Geoscience

Martian meteorites

A rare group of stony meteorites (less than 20 are known) may have come from the planet Mars. Most of these have been found in Antarctica.

They were originally named 'SNC's', after the names of the three meteorite subgroups into which they were first classified: Shergotty (S), Nakhla (N) and Chassigny (C). However, a new subgroup of Martian meteorites was recognised when a meteorite from the Allan Hills in Antarctica (ALH 84001) was also found to be from Mars.

All of the Martian meteorites are igneous rocks, having solidified at or below Mars surface. This makes them difficult to distinguish from many similar rocks on Earth. However there are several pieces of evidence that point to an origin on Mars.

Evidence for a Martian origin

It is believed that all of the Martian meteorites landed on Earth as fragments from large meteorites or asteroids that crashed into Mars between 180 and 1300 million years ago.

The meteorites are most likely to have come from the planet Mars because:

The meteorites are relatively young in age (< 1300 million years) compared to the Solar System and other meteorites (~ 4500 million years).
The mineral grains within them have a different oxygen isotopic composition compared to the Earth and Moon.
Trapped pockets of gases within at least one of the Martian meteorites have the same chemical and isotopic composition as those of the Martian atmosphere sampled by the Viking landers of 1976.
All Martian meteorites are formed from igneous rocks, so they must have formed on an earth-like planet or one of its moons. Venus and Mars are the only other likely candidates. Since the atmosphere on the planet Venus is very dense, it causes any ejected rock produced by impact to melt and vaporise due to frictional heating. This means that the planet Mars is left as the most likely origin of ejected meteorite fragments.

The Dar al Gani 476 meteorite

A Martian meteorite called 'Dar al Gani 476' was found in the Libyan Desert on May 1 st, 1998. It is an achondrite (shergottite) stony meteorite, mainly composed of the minerals olivine and plagioclase, and is believed to have originated as a basalt on the planet Mars. The total known weight of this meteorite is 2.015 kg, but the specimen shown here weighs only 0.358 grams and has dimensions of 1.5 cm x 1.5 cm x 0.2 cm.