The interest in extremophilic microbes intensified after the discovery of the now-famous Martian meteorite known as ALH 84001, a hunk of rock found in the Allan Hills region of Antarctica on December 27, 1984. After it was discovered, this piece of cosmic slag was filed away and forgotten for a decade. It was finally reexamined, however, and determined to be from Mars. A team of NASA scientists then began to probe it, and their examination culminated in the stunning announcement on August 7, 1996, that this particular piece of rock might contain fossils of Martian microbes in its stony grasp.
Of the various lines of evidence used by NASA scientists to arrive at this startling conclusion, the most fascinating were small rounded objects in the meteorite resembling fossil bacteria. And why not? Conditions on the Martian surface today are highly inimical to life: subject to harsh ultraviolet radiation, lack of water, numbing cold. The Mars Pathfinder expedition only seemed to confirm the planet's inhospitality—even for the highly tolerant ex-tremophilic microbes. But what of the Martian subsurface? Perhaps life still exists in the subterranean regions of Mars, where hot hydrothermal liquid associated with volcanic centers could create small oases, a Martian equivalent of Earth's deep biosphere, replete with archaeans.
And even if life is now totally extinct on Mars, what of its past? Since the Viking landing of 1976, scientists have known that the ancient Mars had a much thicker atmosphere and had water on its surface, at least for a brief period of time. Three billion years ago, Mars could have been warmer because of its cloaking atmosphere. Such conditions still would have been too harsh for animal life, but judging from what we now know about the extremophiles on Earth, the early Martian environment would have been quite conducive to colonization by microbes. The extremophiles need water, nutrients, and a source of energy. All would have been present on Mars. It may be that life does not exist on Mars today. Yet there may be a great deal that we can learn about ancient Mars in its fossil record—a fossil record perhaps populated by Martian analogs to Earth's extremophiles. Andrew Knoll of Harvard University has pointed out that for very old rocks, the fossil record may be fuller on Mars than it is on Earth, because there has been little erosion or tectonic activity on Mars to erase the billions of years of fossil records. Knoll has even told us where on Mars to search for fossils: on an ancient volcano named Apollinaris Pater, whose summit shows whitish patches interpreted to be the minerals formed by escaping gases, or in a place called Dao Vallis, a channel deposit on the flank of another ancient volcano where hot water may have flowed out from a hydrothermal system within the Martian interior. Mineral deposits there might yield a rich fossil record of ancient Martian extremophiles.
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