First liquid water may have been spotted on Mars

NASA's Phoenix lander may have captured the first images of liquid water on Mars - droplets that apparently splashed onto the spacecraft's leg during landing, according to some members of the Phoenix team.

The controversial observation could be explained by the mission's previous discovery of perchlorate salts in the soil, since the salts can keep water liquid at sub-zero temperatures. Researchers say this antifreeze effect makes it possible for liquid water to be widespread just below the surface of Mars, but point out that even if it is there, it may be too salty to support life as we know it.

A few days after Phoenix landed on 25 May 2008, it sent back an image showing mysterious splotches of material attached to one of its legs. Strangely, the splotches grew in size over the next few weeks, and Phoenix scientists have been debating the origin of the objects ever since.

One intriguing possibility is that they were droplets of salty water that grew by absorbing water vapour from the atmosphere. Arguments for this idea are laid out in a study by Phoenix team member Nilton Renno of the University of Michigan in Ann Arbor, and co-authored by 21 other researchers, including the mission's chief scientist, Peter Smith of the University of Arizona in Tucson. The study (pdf) will be presented in March at the Lunar and Planetary Science Conference in Houston, Texas.

Widespread water

Gaping canyons and river-like channels attest to the fact that large amounts of liquid water once flowed on Mars. The surface now appears dry, though the changing appearance of some crater gullies over a period of several years has hinted at the existence of subsurface aquifers that occasionally release bursts of water.

Certainly, at Phoenix's landing site in the Martian arctic, it is too cold for pure water to exist in liquid form - the temperature never rose above -20° C during the five-month-long mission.

But salty water can stay liquid at much lower temperatures. And perchlorate salts, which were detected for the first time on Mars by Phoenix, would have an especially dramatic 'antifreeze' effect. An extremely salty mixture of water and perchlorates could stay liquid all the way down to -70° C.

If perchlorates are widespread on Mars at high concentrations, then pockets of liquid water might also be widespread below the planet's surface. "According to my calculations, you can have liquid saline solutions just below the surface almost anywhere on Mars," Renno told New Scientist.

And Phoenix may have already snapped images of water kept liquid thanks to perchlorate salts.

Melted ice

The clumps may have come from ice melted by the lander's thrusters. Phoenix's thrusters cleared away the topsoil at the landing site, exposing an ice layer below.

Laboratory experiments the team carried out on Earth suggest the thrusters would have melted the top millimetre or so of this layer and then could have splashed the melted water onto the lander's leg. If enough perchlorate was mixed into the droplets, they could have stayed liquid during the daytime, though they may have frozen each night.

Alternatively, Renno says the clumps may have come from a thin layer of perchlorate- rich water that was already liquid.

Why does the team think the clumps might be liquid water in the first place? The argument rests on the fact that salt is hygroscopic, meaning it attracts water. So droplets of salty fluid on Mars would tend to absorb water vapour from the atmosphere, explaining why the clumps grew over time. Indeed, at the temperatures and humidity observed at the Phoenix site, the expected growth rate of salty droplets matches the observations, the team says.

Most provocatively, a series of images (pictured here) appears to show one candidate droplet growing after absorbing the liquid from its neighbour - a behaviour the team ascribes to liquid water.

'Convincing story'

Mark Bullock of the Southwest Research Institute in Boulder, Colorado, who has experimented with salty water under Martian conditions but was not involved in Renno's study, is impressed with the results. "I think it makes a pretty convincing story for the existence of exotic brines on the Phoenix lander leg," he told New Scientist.

But Phoenix team member Michael Hecht of NASA's Jet Propulsion Laboratory in Pasadena, California, disagrees. He says the clumps were probably patches of ice that formed and grew from water vapour freezing onto the leg.

Renno counters that ice would be more likely to sublimate than grow on the leg, which would have been warmed by heat leaking from the spacecraft's body. Indeed, the layer of ice exposed beneath Phoenix was observed to vaporise over time.

But Hecht argues that the leg may have been colder than its surroundings. Though there were no temperature sensors on the leg, he says the surface of the ice patch was warmed by direct sunlight, whereas the lander leg was in shadow. Water vapour that sublimated from the ice below Phoenix might have recondensed as ice on its cold leg, he argues.

Too salty for life?

Phoenix, which ran out of solar power five months after landing, is not expected to wake up again, so there is no way to further investigate the bumps on its leg. But Renno hopes to bolster the case for salty droplets with future experiments on perchlorate- rich water under Mars-like conditions. He says those tests should be completed in a few months.

Regardless of their outcome, the discovery of perchlorates in the Martian soil suggests that pockets of liquid water may dot the planet. Could life eke out an existence in such pockets? "It's possible," Renno says, pointing out that there are microorganisms on Earth that can survive extreme conditions, including very salty water.

But it may be difficult. One way to describe salt concentrations is with a number called the water activity, which is 1 for pure water, and smaller for saltier solutions. The most salt-tolerant organism known on Earth is a fungus that can survive down to a water activity of 0.61.

However, to lower water's freezing point all the way down to -70 °C with perchlorates, the necesssary concentration of perchlorate salts would give a water activity of just 0.5. "If you tried to put any kind of life-form you can imagine on Earth in a brine solution of that sort, the water would be sucked out of the cells," mission leader Peter Smith told New Scientist.