Researchers: Mars' features not eroded by running water
AMERICAN GEOPHYSICAL UNION NEWS RELEASE
Posted: March 13, 2002

Scientists have provided new evidence that liquid carbon dioxide, not running water, may have been the primary cause of erosional features such as gullies, valley networks, and channels that cover the surface of Mars. Research suggesting that condensed carbon dioxide found in Martian crust carved these features is reported by Kenneth L. Tanaka and colleagues at the U.S. Geological Survey in Flagstaff, Arizona, and the University of Melbourne, Australia, will appear this month in Geophysical Research Letters, published by the American Geophysical Union.

Using Mars Orbiter Laser Altimeter (MOLA) data, Tanaka and his colleagues constructed elevation profiles of the Hellas basin, which, at 2000 kilometers [1,240 miles] wide and nine kilometers [six miles] deep, is the largest well-preserved impact basin on Mars. By examination of digitally created elevation profiles with 500-meter [2,000 foot] resolution, they found that the volcanic regions of Malea and Hesperia Plana, along the rim of the Hellas basin, are several hundred meters [yards] lower than adjacent rim sectors. Additionally, these areas lack the prominent triangular peaks, called massifs, that are common in nearby areas.

Along the inner slopes of these regions, the researchers found, however, evidence of old massifs covered by volcanic rocks. They are too low to be covered, if there were volcanic activity today. The researchers suggest as an explanation that prior to volcanic activity, these regions along the rim of the basin resembled nearby areas, but were eroded to their present-day elevations following the emplacement of the volcanic rocks.

Tanaka and his colleagues propose a "magmatic erosion model" to explain the features of the volcanic areas of Malea and Hesperia Plana, suggesting that they underwent catastrophic erosion associated with explosive eruptions of molten rock. They suggest that liquid in the Martian crust was heated when molten rock, or magma, rose to the surface. As the liquid was heated, it expanded, until the pressure of overlying material was too great, and an explosive eruption occurred, shattering overlying rock, and causing it to move with the magma in an erosive debris flow.

The authors believe that the fluid in the crust along this area of the rim of the Hellas basin was mainly liquid carbon dioxide. A debris flow dominated by carbon dioxide would flow faster and farther than a water-based flow, they say. Also, carbon dioxide is more volatile than water at lower temperatures, and the cold temperatures found on Mars would mean that less carbon dioxide- based magma would be required to produce the observed erosion than magma containing mainly water.

The researchers suggest that this mechanism of erosion can also explain collapse features and channels elsewhere on Mars. They also note, however, that their model is based on a variety of assumptions that must be further tested.

The paper by Kenneth L. Tanaka, Jeffrey S. Kargel, David J. MacKinnon, Trent M. Hare [Astrogeology Team, U.S. Geological Survey], and Nick Hoffman [University of Melbourne], "Catastrophic Erosion of Hellas Basin Rim on Mars Induced by Magmatic Instrusion in Volatile-Rich Rocks," will be published online within the next two weeks and later in the print edition of Geophysical Research Letters. Its citation, which is to the online version, is 10.1029/2001GL13885, 2002.

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