Mars Rock Cools Down, Life Debate Heats Up

The famous martian meteorite, ALH84001, may be cool enough for life again. After NASA scientists claimed last August to have found evidence of past life in carbonate intrusions in the rock, skeptics argued that the chemical composition of the carbonate could only have formed at temperatures of 650 degrees Celsius or above--too high for life. But in tomorrow's issue of Science, two papers fuel the argument with new evidence that the rock formed at comfortably low temperatures.

Both groups see abrupt changes in the properties of the carbonates that would have been washed out at high temperatures, they say. In one study, John Valley, a petrologist at the University of Wisconsin, and his colleagues found that isotopic ratios of several elements vary widely throughout the rock. " > The gradients are very steep," he says. "[At high temperatures], these sorts of gradients diffuse away in hours to days," leaving the rock rather uniform in isotopic composition. He concludes that the carbonates must have formed at relatively low temperatures, perhaps less than 100 degrees Celsius.

Similarly, geologist Joseph Kirschvink and his colleagues at the California Institute of Technology looked at a tiny inclusion in the carbonate, composed of two separate mineral crystals that had been crushed together. After sawing the crystals apart, the team measured their magnetic fields and found that each had a field pointing in a different direction, presumably because the crystals were reoriented when they were crushed. If the rock had ever been heated above about 320 degrees Celsius, its magnetization would have been wiped out. If it had been remagnetized, the fields would have been aligned in the same direction, says Altair Maine, a member of the team. The pattern of the meteorite's magnetic signals thus indicates that it "has a cool history," he concludes.

Ralph Harvey, a geologist at Case Western Reserve University in Cleveland who has argued that the carbonates formed at high temperatures, isn't convinced by either finding. For one, he says, if the carbonate formed rapidly at high temperatures and then cooled, steep isotopic gradients could have been frozen in. As for the magnetic data, he argues that the crystals might have been magnetized and then crushed into different orientations long after the carbonate formed.

The heat is still on the life-on-Mars claim, it seems. "Isotope people think it was formed at low temperature, and chemical-composition people high temperature," says Robert Clayton, a geologist at the University of Chicago. "Will anybody budge?"