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Athena Review : Remote Sensing
Detailed images of numerous eroded gullies in impact craters in Mars' southern hemisphere were first documented in 2000 by the Global Surveyor satellite (fig.2). These features have been restudied through new imagery from the visible light camera on NASA's Mars Odyssey spacecraft (fig.1), which show that liquid water from melting snow packs is the likely cause of the erosion tracks. Dr. Philip Christensen, the principal investigator for Odyssey's camera system and a professor from Arizona State University in Tempe, proposes in the Feb. 9, 2003 issue of Nature that gullies seen at the edge of impact craters were formed by trickling water from melting snow packs, not by underground springs or pressurized flows, as had been previously suggested. Water, normally subject to rapid evaporation in the red planet's thin atmosphere, is sheltered along the interior sides of the impact craters, as discussed in Christensen's paper
The impact crater imaged in 2001 by the Odyssey spacecraft in the southern mid-latitudes of Mars showed eroded gullies on its cold, pole-facing northern wall (fig.1). What Christensen terms "pasted-on terrain" occurs as a smooth deposit of a volatile (evaporating) material judged to be snow.
The tracks or gullies in the top right-center appear to emerge from beneath and within a gradually disappearing blanket of snow. The current snow pack in this crater (located at 43 degrees south, 214 degrees east) sems to remain only on the coldest part of the crater wall at top.
On the less-shaded, warmer sides of the crater (left), the snow cover has completely disappeared, leaving the gullies exposed. The image shows an area 14.8 m (9.2 miles) by 21.6 m(13.4 miles).
The unique mid-range resolution of the visible light camera in Mars Odyssey's thermal emission imaging system was critical for the insight, because of its wide field of view.
[Fig.1 (left): Crater image with "pasted-on terrain" and erosion gullies, from Mars Odyssey (NASA/JPL image PIA04408 ).]
In fig.2, taken from the Global Surveyor in 2000, a more detailed view of gullies and an adjacent snow field (arrow) is seen from an impact crater. The image covers an area of 2.8 km (1.7 miles) by 4.5 km (2.8 miles) and is located at 33.3 degrees south, 92.9 degrees east.
Fig.2 (right): detail of crater from Global Surveyor (NASA/JPL image M09-2875.]
[For more information on NASA and JPL/Caltech, see http://www.jpl.nasa.gov ]
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