Wednesday, 10 July 2013

The Prehistory Of The Red Planet Part Five

Today, the Red Planet's air is a thin, unsavory brew of carbon dioxide, nitrogen with other elements and compounds. Billions of years ago, however, it was rather more interesting. According to scientists led by Professor Bernard Wood from Oxford University, around four billion years ago, Mars had a significant oxygen content in its atmosphere.

The Red Planet, so named because of its rubicund soil and rocks, indicates the presence of iron (III) oxide. Today, Mars is a dead planet. Where nothing moves save the wind. So where did the oxygen making up the oxide come from? Initially, it was put down to natural geochemical processes which were strictly sub-terrestrial. Yet Professor Wood believes that there is another explanation.

An artist's impression of the oxidized volcanic Martian landscape
In its infancy Mars was highly volcanic. Basalt lava flows were common. We can see these preserved remains from space. When black basaltic lava is weathered in an oxygenated atmosphere, it takes on a reddish tint as minerals are oxidized to form iron (III) oxide. We can observe this process happening on Earth, also, in particular on the twisted, moss-covered basalts of Iceland.

The NASA Mars rover Spirit was  investigating 3.7 billion year old volcanic rocks on the Martian surface, which showed signs of having been weathered on the planet's surface and exposed to Oxygen, in the same fashion as Earth's red basalts. Of course the nature of the rocks could be due to sub-terrestrial geochemical processes. To make sure that this was not the case, the scientists examined another type of rock for comparison.

Martian meteorites are the rarest type of material to impact Earth. They are blasted towards us by supermassive volcanic eruptions or ejecta produced by bodies colliding with Mars. They originate deep in the Red Planet's crust and do not, therefore, come into contact with the Martian atmosphere long enough to be affected by it. These showed no signs of oxidation. This suggests that chemical processes involving oxygen were not common in the sub-terrestrial crust and mantle.
A Martian meteorite. Its colour shows that it has not been oxidized

The only explanation, therefore, for the oxidized surface rocks and the non-oxidized sub-terrestrial Martian meteorites is that Mars' ancient atmosphere was once rich in oxygen. 'As oxidation is what gives Mars its distinctive colour, it is likely that the Red Planet was wet, warm and rusty billions of years before Earth's atmosphere became oxygen-rich,' said Professor Wood. What is interesting are the questions this raises for our understanding of Mars' planetary history.

How did oxygen appear in its atmosphere? It is a highly reactive gas and so is easily removed through reactions with molten minerals served up by volcanic eruptions. This was exactly what occurred in the Earth's early years. Our atmosphere only gained oxygen due to the appearance of photosynthetic organisms, which eventually generated the gas at such a rate that its production was equal to its removal from the atmosphere by volcanic activity.

It is possible that Mars gained its oxygen via the action of living creatures, an explanation which has profound implications for research into extraterrestrial life. Yet the biggest mystery is why Mars gained an oxygenated atmosphere so quickly? Both Mars and Earth formed around 4.5 billion years ago. However, the latter only gained an oxygenated atmosphere 2.4 billion years ago almost two billion years after its formation. Mars, on the basis of the geological evidence, had oxygen just 500 million years after its formation.

What is more, this was with a huge amount of volcanic activity on the planet's surface. Exactly what happened billions pf years ago will most likely remain a mystery. We can only glean so much from our telescopes and even the Martian rovers are limited as they will never be able to self-interpret the data they collect and direct action accordingly. To gain proper insight into Mars' prehistory, we will have to visit the Red Planet ourselves.