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SPACE GEOSCIENCE ARTICLE

Life on Mars?
by MIKE BALDWIN

This 4.2 lb. potato-shaped meteorite, called Alh84991, was found in 1984
on the Allan Hills ice field, Antarctica,
by an expedition of the National
Science Foundation's Antarctic
Search for Meteorites program.

06.21.00: ALH84001 and other meteorites, were preserved for study in Johnson Space Center's Meteorite Processing Laboratory. The martian origin of ALH84001 was recognized in 1993.

This 4.2 lb. potato-shaped meteorite, called Alh84991, was found in 1984 on the Allan Hills ice field, Antarctica, by an expedition of the National Science Foundation's Antarctic Search for Meteorites program.
This report is from research material located at Planetary Materials Curation at NASA-JSC (Johnson Space Center).

Allan Hills 84001 is an orthopyroxene cumulate and the only one currently known to be from Mars. It was originally classified as a diogenite, an orthopyroxene achondrite. It is made up of 97-98% orthopyroxene with some plagioclase glass, chromite and carbonate.

The presence of oxidized iron in the chromite (Fe, Cr, Mg spinel) led to the reclassification of this meteorite as a martian meteorite. This was confirmed by using oxygen isotope analysis. ALH84001 is by far the oldest martian meteorite, with a crystallization age of 4.5 Ga (billion years).

It is a sample of the early martian crust. The cosmic ray exposure age of 16 Ma (million years) dates the ejection of ALH84001 from Mars by impact, while the termination of the cosmic ray exposure gives it a terrestrial (Earth) age of 13 Ka (thousand years). The small amount of carbonate in ALH84001 is the center of attention concerning the possibility of life on Mars. These small grains which are barely visible to the naked eye, range up to 200 microns in size. They appear to have formed in fractures inside this igneous rock in the presence of liquid water or other fluid. There is considerable debate about the origin of these carbonates. The debate focuses on the temperature of formation. These grains are the sites of the three types of evidence that examiners in 1996 suggest represent fossil life on Mars: organic molecules; oxide and sulfide biominerals; and nannofossil-like structures.

A more detailed survey of ALH84001 carbonates can be found at
http://www-curator.jsc.nasa.gov/curator/antmet/marsmets/life.htm.

To summarize, 99% of this unique rock provides evidence for the earliest geologic history of Mars, while no more than 1% of it bears witness to the interaction of the rock with martian hydrosphere and atmosphere and possible evidence for life.

 

    


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