The latest research into possible evidence of past Martian life has failed to win over some scientists, who remain skeptical of any claims that a Martian meteorite shows signs of life that once existed on Mars.

The latest salvo in the debate about life on Mars was the publication Tuesday of two papers in the journal Proceedings of the National Academy of Sciences (PNAS). Those papers provided new evidence that magnetite crystals found in the Martian meteorite ALH84001 could only have been formed by living organisms.

Top: Modern magnetotactic bacteria, one showing a chain of magnetite crystals, as seen in the backscattered scanning electron microscope. Bottom: Magnetite crystals and chains of magnetite crystals in the Martian meteorite ALH84001 in the backscattered scanning electron microscope. One conspicuous chain indicated by arrows. The diameter of a single crystal is approximately one-millionth of an inch. Photo: NASA

One group of scientists, led by Kathie Thomas-Keprta of NASA's Johnson Space Center (JSC), found that magnetite crystals in the meteorite were identical in shape to those on Earth that are formed solely by a particular strain of bacteria. Another team, headed by Imre Friedmann of NASA's Ames Research Center, found chains of magnetite crystals that they believe could only have been formed within living organisms.

Some critics of past studies that linked magnetite crystals and other features of ALH84001 with primitive Martian life were not won over by this latest work. "While I welcome the new findings, and find them extremely interesting, I still feel they fall far short of any standard of proof," said Ralph Harvey of the Case Western Reserve University in Cleveland.

"I wish I could tell you that I believe the latest paper sheds new light on the ALH84001 issue," said John Bradley of the Georgia Institute of Technology and MVA, Inc. "In fact, all they do is restate their original hypothesis that some of the magnetites are consistent with a biogenic origin."

Bradley and Harvey, along with Harry McSween of the University of Tennessee, studied magnetite crystals in ALH84001 as early as 1996, within months of the original announcement that evidence for Martian life had been found in the meteorite. They found magnetite crystals shaped in the forms of whiskers and plates that, they concluded, could only have been formed by inorganic processes at high temperatures.

Both Bradley and Harvey disagree with the wholesale rejection of any inorganic processes that could have formed the magnetite crystals and chains of crystals they found. "Neither of the current papers did any tests to explore the alternative hypothesis that these magnetites morphologies or arrangements are inorganic in nature -- instead, they simply state that 'No inorganic process is known to produce similar structures,'" said Harvey. "This is a very weak argument: the truth of the matter is, nobody has really looked."

There is also some evidence, contrary to the claims published in the PNAS papers, that inorganic processes could form magnetite crystals similar to those found in the meteorite. In a paper published in the March issue of the journal American Mineralogist, a group of scientists at NASA JSC and other institutions found an inorganic method that could explain not only the magnetite crystals found in the meteorite, but the carbonates and sulfides also found in ALH84001.

"Chemically pure, single-domain, defect-free magnetite crystals" were formed by decomposing carbonates rich in iron at temperatures of 470 deg C (880 deg F) in a laboratory experiment, the authors of the paper noted. On Mars, the magnetite crystals, carbonate globules, and iron-rich sulfides could have formed when carbonates formed in waters rich in carbon dioxide which were later subjected to a "transient thermal event" such as an impact, they concluded.

Those who have long argued that the meteorite does show evidence of past Martian life remain convinced that the latest papers confirm their earlier work. In a paper recently published in the journal Precambrian Research, Everett Gibson, a NASA JSC scientist and one of the lead authors of the August 1996 paper that ignited the debate, argued that features found in two other Martian meteorites that closely resemble the putative "microfossils" seen in ALH84001 may mean Mars harbored life until relatively recently.

"If the features observed in the two younger Martian meteorites are confirmed to have a biogenic origin, life may have existed on Mars from 3.9 billion years ago to as recently as 165 to 175 million years ago," Gibson said.

Harvey, though, cautioned against trying to read too much into a single rock. "ALH84001 is being studied at a level of detail far beyond any other rock ever studied," he noted. "Not only that, it's exceptionally complex, with a history over 4 billion years old including volcanism, impacts, travel through space...the list goes on and on. We should expect to see a lot of confusing and mysterious things."

"We know by now that closer investigations on previous claims of biological activity in ALH84001 have shown compelling inorganic explanations, including the previous claims for magnetite," he said. "There's no reason to think that further study won't follow the same path and yield similar results."