"These two instruments are going to provide fundamental, quantitative ground truth that we can tie back to our orbital observations," says Jack Mustard, a planetary geologist at Brown University. Spectrometers onboard spacecraft now orbiting Mars can pinpoint signatures of important minerals, such as clays that form in watery environments, but their true nature can only be determined from up close. That is part of the reason for sending Curiosity to its planned landing site within Gale Crater, an equatorial depression where orbiters have spotted water-linked materials such as clays and sulfates. "If by going there with the Mars Science Laboratory we can ground truth that and then apply that to what we're getting from orbit elsewhere, it's just going to be phenomenal," Mustard says.
Gale Crater should have plenty of attractions to keep Curiosity occupied throughout its 23-month baseline mission on Mars. "We are not a life-detection mission, but our goal is to look for habitable environments," Grotzinger says. "In the case of Gale, we have multiple potentially habitable environments." The clay- and sulfate-rich layers within the 150-kilometer-wide crater form the base of a towering mound about five kilometers high. "It's literally a mountain of layers," Grotzinger says, noting that the mound has the relief of California's Mount Whitney and the layering of the Grand Canyon. "It's this stunning repository of information of the history of Mars," he says. "It's like reading a book."
This deposits could hold clues as to how Mars, in its early history, changed from a clay-producing world to a sulfate-producing one. "Mars has undergone remarkable reorganizations of its planetary systems, and that is reflected in its geology," Mustard says. "The first half billion years of its history are dominated by these clay minerals. Then the planet reorganized itself—it went from neutral pH to acidic. It started to dry out and form sulfates. That is captured, we think, by sedimentary minerals at the landing site."
But before Curiosity can dig into the geologic history of Mars, it first has to get there in one piece. NASA has devised an elaborate scheme to park Curiosity on the surface, just seven minutes after the rover and its descent stage scream into the atmosphere at a speed of about six kilometers per second. The tenuous Martian atmosphere will slow the craft considerably, but the final descent will involve first a parachute, then retrorockets and, finally, a hovering platform, or sky crane, that will lower the rover onto the surface on nylon cords. The intricate choreography of that sequence should produce some white knuckles in the planetary science community come August, but Grotzinger insists that the landing plan is on solid ground, having passed multiple independent reviews. "We've worried about that for years," he says, but "honestly, this seems to be one of the things that people are most confident in."