Previous rovers were smaller and could be landed using air bags for cushioning. But those missions left looming questions about climate, geology, and life. To address them, NASA had to launch a more complex, heavier rover and land it in a more interesting but much more dangerous spot.
The rover, called Curiosity, is huge compared to its predecessors, the toy-size Sojourner and the golf-cart-size Spirit and Opportunity. It carries a host of instruments, including devices that measure radiation, detect carbon, and use lasers to vaporize and analyze minerals.
At the landing site, Gale Crater, orbiting spacecraft have previously revealed layers of sediments possibly laid down by a lake. Inside the crater is Mount Sharp, which promises exposed layers of Martian geologic history.
Scientists chose the site in hopes it could address a profound mystery of the Martian climate. Geologic features suggest water flowed there in the distant past, but nobody knows where the water went or why the climate froze up.
"There was a big event in the history of Mars where it goes from being a warm planet, wet planet, that possibly could have been amenable to life, to one that's harsh and extreme," said John Grotzinger, chief scientist for the overall mission, officially known as the Mars Science Laboratory. Solving that mystery might help scientists better understand and model the climate on Earth.
Water is one substance common to all life on Earth, said Grotzinger, a Philadelphia native and now a professor at the California Institute of Technology. On Earth, he said, life exists just about everywhere that has a source of water, energy, and carbon. One of Curiosity's missions is to determine whether Mars ever had those three components of habitability.
Among the mysteries of Mars is not just why it got so cold, but how it ever got warm enough for water to flow, said David Grinspoon, a planetary scientist at the Denver Museum of Nature and Science. Simply adding carbon dioxide to the picture doesn't solve the mystery, he said. Because Mars is farther from the sun, its atmosphere can't hold nearly as much of this greenhouse gas as ours can without starting to form ice clouds, he said. "If you take the early Earth atmosphere and put it on Mars, it would freeze."
Some planetary scientists have tried creating models of past Martian atmospheres with various combinations of gases, vapor, and clouds, he said, but none have quite fit. Others suspect Mars was never all that balmy. Perhaps, they say, the flowing water was intermittent - the result of bursts of energy brought by meteorite impacts.
Exploring flat places can take you only so far in understanding Mars. The new site promises stacks of sediments that date to different eras on the planet. "By driving over the alluvial fan and up the mountain," Grinspoon said, "we will see pages in the book of early Martian history."
He works with an instrument on the rover that will measure radiation levels on the planet's surface. Those measurements will help assess the dangers posed to future astronauts, as well the suitability of Mars for harboring life of its own.
There's almost unanimous agreement that Mars once had conditions suitable for life as we understand it, he said. The period when Mars had liquid water ended about 3 billion to 3.5 billion years ago. That time overlapped with the existence of life on Earth, which goes back at least 3.8 billion years.
If life never arose, he said, scientists will want to know why not.
Curiosity is uniquely equipped to find carbon-containing compounds - key building blocks of life as we understand it, said Caltech's Grotzinger. Scientists have to be careful about drawing conclusions because carbon-containing compounds aren't necessarily biological, he said. In 1996, Grotzinger was among many scientists voicing skepticism when NASA announced signs of life on a Martian meteorite that had landed in Antarctica.
Among the evidence NASA presented for the claim were complex organic compounds called polycyclic aromatic hydrocarbons, which are found in meat that has been charred, among other places. But such compounds can also form in the complete absence of life. To this day, the Mars meteorite work remains inconclusive.
Curiosity, however, can measure carbon isotopes, which can offer information about the past history of that carbon, including clues that it might have been associated with life. Carbon comes in several isotopes - alternative forms that differ by atomic weight. For various reasons, living things on Earth have less carbon 13 relative to carbon 12 than do nonbiological forms of carbon.
Finding carbon on Mars that was "depleted" in carbon 13 would not prove life arose, because there are plausible ways that could happen in the absence of life. But it would be suggestive enough to make it "a very happy day for NASA," Grotzinger said.
The rover is also equipped to detect methane in the atmosphere - something previous missions have hinted at. Though it's possible to account for methane without life, the compound is a strong hint that microbes could still lurk beneath the ground. Recently, skeptical scientists have aggressively attacked those past methane findings. Curiosity could finally settle the matter.
Carbon compounds in the Martian landscape may also tell scientists where the atmosphere went, said Ryan Anderson, a planetary geologist at the U.S. Geological Service in Flagstaff, Ariz. He helped develop an instrument called ChemCam that uses a laser to vaporize small amounts of rock and analyzes light that is given off.
The instrument can zap rocks and measure their chemical components from up to 20 feet away, he said. It is sensitive to lighter elements, including carbon, allowing it to sniff out concentrations of carbonate minerals, which may have trapped carbon dioxide that once warmed the atmosphere.
Measurements by ChemCam can also help determine whether rocks have been exposed to liquid water, he said. The deep crater is exactly the sort of place you would expect a lake to form on the Red Planet, he said, so if there is no trace of a lake there, that would strongly suggest lakes never formed on Mars. "You learn something either way," he said.
Anderson said he won't be disappointed if Curiosity fails to bring us any evidence of past life. "You have to be careful that you don't confuse what you want to see with what you are seeing."
Contact Faye Flam at 215-854-4977, firstname.lastname@example.org, or follow on Twitter @fayeflam. Read her blog at philly.com/evolution.