Twelve unusual meteorites are almost certainly pieces of Mars that were blasted off the red planet by meteoroid impact. They have been called SNC meteorites after the three type samples, Shergotty, Nakhla, and Chassigny, or now simply martian meteorites. All twelve meteorites are igneous rocks crystallized from molten lava in the crust of a parent body. They are, however, distinct from typical igneous meteorites from asteroids in ways that suggest that the SNC meteorites come from a much larger body, a planet. All but one of these meteorites are very young (less than 1.3 Ga) compared to ancient ages for other igneous meteorites (about 4.5 Ga). They also have higher oxygen fugacities and contents of water and other volatiles, contain minerals with ferric iron, and form a distinct trend in oxygen isotopic composition. The conclusive evidence that SNC meteorites are from Mars is the analysis of gases trapped in glass inclusions in EETA79001, which chemically and isotopically match gases measured in the unique martian atmosphere by the Viking lander spacecraft (Figure 1).
|Classificaion: S=shergottite, N=nakhlite, C=chassignite, ALH84001 is none of these.
find-A designates Antarctic meteorites (all recent finds). Year is recovery date for non-Antarctic meteorites and date of martian classification for Antarctic meteorites.
The martian meteorites represent five different types of igneous rocks, ranging from simple plagioclase-pyroxene basalts to almost monomineralic cumulates of pyroxene or olivine. The meteorites and their rock types are listed in Table 1. Photographs of whole rocks and thin sections of a basalt and a cumulate are illustrated below. All of the meteorites solidified near the martian surface by crystallization from a cooling magma. Some of the shergottite basalts have close to magma compositions, while the other martian meteorites are dominated by accumulation of olivine and/or pyroxene. None of the martian meteorites are surface samples in that they have not been exposed to extensive weathering or irradiation by cosmic rays. The martian soil analyzed by Viking appears to be a weathered basalt which could have been of shergottite composition.
The only natural process capable of launching martian rocks to Earth is meteoroid impact. To be ejected from Mars a rock must reach the escape velocity of 5.4 km/sec, which is more than five times the muzzle velocity of a hunting rifle. During impact the kinetic energy of the incoming projectile causes shock deformation, heating, melting, and vaporization, as well as crater excavation and ejection of target material. The martian meteorites show low to moderate degrees of shock that appear to require a special mechanism to boost them to the escape velocity and eject them from Mars. The impact and shock provide an explanation for why the martian meteorites are all igneous rocks. Martian sedimentary rocks, and certainly soil, may not be sufficiently consolidated to survive the impact as intact rocks which might later land on Earth as meteorites.
|EETA79001, a basaltic shergottite, has light-colored xenoliths and dark glasses containing trapped martian atmosphere.||ALHA77005 lherzolitic shergottite has a splotchy dark-light structure.|
|ALH84001 orthopyroxenite has afractured zone where carbonate weathering products are more abundant.||QUE94201 basaltic shergottite is tiny and the newest martian meteorite.|
|Thin section of basaltic Shergottite shows melt texture of pyroxene and plagioclase mineral.||Thin section of dunite Chassigny shows the cumulate texture of olivine in polarized light.|
Volcanism and Impact
Water on Mars