Did Allan Hills have a drop of Martian soda water?

A strangely named meteorite suggests that Mars was not always dry. Heat her Couper and Nigel Henbest report

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. Almost certainly, these were formed millions of years ago when rain washed carbon dioxide from the Martian atmosphere into the planet's crust. The team concludes that water once flowed on the surface of Mars - or just under it. The new finding backs up recent US-research on Martian topography, which suggests that the extensive basins on Mars are old ocean beds.

When will we know about Martian water for sure? Perhaps in a couple of years, when an international flotilla of spacecraft reaches the Red Planet. Clever Russian robot rovers will trek for miles over the Martian sands; French balloons will drag instrument packages over the bumpy surface; and miniature US probes will examine the terrain in detail. They may well give us the final word on the Red Planet's water. And it is possible they will come up with something even more exciting - the discovery of the first fossils on Mars.

The planets Mars is the "star of the month", as described above, and is the only planet you'll see in the evening sky.

Jupiter is rising around 3am in the south-east, in the constellation Scorpion. The Moon is nearby on 23 February.

Just before dawn, look out for Venus - the most brilliant planet - lower down in the south-east. The moon passes Venus on 26 February.

The stars The glorious winter constellations appear in all their finery this month. Heading the cast is the great hunter, Orion. Four bright stars mark out his shoulders and knees, while three more depict his jauntily angled belt. His head is marked by afainter star because, according to one legend, the goddess Diana mistakenly killed him with an arrow to the head while Orion was swimming.

Orion is a helpful guide to the surrounding constellations. Follow his belt to the upper right, and you come to the red giant star Aldebaran. It marks the eye of Taurus, the raging bull. Farther along the same line is the beautiful little cluster of stars, the Pleiades, or Seven Sisters.

To the lower left of Orion's belt is Sirius, the brightest star in the sky. Often called the Dog Star, it lies in Canis Major, the larger of Orion's two hunting dogs. Above Sirius is Procyon, in the little dog (Canis Minor).

Higher still, you'll find the twin stars Castor and Pollux in Gemini. According to myth, they were the offspring of queen Leda, but by different fathers. Castor's father was Leda's husband, Tyndareus, and he was mortal. Immortal Pollux was the son of Zeus, who visited the beautiful Leda on her wedding night. Their sister, by the way, was the Helen whose face launched a thousand ships.

Finally, almost overhead, is Capella, the "little she goat". Three faint stars nearby represent her kids. For some reason lost in the mists of time, these stars form part of the constellation of Auriga, the charioteer. Ancient star maps depict a celestial Ben Hur encumbered by this capricious quartet.

Diary 3 Mercury at inferior conjunction 7 12.54pm Moon at first quarter 12 Mars at opposition 15 12.15pm full Moon 22 1.04pm Moon at last quarter Graphic omitted