While the Spirit and Opportunity rovers search Mars for evidence of a watery past, planetary geologist Jim Head of Brown University is showing that water is very much a part of the planet’s present. Satellites circling the Red Planet have found evidence of convulsive ice ages that continue to reshape the rusty surface.
Images from three Mars-orbiting probes—NASA’s Mars Global Surveyor and Mars Odyssey and the European Mars Express—tell the frozen tale. Vast accumulations of dusty ice appear from the poles to the midlatitudes; in some places the ice has seemingly formed glacierlike tongues more than a half mile wide. At the poles, gamma-ray data from Mars Odyssey show high concentrations of hydrogen—“the icing on the cake,” Head says. Hydrogen generally denotes the presence of ice, a finding corroborated by Mars Express.
Courtesy NASA/JPL/Cornell University
Opportunity found a much different scene at Meridiani Planum. The rover’s microscopic imager is looking for delicately interwoven layers and traces of micron-size minerals—signs that water once flowed—in the exposed bedrock.
Taken together, the evidence points not only to water but also to water on the move. Head and his colleagues see resemblances between surface cracks in the midlatitudes on Mars and those that form on Earth in Antarctica and in the Arctic due to cycles of freezing and thawing over many years. The Mars features may have formed in a similar way as ice was deposited, lost to evaporation or sublimation, and then redeposited near the poles, perhaps as dusty snow. Global climate change is the only latitude-dependent process that accounts for all the observed features, Head says, leading him to conclude that Mars, like Earth, drifts in and out of convulsive ice ages. Scientists did not recognize this before because the planet is now in an interglacial period.
Head does not expect to find the same icy cover in the equatorial regions of Mars where Spirit and Opportunity landed, but he hopes the “scratch and sniff” tests—in which the rovers drill into Mars rocks and compare the surface with the interior—will reveal the climatic history of the region. Opportunity’s mud-brown landing site is especially promising because it contains hematite, an iron-based mineral that can form in the presence of water. The rover landed near exposed bedrock, thrilling Ray Arvidson, deputy principal investigator of the rovers’ science payload. He hopes that close-up measurements will pick up silica grains in the bedrock or other formations indicative of running water. “There may not be a single smoking gun,” Arvidson says, “but the Rosetta stone is there.”