on NASA Mars Lander
Raises Vapor Quandary
(NASA/JPL) — 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.
inserted the four needles of its thermal and conductivity
probe into Martian soil during
the 98th Martian day, or sol,
of the mission and left it in place until Sol 99.
by NASA/JPL-Caltech / University
of Arizona / Texas A&M
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
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.
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.
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.
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."
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.
Phoenix’s confirmation of a hard layer containing water-ice
about 5 centimeters (2 inches) or so beneath the surface.
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.
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.
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
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