Meteors and Micrometeorites

There is a small group of meteorites originating from the same parent body, all of igneous origin, called the SNC (after their type specimens Shergotty, Nakhla and Chassigny). These meteorites have comparatively young crystallization ages equal to or less than 1.3 Ga. One of these meteorites, EETA 79001, was found on Antarctica in 1979. It had, trapped within glass pockets, gas which both compositionally and isotopically matched, in all respects, the make up of the Martian atmosphere as measured by the Viking mass spectrometer utilized for assessing the soil for the presence of organic compounds.

The data provide a very strong argument that at least some particular SNC meteorites comes from Mars. There are now eightteen SNC meteorites known in total. The two SNC meteorites EETA79001 and ALH84001 supply new and highly interesting information. A subsample of EETA79001, excavated from deep within the meteorite, has been subjected to stepped-combustion. The CO₂ release from 200°C to 400°C suggests the presence of organic molecules.

Because Mars had a warm and wet climate in the past, its surface must be covered by both impact generated regolith and sedimentary rocks deposited by running and/or still water. Such consolidated sedimentary hard rocks should be among the Martian meteorites but are not known. It is possible that they did survive escape acceleration from the Martian surface but did not survive terrestrial atmospheric entry because of decrepitation of the cementing mineral. The STONE experiment is aimed at studying the physical and chemical modifications in sedimentary rocks during atmospheric infall.

A basalt, a dolomite (sedimentary rock) and an artificial Martian regolith (80% crushed basalt and 20% gypsum) were embedded into the ablative heat shield of Foton 12 which was launched on September 9 and landed on September 24, 1999. The recovered entry samples were analyzed for their chemistry, mineralogy and isotopic compositions by a European consortium. Atmospheric infall modifications are made visible by reference to the untreated samples. The results suggest that some Martian sediments could in part survive terrestrial atmospheric entry.

Even if the evidence for ancient life in ALH84001 is not firmly established, two SNC meteorites show the presence of organic molecules, suggesting that the ingredients required for the emergence of a primitive life may have been present on the surface of Mars. Therefore, it is tempting to consider that microorganisms may have developed on Mars until liquid water disappeared. Since Mars probably had no plate tectonics and since liquid water seems to have disappeared from the Mars surface very early, the Martian sub-surface perhaps keeps a frozen record of the very early forms of a terrestrial-like life.

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Fig. 1: The famous Mars meteorite ALH84001 that contains indirect evidence of life.

Fig. 2: Photo courtesy NASA. Scanning electron microscope image of ALH84001, showing possible fossilised bacteria (centre).

Fig. 3: Another possible fossilised bacteria at the surface of ALH84001.