Debate over life in Mars rock rekindles
By Ron Cowen
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.
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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.