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Phoenix to go digging on Red Planet

Lander aims to reveal how ice formed on Mars' arctic region

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

While its rover cousins continue to investigate the surface of the Red Planet (as they have since early 2004), the $462 million dollar Phoenix mission 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 its landing site in the Vastitas Borealis near Mars' north pole, Phoenix is designed to 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 it has ever melted and moistened the surrounding soils. This information could shed light on whether this little-studied area of the planet could ever have been habitable for life, though Phoenix's mission isn't to find life itself.

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

Soil and ice
The vast layers of ice underlying the Vastitas Borealis were discovered in 2002, when the Mars Odyssey orbiter detected the signature of water below the top few inches of ruddy dust that coats the planet. Phoenix will provide the first direct look at this frozen subsurface layer from its landing site at 68 degrees north latitude and 233 degrees east longitude.

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

Phoenix's 7.7-foot (2.3-meter) robotic arm will dig down through the soil to the ice layer below, which is expected to be at about -136 degrees Fahrenheit (-93 degrees Celsius). At that temperature "the bonds [in the water] are so strong [that the ice is] as strong as a concrete sidewalk," Arvidson said.

At the end of the robotic arm is a rasp, about the size of your pinky finger, that will rotate down into the ice and kick up tiny pieces into the scoop for analysis by instruments aboard the lander.

One of the key measurements Phoenix is designed to make is the abundance of the different isotopes (which are versions of the same element with different atomic weights) of hydrogen and oxygen in the water ice. The most common form of hydrogen has no neutrons, but one of its isotopes, deuterium, has one neutron. Oxygen commonly has eight neutrons (this is 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 these two elements, "and that should be a signature of the processes involved in making that ice," Arvidson said.

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

"But suppose it's a different isotopic composition — it means that ice was inplaced in some other time, when water in the atmosphere had a different isotopic composition," Arvidson told "So we're trying to get at the past history and the role of water at the high latitudes."

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