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New Gullies on Martian Sand Dune
Image above: 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 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.
Image credit: NASA/JPL/MSSS/ASU
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Rolling Stones Make New Boulder Tracks
Image above: When a boulder rolls down a dusty slope, it can leave behind a trail of depressions. Usually known as boulder tracks, these features have been documented and studied on Earth, the Moon, and Mars. Geologists studying the Moon and Mars can use these tracks to learn about the physical properties of the fine-grained debris encountered by the boulder as it rolled down the slope. Because of the high-resolution capability (0.5 to 12 meters, 1.6 to 39 feet, per pixel) of the Mars Orbiter Camera on NASA's Mars Global Surveyor spacecraft, dozens of boulder track sites have been identified on the red planet. A Mars Orbiter Camera image of one set of boulder tracks in a south mid-latitude crater (located near 35.8 degrees south latitude, 158.4 degrees west longitude) was obtained on Nov. 14, 2003, (left). A second image of the same site, from Dec. 4, 2004, (right) shows that more than a dozen new boulder tracks formed on the crater wall during the intervening time. Mars is an active planet, with geologic changes occurring - at some scale - every day. In this case, some time between mid November 2003 and early December 2004, a suite of boulders became dislodged from the crater wall, then rolled and perhaps bounced their way to the crater floor.
Image credit: NASA/JPL/MSSS
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