Wednesday, 24 January, 2001

Meteorite clue to water on Mars

http://news.bbc.co.uk/low/english/sci/tech/newsid_1135000/1135028.stm

By BBC News Online science editor Dr David Whitehouse

New evidence that running water may have flowed on Mars in the relatively recent past has been put forward by researchers studying a meteorite from the red planet.

Analysis of the so-called Shergotty Martian meteorite suggests that molten rocks from beneath the surface could have carried substantial amounts of water to the surface in the recent Martian past.

Dramatic images of water-carved valleys and gullies on Mars suggest that the planet had a warmer, wetter past. Writing in the journal Nature, Harry McSween - of the University of Tennessee - and co-workers solve the mystery of where the water came from.

It appears to have been released from water-rich magma that rose to the surface of the planet. Since the Shergotty meteorite was blasted off Mars as recently as 175 million years ago, it implies that water could have flowed on Mars in its recent geological past.

Outgassing

Mars was clearly a warmer and wetter world in the past. Abundant features on its surface testify to vast amounts of running water and a thicker, warmer atmosphere to support it. There may have even been oceans.

Much of the water on the young Mars is thought to have spewed out as steam from the planet's hot interior. Volatile substances such as water dissolved in magma would have come fizzing out in the reduced pressure of the surface.

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.

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 Shergotty meteorite contain a lot of water-soluble elements in their interiors, compared to those in its outer skin.

The grains from the rocks 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.

By conducting experiments on molten rock that reproduce the conditions the magma would experience as it rose towards the Martian surface, the researchers conclude that the magma from which the Shergotty meteorite formed must have begun its ascent containing a lot of water, about 1.8 per cent, much more than has previously been believed.

As various components crystallised out of the magma during its ascent to the surface the water would have been driven out to make its way separately to the surface. This is why the Martian surface rocks are water-poor.

The researchers say that however the water got into the Shergotty meteorite, its presence implies that outgassing occurred from magma that erupted onto the Martian surface only 175 million years ago, the recent past on a geological timescale.

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