Mars Global Surveyor
Mars Orbiter Camera

One of the many mysteries associated with martian geology is the origin of gullies found at latitudes poleward of 30°. 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.

We have found one location where a new gully formed on a dune in an unnamed crater in the Hellespontus region of Mars (west of the Hellas Basin; images MOC2-1220a and MOC2-1220b). Based on earlier observations of other dune fields with gullies (e.g., MOC2-1220c), we suspect that these gullies form by a process other than water fluidization. The third figure above (MOC2-1220c) 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 (MOC2-1220d, above).

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.

The first picture, MOC2-1220a, is a mosaic of two Mars Odyssey Thermal Emission Imaging System (THEMIS) Visible (VIS) images; north is approximately up and sunlight illuminates the scene from the upper left. The dark-toned sand dune field is on the floor of a crater located near 49.8°S, 325.4°W.

The second picture, MOC2-1220b, shows portions of two MGS MOC narrow angle images, the first obtained on 17 July 2002 (image number E18-00979), the second on 27 April 2005 (image number S05-01721). During the ~1.4 Mars years (nearly 3 Earth years) that elapsed between the time that the two pictures were acquired, a couple of gullies formed on the dune slip face. It is critical to recognize that the 2002 image was obtained at a time of year when the incident sunlight was coming in from a lower angle, relative to the horizon, than in the 2005 image. In a more technical sense, the 2002 image had an incidence angle (an angle measured from the normal through the surface feature, not from the horizon) of about 72.2°, while the 2005 image had an incidence angle of about 56.6°. If the gullies had been present in 2002, their appearance would be sharper and more pronounced than they are in the 2005 image. The gullies simply did not exist on 17 July 2002. The steep walls of the gully alcove and channels suggests that the sand in this dune is somewhat cohesive, an observation common among martian sand dunes seen by MGS MOC over the past eight years.

The third picture, MOC2-1220c, shows gullies and slip face avalanche features on a dune in Russell Crater near 54.7°S, 347.3°W. This is a sub-frame of MOC image E02-00070, acquired early in the MGS Extended Mission in March 2001.

The fourth picture, MOC2-1220d, shows layers of snow buried in a sand dune in the Victoria Valley, Antarctica. The picture was taken by the MOC Principal Investigator, Michael Malin, during the austral summer of 1982-1983. Active sand dunes in cold regions such as Antarctica and northern Canada will commonly incorporate wintertime snow as new sand avalanches down a slip face and covers the frozen material. A similar process might occur for middle and high latitude dunes on Mars, although in many cases the "snow" will consist mostly of carbon dioxide frost, with minimal water ice.

• 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