Phoenix Probe Says Both Yes and No to Water on Mars

Written by Nancy Atkinson

NASA's Phoenix Mars Lander has a fork-like conductivity probe on it's robotic arm, and results from the instrument are presenting a bit of a quandary for mission scientists. The thermal and electroconductivity probe has sensed humidity rising and falling in the air the near the lander, but when stuck into the ground, its measurements so far indicate soil that is thoroughly and perplexingly dry. "If you have water vapor 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," said Aaron Zent, lead scientist for the probe. While Phoenix has other tools to find clues about whether water ice at the site has melted in the past, the conductivity probe is the main tool for checking for present-day soil moisture.

Preliminary results from the latest insertion of the probe's four needles into the ground, on Wednesday and Thursday, match results from the three similar insertions in the three months since landing. "All the measurements we've made so far are consistent with extremely dry soil," Zent said. "There are no indications of thin films of moisture, and this is puzzling."

In below-freezing permafrost terrains on Earth, that thin layer of unfrozen water molecules on soil particles can grow thick enough to support microbial life. One goal for building the conductivity probe and sending it to Mars has been to see whether the permafrost terrain of the Martian arctic has detectable thin films of unfrozen water on soil particles. By gauging how electricity moves through the soil from one prong to another, the probe can detect films of water barely more than one molecule thick.

Three other sets of observations by Phoenix, in addition to the terrestrial permafrost analogy, give reasons for expecting to find thin-film moisture in the soil.

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