Spiky Probe on NASA Mars Lander Raises Vapor Quandary

September 5, 2008
Phoenix 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 (Sept. 4, 2008). Image credit: NASA/JPL-Caltech/University of Arizona/Texas A&M University

(PhysOrg.com) -- 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 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, Pasadena, Calif.

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 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 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 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 instrument suite.

Provided by NASA

Explore further: China unveils 2020 Mars rover concept: report

Related Stories

China unveils 2020 Mars rover concept: report

August 24, 2016

China has unveiled illustrations of a Mars probe and rover it aims to send to the Red Planet at the end of the decade in a mission that faces "unprecedented" challenges, state media said on Wednesday.

Phoenix probe lands on Mars (Update 4)

May 25, 2008

NASA's Phoenix spacecraft landed in the northern polar region of Mars Sunday to begin three months of examining a site chosen for its likelihood of having frozen water within reach of the lander's robotic arm.

Managing water resources in forest restoration

September 8, 2014

Hundreds of thousands of acres on the Coconino National Forest are slated for thinning during the next 20 years. Two NAU researchers want the forest restoration efforts to result in better water quality and quantity, a shift ...

Cosmic rays detect soil moisture

November 17, 2010

(PhysOrg.com) -- An instrument that uses cosmic rays to measure the moisture content in soil ultimately could prompt major improvements in weather forecasting and irrigation practices, and provide a wealth of new data about ...

Two approaches for optimizing water productivity

April 29, 2013

Agricultural Research Service researchers in Bushland, Texas, are helping farmers make the most of their water supplies in a region where they depend on the Ogallala Aquifer, a massive underground reservoir under constant ...

Recommended for you

STEREO—10 years of revolutionary solar views

October 26, 2016

Launched 10 years ago, on Oct. 25, 2006, the twin spacecraft of NASA's STEREO mission – short for Solar and Terrestrial Relations Observatory – have given us unprecedented views of the sun, including the first-ever simultaneous ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.