The 1970's Viking mission to Mars provided much of our information about the geology and weather of the red planet. Viking had two orbiters which remotely mapped the surface of Mars, and two landers which analyzed rocks and soils. The Viking mission did not return a Mars rock sample to Earth. Further crucial information about Mars comes from an unexpected source - meteorites that arrived on Earth unaided by technology. They are twelve unusual meteorites that are almost certainly pieces of Mars blasted off the planet by meteoroid impact.
The twelve meteorites are unusual igneous meteorites (SNC achondrites named Shergotty, Nakhla, Chassigny are type examples). Most martian meteorites are 1.3 billion years old or less, much younger than typical igneous meteorites from asteroids which are 4.5 billion years old. They also have higher contents of volatiles than igneous meteorites. The conclusive evidence that the SNC meteorites originated on Mars comes from the measurement of gases trapped in one meteorite's interior. The trapped gases match those that Viking measured in the martian atmosphere.
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| EETA79001, a basaltic shergottite, has light-colored xenoliths and dark glasses containing trapped martian atmosphere. | ALHA77005 lherzolitic shergottite has a splotchy dark-light structure. |
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| ALH84001 orthopyroxenite has afractured zone where carbonate weathering products are more abundant. | QUE94201 basaltic shergottite is tiny and the newest martian meteorite. |
Martian Meteorites |
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| Name | Classification | Mass (kg) | Find/Fall | Year |
| Shergotty | S-basalt (pyx-plag) | 4.00 | fall | 1865 |
| Zagami | S-basalt | 18.00 | fall | 1962 |
| EETA79001 | S-basalt | 7.90 | find-A | 1980 |
| QUE94201 | S-basalt | 0.012 | find-A | 1995 |
| ALHA77005 | S-lherzolite (ol-pyx) | 0.48 | find-A | 1978 |
| LEW88516 | S-lherzolite | 0.013 | find-A | 1991 |
| Y793605 | S-lherzolite | 0.018 | find-A | 1995 |
| Nakhla | N-clinopyroxenite | 40.00 | fall | 1911 |
| Lafayette | N-clinopyroxenite | 0.80 | find | 1931 |
| Gov. Valadares | N-clinopyroxenite | 0.16 | find | 1958 |
| Chassigny | C-dunite (olivine) | 4.00 | fall | 1815 |
| ALH84001 | orthopyroxenite | 1.90 | find-A | 1993 |
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.
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.
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.
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.
An in-depth look at martian meteorites.
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