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.
Figure 1: The famous Mars meteorite ALH84001 that contains indirect evidence of life.
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.
Figure 2: Photo courtesy NASA. Scanning electron microscope image of ALH84001, showing possible fossilised bacteria (centre).
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.
Figure 3: Another possible fossilised bacteria at the surface of ALH84001.