There are no white cliffs of Mars, scientists are reporting, casting more doubt on the theory that Mars once had a warm, wet, Earthlike climate favorable for life.
NASA's Mars Global Surveyor spacecraft, in orbit around Mars, has been measuring the glow of infrared light from the rocks below, looking for patterns of colors that identify different minerals. In particular, scientists have been interested in minerals known as carbonates, which form only in the presence of water. On Earth, the white cliffs of Dover in England are a notable example of carbonates.
In today's issue of the journal Science, the researchers who run the infrared instrument report that Global Surveyor has detected small concentrations of carbonates in Martian dust, 2 percent to 5 percent by weight, but none of the large deposits that would probably form at the bottom of a lake or an ocean.
"I would say it's extremely unlikely Mars had large bodies of warm, standing water that were exposed to the atmosphere for a long period of time," said Dr. Philip R. Christensen, a professor of geological sciences at Arizona State University and senior author of the article. "It's reasonably unlikely that massive carbonates exist, and we haven't seen them."
Many planetary geologists are now moving toward the view that Mars has been cold throughout its 4.5-billion year history and that the considerable quantities of ice known to exist there have been in a frozen state almost all of that time.
"There is a significant shift that has been going on in the last year or two," said Dr. Michael H. Carr of the United States Geological Survey in Menlo Park, Calif. "I find it easy to believe early Mars was cold and it never got much above freezing. There are a lot of reasons for thinking that."
In this view, the early environment on Mars, when it did possess a significant atmosphere, may have resembled Antarctica's. However, if Mars has always been freezing cold, that leaves scientists having to explain how a vast network of what appear to be gullies, dry river beds and canyons formed on the Martian surface.
Those water-worn features had led some to speculate that Mars long ago had a thick atmosphere of carbon dioxide that trapped enough heat to create balmy conditions. Some even saw geological features that they interpreted as shorelines of an ancient ocean that had covered much of Mars' northern hemisphere.
Since then, scientists have found that realistic climate models do not generate that much global warming on Mars, and high-resolution photographs by Mars Global Surveyor have failed to convince most people that the supposed shorelines are actually shorelines.