One year into a 23-month mission, NASA's Mars rover Curiosity has assured its place in the history of planetary exploration as the most ambitious and one of the most successful attempts to date to explore the surface of another planet.
Even before the rover and its package of 10 science instruments wrapped up their first year of measurements, Curiosity's data allowed the mission's science team to answer the broadest question it had designed the rover to answer: Did Mars ever have an environment hospitable to microbial life?
Its testing ground: Gale Crater and the layered foothills of Mt. Sharp, a wind-smoothed mountain whose summit rises 18,000 feet from the crater floor.
But Curiosity didn't have to scale the foothills of Mt. Sharp to find the answer. It lay in the rover's own backyard. Rock formations within several hundred yards of the rover's landing site provided the answer: a resounding "yes."
The science team already had reason to suspect this might be the case, judging from the images and other data from orbiters NASA had sent in its initial quest to "follow the water" in the hunt for habitability. The images suggested that water flowed through – and perhaps pooled in – the crater billions of years ago.
Researchers picked Curiosity's landing site on the basis of what looked to be cemented deposits exposed on the surface. Landing on or near these rocks would give them an early look at potentially revealing rock formations, provide scientifically interesting targets even as instruments were still undergoing their on-Mars tests, and serve as a hedge in case the rover malfunctioned later in the mission.
After a remarkable "sky crane" landing system put Curiosity's six wheels on the surface, the rover provided spectacular confirmation of once-flowing water in the crater, as well as of standing water.
It came in the form of rock outcroppings, themselves conglomerations of naturally cemented pebbles, which represent the jackpot find so far, according to Darby Dyer, a planetary scientist at Mount Holyoke College in South Hadley, Mass.
"We know that Mars is largely a volcanic planet but that there's been a lot of reworking" of the original volcanic rock, says Dr. Dyer, a member of the team gathering and analyzing data from Curiosity's ChemCam, a laser-spectrometer combo mounted on the rover's six-foot-tall mast. Of particular interest are so-called sedimentary rocks, which form in the presence of water.
"This is the first mission where we've seen the whole spectrum of sedimentary rocks," she says. These rocks are common on Earth, but researchers have never had close-ups of these same rock types on Mars.
Having those in hand represent "the most exciting thing on the mission, by far," she says.
The rocks speak to a time in Mars' distant past when water flowed for prolonged periods through Gale Crater – certainly longer than weeks or months, according to a formal analysis of the outcroppings published in the journal Science on May 31.
Water at least three feet deep and flowing at a pace of more than 1.6 miles an hour would have been needed to keep the largest pebbles in the conglomerate rolling along, noted a team led by Rebecca Williams, a senior scientist with the Planetary Sciences Institute in Tucson, Ariz., in the paper. The researchers deduced the long duration of the flow from the rounded edges of the outcroppings themselves.
Other analyses indicate that the soil and water chemistry at the time would have been mild enough to allow microbial life to thrive – a low salt content and clay minerals that speak of water perhaps fresh enough to drink.
Yet evidence for liquid water, widely seen as crucial for organic life, was only one leg of a triangle of potential habitability Curiosity revealed, notes John Grotzinger, a planetary scientist at the California Institute of Technology in Pasadena and the mission's lead scientist.
The rover also showed that all of the key chemical building blocks for life – oxygen, hydrogen, sulfur, phosphorus, and carbon – were present on Mars.
But organisms also need a source of energy, he continues. Researchers on Earth have uncovered remarkable communities of creatures in the deep ocean that draw their energy from chemicals that well up from under the crust at searingly hot hydrothermal vents or even at cold seeps, which release methane.
The team found minerals with characteristics that would have allowed them to serve as meals for microbes, he says.
Even Curiosity's observations of the atmosphere's chemical make-up appear to add weight to a more-habitable past for the red planet.
The measurements present a definitive confirmation of early measurements indicating that over time, Mars lost its initial inventory of lighter elements to space. Without an Earth-like global magnetic field to serve as a deflector shield, charged particles that make up the solar wind were able to strip the atmosphere of lighter elements over time as the wind flowed past the planet.
A denser atmosphere in the past would have helped the planet retain its surface water for prolonged periods.
In addition, Curiosity took measurements of radiation from the sun and from the rest of the galaxy during its 325-million-mile cruise to Mars. The upshot for a human mission: Just the trip to and from the red planet would lead to astronaut radiation exposures that approach or exceed current career limits, based on shielding currently being built into NASA's crew-exploration vehicle.
On July 4, the rover began its five-mile, multimonth trip to Mt. Sharp's foothills, the mission's ultimate destination. Using photos taken from orbit, the science team has identified potential spots along the way that appear to merit closer study.
The first of these is less than a kilometer from Curiosity's current location, says mission manager Rick Welch. Curiosity should reach the location within the next four to six weeks. If the site turns out to be as interesting up close as it looks from afar, researchers may dally there for a few weeks to explore it before resuming Curiosity's trek.
If Curiosity already has hit a towering home run with its results so far, what's left?
"We'd like to get more of them on the scoreboard," Dr. Grotzinger says. The team has sampled only one kind of potential habitat, he explains. The layers in the foothills of Mt. Sharp represent increasingly recent periods of time with altitude. Minerals detected from orbit suggest that they may represent other sorts of habitats that flowing or standing water on the crater floor might have erased there.
Assessing a range of potential habitats on Mars will serve to guide future missions that aim to return samples or look for evidence of ancient life.
Indeed, on July 9 NASA announced that its next rover mission to Mars would aim to hunt specifically for signs of past life on the red planet. And it would cache samples that later could be returned to Earth. The announcement follows a December announcement from John Grunsfeld, NASA's associate administrator for science, that the agency would launch another rover to Mars in 2020. In February the agency appointed a science panel to give the mission some broad definition.
Instead of drilling into rock and sampling the resulting powder, as Curiosity does, planers envision a drill for the 2020 rover that would extract intact core samples, which not only contain information about the chemical composition of individual layers in a sample, but also give researchers a relative sense for the age of any layer bearing interesting chemistry.
In addition, the new rover – to be built largely from Curiosity's blueprints – is expected to carry on-board labs and analytical tools able to detect fossilized cells, as well as minerals and organic chemicals produced via biological activity.