August 20, 2012

Frank Morring, Jr., and Guy Norris Pasadena, Calif.

As the sky-crane descent-stage hovered over the landing site, lowering the rover Curiosity to the surface of Mars on cables, its thrusters blasted deep enough to give investigators an unexpected chance to look all the way down to bedrock. But the excavation zone is also giving mission planners pause as they consider where Curiosity should start its exploration.

It remains to be seen where the rover will drive first. “We probably aren't going to know for a week or two, until we have completed CAP1B [commissioning phase], and we don't have to make that decision at this point,” says John Grotzinger, mission chief scientist.

The choice is boiling down to three main options, the first of which is to study the excavation zone created by the descent thrusters' blast. Grotzinger says the Jet Propulsion Laboratory (JPL) team is “concerned about the level of contamination—not just the hydrazine [rocket fuel] and heating, but also the fractured rock. The team has thought about it and we think the first sample ought to be Mars, rather than something we did to alter it.”

A second option is an area of interesting morphology some 500 meters (1,640 ft.) to the east of Curiosity, where images indicate the convergence of three separate forms of terrain. This could present a target-rich environment where the fluvial features indicative of an ancient watery past give way to “everything else downstream” says Grotzinger. “We are pretty sure that mantling material is obscuring that, and there's lots of interest in answering that question.”

The third option is simply to characterize the landing site and then “get going,” he adds. Curiosity's overall transit speed between areas of interest is going to be slow at first, while checks continue and operators become familiar with the rover. However, Grotzinger says that although the rover incorporates better autonomous systems, these are not “quantum” improvements over the Mars Exploration Rovers. “I honestly don't think we will move much faster than Spirit or Opportunity,” he says.

The JPL team expects Curiosity to take about one year to navigate through dark-colored active sand dunes and around obstacles to make it to a target area “a few hundred meters up in elevation” on the slopes of the 5.5-km-tall (18,000-ft.) Mount Sharp, according to MSL Deputy Project Scientist Ashwin Vasavada. The rover is expected to drive “something like a football field a day” at top speed, and will “have to find a gap in the dune field to give us safe passage.” There will also be “some challenges that will slow us down,” Vasavada adds.

And the team scientists will want to stop along the way to explore interesting terrain. “Anytime we characterize something, it's probably a few weeks of activity, because it might mean acquiring a sample, for example,” Vasavada says. “[It] takes a few weeks to find a place to get the sample, deploy the arm, acquire the sample and process it in our lab.”

Scientists started mapping the terrain inside Gale Crater even before Curiosity landed, using high-resolution data from cameras in orbit around Mars. Dawn Sumner, a science team member from the University of California at Davis, says she and her colleagues divided the target landing and roving area into 1-mi. squares and are inventorying the features in each using the overhead data.