Ice or Lava Sea on Mars? A Transatlantic Debate Erupts

The déjà vu was palpable when U.S. planetary scientists heard the news last week that a frozen sea the size of the North Sea had been found on Mars. “We went through all this 7 years ago when [Mars Global Surveyor] first imaged these terrains,” says planetary geologist Alfred McEwen of the University of Arizona, Tucson. “Our immediate reaction then was, ‘Gosh, that looks like frozen ice.’ But we quickly realized it had to be lava.”

“It can't be lava,” says volcanologist John Murray of the Open University in Milton Keynes, U.K. He and his teammates running the High Resolution Stereo Camera (HRSC) onboard the European Space Agency's Mars Express orbiter reported at the First Mars Express Science Conference last week that the “lava” is actually an ice floe-covered sea frozen in place. That would make the Elysium Plains a fetching place to land and look for microfossils of martian life, Murray notes.

Everyone agrees that both water and lava have gushed from the ground in the vicinity of Elysium (Science, 30 November 2001, p. 1820). McEwen and others had traced lava and water flows back to the great ground cracks of the 1000-kilometer-long Cerberus Fossae. 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. Murray and his colleagues now see signs that about 5 million years ago such a gusher did not just seep into the ground but pooled to a depth of 45 meters over an area about 850 kilometers across. Once its surface froze, they say, the waters moved again, breaking the ice into floes now locked into a frozen sea that has become buried under a protective layer of volcanic ash and sediment.

In a paper to be published 17 March in Nature, Murray and his colleagues will detail the ice signs they see in the images, which are among the first European data returned from another planetary body. A pivotal claim is that the level of the putative sea has dropped since its surface froze. Mapping elevations using HRSC stereo imaging as well as laser altimetry from Mars Global Surveyor, they find that flood material inside as well as outside some craters has sunk about 15 meters below the crater rim. Floodwaters could have seeped or sublimated away, says Murray, but lava could not. In addition, “the edge [of the flow] ties in well with a sea rather than lava,” says Murray. Where HRSC has looked, he sees a beach swept by turbulent flows, a high-water mark, and the final sunken level with pack ice at the bottom.

American Mars geologists, who have dominated the field by dint of returning almost all the previous data from Mars, aren't persuaded. “I think it's unlikely they're right,” says Michael Carr, planetary geologist emeritus at the U.S. Geological Survey (USGS) in Menlo Park, California. McEwen and many others feel more strongly than Carr that they were right the first time. “We've been studying these lavas for 7 years,” McEwen says. “Put aerial photos of Iceland [lava flows] side by side with Mars, and you can't tell the difference.”

Martian lavas could look so much like sea ice because similar processes shape both. But on Mars, McEwen sees—among other volcanic features—small edifices that disgorged the lavas and steep-sided levees at the flow edges like the ones lavas form on Earth. “What we're talking about is a sea of lava,” says planetary volcanologist Laszlo Keszthelyi of USGS in Flagstaff, Arizona. The apparently sunken lava may just be the result of lava withdrawing beneath a solid crust, he says.

Resolution of the matter will likely require targeting the exact areas HRSC imaged with the camera and ground-penetrating radar on Mars Reconnaissance Orbiter, due for launch this August. Until then, water or rock may remain in the eye of the beholder.