Besides Earth, the only inner-solar system body with thermal and atmospheric conditions capable of harboring life at least in the past when it was likely to have had a more substantial atmosphere is Mars, the surface of which appears to have been marked by ancient flowing water. One of the early targets of United States and Soviet space efforts was exploration of the martian surface and the search for life there. The instrumentation of the first successful martian landings, during the Viking mission in 1976, was particularly designed for in situ experiments aimed at checking the martian soil for the presence of organic matter and of life. The outcome of these experiments was ambiguous, and it became clear that, to find remnant water and organic compounds of biological or nonbiological origin, it would be necessary to search below the ultraviolet-irradiated surface. Such experiments consequently play a central role in the future Mars missions planned by the United States and Russian space agencies.
Meanwhile, straying asteroids and comets have proved Lord Kelvin’s prediction that “when two great masses come into collision in space, it is certain that a large part of each is melted, but it seems also quite certain that in many cases a large quantity of debris must be shot forth in all directions, much of which have experienced no greater violence than individual pieces of rock experience in a landslip or in blasting by gun power.” Non-destructive escape of such debris is more likely if the gravitational field is relatively low, as is true of Mars. In the 1960s, it was realized that three meteorites, Shergotty, Nakhla and Chassigny, had unusual petrological features and solidification ages, defining the ‘SNC’ type of meteorite; they were suspected to derive from other planets, most likely Mars because of its vicinity and the likelihood of Earth intersecting their orbits and capturing them. The Viking mission revealed that, compared with Earth, the Mars atmosphere is strongly enriched in 15N relative to the lighter isotope 14N, which can slowly leak away from the relatively weak gravitational field of Mars. This has provided an isotopic signature that has proved useful in identifying the origin of the SNC meteorites /The Antarctic ice fields provide a propitious environment for the collection, preservation and delivery of fallout from the atmosphere and outer space. Meteorites falling in the high-altitude region of accumulating snow become imbedded in the compacting ice, travel downhill towards the ocean in the slow-moving ice sheet, and are gradually exposed in low-altitude areas as the stagnating and rising ice pushed up by obstacles such as mountain ranges — is slowly evaporated by the dry, gale-force winds. Some of the hundreds of meteorites collected in the Antarctic are of the SNC type, and so probably of martian origin. Most resemble rocks formed on Earth, but within some SNC meteorites for example, Elephant Moraine 79001 and Nakhla shock melted glass inclusions and veins with cavities containing trapped gases were discovered, which were found to have the isotope signature characteristic of the martian atmosphere. Since this discovery, it has been assumed that all the SNC meteorites came from Mars, including a rock of ∼4 kg found in 1984, named Allan Hills 84001 it is this object that is the focus of all the recent excitement about the possibility of there having been life on Mars /
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