Next month, three robotic geologists head for the red planet to look at the history of water there and search for signs of life.
Mars beckons, and planet Earth is set to respond.Skip to next paragraph
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On June 2, the European Space Agency is set to launch Mars Express/Beagle 2 to the red planet, followed by a NASA mission that involves sending two rovers on June 5 and 25.
These robotic geologists are designed to scrutinize soil and rocks for clues to the history of the planet's climate. Together, the missions represent a vital step in the quest to answer the question: Did Mars ever offer an environment capable of nurturing life?
Some planetary scientists say Mars exploration is entering a pivotal period. "This decade is really critical," says Daniel McCleese, chief scientist for Mars exploration at the California Institute of Technology's Jet Propulsion Laboratory in Pasadena. "From my point of view, the missions that run out through 2009, as an ensemble, are going to provide the smoking gun one way or the other."
Yet even if the decade ends with a discouraging answer for astrobiologists, a "dead planet" sign hammered into a Martian plain would still leave scientists with an intriguing puzzle. Call it the Goldilocks conundrum, where Mars is too cold and dry, Venus is too hot and its rains too acidic, and Earth is just right for the emergence of life.
"What caused the three terrestrial planets Venus, Mars, and Earth to have formed from similar material and yet departed from one another climatologically?" Dr. McCleese asks. The outcome potentially has left "the zone of life in the solar system so much narrower than it plausibly could have been."
The answer to this broader question, he continues, grows more important as
astronomers find more planets around other stars and try to gauge whether these solar systems have the potential to host life. To answer that question, he says, "we are going to have to fall back on what we know about our own solar system."
Of the two missions, Europe's is aimed directly at the question of whether life ever existed - or still exists - on Mars.
If all goes well, Mars Express, the interplanetary delivery truck for a lander named Beagle 2, will arrive at Mars in December. Mars Express will remain in orbit around the planet, taking its own set of measurements. The lander is aimed at Isidis Planitia, a basin near the Martian equator filled with soils deposited over time by air, ice, or liquid water - vital to organic life.
Instruments will take tiny core samples from neighboring rocks and analyze them for specific ratios of two forms of carbon - ratios that would signal biological activity. The US Viking missions in the 1970s tried to look for evidence of life in Martian soils but came up empty-handed.
NASA, for its part, has taken an indirect approach to the question of life on Mars by first trying to reconstruct the history of the planet's climate. The basic issue is whether Mars ever had a climate warm enough to permit water to persist on the surface long enough to allow simple life forms to emerge.
The two rovers, which for now carry the prosaic names Mars Exploration Rover (MER) A and B, will be landing on sites selected last month for intriguing geology revealed by two US orbiters currently circling the planet. MER-A is set to touch down on Meridiani Planum, a plain that appears to be littered with hematite, an iron oxide that on Earth often forms in the presence of water.
MER-B is aimed at Gusev Crater, a sediment-filled basin that has a river channel running through it. Guided by scientists on Earth, but able to operate on their own if necessary, the rovers will roll over the planet's surface, analyzing rocks for evidence of minerals that form in the long-term presence of water.