Post by Matt Chojnacki, Devon Burr and Jeff Moersch, Earth and Planetary Sciences Department and Planetary Geosciences Institute, University of Tennessee Knoxville, USA
Aeolian transport of sand-sized particles on planetary surfaces is both enhanced and inhibited by the presence of topography. Mountainous topography at all length scales significantly affects dune location, size, shape, and orientation [Pye and Tsoar, 1990]. The surface of Mars has both abundant sand dune populations and substantial topographic relief. Perhaps the best example of the relationship between topography and relief is the ancient rift valleys of Valles Marineris, with over ~21,000 km2 of dune fields found within the ~10 km deep rift system [Chojnacki and Moersch, 2009]. This system of interconnected-chasms, provides natural sinks where wind blown sediment can accumulate. Additionally, the substantial relief and the resulting atmospheric pressure gradient significantly influence the regional meteorology [Rafkin and Michaels, 2003].
Dune fields in Valles Marineris can be broadly divided into two classes: floor- and wall-related dune fields. The “wall dunes” class dune fields are interpreted as climbing and falling dunes [Chojnacki et al., 2010]. On Earth, climbing dunes are formed when migrating dunes encounter and ascend a substantial slope or cliff (>10°), where there is no major wind flow blockage [Pye and Tsoar, 1990]. Falling dunes, found on the downwind side of large topographic highs, are formed by unidirectional down slope winds and gravity [Greeley and Iversen, 1985].
Climbing Dunes of Valles Marineris:
The most spatially extensive climbing dunes are found in a series of dark-toned bed forms draped over the preexisting topography of spur-and-gully walls at the southern edge of Melas Chasma, in central Valles Marineris (Image 1). Distinct slip faces on the uphill side of the dunes, both within gullies and atop spurs are visible in images of the area (e.g. Image 2). This dune morphology is consistent with a unidirectional wind regime driving sand-sized particles southward across the chasma floor and up the ~15° wall slope [Chojnacki et al., 2010]. High resolution image
Falling Dunes of Valles Marineris:
East Coprates Chasma hosts multiple occurrences of falling dunes found several kilometers above the Valles Marineris canyon floor (Image 3). These falling dune fields are perched high above the chasma floor and located mostly within gullies. Presumably, down slope winds are funneled down gullies assisting gravity to develop these falling dunes (see below).
These intriguing aeolian morphologies are the result of a wind environment largely dictated by the micro- and mesoscale topography and the large relief of Valles Marineris. For example, falling dunes found in Coprates Chasma often have horns pointing upwind (Image 3-5a). In contrast, the majority of terrestrial and Martian crescent-shaped dunes that occur on relatively flat topography have horns pointing downwind [Greeley and Iversen, 1985; Pye and Tsoar, 1990]. Several falling dunes in the U.S. Great Basin have strikingly similar arcuate, lobate morphologies and orientations (Image 5). This morphology suggests slip face advancement occurs at a greater rate towards the center of the dune as compared to the flanks, presumably due to a higher velocity and/or more consistent airflow over the center of the dune [Chojnacki et al., 2010].
Chojnacki, M., J. Moersch, and D. Burr (2010), Climbing and falling dunes in Valles Marineris, Mars, Geophysical Research Letters, 37, doi:10.1029/2009GL042263. [abs]
Chojnacki, M., and J. E. Moersch (2009), Valles Marineris Dune Fields: Thermophysical Properties, Morphology, and Provenance, in Lunar and Planetary Institute Science Conference Abstracts, vol. XI, p. Abstract 2486.
Greeley, R. and Iversen, J.D., 1985. Wind as a geological process on Earth, Mars, Venus and Titan. Cambridge Planetary Science, 4,. Cambridge University Press, Cambridge, 333 pp.
Koscielniak, D. E. (1973), Eolian deposits on a volcanic terrain near Saint Anthony, Idaho, M.A. thesis, State Univ. of N.Y., Buffalo, N.Y.
Pye, K., and H. Tsoar (1990), Aeolian sand and sand dunes, Unwin Hyman, London.
Rafkin, S. C. R., and T. I. Michaels (2003), Meteorological predictions for 2003 Mars Exploration Rover high-priority landing sites, J. Geophys. Res., 108(E12), doi:10.1029/2002JE002027.[abs]