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.

    These 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 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 (1.3 Ga or less) 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).

    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 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.

Why aren't they red?

    The oxidized iron produced by weathering in surface rocks gives Mars its red color, but the less-weathered igneous rocks just below the surface are gray or black. None of the martian meteorites are weathered surface samples, but are all igneous rocks crystallized from molten lava near Mars' surface. They include five different rock types, which do not all appear to be geologically related to each other.

How did they get here?

   Meteoroid impact is the only natural process capable of launching martian rocks to Earth. 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. An impact capable of ejecting the martian meteorites into space would have left a crater of 10-100 km. The meteorites spent several million years in space before landing at various sites on Earth.

What do they tell us about Mars?

   Martian meteorites tell us about several processes occuring at various times throughout Mars' history. The story begins with mars' differentiation into core, mantle and crust very soon after planet formation at 4.5 billion years ago. The oldest martian meteorite crystallized from a magma soon thereafter. The younger martian meteorites show us that igneous volcanism continued until at least 1.3 billion years and probably 170 million years. Impacts occurred on the surface throughout Mars' history.

    Many of the martian meteorites show some evidence of interaction with liquid water. Some have igneous minerals with a little water, but most have alteration products (especially salts and clays) caused by weathering. Studies of the lightest elements that make up the atmosphere tell us that Mars' atmospheric evolution was very different from Earth. Some of the lightest gases from Mars' atmosphere were lost to space throughout time.

What don't they tell us?
Why do we need sample returns?

   We do not know where the meteorites came from on the surface of Mars, therefore we can't use them for "ground truth" for remote sensing studies. We can only infer that one is from the old cratered terrain of the southern highlands and that the rest are from the young volcanic terrain in the northern plains.

    The martian meteorites are all igneous rocks and do not tell us as much about mars' water and atmosphere as we could learn from studies of old sediments and soils. Igneous rocks are not the best candidates for searching from martian life. Sample return missions directed to both old sedimentary rocks and young volcanic rocks are needed for "ground truth" and to better understand volatiles and possible life on Mars.