[meteorite-list] Spiky Probe on Phoenix Raises Vapor Quandary
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 4 Sep 2008 18:04:29 -0700 (PDT)
Spiky Probe on NASA Mars Lander Raises Vapor Quandary
Jet Propulsion Laboratory
September 04, 2008
TUCSON, Ariz. -- 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.
"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, Calif., lead scientist for Phoenix's thermal and
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,
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
Another is Phoenix's confirmation of a hard layer containing water-ice
about 5 centimeters (2 inches) or so beneath the surface.
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
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 ice layer.
"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 inished looking yet."
The thermal 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
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
For more about Phoenix, visit: http://www.nasa.gov/phoenix or
Media contacts: Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
guy.webster at jpl.nasa.gov
Dwayne Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown at nasa.gov
Sara Hammond 520-626-1974
University of Arizona, Tucson
shammond at lpl.arizona.edu
Received on Thu 04 Sep 2008 09:04:29 PM PDT