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.
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.
The missions come at a time when planetary scientists are conducting a fundamental makeover of their notions regarding the planet's geological history.
"We're tearing down the framework of our earlier understanding of Mars and in its place we have yet to build a complete new structure," McCleese says.
The effort began with the Mariner missions in the mid 1960s and the Viking lander-orbiter missions in the mid-1970s. It blossomed with the arrival of the Mars Global Surveyor (MGS) and Mars Odyssey orbiters in 1999 and 2001. The duo has revealed topography and surface mineral deposits with a stunning level of detail, revealing features like valleys, mesas, gullies, and other signs of erosion that look deceptively Earthlike.
Indeed, data from the Mars Global Surveyor "has turned Mars geology upside down," says Harold McSween, a geologist at the University of Tennessee. "We've always had this view that Mars geology was kind of simple. Mars was a bunch of lava flows, and they just lie there soaking up impacts from meteors."
Yet using the MGS camera they designed, Michael Malin and Kenneth Edgett, with Malin Space Science Systems in San Diego, have "seen layered rock, which they interpret as layers of sediment, everywhere at high resolution," McSween says. "Many of these are deeply eroded," presenting planetary geologists and climatologists with a more complicated story to decipher.
McCleese says most of his colleagues support the notion that early in its history, the planet had a sufficient heat-trapping atmosphere to allow water to collect, flow, and persist on the surface. Many of the large valleys and erosion features they see today probably formed during this period, they argue. Other evidence from remote-sensing studies of the planet's core suggest that, early on, Mars had a strong magnetic field that might have shielded budding life forms on the surface from the bath of space-based radiation enveloping the planet.
Yet many of the features the cameras are capturing also could have formed under more frigid, dry, ice-capped conditions similar to those astronomers see today.
A team led by Marc Kramer, a hydrologist at NASA's Ames Research Center at Moffett Field, Calif., looked again at old Martian crust that in earlier, less detailed images appeared to sport basins and dead-end rivers, which were thought to have formed through groundwater undermining the surface.
Using the more detailed Mars Global Surveyor images and comparing what they saw to hydrological models, team members reported in January that the features actually form drainage networks similar to those found in the Great Lakes region of the US. The origin of such features in the Great Lakes has been traced to glacial melt and gravity rather than large erosive flows of water from rain.
Before he and his colleagues undertook their study, "I was of the mind-set that precipitation may have driven these older features," Dr. Kramer says. "It's still a plausible alternative, but the cold model also can drive the kind of features we're seeing."
These and other competing ideas are providing the intellectual fuel for June's assault on Mars.
The deluge of new data since 1997 is opening "an awful lot of doors to understanding Mars, but they also can leave us scratching our heads," says John Grant, a researcher at NASA-Ames and a member of the MER site selection committee.
"Now we've got the opportunity to actually go to a couple of places where there's been a detailed evaluation of the surface. We can actually start testing hypotheses."