Meteorite clue to water on Mars

The surface of Mars shows apparent evidence of water

By BBC News Online science editor Dr David Whitehouse

Scientists have put forward new evidence that running water may have flowed on Mars in the relatively recent past.

The evidence comes from a study of the Shergotty meteorite, one of only 15 lumps of rock found on Earth which are known to have originated on the Red Planet.

The analysis suggests that molten rocks from deep within Mars could have carried substantial amounts of water on to the planet's surface.

Dramatic images of what appear to be water-carved gullies and water-deposited sediments on Mars have already indicated that the planet probably had a much warmer and wetter past. Now, the new meteorite study by Harry McSween, of the University of Tennessee, and co-workers provides a possible source for all the water.

And since the Shergotty meteorite was blasted off Mars as recently as 175 million years ago, their work implies that this water could have flowed on Mars in the very recent geological past.

Process of outgassing

There are abundant features on the Martian surface that testify to the vast amounts of water that must have run across the planet some time in the past. It is quite possible Mars once had oceans.

The meteorite suggests Mars was wetter

Much of the water on the young Mars is thought to have spewed out as steam from the planet's hot interior. All sorts of volatile substances dissolved in magma would have come fizzing out of molten rocks as they hit the reduced pressure that existed on the planet's surface - water included.

But there is a problem with this theory. Samples of volcanically formed rocks from Mars show little sign of having solidified from magma that was once rich in water.

Now, in a series of experiments, McSween's team show that this evidence of a low-water content in Martian magma may be misleading. They find that the mineral grains in the interior of the Shergotty meteorite contain a lot of water-soluble elements compared with those in its outer skin.

Separate journey

The grains from the rock's interior are thought to have come from deep inside Mars, whereas the water-poor crust of the rock would have come from nearer the surface.

The team conducted experiments on molten rock to reproduce the conditions magma would have experiences as it rose towards the Martian surface. From this, the researchers were able to show that the magma from which the Shergotty meteorite formed must have begun its ascent containing a lot of water, about 1.8%. This is much more than had previously been believed.

The researchers think that as various components crystallised out of the magma during its ascent, the water would have been driven out to make its way separately to the surface. This is why the Martian surface rocks appear water-poor.

Because the Shergotty meteorite is 175 million years old, the water that outgassed from its parent rock must have gushed on to the planet's surface at that time. This supports the notion that water was running on Mars in the recent geological past.

The substantial amounts of water released from magmatic outgassing would have caused surface erosion, also suggesting that many of the fluvial features we see on the surface of Mars have a much more recent origin than was once thought.