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Animation: New Gullies on Martian Sand Dune> | Figure 1: Left and Right Narrow-Angle Image | |
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Figure 2: Context View | Figure 3: New Boulder Tracks on Slope; North is Down> | Figure 4: Snow Incorporated into Antarctic Dune |
One of the many mysteries associated with martian geology is the origin of
gullies found at latitudes poleward of 30 degrees latitude. Most of these
gullies are found within craters or other depressions, and appear to be
related to the bedrock. Several hypotheses have been proposed for their
origin, including groundwater seepage and melting at the base of a
dust-mantled snow pack.
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, are not conducive to such
processes. An alternative process of gully formation on these sand dunes
involves frozen carbon dioxide trapped in the winter by windblown sand,
then subliming rapidly enough for the escaping carbon-dioxide gas to make
the sand flow as a gully-cutting fluid.
As part of extended-mission science investigation using the Mars Orbiter
Camera on NASA's Mars Global Surveyor spacecraft, the camera team is
re-imaging many locations where previous observations revealed gullies.
The intent is to see if gully-forming processes are operating on Mars at
the present time.
The team has 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. This pair of narrow-angle images (figure 1) from the Mars Orbiter Camera
shows the dune as it appeared on July 17, 2002, (left) and as it appeared
on April 27, 2005, (right). The nearly three Earth years of intervening
time amount to about 1.4 Mars years. During this period, 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. 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 July 17, 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 the Mars
Orbiter Camera over the past eight years.
Wider context for the dune is shown in a mosaic of two images from the
Thermal Emission Imaging System on NASA's Mars Odyssey orbiter (figure 2),
encompassing the dark-toned sand dune field on the floor of a crater
located near 49.8 degrees south latitude, 325.4 degrees west longitude.
In this image, north is approximately up and sunlight illuminates the
scene from the upper left.
Based on earlier observations of other dune fields with gullies,
camera-team scientists suspect that these gullies form by a process other
than water fluidization. An image of a dune in Russell Crater, taken by
the Mars Orbiter Camera in March 2001, (figure 3) 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 and 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. An example is the layering of snow
buried in a sand dune in Victoria Valley, Antarctica, seen in a photograph
taken by Michael Malin during the austral summer of 1982-1983 (figure 4).
Active sand dunes in cold regions such as Antarctica and northern Canada
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"
would consist mostly of carbon-dioxide frost, with minimal water ice.
What would happen to carbon-dioxide frost incorporated into a martian sand
dune? On surfaces that receive early and direct sunlight, the sand would
heat and the carbon-dioxide 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 carbon dioxide frost would sublime rapidly. This rapid
formation of carbon-dioxide gas may act to fluidize overlying sand,
causing it to flow rather than avalanche, and thus create a gully.
The Mars Orbiter Camera was built and is operated by Malin Space Science
Systems, San Diego, Calif. Mars Global Surveyor left Earth on Nov. 7,
1996, and began orbiting Mars on Sept. 12, 1997. JPL, a division of the
California Institute of Technology, Pasadena, manages Mars Global Surveyor
for NASA's Science Mission Directorate, Washington.