Image: Science

Martian life could have reached Earth inside a meteorite, according to a new study, which looks not at fossils or microbes but at magnetic fields. When ALH84001 was first discovered in the Allan Hills of Antarctica in 1984, it appeared to be just another Martian meteorite. After analyzing it more closely, however, a team of scientists at NASA and Stanford University declared in 1996 that it bore evidence of past life on Mars--namely, tiny mineralized structures, which they believed to be fossilized, primitive bacteria-like organisms. This spectacular finding didn¿t go unchallenged for long, and the potato-size meteorite has been at the center of controversy ever since.

Now ALH84001 has become the focus of a new question. A study of the famed rock published in today¿s issue of Science examines whether life--if it existed--could have traveled from Mars to Earth inside meteorites. Many scientists have argued that thermal sterilization would prevent such a trip: when a meteorite is ejected from Mars and later enters Earth¿s atmosphere, its surface reaches several thousand degrees--temperatures hot enough to kill any living thing. The new study, however, suggests that a meteorite's core may stay sufficiently cool.

Researchers from the California Institute of Technology, led by Benjamin Weiss and Joseph Kirschvink, looked at minuscule patterns of magnetization within the rock as it was heated and cooled. As molten rock cools off, it normally magnetizes in the direction of the local geomagnetic field, much like a compass. In the case of ALH84001, however, the researchers found no regular pattern of magnetization. Instead they discovered several heterogeneous magnetic fields within the rock that were probably caused by fractures it incurred while still on Mars. This finding is important because it indicates that the core of the meteorite never endured the lethal temperatures its surface felt during the interplanetary trip.

"The heat doesn¿t move very quickly in the meteorite," Kirschvink explained. "As the surface of the meteorite heats up, the melting surface is blown away, so it carries the hot material away from the meteorite." According to Kirschvink, the extreme heat never reaches more than a few millimeters into the rock. His group concluded from their measurements that the temperature at ALH84001's core never exceeded 40 degrees Celsius, or about 100 degrees Fahrenheit, en route from the Red Planet.

If heat didn't pose a threat to life, what of the voyage through space? According to Weiss, the European Space Agency conducted tests with bacteria to see how long they would survive in a near vacuum, exposed to subzero temperatures and ultraviolet radiation in space. "After six years they were still alive," Weiss said. Only one in every 10 million meteorites makes the trip from Mars to Earth anywhere near that quickly; most spend millions of years in space. "About a billion tons of Martian rocks have been transferred [to Earth] throughout the history of the planet," Weiss notes. And "once every million years there was a meteorite impact on Mars that was large enough to free enough mass to transport life to Earth." There is no evidence that life-forms from Mars have reached Earth, Weiss said but added: "If there were microorganisms on Mars, then it is probable that they would have made it here."