ALH84001

Sample Summary - ALH84001

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 consists of 97-98% orthopyroxene with minor plagioclase glass, chromite and carbonate. The presence of oxidized iron in the chromite (Fe, Cr, Mg spinel) led to the reclassification as a martian meteorite. This was confirmed by oxygen isotope analyses. There is little other connection between ALH84001 and the other martian meteorites.

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 from Mars by impact, while the termination of that exposure gives a terrestrial 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 (see LPI and ANSMET web sites). These grains are the sites of the three types of evidence that McKay et al (1996) suggest represent fossil life on Mars: PAHs organic molecules; oxide and sulfide biominerals; and nannofossil-like structures. A more detailed survey of ALH84001 carbonates is given below. To summarize, 99% of this unique rocks 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.