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Mars: A geologist’s perspective

By David Rothery


So Mars is ‘Place of the Year’! It has the biggest volcano in the Solar System — Olympus Mons — amazing dust storms, and the grandest canyon of all — Valles Marineris. Mind you, the surface area of Mars is almost the same as the total area of dry land on Earth, so to declare Mars as a whole to be ‘place of the year’ seems a little vague, given that previous winners (on Earth) have been islands or single countries. If you pushed me to specify a particular place on Mars most worthy of this accolade I would have to say Gale crater, the location chosen for NASA’s Curiosity Rover which landed with great success on 6 August.

This was chosen from a shortlist of several sites offering access to layers of martian sediment that had been deposited over a long time period, and thus expected to preserve evidence of how surface conditions have changed over billions of years. Gale crater is just over 150 km in diameter, but the relatively smooth patch within the crater where a landing could be safely attempted is only about 20 km across, and no previous Mars lander has been targeted with such high precision.

Perspective view of Gale crater. Curiosity landed in the ellipse within the nearest part of the crater. Image Credit: NASA

The thing that makes Gale one of the most special of Mars’s many craters is that its centre is occupied by a 5 km high mound, nicknamed Mount Sharp, made of eroded layers of sediment. To judge from its performance so far, the nuclear powered Curiosity Rover looks well capable of traversing the crater floor and then making its way up Mount Sharp layer by layer, reading Mars’s history as it goes. The topmost layers are probably rock made from wind-blown sand and dust. The oldest layers, occurring near the base of the central mound, will be the most interesting, because they appear to contain clay minerals of a kind that can form only in standing water. If that’s true, Curiosity will be able to dabble around in material that formed in ponds and lakes at a time when Mars was wetter and warmer than today. It will probably take a year or so to pick its way carefully across ten or so km of terrain to the exposures of the oldest, clay-bearing rocks, but already Curiosity has seen layers of pebbly rock that to a geologist are a sure sign that fast-flowing rivers or storm-fed flash-floods once crossed the crater floor.

Layers at the base of Mount Sharp that Curiosity will analyze. Image credit: NASA/JPL-Caltech/MSSS

The geologist in me wants to study the record of changing martian environments over time, because I like to find out what makes a planet tick. However the main reason why Mars continues to be the target for so many space missions, is that in the distant past — when those clay deposits were forming – its surface conditions could have been suitable for life to become established. Curiosity’s suite of sophisticated science instruments is designed to study rocks to determine whether they formed at a time when conditions were suitable for life. They won’t be able to prove that life existed, which will be a task for a future mission. If life ever did occur on Mars, then it might persist even today, if only in the form of simple microbes. Life probably will not be found at the surface, which today is cold, arid and exposed to ultraviolet light thanks to the thinness of its atmosphere, but within the soil or underneath rocks.

Finding life — whether still living or extinct — on another world would offer fundamental challenges to our view of our own place in the Universe. Currently we know of at least two other worlds in our Solar System where life could exist — Mars and Jupiter’s satellite Europa. It has also become clear that half the 400 billion stars in our Galaxy have their own planets. If conditions suitable for life occur on only a small fraction of those, that is still a vast number of potential habitats.

So, are we alone, or not? We don’t know how common it is for life to get started: some scientists think that it is inevitable, given the right conditions. Others regard it as an extremely rare event. If we were to find present or past life on Mars, then, provided we could rule out natural cross-contamination by local meteorites, this evidence of life starting twice in one Solar System would make it virtually unthinkable that it had not started among numerous planets of other stars too. Based on what we know today, Earth could be the only life-bearing planet in the Galaxy, but if we find independent life on Mars, then life, and probably intelligence, is surely abundant everywhere. As the visionary Arthur C. Clarke put it: “Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying.”  Terrifying or not, I’d like to know the answer. I don’t think Mars holds the key, but it surely holds one of the numbers of the combination-lock.

David Rothery is a Senior Lecturer in Earth Sciences at the Open University UK, where he chairs a course on planetary science and the search for life. He is the author of Planets: A Very Short Introduction. Read his previous blog post: “Is there life on Mars?”

The Very Short Introductions (VSI) series combines a small format with authoritative analysis and big ideas for hundreds of topic areas. Written by our expert authors, these books can change the way you think about the things that interest you and are the perfect introduction to subjects you previously knew nothing about. Grow your knowledge with OUPblog and the VSI series every Friday!

Oxford University Press’ annual Place of the Year, celebrating geographically interesting and inspiring places, coincides with its publication of Atlas of the World — the only atlas published annually — now in its 19th Edition. The Nineteenth Edition includes new census information, dozens of city maps, gorgeous satellite images of Earth, and a geographical glossary, once again offering exceptional value at a reasonable price. Read previous blog posts in our Place of the Year series.

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