Martian Meteorites: Groups Various groups of meteorites have been found that come from Mars. They have presumably been launched into space due to an impact on the martian surface and, following a (long) cruise through interplanetary space, then happened to fall on Earth. The full list comprises 25 such meteorites known in mid-2001. The resons why we can be relatively sure that they do indeed come from Mars, are described in a true scientific detective story. More information about these and other meteorites will be found at the website of The Natural History Museum in London The main groups of martian meteorites are described below. Shergotty Shergottites are the youngest of the martian rocks, crystallising between ~ 150 and 325 million years ago. The shergottites are divided into two groups, the basaltic (16 specimens) and the lherzolitic shergottites (3 specimens). The division is on the basis of mineralogy, and thus igneous history. The basaltic shergottites are fine-grained pyroxene-plagioclase cumulate rocks formed by crystal accumulation in an extrusive lava flow. Almost all the plagioclase has been converted to maskelynite, by shock pressures up to ~ 60 GPa. The lherzolitic shergottites are coarse-grained olivine-pyroxene cumulate rocks formed at depth. Again, plagioclase has been converted to maskelynite. Nakhla Nakhlites are a group of 3 meteorites with closely-related petrogenetic histories. They are igneous cumulates, with crystallisation ages around 1300 million years. Their mineralogy comprises the clinopyroxenes augite and pigeonite, plus olivine, with minor feldspar, ilmenite and chlorapatite. The mineralogy and petrology is consistent with formation either as a surficial lava flow, or from a sill or dyke emplaced close to the martian surface. All three known nakhlites show secondary features resulting from interaction between the magma and liquid water, either during emplacement or through alteration following crystallisation. The first reference (in 1975) to secondary alteration in Nakhla was a description of a complex intergrowth of oxides and hydrated silicates, resulting from the breakdown of olivine and pyroxene grains. A decade after this discovery, on the basis of acid-dissolution and combustion behaviour, the presence of carbonates was inferred in Nakhla. Since then, the alteration products that have been identified petrographically in nakhlites include carbonates with a variety of mineralogies, sulphates and clay minerals. Many of the carbonates are 13C-enriched; their carbon isotopic composition indicates that the CO2 from which they were formed was in contact with martian atmospheric CO2. Fluid circulation within the crust must therefore at some juncture have neared the surface. Chassigny Chassigny is the only member of its sub-group. Like the nakhlites, it crystallised around 1300 million years ago. It is almost completely composed of olivine, and is an olivine-chromite cumulate. It might be associated with the nakhlites. ALH 84001 The ALH 84001 meteorite is a coarse-grained cumulate of orthopyroxene, which formed at depth below the martian surface. It differs from its relatives in that it is much older, having crystallised around 4400 million years ago, not long after Mars cooled. ALH 84001 has had a complex history of shock and thermal alteration, much of which occurred when it was buried. At some stage, it was excavated to the surface, following which it suffered alteration by fluids, resulting in the production of carbonates and clay minerals. Since only a few hydrated minerals have been identified amongst the alteration products in ALH 84001, it has been proposed that the carbonates were produced at the surface of Mars in a region of restricted water flow, such as an evaporating pool of brine. The hypothesis satisfactorily accounts for the chemical and isotopic characteristics of the carbonates, and is also a mechanism that is compatible with an environment in which micro-organisms might survive. ALH 84001 is the martian meteorite in which a team of scientists in the USA identified potential martian microfossils.

Meteorite EETA79001 found in Antarctica