Mars 2008: A Red Planet Odyssey

Dec. 28, 2007 -- At the tail of 2007, the intrepid robotic explorer Spirit was beating a path to a safe spot to spend the Martian winter. It was a hard year, and the small, six-wheeled rover still bears the scars of a planet-wide dust storm that nearly ended its mission.

Spirit and a twin named Opportunity have been pawing the dead Mars terrain for signs of past water, a key factor in scientists' ongoing quest to determine if Earth's neighbor ever hosted life.

The dust storm left Spirit so caked with debris that sunlight has a hard time getting through to the solar power cells that charge the rover's batteries. The past three weeks were particularly tense because the sun is not rising very high in the sky as the planet's southern hemisphere pivots into winter.

Still Going, and Going

But Spirit, which has already defied the odds, pulled through again, settling onto a northern-facing slope of a rock formation known as Home Plate in Gusev Crater. That it continues to operate at all, four years after arriving on Mars for what was designed to be a three-month mission, is most surprising.

Spirit's prowess has been matched by sister rover Opportunity, which is exploring a crater halfway around the planet. The dust storm shut it down for weeks as well, but Opportunity recovered quicker thanks to brisk winds that instead of depositing more sand blew its solar panels clean.

"The fact that these two rovers are going on their fourth year is amazing," said Mars researcher Ray Arvidson with Washington University in St. Louis.

Both rovers found mineralogical evidence that Mars had water on or near its surface at some point in its past. They will continue to pry out secrets even as scientists prepare for their first direct look at what remains of the planet's water today.

On May 25, a new probe named Phoenix is due to touch down on Mars' unexplored north pole. Its landing spot has been carefully selected and timed to coincide with the seasonal spring thaw. Scientists are counting on scratching through the ground cover and finding what may be a thick layer of water ice beneath the frozen surface.

"The first major discovery we think we'll see is ice," said Arvidson, a Phoenix co-investigator. After that, he added, "we don't know what we're going to find."

Phoenix will bake bits of Martian ice in tiny ovens to determine the isotopic composition of the water, information scientists can use to learn how recently the water was locked into the ground. If the isotopic ratios are similar to atmospheric composition, the water likely was a fairly recent phenomenon in a geologic sense, a mere 100,000 years old or so.

But if the ice samples show a disequilibrium with atmospheric measurements, then the water may be ancient, leftover remains of a long-vanished sea.