080905 Phoenix’s vapour quandary

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Phoenix’s vapour quandary
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 05, 2008

Although Phoenix has sensed a rise and fall in humidity in the air around the lander, the soil itself is found to be thoroughly and perplexingly dry.

"If you have water vapour in the air, every surface exposed to that air will have water molecules adhere to it that are somewhat mobile, even at temperatures well below freezing," says Aaron Zent, lead scientist for Phoenix's thermal and electro-conductivity probe that made the measurements.

The four needles of the thermal and conductivity probe were inserted into undisturbed Martian soil yesterday, but found it to be surprisingly dry. Image: NASA/JPL-Caltech/University of Arizona/Texas A&M University.

The results from the latest insertion of the probe's four needles into the ground on Wednesday and Thursday match results from other measurements of the soil made so far in the mission, but it is still perplexing that the soil is found to be so dry.

"There are no indications of thin films of moisture, and this is puzzling," says Zent. The results are in direct contrast to below-freezing permafrost terrains found on Earth, where thin layers of unfrozen water molecules on soil particles can grow thick enough to even support microbial life.

Three other sets of observations by Phoenix give reasons for expecting to find thin-film moisture in the soil. One is the conductivity probe's own measurements of relative humidity when the probe is held up in the air, which has recorded a humidity transition from near zero to almost 100 percent with every day-night cycle, suggesting there is a lot of moisture moving in and out of the soil.

Another line of evidence is the confirmation of a hard layer of water-ice about 5 centimetres beneath the surface. And the decrease in clumpy cohesiveness of the soil experienced by the robotic arm scoop after being exposed to the air for a couple of days implied that icy material was subliming away, and could possibly be derived from a thin-film of moisture in the ground.

There could be an explanation for the negative results so far, however: the four successful soil insertions have all been into undisturbed soil and so the next line of investigation will be to scoop away some soil and insert the probe closer to the subsurface ice layer.

"There should be some amount of unfrozen water attached to the surface of soil particles above the ice," says Zent. "It may be too little to detect, but we haven't finished looking yet."

The thermal and conductivity probe is the main tool for checking for present-day soil moisture, by measuring how fast heat and electricity moves from one needle to an adjacent one through the soil or air between the needles, but Phoenix also has other tools to determine if water ice has melted in the past and to identify minerals in the soil, such as the Thermal and Evolved Gas Analyser, and other instruments in the Microscopy, Electrochemistry and Conductivity Analyser suite.