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The Winds of Mars

June 25, 2012

The atmosphere of Mars is nearly non-existent. The atmospheric pressure at the Earth's surface is 14.7 pounds/square-inch (psi). This is 100 kilopascals, but I still think about the natural world in English units, reserving metric for the laboratory. The atmospheric pressure on Mars is a little less than a tenth psi, or 600 pascals, and the atmosphere is nearly all carbon dioxide (95%), with a little nitrogen and argon.

Quite surprisingly, the atmosphere holds a lot of suspended dust, so much so that dust storms can obscure Olympus Mons, a large volcanic mountain. Olympus Mons is the tallest mountain in the Solar System. At 14 miles (22 km), it's about three times the height of Mount Everest. Data from the Mars Exploration Rovers (Spirit and Opportunity, show that the suspended dust particles are about 1.5 μm in size.
Olympus Mons topography (Stephanie Albornoz)Now, that's a mountain!

A topographic rendering of Olympus Mons, with a scale in meters.

(Image by Stephanie Albornoz, modified, via Wikimedia Commons).

This particle size is as large as the tenuous atmosphere will hold, but there's an underlying spectrum of larger sand particles near the surface. Sand grains are defined to be particles of size ranging from a few tens of μm to about a millimeter. Where there's sand, there will also be sand dunes. I wrote about terrestrial sand dunes in a previous article (Sand Dunes, February 14, 2012).

A Martian panoramaThe Martian landscape is rocks (about one per square meter) and sand.

(NASA image via North Carolina State University).

Sand dunes were found on Mars quite some time ago, but scientists weren't certain whether they were relics of past processes, or a feature of an active Martian surface. Says Jean-Philippe Avouac, the Earle C. Anthony professor of geology at Caltech,
"For many years, researchers have debated whether or not the sand dunes we see on Mars are fossil features related to past climate, since it was believed that the current atmosphere is too thin to produce winds that could move sand."[1]
The sand can be thick, up to 200 feet (60 meters) in places.[2] The winds of the thin Martian atmosphere must blow about ten times faster than those on Earth to lift sand from the surface.[3]

Such high winds do occur on Mars, although rarely.[3] Interestingly, the thin atmosphere and low Martian gravity will keep the particles airborne for quite some time. After particles are aloft, it takes only a tenth as much wind to keep them there than to get them airborne in the first place.[3] When these particles finally alight, they can move other particles along the surface, a mechanism that causes migration of sand dunes.

NASA's Mars Reconnaissance Orbiter imaged sand dunes and surface ripples moving across the Martian surface, and a team of scientists at the California Institute of Technology has used advanced image processing techniques to measure these movements.[1] They applied their COSI-corr software (Co-registration of Optically Sensed Images and Correlation) to images taken by the High Resolution Imaging Science Experiment (HiRISE). The software can estimate sub-pixel movement from two images taken of the same region.

Topographic features of Martian sand dunesA topographic representation of sand dunes at Nili Patera.
Blue is less than 75 cm displacement; red is 4.5 meters or more displacement.
(California Institute of Technology Image).

The software compared images taken over a 105-day period at Nili Patera, just north of the Martian equator.[3] It found that sand ripples moved up to 4.5 meters (15 feet) during that period (see figure).[1] The movement was very similar to that seen for dunes in the dry Victoria Valley in Antarctica.[1]

The measurements reveal that 1,500 liters (2 cubic yards) of sand blow through each meter-wide stretch of land each year.[2] The spectrum of dune motion is such that some dunes will migrate a distance equal to their length in 170 years, while others will take a thousand years.[3] The Caltech work is published in an article in Nature.[4]

In an attempt to put these Martian winds to good use, a research team from North Carolina State University has designed a wind-driven, "tumbleweed," Martian rover (see figure).[5-7] As can be expected, a lot of computer simulation went into the design. Larger diameter and lower weight tumbleweeds have better performance, and these rovers will generally bounce along the surface, rather than roll, because of the rocky terrain.

Model of a tumbleweed roverModel of a tumbleweed rover.

(NCSU image).

These would not be small tumbleweeds. In order for these rovers not to get stuck between rocks, they need to be six meters in diameter; that is, as large as a truck.[5] Although they would only go where the winds take them, they could do that quickly, and without a need for propulsive power. The research was supported in part by NASA.[5]

References:

  1. Katie Neith and Deborah Williams-Hedges, "Technology Developed at Caltech Measures Martian Sand Movement," California Institute of Technology Press Release, May 9, 2012.
  2. Brid-Aine Parnell, "Supersize shifting sand dunes stalk surface of Mars," Register (UK), May 10, 2012.
  3. Nola Taylor Redd, "Surprise: Martian sand dunes are speedy," Christian Science Monitor, May 9, 2012.
  4. The Flowing Sands of Mars, University of Arizona Press Release, May 9, 2012.
  5. Matt Shipman, "Rock and (Not) Roll: Study Eyes How To Keep A Mars Tumbleweed Rover Moving On Rocky Terrain," North Carolina State University Press Release, May 23, 2012. N. T. Bridges F. Ayoub J-P. Avouac, S. Leprince, A. Lucas & S. Mattson, "Earth-like sand fluxes on Mars," Nature, vol. 485, no. 7398 (May 17, 2012), pp. 339-342.
  6. Alexandre E. Hartl and Andre P. Mazzoleni, "Terrain Modeling and Simulation of a Tumbleweed Rover Traversing Martian Rock Fields," Journal of Spacecraft and Rockets, vol. 49, no. 2 (March–April, 2012), p. 401ff.
  7. Tumbleweed Team Web Site.
  8. David S. F. Portree, "Mars: A World for Exploration (1959)," Wired, May 8, 2012.

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