SOLAR SYSTEM/Meteorites 235 were first thought to come from Mars because of the presence of oxidised iron and hydrated minerals Later in the twentieth century, entrapped gases in these meteorites were found to be similar to the Martian atmosphere sampled by Viking missions The ages of formation of these meteorites (see below) are not those of the asteroidal meteorites (ca 4550 Ma), but fall into two groups – Nakhlites 180 and Shergottites 1300 Ma (equivalent to Earth’s Jurassic and midProterozoic) The widely accepted source of these meteorites is Mars – the source must surely be a planet, and the mechanism the spalling off the surface by large impacts (there are theoretical objections to volcanic ejection) However there are problems: the trapped atmosphere should be the planet’s atmosphere 180 and 1300 Ma ago, not the present atmosphere, and atmosphere’s change with time: also, why are the 26 SNC meteorites recovered to date all a limited range of familiar igneous rocks – Mars is a very diverse surfaced planet? A hypothetical geological history of Mars has been built up by scientists on the basis of these 26 meteorites, an edifice which direct exploration may surely demolish? The joker in the pack is the famous ALH 84001 from Antarctica, a unique orthopyroxenite, which has a formation age similar to the asteroidal meteorites and contains the famous putative microfossils, the evidence about which seems now to favour inorganic rather than organic origin Lunar Achondrites Lunar achondrite meteorites (Figure 14) so completely match lunar surface rock samples obtained by Apollo and Luna missions that there is no doubt as to their provenance First found in Antarctica, they have been later recognized in an existing collection from Western Australia and also new finds in the Libyan desert Volcanic ejection can be ruled out; isotopic evidence suggests that all were spalled off by geologically quite recent and relatively minor impacts on the surface of the Moon, but here there is a glaring unresolved problem There is widespread scientific acceptance of a major impact bombardment of the Moon 3.9 Ma ago, forming innumerable and immense craters: this must have hurled vast volumes of rock out into space, sampling deep below the regolith and surficial breccia (which is all that has yet been directly sampled), there is no trace of this material in the varied log of meteorites Where has it gone? Cratering and Tektites Meteorites normally land with little effect on the ground – even the 11 tonne Mundrabilla iron left no Figure 14 Lunar sourced achondrite meteorite, ALH 81005 from Antarctica, discovered in 1981, after the first such discovery in 1979 by Japanese scientists in the Yamato Mountains The structure is that of the lunar regolith breccias and a large white fragment of highlands anorthosite is visible The cube has sides of cm length (from McCall 1999, reproduced with permission of Palgrave Macmillan) dent in the limestone surface – but multiple showers may produce small, simple craters (the 1947 SikhotAlin shower produced 106 associated with nickel-iron fragments) Larger masses have, in the quite recent geological past, produced kilometre-scale simple craters associated with nickel-iron (e.g., Canyon Diablo, Arizona; Wolfe Creek, Western Australia) and about 170 larger simple craters and more complex ring structures in the geological record are attributed to impact explosion processes involving larger masses, even asteroids The largest, at 180 km diameter (Chicxulub, Yucatan, Mexico) has been associated with the Cretaceous-Tertiary boundary extinction of life (see Impact Structures) Only geochemical traces of the impactor have been discovered at such sites Tektite showers were associated with a very small minority of such structures, but tektites are not meteorites, but are glassy objects melted from the impacted surface rocks, and spread over strewn fields at long distances from the impact sites (see Tektites) Fossil Meteorites The only recorded case of a meteorite being recorded in ancient rocks relates to limestone strata at a quarry near Goteborg, Sweden, where there are 12 horizons crowded with ordinary chondrite meteorites, which must have been derived from rains of stones 480 Ma ago, in the Ordovician, the stones falling onto the limey mud bottom of shallow sea Meteorites not fall repeatedly at the same place because of the Earth’s rotation and this repetition is astonishing, as it implies repeated globally spread rains of meteorites over a period of about 1.75 million years