June 9, 2011 --
The most sophisticated Mars rover ever developed is about to be shipped to the launch site as NASA homes in on a crucial final decision: where to land. The $2.5-billion Mars Science Laboratory, nicknamed Curiosity, is intended to determine if Mars, the most Earth-like place in the solar system, has or ever had conditions to support life. Scientists last week finalized their list of four prospective landings sites. NASA is expected to make the call in July.
Curiosity missed its first launch window because engineers were still working on an elaborate sky crane, a landing unlike any other ever designed for planetary missions. The rover, which is about the size of a small car, will touch down within a 20- by 25-kilometer (12.4- by 15.5-mile) targeted area. In comparison, the 1997 Mars Pathfinder had a landing target that was 200 to 300 kilometers. Being able to land on such a relatively small patch of soil hasn't made things easy for the landing site selection team. In the past, lots of scientifically interesting sites were eliminated because of concerns the rover would survive the landing. "That really helped narrow things down," says Mars geologist Matt Golombek, with NASA's Jet Propulsion Laboratory in California.
Eberswalde Crater stands out among the four contenders by a single feature: the buildup of sediment left by flowing water. "If you want a site that probably has the highest chance of preserving organics and biosignatures that might have existed, this is the place," Golombek told Discovery News. "It's just a spectacular example where water came and built up a sediment." Finding ancient Mars life is not the goal of the mission, but as Golombek points out: "We have the instruments to detect organics if it were there. If they were there, that would be pretty amazing." As alluring as that prospect is, Eberswalde has an equally onerous drawback: a lack of diversity. If the deposits turned out to be nothing more than clay-dusted rocks, the mission would be largely a bust.
Mawrth Vallis contains an open book of Mars' history. The exposed valley walls date back some 3.7 billion years. "There's this stack hundreds of meters thick, with exposed sedimentary rocket and lots of clay minerals," said Golombek. The clays, known as phyllosilicates, form in the presence of water, believed to be a necessary ingredient for life. But Mawrth also presents a significant puzzle. Scientists don't know how it formed, so any water, for example, that once flowed in the area could have been too acidic for life to blossom. "Certain places with water are more conducive to having all the things you need for life than others," said Golombek.
The rim of the 150-kilometer (93-mile) wide Holden Crater is shot with gullies, some of which tail off into fan-shaped deposits of things that were once in water. The finely layered clays are Holden's biggest, but not its only draw. The crater also has ancient rocks known as breccia, a stew of rock fragments broken, tumbled and cemented together due to landslide, flashflood, meteorite impact or other sudden and dramatic event. Holden's breccia date back to the planet's earliest years, a time when the planet was wetter and possibly suited for life.
Gale Crater is another giant, though this one sports an unusual feature: a mound of debris rising about five kilometers (three miles) above the crater floor -- twice as tall as the stack of rocks exposed in Earth's Grand Canyon. Its layered deposits include both clays and sulfates, the only site among the four contenders that have both types of material available, says Golombek. Scientists don't know how the mound formed, but it may be the eroded remnant of sediment that once completely filled the crater.
NASA needs to pick a landing site in the next month or so to give spacecraft control teams time to build the software that will guide the rover's first steering maneuver, following launch in November. The NASA Inspector General issued a report Wednesday saying the agency has resolved most of the technical problems that caused Curiosity to miss its 2009 launch opportunity, but warns that a few hurdles remain. Those include possible contamination issues in the system that will be used to collect and analyze rock and soil samples and concerns about the flight software. The rover is due to be shipped from the Jet Propulsion Laboratory in California to Florida's Kennedy Space Center on June 22.
