Martian meteorites: How Did a Piece of Mars Get to Earth?

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 at least one hundred forty unusual martian meteorites that are almost certainly pieces of Mars blasted off the planet by meteoroid impact.

Why are they from Mars?

The one hundred forty 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.

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.

Martian Meteorites List (142)

An up-to-date List of Martian Meteorites created by Dr. Tony Irving of the University of Washington, sponsored by the International Meteorite Collectors Association, IMCA Inc. The number of terrestrially-unpaired meteorites stated here is a best estimate based on available information and is subject to change. It is very difficult to properly assess the pairing relationships among the most common Enriched Mafic Shergottites, so the actual number of unpaired specimens may be slightly lower than stated.

For a current list of classified specimens, click here

How did the martian meteorites 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.

Earth Science and Solar System Exploration Division, Johnson Space Center

Scientists think that martian meteorites found on Earth have been ejected from the red planet by asteroid impacts. Calculations suggest that there are two ways this can happen. First, an asteroid at least ten kilometers (six miles) wide could fall straight down to the martian surface. The impact crater left behind in this event would be round and at least a hundred kilometers wide.

It’s equally possible that a smaller asteroid, perhaps one kilometer in diameter, might be able to knock material free from Mars, if it approached the martian surface at a very low angle. In this case, the asteroid would swoop in more like an airplane landing than a rock falling straight down. Its impact crater would be elliptical and smaller, as small as ten kilometers wide.

Martian meteorites ALH84001 found in Allan Hills region of Antarctica

No one can say exactly what trajectory ALH84001 took when it spalled off the planet. Scientists will only say that it drifted among the planets of the inner solar system until it encountered Earth some 13,000 years ago. But, in light of the two possibilities suggested above, scientists now think they may know the place on Mars where ALH84001 originated. This is important because identification of the meteorite’s site of origin may have a bearing on future Mars missions.

Planetary scientist Nadine Barlow of the University of Central Florida led the search for the source crater. With the help of a comprehensive crater catalog that she had compiled while doing her graduate work at the University of Arizona in the mid-1980s, and taking into account a number of distinctive characteristics of the meteorite itself, Barlow was able to narrow the search to two possible source craters on Mars.

Because of its great age, the meteorite must have come from the most ancient of martian terrain. Because the meteorite was chipped off Mars only 16 million years ago – very recently on a geologic timescale – Barlow reasoned that the source crater must still exhibit young features, such as a sharp crater rim and an unweathered blanket of ejected material surrounding the crater.

Also, ALH84001 shows evidence of a shock event that preceded its ejection from Mars. In other words, while it was still a part of the martian crust, an asteroid or comet impact may have occurred in the same vicinity. Thus older craters may be found near the source crater. And there is one more clue to the meteorite’s origin. The presence of carbonates in the meteorite suggests that the impact site was located in a watery region.

Barlow’s study used her own crater catalog to identify two possible sites of origin for ALH84001. Both are small, elliptical types located in the heavily cratered southern highlands of Mars. The first crater is located in the Sabaeus Sinus region of Mars just south of the planet’s equator. It is located less than 10 kilometers from the second candidate, an older crater in an area that also shows some possible evidence of ancient river and flooding activity.

Why aren’t martian meteorites 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.

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 Martian meteorites tell us and 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.