Two recent studies have enlivened the argument that an ancient meteorite contains fossils of bacteria from Mars. But several scientists say the reports fall short of proving that the Mars rock, ALH84001, harbors biological remains.
The rock became famous in 1996 when researchers reported several lines of evidence indicating that it harbors relics of life. Most dramatic were tiny worm-shaped features that the team suggested could be fossils of bacteria (SN: 8/1/96, p. 84). Other lifelike features within the meteorite include carbonate globules and mineral grains of magnetite within those globules. Although magnetite can form inorganically in geologic settings, some bacteria on Earth make magnetite crystals that serve as tiny compass needles to navigate through water and sediment.
Now, the same researchers and their new collaborators have homed in on magnetite structure. Studying single crystals from the Mars rock with an electron microscope, the team finds that about 25 percent have a shape identical to that produced by the terrestrial aquatic bacterium called MV-1. Beyond having the intriguing shape, known as a truncated hexa-octahedral, the Martian magnetites are as chemically pure as those made by MV-1, the team notes.
“We have seen a population of magnetites in the ALH84001 carbonate globules that are indistinguishable from magnetites that are formed by bacteria on Earth,” says study coauthor Kathie L. Thomas-Keprta of Lockheed Martin in Houston. No crystals of this shape and purity have ever been detected in the absence of life, she adds.
Extensive testing has convinced the team that terrestrial bacteria or magnetite hadn’t penetrated the Mars rock since its fall to Earth some 13,000 years ago. They describe their work in the Feb. 27 Proceedings of the National Academy of Sciences (PNAS).
“Nobody disagrees with the conclusion that what they see is consistent with bacterial magnetite,” says microscopist John P. Bradley of MVA Inc. in Norcross, Georgia, who has studied magnetite in meteorites. Still, he adds, the crystals could have formed by some unknown inorganic process.
In a second article in the Feb. 27 PNAS, E. Imre Friedmann of Florida State University in Tallahassee and his collaborators report that magnetite chains in the Mars rock closely resemble the string-of-pearl chains made by many bacteria on Earth.
Bradley contends that the chains could have formed in ALH84001 by a blast of heat, perhaps when the rock was chipped off Mars. Friedmann counters that if chains did form in this way, they wouldn’t have the uniformity of those in bacteria.
In the March/April American Mineralogist, Houston scientists describe an experiment in which they heat artificially produced carbonate globules, which then decompose into magnetite grains. They claim that the grains resemble those found in ALH84001, but Thomas-Keprta disagrees.
“We’ll let the scientific community hash it out,” Thomas-Keprta says.