WASHINGTON (AP) - A theory that microbes once lived on Mars is boosted by two new studies of a rock that was blasted away from the red planet and eventually landed on Earth.
Researchers at the University of Wisconsin, Madison, and at the California Institute of Technology said the new studies do not prove that Martian microbes once lived in the rock. But they remove one challenge based on the temperature history of the potato-size chunk of Mars.
"We have ruled out the high temperature hypothesis" that would have made life impossible, said John W. Valley of the University of Wisconsin. "I still don't have final answers. There should still be skepticism."
Wisconsin scientists determined the range of temperatures the rock was exposed to by analyzing the ratios of carbon and oxygen isotopes. At Cal Tech, researchers traced the temperature history by measuring magnetic fields within the rock. Both studies will be published Friday in the journal Science.
NASA scientists last summer claimed that small globules of carbonate found inside a Martian meteorite were the fossilized remains of microbes or bacteria that lived on the red planet more than 15 million years ago.
Based on a microscopic and chemical analysis of the globules, the NASA team theorized that the microbes lived and died in the rock, leaving behind organic chemicals and fossilized remains. The rock was then blasted from the Mars surface by a meteorite impact, spent thousands of years wandering in space and then fell to Earth in the Antarctic. The rock was recovered from an ice field and identified by chemical composition as coming from Mars.
A major challenge to the theory has been that the carbonate globules actually formed by inorganic processes at temperatures of more than 1,200 degrees, far too hot for life.
But the new studies show that temperatures of the globules never exceeded 212 degrees - scalding, but still within the living range of known life forms.
"Our work shows that there are no show stopper lines of evidence in the temperature," said Valley. There are other reasons to be skeptical, however, he said, "and it will be difficult to convince the world one way or the other."
"Our results don't prove there was life," said Joseph L. Kirschvink, head of the Cal Tech team. But the finding proves that the possibility of life cannot be eliminated because of temperature, he said.
The Cal Tech team determined the temperature history of the rock by measuring the magnetic field direction of tiny parts of the samples. The magnetic field direction in a rock will change slightly each time it is heated and cooled.
"To make the measurement, we had to saw apart a specimen the size of a grain of sand," said Altair T. Maine, a member of the Cal Tech team.
Kirschvink said his team found that after the rock cooled from a melt some 4 billion years ago, it was never again heated to a temperature lethal to all life.
The Cal Tech study also showed that early in the history of Mars, the planet had a magnetic field similar to that of Earth. Kirschvink said this means the planet probably had an atmosphere. A strong magnetic field allows a planet to retain an atmosphere.
Over billions years, however, Mars has lost its magnetic field and most of its atmosphere, he said.
Kirschvink said the magnetic studies also show that the sampled part of the Mars meteorite never heated up as it entered the Earth's atmosphere and smashed into the Antarctic. This suggests that microorganisms could survive a trip from Mars to Earth.
"An implication of our study is that you could get life from Mars to Earth periodically," he said. "In fact, every major impact could do it."
Earlier studies had suggested a Mars origin of life and Kirschvink said his studies do not rule out this possibility.
Kurt Marti, an expert on the chemistry of the solar system at the University of California, San Diego, said the two new studies may lay to rest temperature challenges to the Mars life theory, but he said there are other objections.
"These all have to be addressed one by one," he said. "Until that is done, we have to be careful about accepting or rejecting this theory."
Among the theory's other problems: the need for chemical evidence of life based on carbon isotope ratios, and better physical evidence that the carbonate globules are, in fact, fossils.
Valley said he hopes to start soon an analysis of the carbon isotopes.