Spiky Probe on NASA Mars Lander Raises Vapor Quandary

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Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
guy.webster@jpl.nasa.gov

Dwayne Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown@nasa.gov

Sara Hammond 520-626-1974
University of Arizona, Tucson
shammond@lpl.arizona.edu

News Release: 2008-171 September 4, 2008

Spiky Probe on NASA Mars Lander Raises Vapor Quandary

TUCSON, Ariz. -- A fork-like conductivity probe has sensed humidity rising and falling
beside NASA's Phoenix Mars 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 of NASA Ames Research Center, Moffett Field, Calif., lead
scientist for Phoenix's thermal and electroconductivity probe.

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.

"Phoenix has other tools to find clues about whether water ice at the site has melted in the
past, such as identifying minerals in the soil and observing soil particles with
microscopes. The conductivity probe is our main tool for checking for present-day soil
moisture," said Phoenix Project Scientist Leslie Tamppari of NASA's Jet Propulsion
Laboratory, Pasadena, Calif.

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."

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.

One is the conductivity probe's own measurements of relative humidity when the probe is
held up in the air. "The relative humidity transitions from near zero to near 100 percent
with every day-night cycle, which suggests there's a lot of moisture moving in and out of
the soil," Zent said.

Another is Phoenix's confirmation of a hard layer containing water-ice about 5
centimeters (2 inches) or so beneath the surface.

Also, handling the site's soil with the scoop on Phoenix's robotic arm and observing the
disturbed soil show that it has clumping cohesiveness when first scooped up and that this
cohesiveness decreases after the scooped soil sits exposed to air for a day or two. One
possible explanation for those observations could be thin-film moisture in the ground.

The Phoenix team is laying plans for a variation on the experiment of inserting the
conductivity probe into the soil. The four successful insertions so far have all been into an
undisturbed soil surface. The planned variation is to scoop away some soil first, so the
inserted needles will reach closer to the subsurface ice layer.

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

The thermal and electroconductivity probe, built by Decagon Devices Inc., Pullman,
Wash., is mounted on Phoenix's robotic arm. The probe is part of the lander's
Microscopy, Electrochemistry and Conductivity instrument suite.

The Phoenix mission is led by Peter Smith at the University of Arizona with project
management at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and development
partnership at Lockheed Martin in Denver. International contributions come from the
Canadian Space Agency; the University of Neuchatel, Switzerland; the universities of
Copenhagen and Aarhus in Denmark; the Max Planck Institute in Germany; and the
Finnish Meteorological Institute.

For more about Phoenix, visit: http://www.nasa.gov/phoenix or
http://phoenix.lpl.arizona.edu.

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