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Martian meteorite

Mars meteorites include three rare groups of achondritic (stony) meteorites (16 objects total) with isotope ratios that are said to be consistent with each other and inconsistent with the earth. It should be pointed out, however, that the isotope ratios do not actually match Mars ratios especially well, to the extent that Mars ratios are known, although they do differ substantially from Earth isotope ratios and from what is known of Lunar ratios.

All the meteorites are igneous rocks. Lherzolitic shergottites (one from Antarctica, 2 from California) are identified by their Deuterium/Hydrogen ratios. The crystals appear to be 154-187 million years old and they appear, from cosmic ray analysis, to have spent 2.5 to 3.6 million years in space. There are also basaltic shergottites, some of which appear (from the presence of hydrated carbonates and sulfates) to have been exposed to liquid water prior to injection into space. One of the shergottites, known as ALH84001, is much older than the others - about 4.5 billion years. In this respect, it resembles a typical meteorite rather than the other shergottites. The other two types of Martian meteorites are Chassignites and Nakhlites. All appear to have been on earth no more than 35,000 years. Together, these objects are called SNC meteorites.

Although common wisdom is that the SNC meteorites are from Mars, the Mars origin theory does have some problems. The isotope ratios are not an especially good match. A widely published graph showing a near-perfect match is alleged to be constructed from examples selected to "prove" the hypothesis and to be a poor representation of the real data. For example, the Ar40/Ar36 ratio for one meteorite (1650) is almost exactly half-way between Earth (300) and Mars (3000). Carbon Dioxide, the predominant gas in the current Mars atmosphere, is unaccountably rare in the trapped gases in the SNC meteorites. The SNC meteorites do not show shock artifacts that would be expected in small objects ejected with enough velocity to escape Mars. The majority of SNC meteorites are quite young by geologic standards and seem to imply that volcanic activity was present on Mars only a few hundred million years ago. Cosmic ray traces in the meteorites indicate relatively short stays (3 to 3.5 million years) in space. It is asserted that there are no large young craters on Mars that are candidates as sources for the SNC meteorites (How does one identify a young crater on Mars?).

Possible evidence of life has been hypothesized in three meteorites.

In August 2002, a NASA team led by Thomas-Keptra published a study indicating that 25% of the magnetite in ALH84001 occurs as small, uniform-sized crystals in a crystal form that, on Earth, is associated only with biologic activity. The remainder of the material appears to be normal inorganic magnetite. The extraction technique did not permit determination as to whether the possibly biologic magnetic was organized into chains as would be expected.

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