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.

"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."