New data from Mars' surface is
puzzling NASA scientists: A fork-like probe on the Phoenix Mars Lander has
sensed changes in humidity in the Martian air, but finds the dirt below
perplexingly dry.
The measurements, the latest of
which were taken over the last few days, indicate that water vapor is settling
on or in the Martian dirt then being released back into the air on a daily
cycle. So mission scientists have expected to find water molecules sticking to
the Martian surface.
"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 the NASA Ames Research Center in
Moffett Field, Calif., and the lead scientist for Phoenix's thermal and electroconductivity probe, which is making the humidity
measurements.
Phoenix's mission
on the red planet, which began after a May 25 landing there, is all about
the tantalizing search for Martian water. The lander
has already confirmed
the presence of a subsurface layer of water ice, first detected in 2002 by
NASA's Mars Odyssey orbiter, which is still circling the planet.
The Phoenix spacecraft is searching for clues
that this layer was once liquid — a sign of Mars' past potential habitability. Phoenix is also trying to
characterize the current behavior and cycling of water on Mars.
"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," said Phoenix project scientist Leslie Tampari
of NASA's Jet Propulsion Laboratory in Pasadena, Calif. But, she added, "the conductivity probe is the main tool for checking for
present-day soil moisture."
Mission scientists have several reasons to
think that the water vapor detected in the Martian air should be present as a
thin layer of moisture on the ground.
One is that in similar areas of
below-freezing permafrost on Earth, a thin layer of unfrozen water is detected.
One of the goals in sending the conductivity
probe aboard Phoenix
was to see if a similar thin film of moisture was present in the Arctic plains
of Mars, where the spacecraft landed.
Another is the probe's measurements
of relative humidity in the air on Mars.
"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.
The discovery of the water ice layer
and the apparent sublimation (or transition directly into the gas phase) of ice
samples gathered in Phoenix's
scoop are two more signs that moisture should be collecting on the surface
dirt.
But so far, the four insertions
of the lander's probe, the most recent of which
occurred on Wednesday and Thursday, haven't turned up any water.
"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."
The probe can detect water in the
dirt by gauging how electricity moves through the surface between the four
needles of the probe. The instrument can detect films of water barely more than
one molecule thick.
Despite the failure to find a thin
film so far, the Phoenix
team isn't giving up. They are planning a variation of the insertion experiment
that will involve scooping away a bit of dirt before inserting the probe to
reach closer to the subsurface ice layer.
"There should be the same
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."
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