Mars Rocks: What the geology of the Red Planet reveals about its ancient atmosphere

The differences in the chemical composition between meteorites from Mars and the surface rocks on the planet have long puzzled geologists and made the modeling of their different sources and formation mechanisms tricky. However, recent work by the Experimental Petrology Group at the University of Oxford have found a (fairly) simple solution: the conditions under which Martian rocks are formed have changed over the planet’s four billion year history.

Although there are similarities between meteorites from the Red Planet and its surface rocks, there are marked differences in the quantity of certain elements. For example nickel is five times more abundant in the surface rocks which were examined by the NASA’s rover ‘Spirit’ than the meteorites found on the Earth’s surface. Chemical composition is not the only difference however, as dating has discovered that the surface rocks are around 2 billion years younger than the meteorites found on Earth.

There are several possible reasons for these apparent chemical differences. For example, the meteorites may originate from geologically different regions of Mars. However, rather than there being a range of volcanic products across the entirety of the Martian surface, it may be that the composition of volcanic products has changed over time.

So what could cause a change in composition? The composition of lavas reflects the temperature, pressure and depth of the source region within the crust and the oxidation state under which they formed. The new study, which has recently been published in Nature, suggests that the wide range of compositions could be the result of a single source which produces different lavas under different oxidation conditions. This new study allows a more coherent geological model of Mars to be made which can explain the formation of the Martian lavas.

If the early atmosphere on Mars was more oxygen rich than it is today, this would suggest that the early planet was quite rusty – with warm and wet conditions. The mechanism for causing oxidation of the crust, allowing these oxidized lavas to form, is inferred to be the recycling process of subduction. However, the incorporation of oxidized material into the crust would be a fairly shallow process, and so the deeper source responsible for the production of the younger rocks with a lower oxygen content would have been unaffected.

The implications of an oxygen rich environment will no doubt create questions regarding the possibility of life on Mars.