PASADENA, Calif. – The prospect of life on Mars has charged the public imagination for more than a century, ever since astronomers first spied what they thought were canals dug to irrigate the planet's ruddy surface.
But after spacecraft and Earth-based telescopes began taking a closer look at the planet, evidence of the canals — and the Martians who presumably created them — quickly vanished.
Instead, the scrutiny showed Mars to be a dusty, frigid world, shrouded by an atmosphere too thin to breathe, bombarded with radiation and largely dry beyond the ice that caps its poles. It seemed altogether hostile to life as we know it.
But ongoing scientific spadework continues to turn up evidence that suggests that long ago Mars was a wetter, if not warmer, world where rivers large enough to carve canyons the size of the United States flowed across its surface. Life, even if just tiny microbes, could have thrived in such a place.
Beginning late Christmas Eve, a small armada of exploratory spacecraft will reach the Red Planet, some attempting to enter orbit, others to land — a very risky business because of the engineering and physical challenges that await the robotic probes. Together, they represent one of the most ambitious efforts yet to resolve the contradictions that persist in alternately intriguing and beguiling scientists.
"There is no consensus and a lot of contradictions," said Michael Carr, a planetary geologist with the U.S. Geological Survey who has played a role in nearly every past mission to Mars.
A British spacecraft, the Beagle 2 (search), is scheduled to land on Mars Dec. 24. That same day, Europe's Mars Express (search) should enter orbit around the planet. Mars Express successfully released Beagle 2 on Friday, after carrying it piggyback most of the way to Mars.
Spirit, the first of NASA's identical robot explorers, is expected to land Jan. 3. Its sibling, Opportunity, is scheduled to settle on the opposite side of the planet Jan. 24.
The odds of all four spacecraft succeeding are slim.
Since 1960, roughly two-thirds of the three dozen spacecraft sent to Mars have failed, including two 1999 NASA missions, the Climate Orbiter (search) and Polar Lander (search). Most have been lost on launch or arrival, the most perilous portions of any mission.
The most recent failure was the Japanese satellite, Nozomi (search), which failed to enter orbit around Mars earlier this month.
NASA's back-to-back 1999 failures prompted the American space agency to tighten oversight of the design, construction, testing and launching of its spacecraft, including this year's batch.
It's also taken pains to publicly stress the risks of dispatching two landers to Mars.
"Landing on Mars is very, very, very difficult," said Ed Weiler, NASA's associate administrator for space science. "The fact that the world has failed most of the time it's gone there is indicative of that."
Despite the odds, NASA has had three successful landings on Mars: the twin Viking landers reached the planet in 1976, undertaking a direct search for life but producing results that were inconclusive; and the 1997 Pathfinder mission.
Two other NASA spacecraft, the Mars Global Surveyor and the 2001 Mars Odyssey, are already at the planet. There, from high on orbit, they continue to pile on the discoveries. Many of these findings address the question of whether water was present in the Martian past. Little of the evidence, however, offers a definitive answer.
In October, a team of scientists reported Odyssey had detected on the surface of Mars copious amounts of a mineral that's easily weathered away in the presence of water. That suggested Mars has been a dry wasteland for eons.
Weeks later, a second team reported evidence to the contrary, after Global Surveyor beamed back glossy images that show features apparently created by the meandering flow of rivers.
The case for life on Mars routinely undergoes similar setbacks and advances. The two studies are just the latest in some half-dozen "gotcha" moments in Mars science in recent years, said Daniel McCleese, chief scientist of the Mars exploration program at NASA's Jet Propulsion Laboratory.
"We are now in a position where the smoking gun for past, persistent water on Mars depends on who you talk to and what day of the week it is," he said. "The case is being made on both sides. That's the nature of science."
Meanwhile, gains in understanding Mars are also being been made on Earth and include what some believe is compelling evidence for Martian life.
In 1996, a team of scientists announced a meteorite found in Antarctica that was believed to have been blasted from Mars contained microscopic fossils of ancient bacteria. Although many scientists question the claim, it's further energized the search for life.
And new understanding of the tenacity of terrestrial microbes on Earth has scientists thinking Mars might not be too harsh for life after all.
Britain's Beagle 2 lander is designed to seek out organic material in the Martian soil, which could suggest the presence of such forms of life. Its mission also is to sample the atmosphere for traces of methane, a telltale byproduct of many biological processes.
The NASA rovers weren't designed to look for life. Nor will they look for water, the necessary ingredient of life as we know it. Instead, they'll look for minerals in the rocks on Mars that could suggest, on the one hand, the past presence of water and, on the other, the possibility that it allowed the planet to harbor life.
"We need a proxy for the proxy," Firouz Naderi, manager of the Mars exploration program at JPL, said of the indirect search for life.
Such evidence could suggest Mars was a warmer, damper and all-around more hospitable place billions of years ago — just as life first stirred here on Earth.
"It immediately raises questions: If the conditions were right, if it happened on Earth, could it have happened on Mars?" said Steve Squyres of Cornell University, the principal scientist on NASA's current mission.
The $820 million pair of rovers and the array of instruments they carry should help reconcile the conflicting views of Mars, he said.
"The mineralogy and topography are telling you different stories. The only thing to do is get down there and look," he said.
Even so, the results likely will not be definitive, said Carr, of the U.S. Geological Survey.
"I am sure at the end of these missions there will be an argument and there will be two camps, just as there are now," he said.