Planets

Curiosity Finds Unique Ripples in Mars' Dunes

Though both Mars and Earth possess wind-blown sand dunes with very similar characteristics, it seems Martian dunes have a little something extra.

Mars is a planet shaped by aeolian -- or "wind-driven" -- processes. So it probably doesn't come as a surprise to know the Red Planet also sports some pretty big sand dunes.

From afar, these dunes strongly resemble the dunes we have on our planet. But in a new study carried out by NASA's Mars rover Curiosity, an active dune field on Mars has revealed that, though many of the processes that shape Martian dunes are the same processes that shape terrestrial dunes, there's an extra ripple that can only form in Mars' atmosphere.

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"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 graduate student Mathieu Lapotre, of Caltech in Pasadena, Calif., in a NASA statement.

On both Earth and Mars dunes can be as large as a football field and consist of a gently-sloping upwind face and a steep downwind face that is shaped by continuous sand avalanches as the prevailing wind keeps pushing material over the apex of the dune. Classical arc-shaped barchan dunes can often result on both planets and Mars satellites have captured some stunning observations of these types of dunes from orbit. Just look at them, they're amazing.

On Earth, the surfaces of these dunes are often rippled with peaks and troughs spaced around 30 centimeters (12 inches) apart. These rows of ripples are created by wind-carried grains of sand colliding with stationary grains, eventually creating a corrugated texture on dunes covering sandy deserts and beaches.

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Until Curiosity started its approach to the active dark Bagnold Dunes six months ago on the northwestern slopes of Mount Sharp, scientists didn't know whether these small-scale "impact ripples" existed. From orbit, larger ripples measuring around three meters (10 feet) from peak to peak could be seen and it was generally assumed that these larger-scale ripples were equivalent to Earth's impact ripples, only much larger owing to the thin Martian atmosphere and lower gravity.

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But when Curiosity arrived at Bagnold, the rover didn't only see the 10 feet-wide ripples, but it also saw the small-scale ripples just like Earth's impact ripples.

"As Curiosity was approaching the Bagnold Dunes, we started seeing that the crest lines of the meter-scale ripples are sinuous," said Lapotre, who's also science team collaborator for the Curiosity mission. "That is not like impact ripples, but it is just like sand ripples that form under moving water on Earth. And we saw that superimposed on the surfaces of these larger ripples were ripples the same size and shape as impact ripples on Earth."

So it turns out that Mars dunes have an added complexity that could only be proven by rolling up close and taking photos. Mars dunes have the small impact ripples, plus medium-sized "sinuous ripples" that can be resolved from space.

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Interestingly, though Earth's dunes don't possess sinuous ripples, they can form underwater -- on a riverbed, for example. Rather than particles colliding, these sinuous ripples are created as flowing water drags particles, causing them to settle in a rippled pattern.

Lapotre, who is lead author of a study that was published on July 1 in the journal Science, thinks that the Martian sinuous ripples are being driven in a similar way, but it's the Red Planet's thin atmosphere that's dragging the particles to form the medium-sized ripples on the sand dunes. Lapotre's team have nicknamed them "wind-drag ripples."

"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."

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But after studying observations (carried out by Curiosity and NASA's veteran rover Opportunity) of Mars' sandstone dating back to 3 billion years ago, the researchers found evidence of these wind-drag ripples preserved in the material of the approximate same size as the ripples that exist in today's Martian dunes. This means the planet lost most of its atmosphere early in its geological history and for the past 3 billion years the atmospheric pressure has remained fairly constant -- a finding that fits with other Mars atmosphere evolution models.

"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.

It's pretty amazing to think that a fairly simple observation of an active sand dune on Mars can reveal so much about Mars' current and ancient atmospheric conditions. But as the sophisticated wheeled robot continues its quest to seek out past and present habitable environments, and this is all in a day's work.

GALLERY: The Beautiful Dunes Of Mars

Mars plays host to a huge number of dune fields -- regions where fine wind-blown material gets deposited to form arguably some of the most beautiful dunes that can be found on any planetary body in the solar system. Using the powerful High-Resolution Imaging Science Experiment (HiRISE) camera on board NASA's Mars Reconnaissance Orbiter, planetary scientists have an orbital view on these features that aid our understanding of aeolian (wind-formed) processes and Martian geology. Here are some of our favorite Mars dunes as seen by HiRISE. Pictured here are shell-like "barchan dunes" in the ancient Noachis Terra region of Mars.

Special thanks to Ari Espinoza of the HiRISE team at the University of Arizona for helping to compile this list.

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Dunes of many shapes, sizes and formation processes can be found on the Red Planet. Shown here are elegant "linear dunes" with deposits of larger rocks and possibly ices in their troughs.

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These slug-like dark dunes are striking examples of "dome dunes" -- elliptical accumulations of fine material with no-slip surfaces. These domes contrast greatly with the often jagged appearance of barchan dunes. Found at the bottom of Proctor Crater, they are darker than the surrounding crater floor as they are composed of dark basaltic sand that was transported by the wind.

READ MORE: Slug-Like Dunes on Mars

Looking like a wind-blown silk sheet, this field of "star dunes" overlays a plain of small ripples, another aeolian feature. The ripples move more slowly across the bottom of Proctor Crater, so the large dune field will travel over the smaller ripples. Dunes are continuously evolving and moving with the wind, ensuring that the Martian surface is never static.

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These "transverse dunes" are undergoing seasonal changes. Likely entering Mars summer, this region of dunes is stained with pockets of subliming ices -- likely carbon dioxide. As the ices turn from solid to vapor, dune material slumps, revealing dark, sandy material underneath.

Resembling the mouths of a shoal of feeding fish, this is a group of barchan dunes in Mars' North Polar region. Barchan dunes betray the prevailing wind direction. In this case, the prevailing wind is traveling from bottom right to top left; the steep slope of material (plus dune "horns") point to the downwind direction. The HiRISE camera monitors barchans to see if they move between observing opportunities, thereby revealing their speed of motion across the Martian plains.

This is the same barchan dune field, zoomed out, a "swarm" of dunes covering the plains.

Not all barchan dunes "behave" and form neat "horny" shapes. They can become muddled and overlapping, creating "barchanoid dunes," as shown here.

This very fluid-looking collection of barchans is accompanied by a wind-blown ridge in the Hellespontus region of Mars but...

...only when zoomed out does the true nature of this fascinating region become clear. The prevailing wind is eroding the mesas (small hills) to the right of the image, carrying fine material downwind (from right to left), creating a startling pattern of barchans and a viscous-looking trail of sandy ridges across the plains.

The band Train sang about the "Drops of Jupiter" -- what about the "Drops of Mars"? Sure, they're not made of any kind of fluid, but they do make for incredibly-shaped dunes. These raindrop-shaped dunes are found in Copernicus Crater and are known to be rich in the mineral olivine, a mineral that formed during the wet history of Mars' evolution.

READ MORE: Mars' 'Raindrop' Sand Dunes Swarm

These craggy-looking dunes are old barchanoids eroding away through seasonal processes (sublimation of sub-surface ices) and the persistent Martian wind.

These linking barchan dunes are at the leading edge of a dune field -- grains of dust have been blown across a plain, deposited and left to accumulate in elongated arrow shapes.

Dome-shaped dunes and barchans seem to "reach out" and touch their downwind partners with slumped material.

Barchan dunes inside Arkhangelsky Crater in the southern hemisphere of Mars reveal a wind direction from top left to bottom right. Note the tracks of Martian dust devils over the dune slopes.

For more on the HiRISE camera, see the HiRISE website and regularly updated Tumblr page. For more on Mars dune definitions, check out the USGS Mars Dunes site.