August 6, 1996 (a Tuesday)

The ALH84001 Meteorite

On this date, NASA Administrator Daniel Goldin announced the discovery of evidence of a primitive bacterial life form on Mars. The evidence came from a tiny putative fossil found on a meteorite in Antarctica thought to have come from Mars billions of years ago. The meteorite, called ALH84001, was found in 1984 in Allan Hills ice field, Antarctica, by an annual expedition of the National Science Foundation’s Antarctic Meteorite Program. It was preserved for study in JSC’s Meteorite Processing Laboratory and its possible Martian origin was not recognized until 1993.

The indication of life hinges on three important pieces of evidence, all discovered within mineralized fractures in the meteorite in close proximity to each other:

  1. hydrocarbons which are the same as breakdown products of dead microorganisms on Earth,
  2. mineral phases consistent with by-products of bacterial activity, and
  3. tiny carbonate globules which may be microfossils of the primitive bacteria.

Based on age dating of the meteorite, the following scenario has been proposed:

  1. The original igneous rock solidified within Mars about 4.5 billion years ago, about 100 million years after the formation of the planet (based on isotope ages of the igneous component of the meteorite).
  2. Between 3.6 and 4 billion years ago the rock was fractured, presumably by meteorite impacts. Water then permeated the cracks, depositing carbonate minerals and allowing primitive bacteria to live in the fractures.
  3. About 3.6 billion years ago, the bacteria and their by-products became fossilized in the fractures (based on isotope ages of the minerals in the fractures).
  4. 16 million years ago, a large meteorite struck Mars, dislodging a large chunk of this rock and ejecting it into space (based on the cosmic ray exposure age of the meteorite).
  5. 13,000 years ago, the meteorite landed in Antarctica.
  6. The meteorite, ALH84001, was discovered in 1984 in the Allan Hills region of Antarctica.

References:

  • S. McKay, E. K. Gibson Jr., K. L. Thomas-Keprta, H. Vali, C. S. Romanek, S. J. Clemett, X. D. F. Chillier, C. R. Maechling and R. N. Zare. “Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001D, Science 273 (5277): 924-930 (16 August 1996).
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