Martian sand traps and elongated dunes

Elongated dunes on Mars
Click for full image.

Cool image time! The photo to the left, rotated, cropped, and reduced to post here, was taken on June 23, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). I was attracted by the uncaptioned image’s title, “Elongating Linear Dunes at Meroe Patera.” What are elongating linear dunes?

The photo shows two such dunes, stretching out to the southwest away from the pile of sand that abuts the cliff to the northwest. Unlike most dunes, which usually form and travel in groups, these for some reason form single straight lines extending for some distance.

I contacted the scientist who requested this image, Joel Davis of the Natural History Museum in London, hoping he could answer some questions about these strangely shaped dunes, and discovered that he was studying this exact subject for a paper since published. As noted in the paper’s introduction,
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The Martian North Pole

The Martian North Pole

Since the very beginning of telescopic astronomy, the Martian poles have fascinated. Their changing sizes as the seasons progressed suggested to the early astronomers that Mars might be similar to Earth. Since the advent of the space age we have learned that no, Mars is not similar to Earth, and that its poles only resemble Earth’s in a very superficial way.

Yet, understanding the geology and seasonal evolution of the Martian poles is critical to understanding the planet itself.

This post will focus on the Martian north pole. The map on the right of the north polar regions is based on many satellite images supplemented by a lot of research by planetary scientists. The black circle in the middle is an area with relatively poor image coverage. The green areas are regions of higher elevation where the bulk of the permanent ice cap is located, surrounded by the blue northern lowlands that cover much of Mars’s northern hemisphere and are thought to have once harbored an intermittent ocean.

Olympia Undae dune field
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The reddish regions encircling the permanent ice cap are large seas of sand dunes, with Olympia Undae the largest and most sand-dune-packed. The image on the right, posted initially here on March 25, 2016, was taken by Mars Odyssey and shows the endlessness of this dune sea. Olympia Undae, spanning 120 degrees of longitude, is about 700 miles long, making it bigger than the Grand Canyon. As I noted in that post, “Just imagine trying to travel though this area. It is the epitome of a trackless waste. And without some form of GPS system getting lost forever would be incredibly easy.”

The polar cap itself, surrounded by those sand seas, is 600 miles across and a little less than 7,000 feet deep. It is made up of many seasonal layers, like the icecaps on Earth, with the bulk a mixture of water ice and cemented dust and sand. The very top layers, dubbed the residual icecap, is about three to six feet thick made up of frozen water having a volume about half of Greenland’s icecap. While this water could evaporate away, data suggests it is, like the icecaps on Earth, in a steady state, neither gaining or losing volume with each Martian year.

Above the residual icecap of water is the seasonal icecap made up of carbon dioxide. Unlike the other layers, this seasonal cap of dry ice, also less than six feet thick, comes and goes with the seasons. During the Martian summer it is gone, the carbon dioxide having sublimated away into the atmosphere. As the weather chills however that carbon dioxide begins to freeze again, falling as CO2 snow on the surface at the poles to create a thin cap of dry ice extending down to about 60 degrees latitude and covering practically everything seen in the first map above.

These facts suggest that future Martian colonists will have an interest in this region. While harsher than the rest of the planet, the conditions at the poles are not so much different that it will be impossible to work here. And here they will find a ready supply of carbon dioxide to help their plants grow, as well as a ready supply of water, all easily mined and near the surface.

In order to understand how this dry ice cap comes and goes, scientists have been using the high resolution camera of Mars Reconnaissance Orbiter (MRO) to repeatedly monitor some of the same locations in these sand seas to track the seasonal changes. In my routine review of the new images downloaded from MRO in May, I came across more than a dozen such images, all of which had been requested by Dr. Candice Hansen of the Planetary Science Institute in Tucson, Arizona, and taken just as the Martian winter was ending and spring was beginning. As she explained to me, “The images I’m requesting now follow-up on many of our earlier study sites so that we can study interannual variability. We’re also looking at more places to get a sense of what is similar/different depending on where you are.”

Below are two of these recent images, showing one example of the springtime changes that can be seen on these dunes.
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Gale Crater dunes: dry and volcanic in origin

Using data from orbit and from the rover Curiosity, scientists have determined that the material in the dunes in Gale Crater that Curiosity has visited are very dry and volcanic in origin.

This dryness is in contrast with the underlying ground, which shows evidence of water. The data also suggests that the material either came from multiple volcanic sources producing different compositions, or some of the sand was somehow changed at a later time.

In other words, the sand in the dunes came from elsewhere.

Ghost dunes on Mars

Scientists have found Martian pits formed by the leftover remains of dunes that long ago blew away.

Scientists have discovered hundreds of crescent-shaped pits on Mars where sand dunes the size of the U.S. Capitol stood billions of years ago. The curves of these ancient dune impressions record the direction of prevailing winds on the Red Planet, providing potential clues to Mars’s past climate, and may hold evidence of ancient life, according to a new study detailing the findings in the Journal of Geophysical Research: Planets, a publication of the American Geophysical Union.

Ghost dunes are the negative space left behind by long-vanished sand dunes. Lava or water-borne sediments partially buried the dunes and hardened, preserving the dunes’ contours. Wind subsequently blew sand off the exposed tops and scoured it out from inside, leaving a solid mold in the shape of the lost dune.

The claim that these geological features “may hold evidence of ancient life” is pure hyperbole, and absurd. However, the features are important because they will help date the sediment or lava flows around them, while also providing markers to help understand the history of the Martian climate.

Dunes on Pluto?

Dunes on Pluto

Cool image time! Scientists reviewing images taken by New Horizons when it flew past Pluto in 2015 have discovered what appear to be dunes of methane on the icepack of nitrogen of Sputnik Planitia. The image on the right, cropped to post here, shows these dunes. You can see the full image if you click on it.

Following spatial analysis of the dunes and nearby wind streaks on the planet’s surface, as well as spectral and numerical modelling, scientists believe that sublimation (which converts solid nitrogen directly into a gas) results in sand-sized grains of methane being released into the environment.

These are then transported by Pluto’s moderate winds (which can reach between 30 and 40 kmh), with the border of the ice plain and mountain range providing the perfect location for such regular surface formations to appear.

The scientists also believe the undisturbed morphology of the dunes and their relationship with the underlying glacial ice suggests the features are likely to have been formed within the last 500,000 years, and possibly much more recently.

There remains a lot of uncertainty here. The features do look like dunes in the image, but it is also possible that other phenomenon not yet understood could have caused this pattern on the icepack surface. Also, the resolution of the image is not sufficient to really see detail at this level. A different process on the surface could be fooling our eyes.

Nonetheless, the scientists hypothesis makes sense, and fits the data known. It also demonstrates again that, even billions of miles from the Sun, in as alien an environment we can imagine, the planet Pluto is an active and complex place.

Mars rover update: July 12, 2017

Summary: Curiosity looks at some big dune ripples, then creeps up hill. Opportunity tests its wheels.


For the overall context of Curiosity’s travels, see Pinpointing Curiosity’s location in Gale Crater.

The interior of a dune ripple

Since my last update on June 23,, Curiosity has worked its way around and, for a few days, even into the small sandy field at the base of Vera Rubin ridge. The scientists noticed that the sandy here had a series of large ripples, and they wanted to take a close look at at least one. The image on the right, cropped to show here, was taken shortly after they had the rover drive through one ripple in order to expose its interior. You can see the robot arm directly above the cut created by the rover’s wheels. On the cut’s wall several distinctly different toned layers are visible. A close look reveals that they are wavy, and probably indicate numerous and repeated overlays as the wind brushes a new layer of dust on top of old layers, time after time. The different tones indicate a change in the material’s composition, which could reveal something about some past events in either Mars’ weather or geology.

In order to decipher this information, however, they will need to be able to date the layers, and figure out when each tonal change happened. I am not sure Curiosity can do this, especially since they have not scooped up any of this dust for later analysis.

They are now approaching Vera Rubin Ridge, and should climb up onto in the coming weeks. At that point they will move off the Murray Formation, where they have been since March 2016, made up of dried and ancient crushed mud, and up onto a lighter, yellowish layer of rock, dubbed the Hematite Unit. This October 3, 2016 press release. gives a good outline of the geology of these regions.
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Weird dunes on Mars

Weird dunes on Mars

Cool image time! The image on the right, cropped and reduced in resolution to fit here, shows an area of inexplicable dark dunes located in Mars’ high northern latitudes. Located in a circular depression (whose outline can be seen across the top and left side of the image), geologists only partly understand the processes producing these dunes. As the noted on the release webpage:

However, a circular depression (probably an old and infilled impact crater) has limited the amount of sand available for dune formation and influenced local winds. As a result, the dunes here form distinct dots and dashes. The “dashes” are linear dunes formed by bi-directional winds, which are not traveling parallel to the dune. Instead, the combined effect of winds from two directions at right angles to the dunes, funnels material into a linear shape. The smaller “dots” (called “barchanoid dunes”) occur where there is some interruption to the process forming those linear dunes. This process is not well understood at present and is one motivation for HiRISE to image this area.

Be sure to look at the full image, as it covers a wider area and shows dunes that travel in all directions, forming mazelike patterns that no theory presently explains.

The Winds of Mars

changing martian dunes
Images taken 1363 days apart.

In two different papers published in two different journals in the past month, scientists have concluded that — despite the thinness of the planet’s atmosphere — the dunes and sands of Mars are being continually shaped and changed by its winds. In both papers the data from which this conclusion was drawn came from high resolution images taken by the HiRISE camera on Mars Reconnaissance Orbiter.

What is especially interesting about this conclusion is that the climate models that had been developed for the Martian atmosphere, combined with wind measurements gathered by the various Martian landers, had all suggested that the kind of strong winds necessary to move sand were rare. To quote the abstract of the paper published on Monday in the journal Geology, Bridges, et al,

Prior to Mars Reconnaissance Orbiter data, images of Mars showed no direct evidence for dune and ripple motion. This was consistent with climate models and lander measurements indicating that winds of sufficient intensity to mobilize sand were rare in the low-density atmosphere.

Similarly, the second paper, Silvestro, et al, published on October 22 in Geophysical Research Letters, stated that

results from wind tunnel simulations and atmospheric models show that such strong wind events should be rare in the current Martian atmospheric setting.

Yet, both studies found significant evidence that such winds do occur on Mars, and are moving sand in many different places.
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