If you're hunting for places where Mars once might have hosted life, it's tough to beat Gale Crater – a 96-mile-wide dent in the Martian crust and the target for NASA's Mars Science Laboratory mission.
Early Monday morning, the mission's one-ton, Mini Cooper-sized, $2.5 billion rover named Curiosity is slated to touch down inside the crater in what scientists say will be the most pinpoint, harrowing landing ever attempted on the Martian surface.
[Editor's note: The original version of this story gave the incorrect weight for the Curiosity rover.]
While the risks are high – NASA refers to the landing as “seven minutes of terror” – so is the potential payback as the Mars Science Laboratory team tries to answer the question: Did Mars ever have the conditions that would have allowed life to emerge?
The answer is intimately tied to the presence of water on the Martian surface early in the planet's history. Water is essential for life to gain and maintain a foothold on any planet, researchers say.
The Gale Crater straddles the boundary between the planet's southern, crater-pocked highlands and the smoother northern lowlands, according to John Grotzinger, a planetary scientist at the California Institute of Technology in Pasadena, Calif., and the mission's project scientist.
Since water flows downhill, "we think billions of years ago water flowed across that surface ... and there's Gale Crater, a little bowl capturing any water that may have been present there," he said during a recent prelanding briefing. "Gale is one of the lowest places on Mars. And if you don't know anything else in advance, that's where you want to go to find evidence of water."
But Gale Crater speaks to more than Mars' early history. It also speaks to billions of years of climatic change that have swept the Martian surface.
It's as though a cosmic kindergartner filled a bowl with soil, then changed his mind and started to empty it again around the edges – leaving a central summit and exposing rock formations at its base that the rover will analyze.
"When it comes right down to it, we don't know much about the crater and the rocks inside it," says Ralph Milliken, a planetary scientist at Brown University in Providence, R.I., and a member of the Mars Science Laboratory's science team.
The ballad of Gale Crater is thought to begin some 3.5 billion to 4 billion years ago, when a chunk of space rock roughly the size of Manhattan slammed into the Martian surface, explains Philip Christensen, a planetary geologist at Arizona State University in Tempe who has focused much of his research on Mars.
The collision occurred at a time when Mars is thought to have had a thicker atmosphere, plate tectonics, and periods when the climate was warm and wet enough to allow water to flow on its surface.
The collision left a crater that at 96 miles wide and 2-1/2 miles deep would have swallowed Rhode Island and most of Connecticut. It would have also generated "a lot of heat on impact," Dr. Milliken adds. "If Mars had water back at that time, and we think it did, it could have set up hot fluids circulating through the crust."
Such fluids appear on Earth at deep-sea hydrothermal vents and in places such as Yellowstone National Park, for instance: both locations where scientists have uncovered unique forms of life able to withstand extreme temperatures and what would be fatal chemical soups to humans.
The collision also would have formed a small peak in the middle of the crater – but nothing that matches the gently sloped, 18,000-foot mountain that has informally been dubbed Mt. Sharp.
If water flowed through the crater, it could have deposited sediments. But the leading candidate for building the mountain is wind, which deposited sediments that may have filled the crater, Dr. Christensen says. The mountain represents the eroded remains of that enormous volume of rock and soil.
If that was the case, the rover Curiosity may be able to uncover evidence for a crater once brimming with sediment, Milliken says.
Images of the mountain from the spacecraft now orbiting the red planet show that rock layers in the bottom half of the mountain are fairly flat, suggesting that they indeed may have spanned the crater wall to wall. The top half of the mountain, however, displays slanted layers that taper at the top, suggesting that the layers formed along the slope rather than extending across the crater.
"Whether or not all the layers of rock in that entire ... pile of material extended all the way to the crater walls? We don't know that for sure," he says.
Researchers have proposed a couple of ways that wind alone could have formed Mt. Sharp. Whatever the mechanism, the beauty of Mt. Sharp is that it has two or three distinct types of sedimentary rock. The types change with altitude, serving as signposts of larger environmental changes taking place across the planet over time.
The bottom layers near the crater floor that are the main targets for study "have minerals in them that form in water," says Christensen – minerals such as clays, sulfates, and gypsum. "The current thought is that when this crater first formed, maybe in the bottom of it there was a standing lake, maybe there was ground water. But there was enough water that it formed sediments that are full of minerals that form in water."
In addition, bluffs at Mt. Sharp's base that exhibit this layering also have channels cut into them, suggesting water at some point repeatedly flowed down the mountain's flanks. Curiosity is slated to examine some of these channels and hunt for evidence of organic compounds there that would suggest one-time hospitality suites for at least simple forms of life.