Ice or lava? Experts debate the nature of this region near the Martian equator.

Frozen Sea or Lava Field?

Dick writes about Earth and planetary science for Science magazine.

Scientists analyzing images from the European Mars Express orbiter announced the presence of a frozen sea on Mars this week, raising hopes that signs of life could still be found on the red planet. But not so fast, say some U.S. planetary scientists, who note that the "sea" was identified as a lava flow 7 years ago.

Scientists agree that both water and lava have repeatedly gushed from the ground in a region near the Martian equator known as Elysium (Science, 30 November 2001, p. 1820). Apparently, rising magma intersected subterranean water and drove it through the cracks to the surface, carrying with it any debris of life past or present.

New images from the Mars Express orbiter suggest that the water from one gusher did not just seep back into the ground, says volcanologist John Murray of The Open University in Milton Keynes, United Kingdom. The water pooled to a depth of 45 meters over an area about 850 kilometers across. Once its surface froze, Murray says, the water moved again, breaking the ice into floes now locked into a frozen sea topped by a protective layer of volcanic ash and sediment.

Murray and his colleagues, who run the orbiter's High Resolution Stereo Camera (HRSC), support their claims with HRSC stereo imaging and laser altimetry from Mars Global Surveyor. The data, presented at a meeting in Noordwijk, The Netherlands, indicate that flood material now sits about 15 meters below crater rims. But the presence of material inside the craters indicates that the flood crested as much as 30 meters higher, the team argues. Water could have seeped or sublimated away to lower the surface, says Murray, but lava could not. In addition, he notes, the edge of the flow looks like a beach, not the edge of a lava flow. The team will report its findings in more detail in the 17 March issue of Nature.

American Mars geologists aren't persuaded. The Martian lavas look like sea ice because similar processes shape both, says planetary geologist Alfred McEwen of the University of Arizona, Tucson. And planetary volcanologist Laszlo Keszthelyi of the United States Geological Survey in Flagstaff, Arizona, notes that the sunken surface may just be the result of lava draining away beneath a solid crust.

Resolution of the matter will likely require targeting the exact areas HRSC imaged with Mars Global Survey and perhaps Mars Reconnaissance Orbiter, due for launch this August. Until then, water or rock will be in the eye of the beholder.

Related sites
Murry et al.'s Lunar and Planetary Science Conference (14-18 March 2005) abstract
John Murray's homepage
Alfred McEwen's homepage
Laszlo Keszthelyi's homepage

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