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Mercury, September/October 1996 Table of Contents

(c) 1996 Astronomical Society of the Pacific

A certain potato-size rock has caused quite an uproar. When a colleague showed me the first CNN report, I was pretty doubtful. Organics in a meteorite? What's new about that? But as I tracked down the press releases, I realized I might be living the moment that had inspired me into planetary science: the moment when we found out there was life on Mars.

The potato in question is a 4.2-pound meteorite, Allan Hills 84001, the oldest of 12 meteorites whose composition indicates they were once pieces of Mars. Allan Hills is our only material record of events on the Red Planet billions of years ago, when the river valleys seen by the Mariner and Viking spacecraft formed [see "The Surface of Mars," January/February 1983, p. 2]. Within the meteorite, a team of researchers led by David McKay of the NASA Johnson Space Center found evidence for microorganisms:

• The meteorite contains complex organic compounds, the first to be identified in a martian meteorite. The compounds, polycyclic aromatic hydrocarbons, are produced when dead microbes decompose. These organics can also be produced during star formation; indeed, they appears in most meteorites. But spectra show that the hydrocarbons in Allan Hills differ from those in other meteorites.
• The meteorite also contains magnetite and iron sulfide, minerals that on Earth are associated with sulfur-eating bacteria. Whereas magnetite forms by oxidation, iron sulfide forms by the opposite process, reduction. The proximity of these two processes is characteristic of biological activity, and difficult to produce by other means.
• The organics, magnetite, and iron sulfide are concentrated in the outermost layers of small globules of carbonate, a mineral that on Earth is typically the product of microorganisms. At the least, the presence of carbonates confirms that Mars once had running water and thicker air. According to an analysis of the carbon and oxygen isotopes in the carbonate, the globules formed 3.6 billion years ago in water at a temperature between 0 and 80 degrees.
• On the surface of the globules, electron microscopes spotted egglike and wormlike shapes that look like terrestrial fossils of the smallest bacteria. Most of these shapes are roughly 50 nanometers across.

The Allan Hills study seems to have avoided the pitfalls, but only further testing will tell. Do the sulfur isotopes in the meteorite bear the telltale signature of life? A new University of New Mexico study has suggested they don't. Are the carbonates indeed 3.6 billion years old, and did they really form at comfortable temperatures? Maybe not, other studies have asserted. Do the bacteria-like structures have cell walls? Do they contain amino acids? Are any of them caught in the act of cell division? Ultimately, the answers may depend on retrieving more rock samples from Mars, a mission that NASA administrator Dan Goldin said the space agency may now launch as soon as 2001.

On Earth, paleontologists have found microfossils and carbon- isotope signatures in the oldest sedimentary rocks, 3.8 billion years old. Older rocks have been destroyed by geologic activity, wiping out our record of the origins of life on Earth. But this record is preserved on Mars. Our genealogy may lie there.

Which characteristics of Earthly life are fundamental, and which are an accident of historical circumstance? If microbial Martians are similar to our bacteria, even though they arose independently, there must be a certain inevitability about what we are [see "The Copernican Revolution Comes Around," p. 14]. If they are identical to our bacteria, then life must have emerged on one planet and been carried by meteorites to the other. We may, as Stanford chemist Richard Zere said on "Nightline," be the descendants of Martians.

If life arises wherever it can -- wherever there are water, organics, and energy -- we live in a universe teeming with living beings. Astronomers have thought so for years, but only this year have the speculative vapors condensed: first, planets around other Sun-like stars [see "In the Wink of a Star," July/August, p. 20], and now, life on another world. Aug. 6, 1996 was the day the universe came alive.