Somewhere in astronomer heaven, Percival Lowell must be smiling. A century after Lowell tr

ained his telescope on Mars and claimed he saw canals built by intelligent beings, scientists are once again in the grip of the Red Planet's most seductive mystery—the possibility that life has existed there.

Did a microscopic race of Martians leave their traces inside a potato-size rock that fell on Antarctica 13,000 years ago？ Even as a new wave of Mars exploration begins this month— NASA's Mars Pathfinder lander arrives on July 4—researchers on Earth have begun a massive effort to answer that question, which has become one of the most controversial in science. No doubt Lowell, whose name has come to be synonymous with scientific wishful thinking, would want to know how it all turns out.

So will the hundreds of researchers who gathered in March at NASA's Johnson Space Center near Houston to attend the annual Lunar and Planetary Science Conference. Presentations at the meeting usually concern such topics as ancient lunar volcanism, or the icy satellites of Jupiter, or the surface composition of a distant asteroid. The subject of extraterrestrial life is rarely mentioned. This year, however, the star of the conference was the meteorite officially designated ALH84001: No less than 37 papers were devoted to it, and to the claims by a research team led by the space center's Dave McKay that the rock contains signs of ancient martian microbes.

The first reports of those claims last August jolted the small community of meteorite researchers. Some feared they were about to witness a scientific fiasco that would capsize not just the McKay team's careers, but their own. "We're all very dependent upon NASA," explains Alan Treiman of Houston's Lunar and Planetary Institute. "I was really worried that if this turned out to be (similar to the controversy over) cold fusion, that NASA was going to go down in disgrace. And that we were going to go with it." But it soon became clear that the debate about ALH84001 bears no resemblance to the one that sank cold fusion. Indeed, the McKay team's report in the journal Science won high praise from such respected scientists as Edward Anders, a University of Chicago professor who is considered the dean of meteorite science. He wrote that the work "sets a new standard for the study for the extraterrestrial materials."

At the same time, Anders and others have been critical to the way the McKay team interprets the three things it saw: first, molecules called PAHs (short for polycyclic aromatic hydrocarbons), which McKay and his colleagues believe were formed from the decay of simple organic matter; second, tiny crystals of iron oxide and iron sulfide, which the team says are identical to grains secreted by certain types of terrestrial bacteria; and finally oblong structures that the team tentatively calls fossil "nanobacteria." The strongly worded conclusion of the McKay team's report—that although each individual finding can be explained nonbiologically, taken together they represent compelling evidence for fossil life—strikes many scientists as wildly overreaching. UCLA's William Schopf, who pioneered the study of fossil bacteria on Earth, an effort fraught with false alarms, has summed up the attitude of many skeptics with a quote from Carl Sagan: "Extraordinary claims require extraordinary evidence."

Gathering evidence from ALH84001 has required a technological assault that Percival Lowell could never have imagined. Using electron microscopes and other state-of-the-art instruments, scientists have analyzed chips the size of rice grains, examining features measured in billionths of a meter (nanometers). At that scale, says McKay team member Chris Romanek of the University of Georgia, touring ALH84001 is "like walking in a jungle." Says the University of Tennesee's Harry McSween: "This is a complex rock. After all, it's 4.5 billion years old, and it's resided on mo

A．analyzing the extraterrestrial materials.

B．informing readers of the debate on life on Mars.

C．describing surface of Mars.

D．reporting the NASA conference on the meteorite ALH84001.