shows selected moons of our solar system at their correct relative sizes to each other and to Earth. Pictured are Earth's Moon; Jupiter's Callisto, Ganymede, Io and Europa; Saturn's Iapetus, Enceladus, Titan, Rhea, Mimas, Dione and Tethys; Neptune's Triton; Uranus' Miranda, Titania and Oberon and Pluto's Charon. Credit: NASA.

This is an illustration of some of the most significant moons of our solar system at their correct relative sizes to each other and to Earth. Pictured are Earth’s Moon; Jupiter’s Callisto, Ganymede, Io and Europa; Saturn’s Iapetus, Enceladus, Titan, Rhea, Mimas, Dione and Tethys; Neptune’s Triton; Uranus’ Miranda, Titania and Oberon and Pluto’s Charon. Credit: NASA.

Since the dawn of mankind, across all cultures, we’ve always had two muses: the sun and the moon. We’ve sung epics, made art, even performed blood sacrifice, all in their name. But just like we now know the sun isn’t unique — rather one among billions and billions of stars — so is our lunar muse actually not all that special. Throughout the solar system, there are 187 known moons or 196 if you count those belonging to the dwarf planets.

Considering all of this, you might feel cheated. All of those moons, and we only got one? Well, some planets are more fortunate than others, but at least we’ve got something. The terrestrial planets, Mars, Earth, Venus, and Mercury only have three moons between them. Venus and Mercury have none while Earth has the Moon and Mars has Phobos and Deimos. However, it’s the Jovian planets that are teeming with moons. At the latest count, Jupiter has 81, Saturn has over 60, Uranus has 27, and Neptune has 14.

Why are terrestrial planets so sparsely populated with moons, in stark contrast to the Jovian planets? There are many reasons but the primary one has to do with the sun’s gravitational tug. Mercury and Venus, the two closest planets to the sun in the solar system, simply have a too weak gravitational pull to grab a passing object and keep it in its orbit. Likewise, these planets can never hold enough debris rings in orbit to eventually coalescence into a natural satellite. Earth and Mars managed to do it, but they’re the outermost planets in the inner solar system. The farther from the sun you are, the easier it is to capture or maintain a satellite.

Now, let’s have a look at some of the most notable moons in the solar system.

Moons of the inner solar system

The Moon

The Moon.

Credit: Pixabay.

Closest to home and our hearts, the moon is one of the biggest in the solar system, with a radius of 1,737 km. With a density of 3.3464 g/cm³, it’s also the second densest moon in the solar system after Jupiter’s Io. The moon only has 0.273 of the Earth’s size and 0.0123 of its mass.

It formed in the aftermath of a giant impact between proto-Earth and a planetary-sized body we call Theia. The giant impact hypothesis was first laid on the table in the mid-1970s when astrophysicists proposed the moon was formed by a grazing collision between the proto-Earth and a Mars-sized body called Theia. This eventually became the leading hypothesis that explained how our sole natural satellite came to be. But then in 2001 scientists reported that the isotopic compositions of a variety of elements collected from both terrestrial and lunar rocks are nearly identical.

An influential paper published in 2016 in Nature showed that lunar rocks are enriched by about 0.4 parts per thousand in the heavier isotope, potassium-41. The only process that would lead to this sort of event, say the researchers, is incomplete condensation of the potassium from the vapor phase during the moon’s formation. In other words, the impact must have completely vaporized both planets, and from that mush of debris, a new Earth and what we now know as the Moon formed.

The moon has a 5° tilt to the plane of Earth’s orbit around the sun. As a result, from our viewpoint on Earth, the moon normally passes either above or below the sun each month at new moon. In late 2015, two planetary scientists – Kaveh Pahlevan and Alessandro Morbidelli – published a paper in which they explain how the moon got this tilt. According to their simulations near-misses between the Earth-moon system and large objects like asteroids gravitationally jostled the moon into a tilted orbit.

Another recent insight into the moon’s geology and history suggests its interior holds a lot of water. With the help of data from India’s Chandrayaan-1 spacecraft that was in lunar orbit from 2008-2009, in 2017, a team at Brown University found lots of water encased in the moon’s mantle. All of this water is, of course, not liquid but rather embedded in the rocky material akin to the water trapped within Earth’s mantle. This study is quite important in relation to the Theia Impact hypothesis. It suggests that some of the vaporized water survived or not all of it drifted into space. Alternatively, the water could have been delivered later by asteroids.

Phobos and Deimos

phobos_deimos

Credit: NASA.

These are Mars’ two moons. Phobos, whose name comes from the Greek phobia (fear), is the larger of the two moons and has the closest orbit to Mars. It’s only 22.7 km across, though, which explains its irregular shape (bigger bodies become naturally spherical due to pressure exerted by gravity). While our moon orbits the Earth at a distance of 384,403 km, Phobos is only 9,377 km above Mars.

Deimos, which in Greek mythology is the twin brother of Phobos and personified terror, is Mars’ second moon. It’s much smaller than Phobos, measuring just 12.6 km across and also orbits its parent planet much farther away than Phobos. At a distance of 23,460 km, Deimos takes 30.35 hours to complete an orbit around Mars.

Moons of the outer solar system

The Galilean moons: Io, Europa, Ganymede, Callisto

Galileo Galilei discovered the first four Jovian moons in the 17th century cementing the Copernicus model of a heliocentric system. Credit: YouTube capture.

Galileo Galilei discovered the first four Jovian moons in the 17th century cementing the Copernicus model of a heliocentric system. Credit: YouTube capture.

Jupiter has more natural satellites than any other planet in the solar system. By the latest count, there are 81 Jovian moons, the last two being officially recognized in June 2017. Called S/2016 J1 and S/2017 J1, these two moons barely measure 1-2 km across. UPDATE 18 july 2018: Astronomers found 12 new Jovian moons. 

By far, the most important Jovian moons are the so-called Galilean moons, in honor of  Galileo Galilei who discovered them in 1610. At the time, writing in Sidereus Nuncius, Galilei asserted that the four observations were planetary bodies. Nevertheless, the findings proved extremely influential for a time when the Copernican system was still out of favor. Galileo’s discoveries brought important evidence to support the idea that not everything revolved around the Earth.

“I should disclose and publish to the world the occasion of discovering and observing four Planets, never seen from the beginning of the world up to our own times, their positions, and the observations made during the last two months about their movements and their changes of magnitude; and I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day . . . On the 7th day of January in the present year, 1610, in the first hour of the following night, when I was viewing the constellations of the heavons through a telescope, the planet Jupiter presented itself to my view, and as I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, namely that three little stars, small but very bright, were near the planet; and although I believed them to belong to a number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic, and to be brighter than the rest of the stars, equal to them in magnitude . . .When on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for there were three little stars all west of Jupiter, and nearer together than on the previous night.”

“I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun; which was at length established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions around Jupiter,” Galilei wrote on March 1610 in Sidereus Nuncius. 

Io, Europa, Ganymede, and Callisto are the solar system’s fourth, sixth, first, and third largest satellites, respectively. Though they’re just 4 of 69 known satellites, they collectively sum 99.999 percent of the total mass orbiting Jupiter, including the ring system.

Io

True color image of Jupiter’s moon Io made by the Galileo spacecraft. Credit: NASA/JPL/University of Arizona

True color image of Jupiter’s moon Io made by the Galileo spacecraft. Credit: NASA/JPL/University of Arizona

Io is the innermost Galilean moon and fourth-largest moon in the solar system, standing 3,642 km in diameter. Like all other moons in the solar system, its name comes from Greek mythology after a priestess that served Hera and later became Zeus’ lover.

The mountain rises 8.6 kilometers, or roughly 5 miles, above the volcanic plain. Io is home to some of the highest mountains in the solar system, including some that tower 10 miles high, far higher than any mountain on Earth. Credit: NASA/JPL/University of Arizona

The mountain rises 8.6 kilometers, or roughly 5 miles, above the volcanic plain. Io is home to some of the highest mountains in the solar system, including some that tower 10 miles high, far higher than any mountain on Earth. Credit: NASA/JPL/University of Arizona

What’s particularly interesting about Io is that it’s very geologically active. The interior of Io is continuously heated, which leads to many active volcanoes on its surface — around 400 active volcanoes by the most recent count. The moon’s surface is also dotted by more than 100 mounts, some taller than Mount Everest. 

Europa

Jupiter's icy moon Europa. Credit: NASA.

Jupiter’s icy moon Europa. Credit: NASA.

Europa is the second innermost moon of Jupiter. It’s named after the mythical Phoenician noblewoman who was courted by Zeus and became the queen of Crete. It’s the smallest of all the Galilean moons, at just 3121.6 kilometers in diameter. Don’t let its size fool you, though.

Many scientists believe that Europa is the best place in the solar system to look for alien life, which might stay hidden beneath the moon’s blanket of ice.

Europa’s surface temperature at the equator never rises above minus -160 degrees Celsius (-260 degrees Fahrenheit). At the poles of the moon, the temperature never rises above -220 C (-370 F). The subsurface, however, is a whole different story. Beneath the dozens of miles of thick ice, scientists think Earth-like salty and liquid oceans can be found.

“Europa’s ocean is considered to be one of the most promising places that could potentially harbor life in the solar system,” said Geoff Yoder, acting associate administrator for NASA’s Science Mission Directorate in Washington. “These plumes, if they do indeed exist, may provide another way to sample Europa’s subsurface.”

The plumes, which can rise 200km above the Europa’s surface and which Yoder was referring to, were first observed jetting from the moon’s surface in 2013. In April 2017, NASA announced the discovery of hydrogen molecules in the plumes spewing off one of Saturn’s icy moons called Enceladus. This suggests that there are hot spots hidden beneath the ocean which can provide enough energy for life to both appear and thrive, given how common hydrothermal vents or underwater geysers are on Earth’s ocean floor.  A similar plume was identified gushing out of Europa, suggesting similar things occur as on Enceladus.

Whether or not life exists on Europa might not be a rhetorical matter. NASA plans on sending three different instrument suites which will collect samples and analyze them, looking for native life forms. It’s not clear when this will happen.

Besides Earth, Europa seems to be the only other place in the solar system that has plate tectonics.

Ganymede

Ganymede is Jupiter's largest moon and also the largest moon in the solar system. Credit: Wikimedia Commons.

Ganymede is Jupiter’s largest moon and also the largest moon in the solar system. Credit: Wikimedia Commons.

This moon is the largest moon in the entire Solar System. At 5262.4 kilometers in diameter, it’s actually bigger than the planet Mercury, though it has only half its mass being an icy world much like Europa.

Another notable distinction is that Ganymede has a magnetosphere, likely created through convection within the liquid iron core. Some scientists argue that the presence of a magnetic field is indicative of a subsurface ocean on the moon.

Ganymede is characterized by a mix of smooth, dark regions dotted with craters but also lighter regions where deep grooves are visible.

It’s also striking that this moon has an oxygen atmosphere that includes O, O2, and possibly O3 (ozone), and some atomic hydrogen.

Callisto

The crater-riddled Callisto. Credit: Wikimedia Commons.

The crater-riddled Callisto. Credit: Wikimedia Commons.

It’s the fourth and farthest Galilean moon. At 4820.6 kilometers in diameter, it is also the second largest of the Galileans and third largest moon in the Solar System.

The moon is named after the daughter of the Arkadian King, Lykaon, and a hunting companion of the goddess Artemis.

Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameter. Credit: Wikimedia Commons.

Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameter. Credit: Wikimedia Commons.

Callisto is the most similar satellite to our own moon — heavy cratered and mostly dark in appearance. Since it’s so littered with craters, this tells us that it must one of the oldest among the Galilean moons. One crater, in particular, has a diameter in the order of several thousand kilometers suggesting a massive impact occurred sometime in Callisto’s history. In fact, it’s very surprising the moon survived the ordeal intact. Again, observations suggest this satellite might have a subsurface ocean.

Saturn’s moons

Cassini delivers this stunning vista showing small, battered Epimetheus and smog-enshrouded Titan, with Saturn's A and F rings stretching across the scene. Credit: NASA.

Cassini delivers this stunning vista showing small, battered Epimetheus and smog-enshrouded Titan, with Saturn’s A and F rings stretching across the scene. Credit: NASA.

Like Jupiter, Saturn’s orbit is packed with moons — some 150 moons and moonlets. Most, however, are very small with a high fraction ranging from less than 10 km in diameter to between 10 and 50 km in diameter. Saturn does have a couple of very large moons, all named after the Titans of Greek mythology.

The biggest and most important moon is Titan, discovered by Christiaan Huygens in 1655. Like Ganymede, Titan is bigger than the planet Mercury, at 5150 km in diameter. It also comprises 96% of the mass in orbit around the planet.

Titan is also one of the few moons in the solar system to support an atmosphere. It’s thick, cold, and primarily composed of nitrogen with some methane — quite similar to what we call smog here on Earth.

The dense haze that covers the planet has always made it difficult to observe Titan and for a very long time, we knew nothing about what its surface looks like. Hubble Space Telescope images, as well as those from the Cassini satellite mission to Saturn, have finally allowed astronomers to penetrate the hazy atmosphere of Titan, revealing that it has surface features.

Cassini radar map of Titan's surface. Credit: Cassini Radar Mapper, JPL, ESA, NASA.

Cassini radar map of Titan’s surface. Credit: Cassini Radar Mapper, JPL, ESA, NASA.

We’ve learned, for instance, that Titan is the only place in the solar system other than Earth with liquids on its surface. It’s not water though. Instead, images beamed back by the Huygens lander from the beginning of 2005 suggest Titan is covered rivers and lakes that appear to have contained liquid methane-ethane.

Other notable Saturnian moons include Iapetus (1671), Rhea (1672), Dione (1684), and Tethys (1684) — all discovered by Giovanni Domenico Cassini — and Mimas (1789) and Enceladus (1789), discovered by William Herschel.

Enceladus displays evidence of active ice volcanism: Cassini observed warm fractures where evaporating ice evidently escapes and forms a huge cloud of water vapor over the south pole, and as mentioned earlier, tall plumes gushing out of Enceladus suggest there might be a chance of finding life there.

Sixteen of Saturn’s moons keep the same face toward the planet as they orbit. Called “tidal locking,” this is the same phenomenon that keeps our Moon always facing toward Earth.

Uranus’ moons

Uranus and its five major moons are depicted in this montage of images acquired by the Voyager 2 spacecraft. The moons, from largest to smallest as they appear here, are Ariel, Miranda, Titania, Oberon and Umbriel. Credit: NASA/JPL

Uranus and its five major moons are depicted in this montage of images acquired by the Voyager 2 spacecraft. The moons, from largest to smallest as they appear here, are Ariel, Miranda, Titania, Oberon and Umbriel. Credit: NASA/JPL

Uranus has 27 moons that astronomers know of. Like all other gas giants, Uranus has many small satellites but also a couple of large moons like Miranda, Ariel, Umbriel, Oberon, and Titania, in this very order by size. Their size range varies from 472 km for Miranda to 1,578 km for Titania. Ariel is the brightest while Umbriel is the darkest but all of Uranus’ moons are dark overall. Most are comprised of rock and ice with the notable exception of Miranda which is primarily ice. We’re not talking about water ice, though. The components may include ammonia and carbon dioxide.

Scientists reckon all of these larger moons formed out of the accretion disk which once gravitated around the planet. Alternatively, the material that seeded from the moons could have been debris following a major impact early in the history of Uranus.

Neptune’s moons

A real picture of the peculiar world of Triton taken on October 10, 1999 by Voyager 2. Credit: NASA.

A real picture of the peculiar world of Triton taken on October 10, 1999 by Voyager 2. Credit: NASA.

Neptune has 14 satellites, all of which are aptly named after Greek and Roman deities of the sea. Based on their orbit and proximity to Neptune, these can be divided into two main groups: the regular and irregular moons.

Neptune’s regular moons are Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, and S/2004 N 1 (the only Neptune moon missing a proper name). Neptune’s irregular moons consist of the remaining satellites, including Triton — a strange satellite and the first Neptune moon discovered. William Lassell discovered Triton on October 10th, 1846 while he was attempting to confirm his observation, made just the previous week, that Neptune had a ring.

All of Neptune’s moons, with the exception of Triton which orbits close to the planet, have retrograde and eccentric orbits far away from the planet. In any event, not much is known about satellites belonging to both Uranus and Neptune. No spacecraft has come near them ever since Voyager 2,  which is now close to exiting the solar system.

Pluto and the Kuiper Belt

Pluto-moons

Pluto and its five moons from a perspective looking away from the sun. Approaching the system, the outermost moon is Hydra, seen in the bottom left corner. The other moons are scaled to the sizes they would appear from this perspective. Credit: NASA/M. Showalter.

Pluto is by far the most difficult of the nine historical planets to study. I say historical because Pluto has been demoted to a dwarf planet status in August 2006 by the International Astronomical Union (IAU). But the in July 2015, NASA’s New Horizons missions performed the first spacecraft flyby of Pluto revealing much about it and its surrounding moons.

Pluto has five moons that we know of, four of which have been discovered only in the last decade. These are Charon, Hydra, Nix, Styx, and Kerberos — named after creatures and places from Pluto’s underworld.

Pluto’s system of moons is quite peculiar in many ways. Pluto’s four small moons — Styx, Nix, Kerberos and Hydra — follow near-circular, near-equatorial orbits around the central ‘binary planet’ comprising Pluto and its large moon, Charon. This has prompted some astronomers to call Pluto and Charon a ‘binary-dwarf system’.

This composite image from the Hubble Space Telescope shows Pluto and its largest moon, Charon, at the center. Pluto's four smaller moons orbit this 'binary planet' and can be seen to the right and left. The smaller moons must be imaged with 1000 times longer exposure times because they are far dimmer than Pluto and Charon. Credit: NASA/M. Showalter.

This composite image from the Hubble Space Telescope shows Pluto and its largest moon, Charon, at the center. Pluto’s four smaller moons orbit this ‘binary planet’ and can be seen to the right and left. The smaller moons must be imaged with 1000 times longer exposure times because they are far dimmer than Pluto and Charon. Credit: NASA/M. Showalter.

What’s more, Charon and Pluton are tidally locked to the other meaning the two always present the same face to each other, like the Earth and the Moon. From any position on either of the two bodies, the other is always at the same position in the sky or always obscured. Mathematically, this entails that the each body’s rotational period is equal to the time it takes the entire binary dwarf system to rotate around its common center of gravity.

Styx, Nix, and Hydra are tied together by a three-body resonance, so any chaotic movement exhibited by one of them will spread the chaos further to the others.

“It’s not just a little bit chaotic,” Mark Showalter from the New Horizons team said. “Nix can flip its entire pole. It could actually be possible to spend a day on Nix in which the sun rises in the east and sets in the north. It is almost random-looking in the way it rotates.”

There are four other dwarf planets in the solar system that we know of besides Pluto. These are Eris, Makemake, Haumea, and Ceres, which can be found well beyond Neptune into the outer limits of the solar system. Each has one or a couple of moons. Haumea has two moons, Hi’iaka and Namaka. Eris has one moon called Dysnomia, which is named after the daughter of Eris in Greek mythology. Finally, last year in 2016, astronomers confirmed that Makemake has at least one moon, for now, designated S/2015 (136472).

Bonus: the first moon outside our solar system

Just earlier this week, a team led by David Kipping of Columbia University announced exciting data that suggests they’ve found the first ever exomoon. The statistical confidence that the observed signal is real and not the result of some random aberration was 99.999%. So that’s about as sure as you get but in science, things are almost never certain. The researchers will have the chance to find out for sure once the Hubble Space Telescope directs its lens on the star in question, located some 4,000 light-years away from Earth. If this is confirmed, it will be the first moon we’ve ever seen outside our solar system.

This Cassini image features a density wave in Saturn's A ring (at left) that lies around 134,500 km from Saturn. Density waves are accumulations of particles at certain distances from the planet. This feature is filled with clumpy perturbations, which researchers informally refer to as "straw." The wave itself is created by the gravity of the moons Janus and Epimetheus, which share the same orbit around Saturn. Elsewhere, the scene is dominated by "wakes" from a recent pass of the ring moon Pan. Credit: NASA

Amazing pictures of Saturn’s rings up close and personal

This Cassini image features a density wave in Saturn's A ring (at left) that lies around 134,500 km from Saturn. Density waves are accumulations of particles at certain distances from the planet. This feature is filled with clumpy perturbations, which researchers informally refer to as "straw." The wave itself is created by the gravity of the moons Janus and Epimetheus, which share the same orbit around Saturn. Elsewhere, the scene is dominated by "wakes" from a recent pass of the ring moon Pan. Credit: NASA

This Cassini image features a density wave in Saturn’s A ring (at left) that lies around 134,500 km from Saturn. Density waves are accumulations of particles at certain distances from the planet. This feature is filled with clumpy perturbations, which researchers informally refer to as “straw.” The wave itself is created by the gravity of the moons Janus and Epimetheus, which share the same orbit around Saturn. Elsewhere, the scene is dominated by “wakes” from a recent pass of the ring moon Pan. Credit: NASA

This week the folks at NASA treated us with some of the finest eye candy there is. Some of the closest pictures of Saturn’s rings were beamed back by the Cassini probe on December 18, 2016 with incredible detail. Some of the features pictured are as small as 0.3 miles (550 meters) across, a stunning thing when you realize the images were taken 1.2 billion miles away.

A region in Saturn's A ring. The level of detail is twice as high as this part of the rings has ever been seen before. The view contains many small, bright blemishes due to cosmic rays and charged particle radiation near the planet. Credit: NASA.

A region in Saturn’s A ring. The level of detail is twice as high as this part of the rings has ever been seen before. The view contains many small, bright blemishes due to cosmic rays and charged particle radiation near the planet. Credit: NASA.

“As the person who planned those initial orbit-insertion ring images — which remained our most detailed views of the rings for the past 13 years — I am taken aback by how vastly improved are the details in this new collection,” said Cassini Imaging Team Lead Carolyn Porco, of Space Science Institute, Boulder, Colorado, in a statement. “How fitting it is that we should go out with the best views of Saturn’s rings we’ve ever collected.”

This image shows a region in Saturn's outer B ring. NASA's Cassini spacecraft viewed this area at a level of detail twice as high as it had ever been observed before. And from this view, it is clear that there are still finer details to uncover. Credit: NASA.

This image shows a region in Saturn’s outer B ring. NASA’s Cassini spacecraft viewed this area at a level of detail twice as high as it had ever been observed before. And from this view, it is clear that there are still finer details to uncover. Credit: NASA.

Though it might seem like it, Saturn’s rings are not actually solid disks. Instead, these are comprised of countless ice particles and thanks to the gravitational effect of the planet’s moons, these icy chunks congregate in rings.

The view here is of the outer edge of the B ring, at left, which is perturbed by the most powerful gravitational resonance in the rings: the "2:1 resonance" with the icy moon Mimas. This means that, for every single orbit of Mimas, the ring particles at this specific distance from Saturn orbit the planet twice. This results in a regular tugging force that perturbs the particles in this location. Credit: NASA.

The view here is of the outer edge of the B ring, at left, which is perturbed by the most powerful gravitational resonance in the rings: the “2:1 resonance” with the icy moon Mimas. This means that, for every single orbit of Mimas, the ring particles at this specific distance from Saturn orbit the planet twice. This results in a regular tugging force that perturbs the particles in this location. Credit: NASA.

Cassini has been orbiting Saturn for nearly 12 years. We owe much to this spacecraft and the dedicated staff that operate the mission along the years. Sadly, Cassini’s voyage is soon to come to an end in April 2017 when the craft is scheduled to plunge through the gap between the rings and Saturn itself.