Mars, the red planet is perhaps the best candidate in the Solar System to find extraterrestrial life. Or at least evidence that it once existed there in the past. Not intelligent life of course, but maybe simple bacteria similar to what appeared on earth about one billion years after its formation. Mars must have had water in the past, and it is now most likely frozen beneath the surface as permafrost. A vast amount of evidence has just recently emerged to support the theory that Mars was once much warmer and wetter.

This image of Mars was taken using the Hubble Space Telescope.

Mineral deposits that can only form in water, layered rock beds that appear to be sedimentary in origin, channels that seem to have been cut by flowing water are just a few examples. Today, water cannot exist on the surface of Mars in its liquid form because of two major reasons. First, the temperature on Mars is an average minus 58^oC, however, it dose very occasionally creep just above freezing during midsummer at equatorial latitudes. An example is the Hellas Basin in the southern hemisphere.

Photo of the surface of Mars as seen by Viking 1. (The round thing at the bottom right is part of Viking, not a Martian artifact.)

The second reason is the low pressure. The Martian atmosphere is much thinner than that of Earth’s, the pressure being around 600 Pascals. Water needs a minimum of 610 Pa to become liquid at 0^oC. However in the Hellas Basin, the pressure can sometimes rise to just about this point, if this coincides with the temperature being above freezing, it is just possible that water could remain stable for a brief time. Such an event would be considerably rare, and any water would evaporate very quickly. But, this means that if Mars can only just about hold water on its surface now, then it almost certainly must have been able to do so in the past, particularly during the first billion years or so.

The atmosphere must have been a lot thicker back then, as any planet’s air will tend to dissipate into space over time. In the case of Mars, which is much smaller than the Earth and therefore has less gravity to hold onto its air molecules, the atmosphere has all but been lost to space. There is also the theory of periodic warming due the impact of large asteroids. If an asteroid is big enough, it will melt large portions of the crust, which would be absorbed by the atmosphere, then distribute heat around the planet. This heat might even stay around for a few centuries, if not millennia. It is very difficult to say how often such evens happen, but they may occur a few hundred million years apart (but this is a very vague estimate).Although Mars’s close proximity to the Asteroid Belt may help the planet to capture asteroids, they may also enter orbit and become moons. It is believed that Mars’s two moons, Phobos and Demos may have been asteroids that were captured by Mars. The largest moon, Phobos is in-fact in an unstable orbit and it will hit the planet in about 50 million years time.

The planet Mars is home to the tallest volcano in the entire Solar System. Olympus Mons (see right) is an incredible 27 kilometers high, 550 km wide and has a caldera with a maximum of 85 km across. The volcano also has 2 km high cliff running most of the way around it which is easily visible in the image. Olympus Mons is so wide that if one end was in London, the other would be just about in Glasgow. It is hundreds of millions of years old, and probably billions. It is similar in shape to the Hawaiian volcanoes, shield shaped, but unlike them in other respects. The Hawaiian volcanoes form because they are on top of a hotspot which has been in the same place for over a hundred million years whilst the Pacific plate drifts over it.

Olympus Mons as seen by the Viking orbiter.

But Mars has no plate tectonics, so the volcano remains above the hotspot. Because the gravity of Mars is less than Earth's the Olympus Mons can grow even taller. It is very important to know when Olympus last erupted, if it was recent, that means that Mars is probably geologically active today. Well, in 2004, the Mars Express probe found that there were lava flows as young as 2 million years, the geological blink of on eye. So it now seems that Olympus Mons is probably not extinct as once thought, and it may well erupt again in the future. But we just don't know.

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