Phoenix landed on Mars on May 25, 2008. It was the first mission to land in the northern region of Mars, where previous orbital missions had discovered an ice cap lurking underneath the surface.

The lander confirmed the ice was there. It also analyzed soil samples that may imply a wetter Mars in the past, and studied present-day traces of water in the vicinity of the spacecraft with the Thermal and Electrical Conductivity Probe (TECP).

The TECP "was like the Swiss army knife of instruments," says Aaron Zent of NASA Ames Research Center. It was equipped with a four-pronged fork that could be stuck into the ground to measure soil moisture and temperature. It also had a sensor for relative humidity.

During the day, the martian air was its most humid with 2 Pascals of water vapor pressure, which is 100 to 1000 times less than on Earth. Each night, beginning at 8:00pm (local solar time) the water vapor would begin to disappear, reaching a low at around 2:00am of around one percent of its daytime value.

This was not a complete surprise, what with temperatures dropping 50 degrees Celsius each night. "But nobody expected to see the atmosphere get sucked dry this much," Zent says.

Part of the missing water eventually turned up towards morning as frost late in the mission, but the majority appears to have been absorbed by the dry martian soil.

Caught on film
Zent presented his water cycle results at a recent American Geophysical Union meeting. He and his colleagues have not yet figured out how much water is absorbed into the soil, since the TECP's soil moisture measurements were not conclusive.

However, Zent has a pretty good idea of what must be happening. Water molecules from the air are condensing into thin films on the soil particles. These films are not unique to Mars - they occur whenever the surrounding air contains water vapor, Zent says.

Sometimes the films build up into water droplets (dew) or ice crystals (frost). But on Mars the thin films of water never become solid or truly liquid. Zent calls these martian films "unfrozen water."

"It is not free to flow around like liquid water, but it's more mobile than ice," Zent says. "The thin films do allow some chemistry and can support some biology."

Zent and others are interested in thin films because on Earth they provide tiny microbes a place to live when the temperatures are below freezing.

For instance, in the Antarctica Dry Valleys, the coldest and driest desert on Earth where temperatures hover around minus 20 degrees Celsius, researchers have found the dry dirt is "full of microbes," Zent says. At such low temperatures, the unfrozen water films are only nanometers thick; considerably thinner than the microbes they coat and sustain.

Life in a cold sweat
On Mars, Zent speculates that the soil is too cold (minus 70 degrees Celsius at night) and the films too thin (not much more than a couple of water molecules thick) to support life.

"The films have to be mobile enough to carry nutrient molecules in, and waste molecules out," Zent says. "They probably are not right now."

However, Zent thinks that in times past, Mars may have been more accommodating. There is evidence that Mars' rotation axis was tilted over four or five million years ago (smaller wobbles may have occurred more regularly). During such a high obliquity phase, the poles would have pointed at the sun for half of the year, leading to much higher atmospheric humidity.

"We may get periods when this area could be habitable," Zent says. The increased humidity would have allowed the soil to have thicker films in which life could potentially thrive.

Later, as the martian axis righted itself, Zent speculates that martian bugs, if they existed, could go dormant and wait for the axis to tilt again in their favor.

"If you are a microbe that can live millions of years as a spore, then during these periods you can wake up, fix some genetic damage, and reproduce yourself," Zent says.

Ice migration
Fellow Phoenix scientist Bill Boynton of the University of Arizona is not optimistic that martian films were ever habitable - he thinks the humidity never gets high enough.

He argues that the planet's tilt changes too slowly, allowing martian ice plenty of time to migrate to the coldest region. The coldest planetary region is currently the poles, but during the last high obliquity phase the coldest region on Mars was the equator.

"The cold ice in the low latitudes acts like a cold finger and sucks most of the water vapor out of the atmosphere," Boynton says. "We end up with a similar very dry climate, it is just that the equator is cold and the poles are warm."

Zent agrees that this climate switch will eventually take place, but he thinks it may take several hundred thousand years for the water vapor to be sucked out of the atmosphere. And that may be long enough to sustain a microbial population.

This story was written with reporting help from Leslie Mullen.

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