Did Allan Hills Have a Drop of Martian Soda Water?

Article excerpt

The constellation of Leo - which rises mid-evening this month - is looking distinctly odd at the moment. One of those rare constellations that usually looks like its namesake, it is currently almost unrecognisable as a crouching lion. The reason is that a brilliant, reddish-coloured interloper has wandered in among the stars and distorted its familiar shape. The culprit is the planet Mars. This month it provides the lion with a second heart that is brighter than Leo's own Regulus. On 12 February, Mars stands at "opposition" - opposite the Sun in the sky, and at the closest point to Earth in its orbit. As a result, it shines at its brightest, and is visible in the sky all night long. But because Mars has a very oval orbit around the Sun (Earth's is almost circular), not all oppositions of Mars - which take place roughly every two years - are equally close. This is one of the worst there can be. Mars, 101 million kilometres away, is almost twice as far away as it can be at a favourable opposition.

Consequently, Mars is not putting on a good show this time. Only half the size of Earth, its disc at this opposition is only a quarter the size of far more distant (but far larger) Jupiter. Small telescopes will reveal only a tiny red blob; with a largeramateur telescope you may just glimpse some elusive markings. To see Mars well, you need access to something like the Hubble Space Telescope, which has just taken an astonishing portfolio of images of our neighbour worlds. Mars is revealed as having an enormous cap of frost and cloud covering its north pole.

While Mars may not look that impressive from a distance, it more than makes up for that by being the most Earth-like of all the planets. It has frozen polar caps; a day almost the same length as Earth's, and similar seasons, as well as huge volcanoes andcanyon systems and vast rolling deserts. But the similarity ends when it comes to Mars's atmosphere, which is made of carbon dioxide and virtually non-existent. As a result, Mars suffers mercilessly low temperatures. While the mercury may just touch 20Con a summer's day at the equator, the temperature is usually well below freezing, and can drop to -120C.

The big question about Mars is whether it ever managed to hold on to large amounts of water. Water is the lubricant of life: on our own planet, it was water that helped to start life, and water that has maintained it ever since. If Mars ever had enough water, life may have got started on the Red Planet - only to be nipped in the bud by the Sun's ultraviolet radiation pouring in through Mars's planet-wide ozone hole.

Now, proof that Mars once had water - even, possibly, oceans of warm soda water - has come from an unusual source. Monica Grady of the Natural History Museum/Open University has been making a special study of a meteorite with the unlikely name of Allan Hills. The meteorite's composition singled it out as a member of a rare group that started life on the surface of Mars, but was blasted into space by an unknown catastrophe.

When Grady's team analysed Allan Hills in detail, they found it contained carbonate minerals. …