The researchers--Dennis Reiss and Ralf Jaumann--published their findings in a recent edition of the Geophysical Research Letters. In Berlin, the pair are scientists at the Institute of Space Sensor Technology and Planetary Exploration, German Aerospace Center.

The article's title, "Recent debris flows on Mars: Seasonal observations of the Russell Crater dune field", suggests the intriguing location --Russell Crater--for some of the best images of Martian erosion patterns.

Mudflow?
But such patterns can come from avalanches, wind or shifting geology. How can satellite photos reveal the true cause? At least for the dunes of Russell Crater, however, the researchers conclude that: "liquid water may be stable over a limited period around noon/early afternoon in late spring/early summer under current climatic conditions," --and likely within the last thousand years.

The close-up images are taken from a 3 kilometer patch (1.8 miles), on the downslope side of a dune in Russell Crater [in the Southern Hemisphere, 54.5 S and 12.7 E]. Similar erosion patterns are seen on several smaller dunes in Green Crater and Kaiser Crater--all in the south.

Depending on when Russell Crater is photographed, it may appear shrouded by dust and haze, or show remarkably clear erosion patterns on its sculpted dunes. But the seasonal change shows the effects of temperature (and thawing) on the dune flows.

The summer peak temperature is around 17 degrees Celsius (or 62 degrees Fahrenheit, 290 Kelvin), but drops back below water freezing at night. Reiss and Jaumann concluded that this steep temperature rise in daylight summer and the surface brightness (or albedo) could distinguish what part of the dune was covered in dry ice (frozen carbon dioxide) and what part was covered by frozen water ice.

The key photograph is part of a much larger satellite library, and labelled M1901170, which shows orbital details at a resolution of 2.78 meters per pixel.

Bright Frost
During the Martian autumn and winter, both carbon dioxide (dry ice) and water vapor freeze out of the atmosphere and frost the top few centimeters of the Russell Crater dunes.

When spring arrives, dry ice turns directly to vapor first, particularly since the thin Martian atmosphere is nearly 100 times less dense than Earth's. But for a few hours during summer noon, enough frozen ice may melt to induce liquid water flows, according to the scientists. They suggest that like on the arctic and alpine slopes of Earth, fine-grained soil then may flow downslope from the dune's crest.

The slurry of fine soil, they call debris flows. The debris flows run out the dune base without showing depositional fans.The thin, channel-like tracks show a fine dendritic pattern, and originate from alcoves. But the main channel can stretch up to 2 km long (1.2 miles), with a thickness of 8 to 17 meters.

The researchers also set out to try to answer why the erosion only occurs on one side of the dune, and tends to flow poleward. They concluded that "bright frost remains only at the protected colder, poleward facing dune slopes, the areas in which the erosion features occur...

"The amount of condensed H2O-ice in the near subsurface of the dune is unclear, but it is likely that the H2O-ice would persist longer into the spring on the protected, colder dune slopes."

Unlike other Mars erosion patterns, the distinct embankments are unique. The channels also end without a spill apron, but a few show a dam-like ridge at terminus.

While a full explanation is ambiguous, the scientists note that the two key ingredients--seasonal thawing, sandy soil with slopes--seem consistent with what may be a brief and relatively recent summer noon of liquid water in Russell Crater.

The sinuous erosion in Russell Crater is also distinct from the avalanches seen elsewhere on Martian dunes, according to Reiss and Jaumann. Based on the lack of small craters in the dune fields, the team estimates its surface age at 100 to 10,000 years--a recent soil pattern.

Satellite Images
The episodic melting and freezing leave behind erosion patterns that can be viewed using satellite images from the Mars Global Surveyor (MGS), and its onboard Mars Orbital Camera (MOC).

The Mars Global Surveyor has been orbiting the red planet since Sept. 12, 1997. Its mission has examined the entire Mars surface and provided a wealth of information about the planet's atmosphere and interior.

A new batch of high resolution photos, taken between February and July 2002, were added online and they bring the total number of images in the online gallery to more than 123,800. The images are available from the Mars Orbiter Camera Gallery.

The American Geophysical Union summarized Reiss and Jaumann's findings based on MGS images: "Erosion along a Martian dune slope may be caused by seasonal melting of water ice normally frozen within the fine-grained surface material. ..

"Their analysis shows frost forming at various times on the planet's surface, leading them to estimate that liquid water could become stable for a limited time during the summer in the southern hemisphere. The explanation offers an alternative to theories that propose that frozen carbon dioxide may have produced the flows or that geothermal heating could have reached the surface in the past."

What's Next
All three Mars missions planned for June launch will look for evidence of ancient water. On June 2, the European Space Agency will launch its Mars Express mission from the Russian Cosmodrome. On June 8 and June 25, NASA will launch its two Mars Exploration Rovers on Delta III rockets from Cape Kennedy.

All three landers will blitz the Martian equator around January 2004.

The Mars Orbital Camera experiment on the Global Surveyor is in excellent health and continues to return a wealth of new information every day. The Global Surveyor will support the planned landing missions by observing the sites and monitoring the weather on Mars.

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