Phoenix Site on Mars May be in Dry Climate Cycle Phase

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

Rachel Prucey 650-604-0643
NASA Ames Research Center, Moffett Field, Calif.
rachel.l.prucey@nasa.gov

Lori Stiles 520-626-4402
University of Arizona, Tucson
lstiles@u.arizona.edu

News release: 2008-236 December 15, 2008

Phoenix Site on Mars May Be in Dry Climate Cycle Phase

PASADENA, Calif. -- The Martian arctic soil that NASA's Phoenix Mars Lander dug into this year is very cold
and very dry. However, when long-term climate cycles make the site warmer, the soil may get moist enough to
modify the chemistry, producing effects that persist through the colder times.

Phoenix found clues increasing scientists' confidence in predictive models about water vapor moving through
the soil between the atmosphere and subsurface water-ice. The models predict the vapor flow can wet the soil
when the tilt of Mars' axis, the obliquity, is greater than it is now.

The robot worked on Mars for three months of prime mission, plus two months of overtime, after landing on
May 25. The Phoenix science team will be analyzing data and running comparison experiments for months to
come. With some key questions still open, team members at a meeting of the American Geophysical Union
today reported on their progress.

"We have snowfall from the clouds and frost at the surface, with ice just a few inches below, and dry soil in
between," said Phoenix Principal Investigator Peter Smith of the University of Arizona, Tucson. "During a
warmer climate several million years ago, the ice would have been deeper, but frost on the surface could have
melted and wet the soil."

With no large moon like Earth's to stabilize it, Mars goes through known periodic cycles when its tilt becomes
much greater than Earth's. During those high-tilt periods, the sun rises higher in the sky above the Martian
poles than it does now, and the arctic plain where Phoenix worked experiences warmer summers.

"The ice under the soil around Phoenix is not a sealed-off deposit left from some ancient ocean," said Ray
Arvidson of Washington University in St. Louis, lead scientist for the lander's robotic arm. "It is in equilibrium
with the environment, and the environment changes with the obliquity cycles on scales from hundreds of
thousands of years to a few million years. There have probably been dozens of times in the past 10 million
years when thin films of water were active in the soil, and probably there will be dozens more times in the next
10 million years."

Cloddy texture of soil scooped up by Phoenix is one clue to effects of water. The mission's microscopic
examination of the soil shows individual particles characteristic of windblown dust and sand, but clods of the
soil hold together more cohesively than expected for unaltered dust and sand. Arvidson said, "It's not strongly
cemented. It would break up in your hand, but the cloddiness tells us that something is taking the windblown
material and mildly cementing it."

That cementing effect could result from water molecules adhering to the surfaces of soil particles. Or it could
be from water mobilizing and redepositing salts that Phoenix identified in the soil, such as magnesium
perchlorate and calcium carbonate.

The Thermal and Electrical Conductivity Probe on Phoenix detected electrical-property changes consistent
with accumulation of water molecules on surfaces of soil grains during daily cycles of water vapor moving
through the soil, reported Aaron Zent of NASA Ames Research Center, Moffett Field, Calif., lead scientist for
that probe.

"There's exchange between the atmosphere and the subsurface ice," Zent said. "A film of water molecules
accumulates on the surfaces of mineral particles. It's not enough right now to transform the chemistry, but the
measurements are providing verification that these molecular films are occurring when you would expect them
to, and this gives us more confidence in predicting the way they would behave in other parts of the obliquity
cycles."

The Phoenix mission is led by Smith at the University of Arizona with project management at NASA's Jet
Propulsion Laboratory, Pasadena, Calif., and development partnership at Lockheed Martin, Denver.
International contributions come from the Canadian Space Agency; the University of Neuchatel, Switzerland;
the universities of Copenhagen and Aarhus, Denmark; Max Planck Institute, Germany; and the Finnish
Meteorological Institute; and Imperial College, London. For more about Phoenix, visit:
http://www.nasa.gov/phoenix.

-end-

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