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Mars Orbiter Reveals Likely 'Pseudocraters'
By Kenneth Silber
Staff Writer
posted: 05:15 pm ET
22 October 1999
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mars_pseudocratersNew images from the Mars Global Surveyor's orbiting camera show cone-shaped structures likely to be "pseudocraters" -- volcanic features that give clues to the history and location of water on Mars. Despite their name, pseudocraters (also known as "rootless cones") are a type of crater. However, they result not from impacts but rather from explosions that occur when lava interacts with ice or liquid water. On Earth, pseudocraters are found in Iceland and other volcanic regions. The martian cones, which have diameters less than 275 yards (250 meters), have "key characteristics" typical of pseudocraters, says Alfred McEwen, a planetary scientist at the University of Arizona in Tucson. The cones, he notes, "clearly rest on top of lava flows [but] don't seem to be vents for lava themselves." Also, they are distributed in random clusters, similar to terrestrial pseudocraters. According to McEwen, the apparent pseudocraters are probably "extremely young" in geological terms -- several tens of millions of years. This implies, he adds, that "shallow ice is still there." The image below shows several cone-shaped structures in the Amazonis Planitia region. 
Credit: NASA/JPL/Malin Space Science Systems and University of Arizona. (Image has been rotated 90 degrees from original version, which shows an area 1.9 miles, or 3 kilometers, across.) The following image shows the broader context, covering an area 71 miles (115 kilometers) across. 
Credit: NASA/JPL/Malin Space Science Systems and University of Arizona. (Image has been rotated 90 degrees from original version.)
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