杏吧原创

THERE was great excitement last week when news leaked out that a spacecraft
has seen signs that water might have recently flowed on Mars鈥檚 surface. But
experts are puzzled by how water could be liquid in the frigid Martian climate
and warn that concrete proof will be hard to come by.

鈥淚f these results prove true, that there is water on Mars near the surface,
it has profound implications for the possibility of life on Mars,鈥� NASA鈥檚
associate administrator for space science Ed Weiler told a press conference in
Washington DC last week. 鈥淛ust about any place biologists find liquid water,
organic molecules and energy, they find life, whether it鈥檚 on the surface of the
Earth or 10 000 feet below.鈥�

The debate about water on Mars began in earnest when the Mariner 9 spacecraft
photographed the planet in 1972. The images showed features that looked like
giant flood channels and river valleys. But it seemed impossible that liquid
water carved these in the recent past since the average surface temperature on
Mars is around 鈥�50 掳C and seldom rises above freezing. However, there
could have been liquid water on the planet more than a billion years ago, when
the climate would have been much warmer
(see 鈥淪alty seas鈥�).

But pictures taken since March 1999 by a camera on the Mars Global Surveyor
spacecraft suggest that water has flowed on Mars much more recently, says Mike
Malin of Malin Space Science Systems in San Diego, chief scientist for the
camera. He and his colleague Kenneth Edgett have seen rock formations at around
150 sites that seem to have formed relatively recently, during sudden
floods.

None of the formations has been around long enough to be scarred by craters
or shrouded in wind-blown sand, making them a million years old at most. 鈥淭he
features appear to be so young that they might be forming today,鈥� says
Malin.

The formations are gullies on crater and valley walls. Each is a deep
V-shaped channel, typically a few kilometres long, with a collapsed region at
the top and a pile of debris at the bottom. Their shapes resemble terrestrial
gullies formed by water rather than dry landslides or lava flows. 鈥淥n Earth,
there鈥檚 no question these are created by water,鈥� says Malin.

He and Edgett think the channels form because there is a layer of water more
than 100 metres below the Martian surface that is kept liquid by the pressure of
the overlying rock. When exposed in a cliff face, this water freezes. More water
then builds up behind the dam, until it eventually bursts through the ice. This
flash flood would carve a channel (see Diagram).

鈥淭hese observations are very compelling,鈥� says Maria Zuber, a planetary
scientist at the Massachusetts Institute of Technology. But some things are
puzzling, she adds. The features appear on the coldest regions of Mars, near the
poles and on slopes facing away from the Sun. 鈥淚t鈥檚 so cold on Mars鈥攜ou鈥檙e
a hundred degrees away from the melting temperature. But there might be very
high pressures that make melting occur.鈥�

鈥淚 find it really exciting,鈥� says Steve Squyres of Cornell University in
Ithaca, New York. 鈥淭he pictures make a compelling case that water has leaked out
of the ground recently on Mars. But I think it might be an over-interpretation
to suspect that these things are tapping into a really large, subsurface
reservoir.鈥� Because the gullies are small, he thinks local melting of ground ice
due to the Sun or the planet鈥檚 internal heat might be responsible.

A definite answer will be a long time coming. 鈥淚 don鈥檛 think you can really
test this hypothesis fully without actually going to the place from which the
water is believed to have emerged,鈥� says Squyres. 鈥淏ut nature has played a very
cruel trick on us here, and put these things on the most unimaginably difficult
places to reach on the planet.鈥� For instance, they鈥檙e on steep and probably
crumbly slopes that existing robots couldn鈥檛 navigate. 鈥淚鈥檓 not sure that even a
human in a spacesuit could climb up to these things,鈥� he adds.

In the meantime, the Global Surveyor team plans to check future snapshots of
the same gullies. If they change, this would strengthen the case that water is
still flowing today.