GULLIES forming on Mars have been caught on camera, claims a geologist. After reanalysing photos captured by the Mars Global Surveyor, Nick Hoffman of the University of Melbourne says he can see gullies being carved out during the planet鈥檚 late spring. It is too cold at that time of year for the gullies to have been carved by liquid water, so they must be created by avalanches of carbon dioxide, he says.
If Hoffman is right, it is a serious blow for hopes that liquid water, and possibly life, could exist near the Martian surface.
The Red Planet is famous for its ancient valley networks. Most planetary scientists believe they are remnants of old river systems, and evidence that billions of years ago Mars had a balmy climate that harboured liquid water and maybe even life.
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But the discovery in 2000 of much smaller V-shaped gullies on the edge of channels, craters and canyons has fuelled speculation of more recent Martian life (快猫短视频, 1 July 2000, p 6). Features such as distinct light and dark areas in the gullies 鈥� indicating that the dust has not had time to settle 鈥� suggest that the gullies are young and formed under the current frigid climate. That in turn bolstered the idea that liquid water might exist today, not far below the surface.
Hoffman scrutinised pictures taken every few days on Mars, from early spring to late summer. The photos show that the whole area is covered with snow early in the year. This gradually retreats until, in late spring, snow remains only in the gullies. And this is when Hoffman says he sees evidence of the gullies being carved out. 鈥淟ong thin trails suddenly appear, cutting through the snow down the length of many of the gullies,鈥� he says.
Infrared measurements taken by Surveyor show that at this time of year it is way too cold for even salty water to melt. However, argues Hoffman, the slightly warmer spring temperatures would be enough to rapidly vaporise some of the carbon dioxide snow. The gas would trigger avalanches, carrying sand, dust and rocks down the slopes.
Hoffman says other researchers have so far missed the trails because they are intent on finding evidence for liquid water, and tend to study photos from the end of the summer, when the snow has gone and temperatures are highest.
Planetary scientist Sarah Stewart of the Carnegie Institution in Washington DC thinks Hoffman may be onto something. 鈥淚t鈥檚 a reasonable idea and questioning the dogma about water on Mars is healthy,鈥� she says. Although the gullies look very similar to channels cut by water on Earth, no one knows if CO2 could also create such features. 鈥淣ow someone needs to show, either in the lab or with a model, that you can make these V-shaped channels with gaseous CO2,鈥� says Stewart.
This isn鈥檛 the first time Hoffman has challenged the idea that liquid water existed on Mars. He has previously argued that Mars鈥檚 giant canyons were carved by supercooled CO2 and rubble, triggered by liquid CO2 boiling out from below the surface (快猫短视频, 5 August 2000, p 6).
Despite his scepticism, Hoffman is still enthusiastic about the next generation of Mars exploration rovers, due to be launched later this year. 鈥淲e need to keep collecting data, but we need to stop jumping to conclusions about the existence of water on Mars,鈥� he says. (See also 鈥淩ivers of dust鈥�)