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The Mars mystique

After 50 years of missions to Mars, scientists are unlocking some of the mysteries surrounding a planet that has captivated mankind for millenniums. Will ­humans ever leave a boot print on Mars?

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A year later, after Opportunity landed, it found tiny spheres of minerals sprinkled across the surface and embedded in a layer of sediment that was part of a rocky outcropping the rover was examining. Researchers dubbed the spheres "blueberries," later identified as made of the mineral hematite, which form in water-saturated soil deposits. Last September, the team said it had discovered similar spheres in a formation at Endeavour Crater, but with different compositions. In addition, the rover has detected veins of gypsum and clays in the rocks, further suggesting water in the area early on.

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Curiosity has added to the growing evidence. In its brief sojourn, the rover has already helped scientists identify rounded rocks, as well as rocks bound up in cemented clumps of soil, that point to the presence of an ancient riverbed etched on the floor of Gale Crater billions of years ago.

Comparing timelines for the geological evolutions of Earth and Mars, as well as for the emergence of life on Earth, "it becomes abundantly clear that, yes, there probably was persistent standing water on Mars long enough" for life to have evolved in the same fashion it did on Earth, Dyar says.

Now, Curiosity, with its ability to zap, drill, and analyze rock and soil, while rolling across the surface of Mars (and leaving JPL's initials in Morse code in its tracks), is taking the next step: hunting for potential ancient habitats. Yet even if it comes up dry, the mission holds the promise of writing a new chapter in humanity's Book of Mars, and perhaps even Book of Earth.

"I can't imagine being disappointed scientifically, even if we don't find carbon" or features that strongly indicate that the area was not only habitable, but in fact did support life, said John Grotzinger, the mission's lead scientist, during a prelanding briefing.

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Curiosity's path up Gale Crater's Mt. Sharp is essentially a stroll through geological time and environmental history on Mars – one that represents a strong contrast to that on Earth, where life did evolve.

"Even in the case where life was never present on Mars, I still see [the mission] as an extraordinary opportunity to get a bearing on our own existence on Earth," Dr. Grotzinger said.

Whatever Curiosity uncovers about the past, it still leaves open the possibility that habitats may exist on the red planet today. Indeed, the prospect of extant life seems less far-fetched than it did in the years following the Viking results (see timeline). One of the remarkable discoveries from Odyssey is the presence of vast amounts of frozen slurry below the Martian surface, which is either ice layers tens of feet thick or a mix of ice and soil. The deposits extend from the poles deep into temperate latitudes.

The presence of the ice was confirmed in 2008 by the Phoenix Mars Lander, whose sampling scoop discovered it just beneath the surface at its landing site, informally known as Green Valley, in the north polar region. To the researchers' surprise, the lander also detected snow falling during its six-month mission.

Radar on the Mars Reconnaissance Orbiter uncovered evidence that the lower-latitude deposits may represent the remains of glaciers retreating as the planet undergoes a warming trend – part of long-term swings in the planet's average warmth tied to its tilt and orbit. Shaded by rock and soil on the surface, some of these formations are up to a half-mile thick and extend for miles. Flash melting of remnant glaciers when erosion exposes some of their ice may be responsible for relatively young-looking gullies orbiters have spotted on crater walls.

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