Robot to Dig Martian Arctic

A softtouchdown in Mars' northern arctic plains set for Sunday is just the first stepfor NASA's Phoenix Mars Lander. If the dust clears, solar-power arrays deployand all equipment checks out, Phoenix will then have some digging to do.

While itsrover cousins continue to investigate the surface of the red planet (as theyhave since early 2004), the $462 million dollar Phoenixmission aims to see what's underneath the soil. "Our voyage is down;we dig," said Phoenix principal investigator Peter Smith of the University of Arizona.

At itslanding site in the Vastitas Borealis near Mars' north pole, Phoenix is designedto scoop up samples of Martian soil, as well as the layers of rock-hard ice beneath,in the hopes of shedding light on when and how the ice formed and whether ithas ever melted and moistened the surrounding soils. This information couldshed light on whether this little-studiedarea of the planet could ever have been habitable for life, though Phoenix's mission isn't to find life itself.

"We'reliterally scratching the surface, and it's a stepping stone," Smith said."If we see something that's unexpected and absolutely fascinating andinteresting, I would expect NASA would want other missions, that it would gotake the next step in the polar regions."

Soil andice

The vastlayers of ice underlying the Vastitas Borealis were discovered in 2002, whenthe Mars Odyssey orbiter detected the signature of water below the top fewinches of ruddy dust that coats the planet. Phoenix will provide the firstdirect look at this frozen subsurface layer from its landing site at 68degrees north latitude and 233 degrees east longitude.

"What Phoenix is trying to do is follow the water and validate what we think we discovered fromorbit," said Phoenix landing site working group chairman Ray Arvidson of Washington University in St. Louis.

Phoenix's7.7-foot (2.3-meter) robotic arm will dig down through the soil to the icelayer below, which is expected to be at about -136 degrees Fahrenheit (-93degrees Celsius). At that temperature "the bonds [in the water] are sostrong [that the ice is] as strong as a concrete sidewalk," Arvidson said.

At the endof the robotic arm is a rasp, about the size of your pinky finger, that willrotate down into the ice and kick up tiny pieces into the scoop for analysis byinstruments aboard the lander.

One of thekey measurements Phoenix is designed to make is the abundance of the differentisotopes (which are versions of the same element with different atomic weights)of hydrogen and oxygen in the waterice. The most common form of hydrogen has no neutrons, but one of itsisotopes, deuterium, has one neutron. Oxygen commonly has eight neutrons (thisis called oxygen-16), but one of its stable isotopes has 10 (called oxygen-18). Phoenix's mass spectrometer will measure the ratios of the isotopes of thesetwo elements, "and that should be a signature of the processes involved inmaking that ice," Arvidson said.

Here iswhat those details could reveal about ice on Mars: One theory is that the iceis in equilibrium with the scant amount of water vapor in Mars' atmosphere andfroze out of the air and into the pore spaces between the soil grains. BecauseMars' gravity is weaker than Earth's, it can only hold on to heavier elementsin its atmosphere, so it has a higher ratio of deuterium and oxygen-18 to theirlighter isotopes. If the mass spec examines the isotopic ratios of the waterand the air "and if they're identical, it means that the water in theatmosphere is in contact, in equilibrium with the ice," Arvidsonexplained.

"Butsuppose it's a different isotopic composition ? it means that ice was inplacedin some other time, when water in the atmosphere had a different isotopiccomposition," Arvidson told SPACE.com. "So we're trying to getat the past history and the role of water at the high latitudes."

Signs oflife

The lander alsois set to scoop up samples of soil near the ice layer to look for signs ofpotential habitability. Because the ice has been so cold for so long,"it's been in a deep-freeze, and if there are any organics, they should bevery well preserved," just as food can bepreserved in your freezer, Arvidson said.

The frozenground on Mars today probably isn't too hospitable a place for life, so missionscientists aren't expecting to get to the pole and find "little greenmen," or even "little green microbes" ? instead the lander willlook for conditions that could support them.

Specifically,the instruments on Phoenix will analyze the soil to see if the water icelayer was once ever a liquid water layer.

"Liquidwater changes soil, ice doesn't do much of anything," Smith explained."Ice is like another form of rock. Nothing happens because ice is nearby ?it has to melt."

So if thelander's instruments find evidence of clays, salts or carbonates ? all of whichare transformed by water ? in the soil, that would mean that "the soil waswet with liquid water" or was blown in from somewhere else on the planetthat once had liquid water, Smith explained.

In thesearch for signs of life on Mars, "there's not a magical formula thatwe're looking for," Arvidson said, but there are a few key conditions thatwould increase the likelihood that Mars at least at some point harbored life.

The firstis the ice itself, "because water and habitability kind of gotogether," Arvidson said. Phoenixwill also dissolve soil samples in four teacup-sized beakers that have electrodesto measure the soil's pH (level of acidity) and oxidation potential, which canaffect an organism's ability to carry out certain key biochemical reactions. Itwill also look for certain elements (carbon, hydrogen, oxygen, phosphorus andsulfur) that go hand-in-hand with life, on Earth at least.

Gases givenoff when soil samples are heated in tiny ovens aboard the spacecraft will showwhether any organic compounds, which could be traces of past life, are presentin the soil. But scientists have to make certain that any detected organicsdidn't just make the trip with the lander from Earth.

"If weget a hit like that, we are going to be totally, totally, like, probably fortwo or three days, making sure we haven't goofed in some way," Arvidsonsaid.

"Infact, it's really tough. If we measure organics, the first thing we think is,'It's terrestrial; we brought it with us.' The second thing is that it's fromthe asteroids and comets," Smith agreed. "It would take aconsiderable amount of evidence before we could talk about biology."

Martianweather

When Phoenix's three-month primary mission is completed (likely in September) at the end of thenorthern hemisphere summer on Mars,the lander will switch modes to become a weather station.

The weatherinstrumentation aboard the lander, provided by the Canadian Space Agency,includes a 4-foot (1.2-meter) mast with sensors at three heights that can monitortemperature. A wind telltale at the top of the mast shows the wind directionand speed.

A probethat can measure the moisture level of soil also is designed to measure therelative humidity of the Martian air. Such measurements characterizing theatmosphere at high latitudes have never been made before, Arvidson said.

Phoenix is also equipped with a lidar (for"light detection and ranging") tool that can measure dust and iceparticles in the atmosphere. The tool sends powerful laser pulses verticallyinto the air, which then scatter off the particles, some returning to theinstrument. This information will help scientists track changes in particleabundance and learn how clouds and dust plumes move and form in the Martianatmosphere.

Missionscientists are also hoping that as summer ends and the polar ice cap expands, Phoenix will be able to watch the process. "That would be totally cool,"Arvidson says, since the ice cap formation has never been observed from thesurface. Scientists don?t even know if the white coating observed fromsatellites is frost, snow or slabs of ice.

"Ifwe're lucky, what we'll see is the accumulation of ice, water ice, and dust,and maybe even CO2 [carbon dioxide] ice," Arvidson said.

Eventually,as the sun sets (though it rises and descends in the sky each "sol,"or Martian day, the sun remains about the horizon throughout the northernhemisphere summer above the arctic circle, just as it does on Earth) and thecraft is encased in this advancing ice, it will end its mission for good.

Because nocraft has ever ventured this far north on Mars (the closest was Viking2's landing at 48 degrees latitude), scientists have little idea what toexpect from any of the analyses Phoenix will perform. Whether they'll findsigns of a muddy Martian past or organics is anybody's guess.

"Ican't tell you what we're going to find, because this is really exploration anddiscovery," Arvidson said.

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