The announcement was met by a combination of enthusiasm and skepticism. Skeptics noted that the researchers had to take a "sum of the evidence" approach. William Schopf of the University of California at Los Angeles, an expert on microfossils, told Science magazine "I think it's very unlikely they have remnants of biological activity."
The chain of reasoning that lead McKay's team to conclude that it 
had located evidence of life included these turning points:
- Was the rock really from Mars? (It is one of 12 Antarctic meteorites thought to have originated on Mars, on the basis of their isotopic (defined) composition.)
Mars image at right courtesy of NASA.
- How old was the rock? (Apparently it crystallized, or cooled, from molten rock 4.5 billion years ago, only 100 million years after the formation of the planet.)
- When did "life" enter the rock? (About 3.6 billion years ago, when the rock, still on Mars, was cracked by a major shock -- presumably a nearby meteorite impact. Then a liquid (probably water) flowed into the crack, carrying the telltale globules of carbonate minerals.)
- When was the meteorite ejected from Mars? (About 16 million years ago, judging from how much space radiation changed the atoms in the minerals.)
- How did the rock achieve Mars' escape velocity, 5 kilometers per second? From the impact of another meteorite near the rock's location on Mars.
- What was Mars like 3.6 billion years ago? It was apparently warmer and wetter than today.
Clearly, the striking announcement of possible life on Mars is just the of the step in a long exploration process. Scientists, energized, are returning to study existing samples, and searching for new ones. "There will be excruciatingly detailed studies of ALH 84001 and its possible microfossils to see if they are actually microfossils," says Allan Treiman, a staff scientist at the Lunar and Planetary Institute in Houston.
"Folks will be looking for evidence of cell walls and other cell structures," Treiman adds. "They also will be looking at 'cell' sizes and shapes to see if they look like a population of growing, reproducing bacteria. And there are 11 other martian meteorites that will get studied again within inches of their lives." Proof that life lived on Mars must await an expedition to drill into the planet. Such an effort is being considered for a launch in 2005. Here's a detailed discussion of technical issues regarding the meteorite.
The
Rocky Horror Show, part II
Hear about the newly discovered planet, the one that could, just could, be hospitable to life? So did The Why Files... .
For more speculation on the meaning of extra-terrestrial life, see "Are We Alone?" in the bibliography.
What's the real reason why life chose Mars?
Let's face it: those Martian microbes had it easy. At least they didn't have to make a living in a pot of boiling water. (Earthly microbes can, you know)...
