The SNC meteorites are from Mars

The 14 SNC meteorites are all igneous rocks, either basalts or basaltic cum ulates. They are inferred to be from Mars, based on direct comparison with Martian materials and on consistency with inferences about Mars. Most telli ng is that the SNC meteorites contain traces of gas which is very similar i n elemental and isotopic compositions to the modem Martian atmosphere as me asured by Viking landers on Mars and spectroscopy from Earth. The Martian a tmosphere appears to have a unique composition in the solar system, so its presence in the SNCs is accepted as strong direct evidence that they formed on Mars. Independent of this link, the SNC meteorites must be fi om a plan et with a significant atmosphere because they contain several abundant gas components, one of which carries large isotopic fractionations characterist ic of atmospheric processing. Further, the elemental compositions and oxida tion state of the SNC meteorites are consistent with data from in situ anal yses of Martian soils and rocks, and an quite distinct from compositions of other meteorites, rocks from the Earth, and rocks from the Moon. The range of formation ages for the SNC meteorites, 4.5-0.18 Ga, is consistent with the varied ages of the Martian surface (based on its cratering record) and inconsistent with surface ages on any other solar system body. The extreme chemical fractionations in the SNC meteorites suggest complex internal proc esses on a large planet, which is inconsistent with an asteroidal origin. S ome SNCs were altered by aqueous solutions at < 0.7 Ga, consistent with the recent presence of liquid groundwater in Mars inferred independently from the geology of its surface. There seems little likelihood that the SNCs are not from Mars. If they were from another planetary body, it would have to be substantially identical to Mars as it now is understood. (C) 2000 Elsevi er Science Ltd. All rights reserved.