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One of the last images from Phoenix. Credit: NASA/JPL/University of Arizona

 

Nov 03, 2008
The Phoenix Expires

Winter means the end of life for the Phoenix Mission. Have its mission objectives been fulfilled or is more information necessary?

Temperatures continue to fall as Mars enters its winter months. A frigid -96 degrees Celsius was recorded a couple days ago, with continued drops expected until the northern latitudes begin to precipitate carbon dioxide snow. Eventually, the lander will be buried completely in dry ice, but its functions will cease long before that time as the batteries drain due to a lack of sunlight.

Consensus opinions state that Mars was once a “warm, wet world millions of years ago.” Due to some slow geological changes or alterations in its orbit, Mars changed from a hospitable planet to the cold, barren wasteland that it is today. However, planetary scientists hoped that water might remain in the polar regions because the Odyssey orbiting observatory detected the signature of hydrogen in that region, although it was probably neutrons and gamma rays released by cosmic rays and not water vapor. It is an assumption that the detected hydrogen came from water molecules escaping beneath the surface crust. The hydrogen may have been "implanted" electrically by the solar wind, or molecules other than water were transferred to Mars from another celestial body.

Phoenix landed in the high Martian latitudes on May 25, 2008, while the northern Sun was above the horizon all day. It touched down in what was supposed to be a patch of ice so that the lander could dig several shallow trenches to find and analyze water just below the surface. The experiments were designed to uncover evidence for life by revealing carbon-based chemistry within the soil samples. At present, no biochemistry has been found, despite baking the samples at almost 1000 degrees Celsius so that volatile compounds would be driven off and detected by the spectrographic analyzer.

On June 19, 2008, small chunks of a white substance were seen in the bottom of a shallow excavation dug by Phoenix and remained for two days before disappearing. In a Picture of the Day about the discovery, it was predicted that the material was some kind of mineral—magnesium sulfate perhaps, or silicon dioxide—because those compounds are found in abundance in other areas on Mars. The Martian rovers Spirit and Opportunity both found deposits of white minerals mixed with iron oxides and sulfur.

The fact that the small white pieces vanished means they were not mineral compounds, but the question of whether they were water ice remains open. On July 31, NASA announced that they had discovered water in the soil samples that had been delivered to their ovens. However, readings taken on September 4, 2008, by a conductivity probe stuck into the ground indicated that the soil is completely dry.

The key evidence for water ice on Mars is those small chunks of sublimating material. Mark Lemmon of Texas A&M University, College Station told reporters that the disappearing chunks could not have been carbon dioxide ice because that material would not have been stable for even one day as a solid.

Since four different probe measurements found no conductivity consistent with water in the soil, then it might have been solid carbon dioxide, or dry ice, that was initially dug up by the lander. Carbon dioxide is a dielectric insulator, so no current would flow through it, leaving the prongs of the soil analyzer reading no electricity.

Aaron Zent, team leader for Phoenix's thermal and electroconductivity probe wrote: "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.... All the measurements we've made so far are consistent with extremely dry soil. There are no indications of thin films of moisture, and this is puzzling."

Dry ice melts like regular ice at -57 Celsius if the vapor pressure at its melting point is about five atmospheres. At a vapor pressure of one atmosphere dry ice sublimes at -78 Celsius, which is lower than its melting point, so it changes from solid to vapor because the additional four atmospheres of pressure is missing. The average daytime temperature on Mars during the last three months has been -31 Celsius and the lowest nighttime reading has been -80 Celsius with extremely low pressure of around 8.5 millibars (Earth sea level pressure is approximately 1013 millibars).

Could other environmental factors have influenced the time it takes for dry ice to sublime? Making it difficult to determine the rate of sublimation? If that was the case, then the initial white chunks might have been something other than water, but with no definite recourse to dry ice. Both NASA and Electric Universe proponents must wait for another mission and additional information since Phoenix will cease operations within a few weeks.

By Stephen Smith

___________________________________________________________________________

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