HAS Spirit landed in the mud? Strange marks in the soil close to the rover鈥檚 landing site have got the mission鈥檚 scientists daring to consider the possibility that against all the odds, there is liquid water on 鈥� or just beneath 鈥� the surface of Mars.
If they turn out to be right, it would be the clearest sign yet that lakes and oceans once existed on the planet, increasing the chances that life did too. Intriguingly, the existence of liquid water in the soil could also rule out the only alternative explanation for the positive results that the Viking Mars landers produced when they tested for life in 1976.
The mystery stems from a small, disturbed patch of ground very close to the lander. Dubbed the 鈥渕agic carpet鈥� because of its apparent folds and ripples, the mark was made by the lander鈥檚 airbags scraping across the soil. But its appearance has taken scientists analysing Spirit鈥檚 images completely by surprise. Rather than breaking or cracking as you would expect completely dry soil to do, the surface seems to have flowed and folded, as if it was wet. Science team leader Steven Squyres describes the marks as bizarre. 鈥淚t looks like mud, but it can鈥檛 be mud,鈥� he says.
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Pure water can鈥檛 exist as a liquid at the low temperatures and pressures found on the surface of Mars 鈥� it would either freeze or evaporate into the painfully dry atmosphere. But that鈥檚 not true of brine. If the water contains enough salt, then it could be stable. 鈥淪ome dense brines are fluid down to very low temperatures,鈥� says science team member David Des Marais. He thinks that could explain the strange behaviour of the magic carpet. 鈥淚f it鈥檚 not pure water, brines should be considered. We know there are some brines that are stable under these conditions.鈥� Des Marais suggests that if past bodies of water on Mars evaporated, the remaining liquid would have become more and more salty until it was briny enough to be stable.
The existence of some form of water within the top metre of the Martian soil, even in the equatorial regions where Spirit has landed, was one of the startling findings made last year by the Mars Odyssey orbiter (快猫短视频, 1 March 2003, p 21). Its neutron spectrometer detected hydrogen concentrations of more than 6 per cent in some equatorial areas, which was assumed at the time to be in the form of ice or hydrated minerals.
But those observations would also be consistent with brines that could, at least some of the time, be liquid. And if these briny soils were coated by even a thin veneer of dust, they would have been concealed from Mars Global Surveyor鈥檚 cameras.
The Spirit team hasn鈥檛 detected brine directly, and there are still other explanations for the strange markings near the lander 鈥� an electrostatic attraction between the dust grains may be causing the soil to clump together, for example.
Alternatively, perhaps ice in the soil melted on contact with the airbag, or the airbag itself might have punctured, releasing a tiny amount of moisture. But these scientists aren鈥檛 the first to suggest there might be brine on the Martian surface. Others have proposed that seeping salt water created the thousands of mysterious gullies seen on the steep slopes of crater and canyon walls.
What鈥檚 more, the very first microscope image of Mars dirt, unveiled last Friday, contains puzzling features that have also got the Spirit team thinking seriously about water. The picture shows many hollow spheres or tubes. These too could result from electrostatic clumping, but science team member and geologist John Grotzinger thinks the grains may have been cemented together by some kind of salt, perhaps when dust-coated water droplets slowly evaporated.
That is consistent with the idea that a lake dried up, leaving behind a layer of extremely concentrated brine. Grotzinger says he has seen similar hollow structures in desert soils in the south-west US, formed when capillary action draws tiny amounts of moisture up through the dust, leaving tiny tubes cemented by salts when the water evaporates.
While finding liquid water or signs of it is the central aim of the Mars Rover missions, because of its implications for past life, brine may not be good news for the possibility of life existing on the planet today, says Des Marais. 鈥淎bove a certain salinity, it gets to be a problem for life.鈥�
But any moisture in the soil, however salty, would be good news for the prospects of life according to Gilbert Levin, who designed one of the three life-detection tests on the Viking Mars missions in 1976. His 鈥渓abelled release鈥� experiment mixed Martian soil with a nutrient soup containing radioactive carbon-14. The experiments on both landers produced radioactive gas, as would be expected if alien microbes were metabolising the nutrients.
The prevailing view is that the gas was released not by living organisms but by unusually strong oxidants in the soil. But Levin believes his test did detect Martian life, and he says the new Mars images vindicate his view. 鈥淚f there is liquid water in the soil, the strong oxidant cannot be there, since liquid water, or even just the vapour from it, would destroy the oxidant,鈥� he told 快猫短视频.
If the Spirit mission can prove that liquid water exists on Mars today, even in minute quantities, Levin says that rules out the alternative explanation of the Viking results, and 鈥渋neluctably leads to life on Mars鈥�.
Unfortunately Spirit will not be able to study the magic carpet area directly, because it is too close to the landing platform and so considered too dangerous for the rover to negotiate. But the team believes that whatever accounts for the behaviour of that patch of soil will be found elsewhere in the area. They hope to duplicate the marks by using one of the rover鈥檚 wheels to dig into the soil, or by rotating the wheel sideways to mimic the dragging of the airbag.
Spirit could be close to an amazing discovery. But the team is being cautious until definitive proof is found. Rather than jumping to conclusions about the planet鈥檚 geological past, says science team member Tim Parker, 鈥淚 prefer to let Mars tell us.鈥�