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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.
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).
The 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.
A 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 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:
- Katie Neith and Deborah Williams-Hedges, "Technology Developed at Caltech Measures Martian Sand Movement," California Institute of Technology Press Release, May 9, 2012.
- Brid-Aine Parnell, "Supersize shifting sand dunes stalk surface of Mars," Register (UK), May 10, 2012.
- Nola Taylor Redd, "Surprise: Martian sand dunes are speedy," Christian Science Monitor, May 9, 2012.
- The Flowing Sands of Mars, University of Arizona Press Release, May 9, 2012.
- 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.
- 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.
- Tumbleweed Team Web Site.
- David S. F. Portree, "Mars: A World for Exploration (1959)," Wired, May 8, 2012.
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