Tag Archives: Dunes

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).

Finger-print like pattern observed near al-Idrisi Montes mountain on Pluto. Credit: NASA.

Pluto has frozen ‘sand’ dunes made from methane ice

Finger-print like pattern observed near al-Idrisi Montes mountain on Pluto. Credit: NASA.

Finger-print like pattern observed near al-Idrisi Montes mountain on Pluto. Credit: NASA.

For some time, scientists have been intrigued by strange, regularly spaced ridges, that stuck out of Pluto’s cold, dark plains like thumbprints pressed into ice. Now, a new study suggests that these formations are actually dunes made out of methane “sand,” which is striking considering the dwarf planet’s extremely thin atmosphere.

“When we first saw the New Horizons images, we thought instantly that these were dunes but it was really surprising because we know there is not much of an atmosphere. However despite being 30 times further away from the sun as the Earth, it turns out Pluto still has Earth-like characteristics. We have been focusing on what’s close to us, but there’s a wealth of information in the distant reaches of the solar system too,” said Dr. Jani Radebaugh, Associate Professor in the Department of Geological Sciences at Brigham Young University.

Earth is not the only place in the solar system with dunes. Previously, scientists had spotted them on Mars, Titan, and even on a freaking comet. Now, Pluto, a place few expected could harbor such formations, joins the list of dune-bearing worlds.

“We knew that every solar system body with an atmosphere and a solid rocky surface has dunes on it, but we didn’t know what we’d find on Pluto,” said Dr Matt Telfer, Lecturer in Physical Geography at the University of Plymouth, lead author of the new study published in Science.

The dunes were first spotted by NASA’s New Horizons spacecraft in 2015 when it found methane mounds nestled beside the massive glacier that makes up the western half of Pluto’s Sputnik Planitia, the famous heart-shaped region. The mounds stretch for more than 12 miles and occupy an area equivalent to twice that of Utah Lake.

Pluto's Sputnik Planitia. Credit: NASA.

Pluto’s heart-shaped Sputnik Planitia. Credit: NASA.

The frozen sand whose features resembles earthly dunes is shaped by winds blown on the glacial plain from the direction of a mountain range found along Planitia’s border. Scientists know this because of the shape of the piles of sand whose dark streaks allowed them to retrace the direction from which the wind blew.

This was a surprising observation. With an atmosphere 100,000 times thinner than on Earth and an average temperature hovering at about -230°C, it’s difficult to imagine the wind blowing anything there. The surface patterns recorded by New Horizons don’t lie, though.

Researchers modeled the conditions they saw and concluded that the ‘sand’ is made of nitrogen or methane ice. There’s plenty of nitrogen ice adorning the glacier nearby from the dunes while methane is likely sourced from the snowcaps of nearby mountains like al-Idrisi Montes, which drifts down into the plane.

Once airborne, the particles are pushed by winds that blow between 18 and 24 miles an hour. The winds may be powered by the sublimation of surface ice which turns from solid directly into gas when sunlight hits. The upward force is what drives the piles of particles at the surface.

“On Earth, you need a certain strength of wind to release sand particles into the air, but winds that are 20% weaker are then sufficient to maintain transport. The considerably lower gravity of Pluto, and the extremely low atmospheric pressure, means the winds needed to maintain sediment transport can be a hundred times lower. The temperature gradients in the granular ice layer, caused by solar radiation, also play an important role in the onset of the saltation process. Put together, we have found that these combined processes can form dunes under normal, everyday wind conditions on Pluto,” said Dr Eric Parteli, Lecturer in Computational Geosciences at the University of Cologne.

The researchers also think that the dunes are young, rather than ancient, and might still be active. For instance, Pluto’s heart-shaped central plain has a peculiar polygonal pattern which resembles the surface of gently boiling water. This feature is the result of slow convection in the thick layer or nitrogen and other ices over hundreds of thousands of years, according to one hypothesis. If this is true, the dunes have to be more recent than the action of this convection otherwise they’d be churned apart by it.

Personally, I find it remarkable every time we get to see familiar Earthly features on alien worlds, be them tall mountains, vast deserts, or undulating canals. Sights like these, including these most recent findings on Pluto, remind us that, in some respects, our world isn’t that special at all.

Martian sand ripples as recorded by the Mars Reconnaissance Orbiter from the planet's orbit. Credit: Wikimedia Commons

What Mars’ unique sand dunes can tell us about its past

NASA’s Curiosity Rover found some of Mars’ sand ripples are of a type not seen on Earth. These wind-sculpted sand formations are distinctly made by Mars’ atmosphere and hold secrets about the Red Planet’s past.

Martian sand ripples as recorded by the Mars Reconnaissance Orbiter from the planet's orbit. Credit: Wikimedia Commons

Martian sand ripples as recorded by the Mars Reconnaissance Orbiter from the planet’s orbit. Credit: Wikimedia Commons

“Earth and Mars both have big sand dunes and small sand ripples, but on Mars, there’s something in between that we don’t have on Earth,” said Mathieu Lapotre, a graduate student at Caltech in Pasadena, California, and science team collaborator for the Curiosity mission. He is the lead author of a report about these mid-size ripples published in the July 1st issue of the journal Science.

Both on Mars and on Earth, we can find the same football field-sized dunes, shaped by sand avalanches with steep upwind faces. There are also the smaller sand ripples — rows of sand less than a foot (30 centimeters) apart which on Earth are formed by wind-propelled sand grains colliding with other sand grains on the ground. These are typically referred to as “impact ripples” by scientists.

NASA researchers were thrilled to find these same formations on Mars after the first high-resolution images beamed back from orbit by the Mars Reconnaissance Orbiter. Judging from these images, the ripples were about 10 feet (3 meters) apart on the dunes’ surfaces, but not much detail could be made from that high above.

Two sizes of ripples are evident in this December 13th, 2015, view of a top of a Martian sand dune, from NASA’s Curiosity Mars rover. Sand dunes and the smaller type of ripples also exist on Earth. Credit: NASA

Two sizes of ripples are evident in this December 13th, 2015, view of a top of a Martian sand dune, from NASA’s Curiosity Mars rover. Sand dunes and the smaller type of ripples also exist on Earth. Credit: NASA

Eventually, Curiosity Rover’s wheels touched the Bagnold Dunes on the northwestern flank of Mars’ Mount Sharp. Immediately, NASA researchers back on Earth who studied the new footage found the crest lines of the meter-sized ripples are sinuous. This means Mars’ sand ripples are not at all like the impact ripples seen on Earth, but more like those that form under moving water. Even more amazingly, superimposed on these larger ripples were smaller ripples about the same size and shape as Earth’s impact ripples.

NASA concludes that these meter-sized ripples were built by Martian wind dragging the sand particles akin to how flowing water drags sand particles on Earth. In other words, that’s a totally different mechanism that anything we’ve seen on Earth.

“The size of these ripples is related to the density of the fluid moving the grains, and that fluid is the Martian atmosphere,” he said. “We think Mars had a thicker atmosphere in the past that might have formed smaller wind-drag ripples or even have prevented their formation altogether. Thus, the size of preserved wind-drag ripples, where found in Martian sandstones, may have recorded the thinning of the atmosphere.”

These latest findings investigate modern dunes, but sand ripples preserved in sandstone dating from more than 3 billion years ago were previously investigated by NASA’s Curiosity and Opportunity rovers. Wind-drag ripples about the same size as the ones found at Bagnold Dunes were found, which fits the current narrative that suggests Mars lost its original atmosphere in the planet’s early history. But despite Mars looks nothing like it used to, it still bears many similar features to those found on Earth.

“During our visit to the active Bagnold Dunes, you might almost forget you’re on Mars, given how similar the sand behaves in spite of the different gravity and atmosphere. But these mid-sized ripples are a reminder that those differences can surprise us,” said Curiosity Project Scientist Ashwin Vasavada, of NASA’s Jet Propulsion Laboratory in Pasadena.



Cross bedding explained, on an outcrop from Mars

NASA recently uploaded a strikingly beautiful photograph on their website showing a petrified sand dune on Mars. The image was actually pieced together from several shots taken using Curiosity’s Mast Camera (Mastcam) on August 27th. From end to end, the panorama spans a full 135 degrees of other-worldly awesomeness, with east to the left and southwest to the right.

The panorama of Mars, build using pictures taken by the Curiosity rover.
Image via NASA

The component images in the center and upper portion of the mosaic are from Mastcam’s right-eye camera, which is equipped with a 100-millimeter-focal-length telephoto lens. Images used in the foreground and at far left and right were taken with Mastcam’s left-eye camera, using a wider-angle, 34-millimeter lens.

The structures are part of the Stimson unit on Mount Sharp (or Aeolis Mons, clocking in at 5.5 km height from the valley floor of Gale crater). They don’t look quite like this on Mars – the pictures were white balanced and color corrected to correspond to daytime lighting conditions here on Earth.

Grab your hardhat and pencil, it’s fieldwork time

My educational background is geophysics. I investigate geological structures and physical anomalies tied to geological settings using physical methods – such as ground-penetrating radar, measurements of the Earth’s magnetic field, resistivity and conductivity readings, and a lot of other cool, geeky stuff that girls hanging out in bars really like hearing about.

But as everyone who studies earth sciences, I was also thought about geology and geological processes, and i have a pretty good grasp of the important phenomena that make our planet look, feel, and behave the way it does. So without further ado, let’s take a look at the outcrop.

The sandstone slabs can be easily discerned, and they show a characteristic named cross bedding, very similar to what a geologist would find on Earth. For non-geologists out there, cross bedding means, simply put, that the rocks look “flow-y,” like this:

Image showing cross-bedding, or more accurately cross-lamination (the structures that make up the beds are called laminae when they are less than 1 cm in thickness, and strata when they are over 1 cm thick).
Image via indiana.edu

To understand bedding, you must first get a general idea of how sedimentary geological structures form – sedimentation.

Let’s take a particle, such as a speck of dust, a grain of sand, or even piece of a larger, already formed rock. It can, over time, get dislodged or eroded from their resting place, and become incredibly mobile. This is the first step in how these rocks form – formation of sediment.

Several forces, such as gravity, running water and even wind can then carry and deposit them far away from an initial spot to a sedimentary area, such as the face of the dune that is not swept by wind – this is the second step, transport.

Over time, if enough particles are carried, they solidify under immense pressure and sometimes temperature to form sedimentary rocks – in geology we call this third step lithification.

Now, let’s take our initial spot on the top of a mountain, and our sedimentary area in a wide delta with a river flowing down connecting the two. In the steep mountainous terrain, the river flows fast and erodes pieces of the mountain, taking material – from decently sized rocks and small pebbles all the way to particles as fine as a human hair – on the journey to the delta.

Downriver, as the terrain gets flatter, the waterway widens and becomes lazy, flowing slowly and peacefully to the delta. As the water looses speed, it also looses the energy it needs to carry larger pieces of material, so they drop to the bottom – here sedimentary rocks will be coarser. The lighter particles can still be carried and sediment further away, and here the rocks will be much finer.

In this sample of conglomerate, you can easily see smaller pebbles and stones bound in a natural cement made of fine particles.
Image via geology

In this case, where the sedimentary zone is nice and horizontal and the water flows slowly, there is nothing to disturb the sediment so all packets of rocks will be relatively flat, and of similar thickness.

But if we spice things up a bit, and have our initial spot on the left (and wind-swept) side of a dune, particles will be carried over the top and put to rest on the other sloped side – we will see cross-bedding. This is the phenomena that likely created this dune and the cross bedded sandstone we see in the NASA picture.

Image via itssedimentary.tumblr

If the rocks form in an watery environment, either very close to the surface waves or where water flows in such a way as to disturb the sediment, then too we will see cross bedding. In a way, what you would see in a cross laminated or cross strata packet of rocks in this case is fossilized movement of the medium that transports the material.

Where does it go from here?

On Earth, the cycle of erosion and sedimentation is never truly over. What is now a delta could, with a little tectonic help, become a mountain and feed another sedimentary area on its own.

The packets of rocks could get melted and their minerals would form fresh volcanic rocks, or squished and pulled and turned and heated into metamorphics. These can then become eroded, and the cycle continues. Slowly, too slow for a human to perceive, the rocks move, get transformed, destroyed and formed anew.

Scientific consensus is that Mars is no longer tectonically active. It’s waters are frozen or lost in space, and its atmosphere is barely even there – it’s almost 100 times thinner than on Earth. Sediments will have a hard time being transported by anything other than gravity, so what Mars should experience is just a leveling out, as the terrain seeks to flatten out under the strain of its own weight. But that would happen very, very, very slowly.

Chances are, if you ever reach Mars, those dunes will still be there.