Mapping Mars' Ripples Unveils Dune Mysteries
By combining high-resolution observations of Mars dune ripples and comparing them with sand dunes on Earth, scientists are beginning to understand how surface winds influence the Red Planet.
In the Matt Damon movie "The Martian," the winds on Mars are portrayed as being savage, relentless hurricanes that would give any terrestrial storm a run for its destructive money. However, the realities of the Red Planet's windy nature is a lot more subtle, slowly shaping the aeonian landscape over long periods of time.
But there are many mysteries, principally based around how surface winds influence small-scale structures and how a human presence on the surface may modify those winds.
Though previous Mars missions have been able to image the planet's vast dune fields, watching them slowly evolve while being molded by prevailing winds, it wasn't until NASA's Mars Reconnaissance Orbiter (MRO) arrived in 2006 that we could gain super high-resolution observations of ripples on the individual dunes. Having such a fine resolution view meant planetary scientists could capture long-duration snapshots of individual dunes, and even features on those dunes, changing over time, thereby piecing together Mars' windy nature.
"I have always wondered which way the winds blow on Mars," said Mary Bourke, of the School of Natural Sciences in Trinity College Dublin. "Of course we don't have long-term meteorological stations up there to help me answer that question, so we have previously had to use the shape of sand dunes or the elongation direction of sand streaks to map wind flow, but we miss much of the finer detail there."
Understanding Martian winds is not only an academic curiosity about the formation of sand dunes on another world. As NASA cranks up its Mars exploration efforts, we need to know as much as possible about the Martian atmosphere, and this means gaining a better grasp on how short term and long term trends in weather patterns may impact robotic and future human missions to the Red Planet's surface.
Using data from the High-Resolution Imaging Science Experiment (HiRISE) camera on board the MRO, and comparing the Martian features with features on sand dunes found on Northern Ireland's peninsula of Magilligan, Bourke's team was able to create a 3-D model of Mars surface winds. From this model, the researchers were able to, for the first time, see how large dunes on Mars modify localized wind flow. It's all very well understanding which direction the prevailing winds are blowing, but this research provides valuable information about how a dune-filled landscape on Mars may channel or even redirect winds on the smallest of scales.
In other words, we're looking at the kinds of scales that would impact rovers or even astronauts on the dusty surface. Also, we're looking at the kinds of scales that could be influenced by the presence of a permanent human presence on Mars - if you erect a structure on this wind-driven world or if a landing spacecraft disrupts the dunes, how will the Mars winds be modified?
"We worked with data from the Proctor Crater region on the southern highlands of Mars, and found that ripples on the dunes moved around 1.5 meters per year," said Bourke. "This is a much more accurate insight than the one we had before - we now have a better platform from which to consider how Martian landforms have evolved, and how they will evolve when structures such as spacecraft disturb them in the future."
This fascinating study has been published in the journal Nature Geoscience.
Source: Trinity College Dublin
This HiRISE image shows two classes of aeolian bedforms within Proctor Crater. The relatively bright, small ridges are ripples. From their study on Earth, and close-up examination by the MER rovers (roving elsewhere on Mars), we know that ripples are composed of fine sand (less than 200 microns in diameter) or fine sand coated with coarser sand and granules.
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
and Martian geology. Here are some of our favorite Mars dunes as seen by HiRISE. Pictured here are shell-like "
" in the ancient Noachis Terra region of Mars.
Special thanks to Ari Espinoza of the
for helping to compile this list.
Dunes of many shapes, sizes and formation processes can be found on the Red Planet. Shown here are elegant "
" with deposits of larger rocks and possibly ices in their troughs.
These slug-like dark dunes are striking examples of "
" -- 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.
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
the smaller ripples. Dunes are continuously evolving and moving with the wind, ensuring that the Martian surface is never static.
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.
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,
. For more on Mars dune definitions,