Martian meteorite mineral not made by organisms

University of Hawaiʻi
Tara Hicks, (808) 956-3151
SOEST Outreach Specialist
Ed Scott, (808) 956-3955
Hawaii Institute of Geophysics and Planetology
Posted: May 15, 2002

Claims that tiny crystals of iron oxide in a martian meteorite were formed by organisms on Mars are incorrect, according to a new study published today. Two scientists at the University of Greenwich, London, and the University of Hawaii believe they have found definitive evidence that will end a long-running controversy over the origin of the iron oxide grains.

Since the startling report in 1996 by NASA scientists of possible evidence for life in the Martian meteorite ALH84001, attention has become increasingly focussed on the origin of tiny crystals of an iron oxide called magnetite as they appeared to provide the most compelling evidence for Martian life. The magnetite crystals are remarkably similar in size, shape and composition to the tiny magnetites used by bacteria as compasses to help them navigate through pools of water, and seemingly unlike all other known iron oxides in rocks.

The new study of ALH84001 by David Barber and Ed Scott, which is published in the Proceedings of the National Academy of Sciences USA, shows that the planes of oxygen atoms in the magnetite crystals are beautifully aligned with those in the surrounding carbonate crystal. This proves that the magnetite crystals could not have formed elsewhere and then been deposited in the carbonate, as the NASA scientists argued. They must have formed exactly where they are observed today in the carbonate.

Barber and Scott also discovered oriented crystals of magnesium oxide in the martian carbonate and conclude that the iron and magnesium oxides both formed when the carbonate was hot and had partly decomposed to form carbon dioxide gas. Since the meteorite was heated by an impact on Mars 4.0 billion years old that melted and vaporized many minerals in the rock, Barber and Scott infer that the iron and magnesium oxides formed then. Martian organisms cannot be responsible for the size and shape of any magnetite crystal in ALH84001.

Does this result mean that the most famous meteorite will now be returning to obscurity? Absolutely not, according to Scott. "Meteorites are cosmic gifts that keep on giving, and giving!" ALH 84001 is the oldest rock we have from any planet as it crystallized over 4.4 billion years ago and it surely contains clues to many mysteries about the solar system. For example, if Mars and the Moon were both bombarded by asteroids 4.0 billion years ago, why should we find an ancient meteorite from Mars but no equally ancient lunar meteorites or returned samples from the Moon of comparable size? Can the magnetite crystals in ALH84001 tell us the strength of the magnetic field on Mars 4.0 billion years ago? An ancient Martian rock, even one that has apparently failed to bring evidence of extra-terrestrial life, is a precious gift.

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