Probe Sensed Humidity Variances... But Can't Identify
NASA has a minor Martian mystery on its hands. A fork-like
conductivity probe has sensed humidity rising and falling beside
the Phoenix lander... but when stuck into the ground, its
measurements so far indicate soil that is thoroughly and
"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, CA, 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 in Pasadena, CA.
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
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
"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, WA, 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.