Meteorites from Mars

The infamous ALH 84001 Mars meteorite, with its alleged microbial fossils, is unique in more ways than just its reputed evidence for Martian life. It also falls into a class of meteorite called an orthopyroxenite, of which it is the only member. More common Martian meteorites are the shergottites and the nakhlites, and both types have fascinating histories. The first known shergottite landed near Shergotty in India in 1865, and is a basaltic rock laced with pigeonite, augite and maskelynite, with much of the latter being transformed into plagioclase during the intense impact that dug the rock out of the ground on Mars and gave it enough energy to escape Mars’ gravity and wind up here.

It turns out that there are two types of shergottite from two impact events, one 500,000 years ago and another three million years ago, and they can be identified based on whether they are coarse or fine grained. The fine grained material formed in lava flows, whilst the coarser material is rock that cooled slowly beneath the Martian crust, only to be erupted out onto the surface between 165 and 450 million years ago.

The nakhlites are even more interesting. Only seven are known, with the prototype having fallen in the Nakhla region of Egypt in 1911. Formed almost entirely from the green mineral augite, they also bear testament to the red planet’s watery past, with small amounts of clays, carbonates and sulphates, and in one or two cases iddingsite, which forms from olivine that has been exposed to running water. Nakhlites have been dated back 1.3 billion years, and were wrenched off Mars in an impact some 12 million years ago.

The final known type of Mars meteorite are chassignites, of which only a pair are known, and they are rich in olivine having formed in Mars’ crust. These little chunks of Mars, whilst no substitute for a bona-fide sample–return mission, help to bring the red planet to us.