Here Be Dragons

THE INCREDIBLE SHRINKING MARTIANS
McKay's team does not claim to have found a "clincher"—a single
piece of evidence that would be irrefutable proof of the past existence
of living organisms on Mars. Rather, the team points to a number of
features of the meteorite, mostly to do with the carbonate globules,
that the team believes can collectively best be explained by biological
processes.
First, within the carbonate globules, the researchers found minute
crystals of magnetite (Fe 3 O 4 ). These crystals, measuring only about
50 nanometers across (a nanometer is 10' 9 meters), are similar to
those found in many terrestrial bacteria, where they serve as little
compasses. By orienting themselves with respect to the slanting lines
of the Earth's magnetic field, the bacteria are able to tell "up" from
"down."
Second, the researchers found organic carbon compounds in association
with the globules. These compounds included a class of substances
called polycyclic aromatic hydrocarbons, or PAHS, that the researchers
interpreted as being the decay products of microbes that
became trapped in the forming globules.
Third, and most dramatically, McKay believes that he and his colleagues
have seen the martian microbes' fossilized remains. Using
very high-resolution scanning electron microscopy, McKay and his
colleagues found areas within the carbonate globules that contain
ovoid or tubular structures resembling the tiniest bacteria found on
Earth. A photograph of one particularly striking example—a segmented,
wormlike object resting languidly on a bed of mineral
grains—has appeared in newspapers, in magazines, and on television
shows around the world. This 'creature'—if 'creature' it is—has become
Mars's de facto ambassador to Earth.
McKay's announcement ignited a firestorm of controversy. Some
critics simply asserted their reasons for thinking that McKay's conclusion
had to be wrong. It was pointed out, for example, that Mars lacks
a magnetic field, so martian bacteria would have no use for magnetite
particles. It was also claimed that the carbonate globules must have
formed at temperatures far too high for living things to have existed
inside them. Many other scientists dropped whatever they were doing,
beseeched NASA for a chip of the meteorite, and conducted their own
tests. Among the latter was Jeff Bada, the San Diego origin-of-lifer
whom we met in Chapter i.
Bada (with several colleagues) studied the organic materials in the
meteorite. 13 He looked particularly for amino acids in the carbonate
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