Figure: Dune gully orientation

This MOC image of the "Russell Dunes" shows how dune face morphology varies as a function of azimuthal orientation (gullies on South-West facing slip slopes, sand avalanches on North-West facing slopes).

One of the many mysteries associated with martian geology is the origin of gullies found at latitudes poleward of 30^o. Most of these gullies are found within craters or other depressions, and appear to be related to the bedrock. Several alternative hypotheses have been proposed for their origin, including groundwater seepage and melting at the base of a dust-mantled snowpack.

Some middle-latitude gullies are found on sand dunes. These gullies appear to be different from those found on the slopes of craters, but generally have been interpreted to form by similar processes. In the present martian environment, it is difficult to introduce water to the surface. The temperature and atmospheric pressure may permit water to exist, but the rate of heating of the ground and atmosphere, and the amount of energy available to warm the ground or melt snow, is not conducive to such processes.

As part of the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) Extended Mission science investigation, the MOC team has an on-going effort to re-image locations known from previous observations to have gullies. The intent is to see if gully-forming processes are operating on Mars at the present time.

The figure shows how the morphology of the dune's slip face changes with direction: gullies form on pole-facing slopes (southwest in this case), while normal slip face avalanche features ("avalanches" in the figure) are seen on the equator-facing slopes (northwest in this case). Most of the dunes that have gullies on them are located in the Hellespontus/Noachis regions, and are frost-covered during the winter. Based on experience in Antarctica and other cold regions on Earth, it is known that snow and ice can be incorporated into dunes during winter.

What would happen to CO2 (carbon dioxide) frost incorporated into a martian sand dune? On surfaces that receive early and direct sunlight, the sand would heat and the CO2 frost would sublime over a period of time, undermining the slope and promoting normal sand sliding. On slopes that were initially shaded and later exposed to direct sunlight, heating would be delayed and the CO2 frost would sublime rapidly. This rapid formation of CO2 gas may act to fluidize overlying sand, causing it to flow rather than avalanche, and thus create a gully.

Reference Material:

• Previous MGS MOC releases about dune gullies:
  
  1. Russell Dune Gullies, 8 May 2005
  2. Russell Dune Gullies, 27 April 2003


• The THEMIS images used here have been archived with the NASA Planetary Data System (PDS)
  and can be found here: http://themis-data.asu.edu/


• Other viewing options for some of the pictures in this release:
  1. Pair of MOC images, showing that new dune gullies formed, without annotations
  2. THEMIS mosaic of the dune field, without annotations
  3. Animated GIF of new dune gully

Publication credit:

• Malin, M. C., and K. S. Edgett (2005) 8 Years at Mars #1: New Dune Gullies, Malin Space Science Systems Captioned Image Release, MOC2-1220, http://www.msss.com/mars_images/moc/2005/09/20/dunegullies/.
  
  or
  
  
• Malin, M. C., and K. S. Edgett (2005) 8 Years at Mars #1: New Dune Gullies, Malin Space Science Systems Captioned Image Release, MOC2-1220, NASA/JPL Planetary Photojournal, http://photojournal.jpl.nasa.gov/, catalog number PIA04290.

Credit: NASA/JPL/Malin Space Science Systems

http://www.msss.com/mars_images/moc/2005/09/20/dunegullies/index.html