22. ALH 84001 Carbonate Globules

This thin-section view is about 0.5 millimeters across. The possible traces of ancient life in ALH 84001 are in rounded brownish and clear globules like these. The globules are made of the carbonate minerals siderite (iron carbonate, brownish) and magnesite (magnesium carbonate, clear), and the dark outside rinds are rich in iron oxide and sulfide minerals (magnetite, pyrrhotite, and possibly greigite). These globules formed after ALH 84001 had solidified from lava (at 4.5 billion years ago), but their age is not yet known (estimates range from 4.0 billion years to 1.2 billion years old). It was in these globules that the research team headed by Dr. David McKay of NASA’s Johnson Space Center found the features that might be fossil relics of martian life. Dr. McKay and his group worked for two years before presenting their work for publication. In these rounded globules, they found three kinds of evidence that might be from ancient life on Mars. These three kinds of evidence are (1) mineral grains like some made by Earth bacteria; (2) organic chemical molecules that might be from decomposed organisms; and (3) objects that have the same sizes and shapes as some Earth bacteria.

Allan Treiman (Lunar and Planetary Institute)

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