Tag Archives: moons

Why does Jupiter have so many darn moons?

Credit: Pixabay.

Jupiter is monstrously large, so much so that it is more than twice as massive as all other planets in the solar system combined. Consequently, it has an equally massive gravitational pull that helped it capture a myriad of satellites.

By the latest count, the gas giant has 79 confirmed moons. That’s two moons shy of Saturn’s count of 82 confirmed moons. But the race is still on for which planet has the largest entourage since astronomers keep constantly discovering new ones.

Saturn stole the crown from Jupiter in 2019 when astronomers discovered 20 new moons. But Jupiter isn’t done for — far from it. Twelve new satellites were discovered in 2017 alone around Jupiter by researchers affiliated with the Carnegie Institution for Science thanks to recent advances in large digital cameras and astronomical techniques. What’s more, there are good reasons to believe that Jupiter has, by far, the largest number of moons in the entire solar system.

Orbits of Jupiter’s 71 irregular satellites as of 1 January 2021, each of them labeled. Credit: Nrco0e / Wikimedia Common.

In September 2020, astronomers from the University of British Columbia identified 45 candidate moons with a diameter of over 800 meters. The researchers only surveyed a tiny area of the sky though and when they extrapolated their data, they concluded that there could be more than 600 of these tiny moons orbiting Jupiter.

These candidates are currently under investigation, which will take a lot of time since it takes quite a lot of telescope time to reliably verify their orbits. But while scientists are kept busy with cataloging huge boulders zipping across Jupiter’s motion, it’s perhaps a good time to find out why Jupiter has many moons. What’s so special about it?

Thanks for the free lunch, sucker!

The reason why Jupiter is such a moon magnet has a lot to do with its stupendous mass, equal to more than 300 Earths. Earth’s one and only Moon likely formed billions of years ago after a huge proto-planet slammed into primordial Earth. But most moons, especially around gas giants, don’t have such an exciting history. Like a school bully, Jupiter and other gargantuan planets like it are in the habit of ‘capturing’ rocky objects ranging from smallish asteroids to full-fledged mini-planets with volcanic activity, such as Io.

Jupiter, as well as other distant gas giants in the solar system such as Saturn and Neptune, has another ace up its sleeve. Not only does it have a very strong gravitational pull thanks to its mass, but it is also quite far away from the Sun. It’s about 5 times farther away from the Sun than Earth is, completing a full orbit every 11.86 years.

This great distance allows Jupiter to exert a larger area of influence or control as the Sun’s gravitational influence weakens the farther away you travel from it. With such a wide net cast, it’s no wonder that Jupiter has moons orbiting it as far away as 23.5 million miles, as is the case for Pasiphae and Sinope. Meanwhile, Venus and Mercury, the two closest planets to the sun in the solar system, have no moons at all, while Earth has a measly one to speak of and Mars as two tiny satellites.

Also important, albeit to a lesser degree, is the gas giant’s shape. A cosmic body that is regularly round will have a more stable orbit than a potato-shaped one. Jupiter is almost perfectly round and that may have helped it capture some additional small satellites, especially in its lower orbit.

Big rocks around a gas giant

Montage of Jupiter and the Galilean satellites, Io, Europa, Ganymede, and Callisto, all photographed by Voyager 1.

Of Jupiter’s many moons, collectively known as Jovian moons, the four largest particularly stand out. These moons — Callisto, Io, Europan, and Ganymede — are often called Galilean moons in honor of Galileo Galilei, the Italian astronomer who first discovered them in 1610. The moons themselves are each named after lovers and favorites of the god Jupiter (Zeus); and since 2004, also after their descendants. In the future, however, astronomers may run out of mythological beings to name Jovian moons by.

Three of these four moons are larger than Earth’s Moon and one, Ganymede, is the largest moon in the solar system. In fact, all you need is a pair of good binoculars or a retail telescope to see all four of these largest moons of Jupiter, which are all at least 3,100 kilometers (1,900 mi) in diameter.

Although the four Galilean moons comprise a small proportion of Jupiter’s of 79 confirmed satellites, they collectively sum 99.999% of the total mass orbiting Jupiter, including the ring system. The Galilean moons are also a lot more quirky than their more puny Jovian cousins. For instance, Io is packed with active volcanoes and Europa may harbor life in a liquid ocean covered by thick ice.

All other Jovian moons are less than 250 kilometers (160 miles) in diameter, although most barely exceed 5 kilometers (3.1 miles).

While the Galilean moons are believed to have formed along with Jupiter from its circumplanetary disk — a ring of gas and dust — the outer, irregular moons are believed to have originated from captured asteroids.

And over the course of its long history, Jupiter likely harbored many other moons, which are now long gone. Some were destroyed by mechanical fracturing during the capture or during collisions with other asteroid-like objects, some simply drifted away out of Jupiter’s clutches. You win some, you lose some.

In the dark and gloomy part of the solar system that Jupiter calls home, there is an abundance of large asteroids (with a diameter greater than one kilometer), so it’s reasonable to believe the number of Jovian moons could swell in the future. But in the meantime, scientists are still busy identifying the gas giant’s existing moons. It may not be long before Jupiter reclaims its crown, rising once again to the top as the planet with the most moons.

Mars’ two moons used to be a single moon, says new research

Today, the red planet boasts two moons — Phobos and Deimos. Although we discovered them almost two centuries ago, their story was still a mystery. Both moons are very small and rough-shaped, which just isn’t very moon-y. A new paper suggests this is due to the fact that they aren’t really moons, but the remnants of a one.

An image of Phobos taken by Mars Express HRSC during a close flyby, April 30, 2017. Image credits ESA / DLR / FU Berlin /J. Cowart.

Phobos, at only 22 kilometers in diameter, is really small — around 160 times smaller than our moon. Deimos is even tinier, with a diameter of only 12 kilometers. Furthermore, while the Moon is pretty much spherical, both of Mars’ are shaped very irregularly. You could describe them as potato-shaped quite accurately. In order to understand why, a team of researchers at the Institute of Geophysics at ETH Zurich used computer simulations to track the moons’ orbits back through time.

At one point, these simulations revealed, the two were likely a single body.

A cosmic breakup

“Our moon is essentially spherical, while the moons of Mars are very irregularly shaped—like potatoes,” says Amirhossein Bagheri, a doctoral student at the Institute of Geophysics at ETH Zurich.

“Phobos and Deimos look more like asteroids than natural moons.”

The first theory the team started working with is that the two are in fact asteroids that were captured by Mars’ gravity field sometime in the past. Still, this didn’t really pan out. Captured objects should, as far as we know, have eccentric orbits at random inclinations around their capturer — but neither Phobos nor Deimos does. They actually have almost circular orbits on the equatorial plane of Mars.

So they set out to simulate how their orbits changed over time, hopeful to get to the bottom of things. The simulation showed that if we go back in time far enough, Phobos and Deimos come to share the same orbit. This, the team says, likely means they’re both pieces of a larger, original body. In Greek mythology, Phobos (fear) and Deimos (terror) are the twin sons of Ares, the god of war. Given that the planet was named after the Roman equivalent of Ares — Mars, — this origin story seems quite fitting.

Still, I have massively simplified the work needed to reach these results. The team had to improve on what we know of the interactions between Mars, Phobos, and Deimos. Khan explains that these bodies all exert tidal forces on one another, leading to “a form of energy conversion known as dissipation, the scale of which depends on the bodies’ size, their interior composition and not least the distances between them”.

Luckily, NASA’s InSight mission is hard at work around Mars, and ETH Zurich supplied the electronics for the mission’s seismometer. With the data it supplied, the team refined our interior models of Mars, which in turn allowed them to improve on the equations describing this process of dissipation.

Data obtained from other instruments on other Martian craft also suggested that both moons were made of a very porous (and thus, lightweight) material. The team explains that this is true — their average densities, both at under 2 grams per cubic centimeter, are around half that of Earth (5,5 grams per cubic centimeter, on average).

“There are a lot of cavities inside Phobos, which might contain water ice,” says Amir Khan, a Senior Scientist at the Physics Institute of the University of Zurich and the Institute of Geophysics at ETH Zurich and co-author of the paper, “and that’s where the tides are causing a lot of energy to dissipate.”

Armed with this data and their findings on tidal interactions, they refined the models underpinning the simulations — then ran them a few hundred times for good measure. Depending on the exact parameters fed into the simulation, it shows that Phobos and Deimos were ‘born’ between 1 and 2.7 billion years ago. Until we know exactly what the moons’ physical properties are — especially porosity and water content — this timeframe will have to be our best estimate. A Japanese mission aimed for launch in 2025 will explore Phobos and return samples to Earth, so we might have a much better estimate in a few years.

As far as the original moon is concerned, the team says it orbited Mars at a greater distance than Phobos today. Deimos, owing to its smaller mass, has remained roughly at the distance this original moon was orbiting on. The more massive Phobos, however, has been drawn closer by tidal forces, and this process is still ongoing. The team’s simulations show that Deimos will keep drifting away from Mars (much like Earth’s Moon is doing), while Phobos will keep inching towards the surface. They estimate either an impact taking place in the next 40 million years or so, or for Phobos to break down under the strain of Mars’ gravity as it comes nearer.

The paper “Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor” has been published in the journal Nature.

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.

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.

Proposed ‘geophysical definition’ re-instates Pluto as a planet — and adds 100 new ones in the Solar System

Pluto, along with a host of more than 100 other bodies in the Solar System should be planets, says Kirby Runyon from the John Hopkins University.

Pluto in its frozen beauty, captured by the New Horizons craft.
Image credits NASA.

Eleven years ago, the International Astronomical Union demoted Pluto, who had done nothing wrong and just kept on orbiting as always, to the status of “non-planet”. Many people, scientists and laymen and I, cried bloody murder at the ruling — we had just lost one of the nine Solar planets. And we were not happy.

Kirby Runyon from the Johns Hopkins University would see justice returned to icy, tiny Pluto, and the debate about what a planet actually is finally settled. Runyon is the lead author of a paper making a case for re-instating Pluto as a planet which will be presented next week at a scientific conference in Texas. Even more, he says that a host of moons (including Europa and the Moon) and other bodies (like these guys here) in the Solar System should be planets, too.

What does a planet make

Back in the glory days, Pluto was the smallest of the nine recognized planets, with a diameter just under three-quarters of the Moon’s, and roughly one fifth of Earth’s. This small size eventually led the IAU to demote Pluto in 2006, but Runyon says it shouldn’t have been the case — Pluto “has everything going on on its surface that you associate with a planet. […] There’s nothing non-planet about it.”

Runyon, whose doctoral dissertation focuses on changing landscapes on the moon and Mars, led a team of six researchers from five institutions in drafting a proposed new definition of “planet”, and a justification for the new system of classification, which will be presented at the Lunar and Planetary Science Conference’s poster session. The authors were all members of the science teams part of the New Horizons missions — the spacecraft became the first man-made object to fly by Pluto, and also captured the first close-up images of the former planet.

The one point that led to Pluto’s demotion was that the IAU standard required all planets and their satellites to move alone through their orbits — and Pluto is too tiny to completely accrete all the matter it passes by on its orbit. The other two criteria — that it orbits a star and is kept round by gravity — were met. Principal investigator for the New Horizons mission Alan Stern of the Southwest Research Institute in Boulder, Colorado, has argued that the IAU definition also excluded Earth, Mars, Jupiter, and Neptune, which also share their orbits with asteroids, so if we demoted Pluto why not these four planets as well?

If it looks like a planet and quacks like a planet…

Charon, Pluto’s largest satellite, can also be considered a planet under the new system.

To settle the debate, the team argues that the qualities intrinsic to a celestial body should carry more weight than external factors (such as orbit) when deciding it if is a planet or not. They settled on the definition of a planet as being “a sub-stellar mass body that has never undergone nuclear fusion” and has enough gravitational pull to maintain a more-or-less round shape.

This geophysical definition goes against the grain of the three-element astronomical system the IAU currently employs in that it makes no reference to the body’s surroundings, so deciding if a planet is officially a planet should become much easier and immediately apparent under Runyon’s system. Most planetary scientists are also generally trained as geoscientists more than astrologists, so a geophysical definition might suit them better than the old classification.

This definition of a planet would certainly cast a wide net. While it isn’t extended to stars, black holes, asteroids, and meteorites, it does cover pretty much everything else in our solar system — the number of Solar planets would increase from eight to almost 110. And that whopping increase is actually a good thing, Runyon says, as he thinks it will engage the public in space exploration. The very word “planet” seems to carry a “psychological weight,” he adds, so more planets could help pique public curiosity and instill a yearning for exploration in people.

Io, Jupiter’s innermost satellite, would also become a planet.
Image credits NASA.

“I want the public to fall in love with planetary exploration as I have,” Runyon added. “It drives home the point of continued exploration.”

The team’s definition doesn’t require approval from a central governing body for scientists to start using it — in fact, it’s already been adopted by Planet Science Research Discoveries, an educational website founded by scientists at the University of Hawaii. If you’re one of those who feels the Solar System is the less for Pluto’s demotion, science says you can now have it back. Along with a hundred new members.

So what do you think of the new classification system? Is it a change for the better, a simplification of a cumbersome system — or complete and utter anarchy? Let us know in the comments.


The poster will be on view for a full day on March 21 at the conference sponsored by the Lunar and Planetary Institute, and Runyon will be on hand for at least three hours to answer questions about it according to Johns Hopkins University.

The other authors are Kelsi Singer of the Southwest Research Institute in Boulder, Colorado; Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona; Will Grundy of the Lowell Observatory in Flagstaff, Arizona; Michael Summers of George Mason University in Fairfax, Virginia.

Mars' moons: Deimos (left) and Phobos (right). Credit: JPL-CALTECH/NASA

Mars likely had many moons formed in the wake of a giant asteroid impact

Mars' moons: Deimos (left) and Phobos (right). Credit: JPL-CALTECH/NASA

Mars’ moons: Deimos (left) and Phobos (right). Credit: JPL-CALTECH/NASA

There are many instances where a comparison between Earth and Mars renders many similarities. As far as moons go, Earth takes the cake, though. We have a beautiful natural satellite orbiting the planet that’s about 25% smaller in diameter than Earth. Mars, on the other hand, has two moons which typically should score you more points. These look more like pebbles than moons when you factor in their size in relation to our own moon — Deimos is only 12 kilometers across, while the larger cousin Phobos has 22 kilometers in diameter.

While the size of Mars’ moons is laughable, some scientists believe the Red Planet used to have many more moons. This is the conclusion of an investigation made by the Royal Observatory of Belgium in Brussels, whose researchers tried to track the origin of Deimos and Phobos.

There are generally two schools of thought. One says the two moons are asteroids captured by the planet’s gravity, the second believes these are ‘homegrown’. Neither of the two hypotheses has so far fared well to scientific scrutiny. Calculations could not trace the satellites’ orbits if these indeed were snatched, nor could they reproduce the conditions for a natural formation.

Instead, Pascal Rosenblatt, a planetary scientist, and his colleagues at the Royal Observatory found a ring of material spewed in outer space by Mars’ impact with an asteroid could better explain things. A model of such a collision suggests the impact could have formed large satellites relatively close to the planet. These large moons could have then herded remaining debris in the sparsely populated outer part of the ring to form Phobos and Deimos. The reason we can’t see these large moons today is because they’ve been reclaimed by Mars’ gravity and struck the planet. Something similar is also set to happen to Mars’ largest moon, Phobos, in the future.

“The resulting orbital resonances spread outwards and gather dispersed outer disc debris, facilitating accretion into two satellites of sizes similar to Phobos and Deimos. The larger inner moons fall back to Mars after about 5 million years due to the tidal pull of the planet, after which the two outer satellites evolve into Phobos- and Deimos-like orbits. The proposed scenario can explain why Mars has two small satellites instead of one large moon. Our model predicts that Phobos and Deimos are composed of a mixture of material from Mars and the impactor,” the researchers wrote.

Brightly reflective Enceladus (504 kilometers, 313 miles across) appears above the center of the image. Saturn's second largest moon, Rhea (1528 kilometers, 949 miles across), is bisected by the right edge of the image. The smaller moon Mimas (396 kilometers, 246 miles across) can be seen beyond Rhea also on the right side of the image. (c) NASA

Five of Saturn’s moons aligned [amazing photo]

Brightly reflective Enceladus (504 kilometers, 313 miles across) appears above the center of the image. Saturn's second largest moon, Rhea (1528 kilometers, 949 miles across), is bisected by the right edge of the image. The smaller moon Mimas (396 kilometers, 246 miles across) can be seen beyond Rhea also on the right side of the image. (c) NASA

Brightly reflective Enceladus (504 kilometers, 313 miles across) appears above the center of the image. Saturn's second largest moon, Rhea (1528 kilometers, 949 miles across), is bisected by the right edge of the image. The smaller moon Mimas (396 kilometers, 246 miles across) can be seen beyond Rhea also on the right side of the image. (c) NASA

On July 29th the Cassini orbiter probe captured a stunning glimpse of five of Saturn’s satellites beautifully aligned. Cassini has been sending incredible photos of Saturn and its surroundings since 2004, as well as remarkable insights like the discovery of a salty ocean under one of its moon’s surface. Click the photo for a larger view.