ALH84001, a controversial meteorite that was found by a team of meteorite hunters from the ANSMET (Antarctic Search for Meteorites) program, in December 1984. It weighted almost 5 pounds, and was shaped like a large potato, approximately 6 x 4 x 3 inches.

The rock was originally formed from molten lava, possibly from an ancient martian volcano. Its planetary origin was determined from trace gases found inside it, which match Mars' atmosphere perfectly. It must have left Mars from the explosion of a meteor impact, as no volcanic eruption could be violent enough to throw it away fast enough for it to leave Mars' gravitational field. (Mars' escape velocity is no less than 5,000 kilometers (11,000 miles) per second.) It spent the next 16 million years floating through space, before succumbing to Earth's gravitational pull approximately 13,000 years ago.

The reason for the controversy around ALH84001 is the claims that it contains evidence of biological life. The first theories arose in 1996, when small globules of carbonate minerals were found, and these must have been formed by liquid water. Futhermore, the mineral deposits have been shown to be much older than than life on Earth, perhaps as old as 3,6 billion years old. Additionally, Doctor David McKay and his colleagues discovered ropy tubular structures in the mineral globules, which they interpreted as possible fossil bacteria. However, it has later been discovered that these bacteria could have infiltrated the rock while it was in Antarctica.

Recently, however, renewed evidence of ancient life on Mars were presented. NASA researchers found that the magnetite crystals embedded in the meteorite are arranged in long chains, which they claim could only have been formed by once-living organisms. Otherwise the magnetic "chain" would have collapsed into an iron clump. Their results are reported in the February 27 Proceedings of the National Academy of Sciences.

The full text of their research paper can be found online:

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