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
"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
Media contacts: Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
Rachel Prucey 650-604-0643
NASA Ames Research Center, Moffett Field, Calif.
Lori Stiles 520-626-4402
University of Arizona, Tucson