Explorers Wanted

JUST LANDING AT ALL WAS A VICTORY. The Jet Propulsion Laboratory engineers who built the Mars Exploration Rovers remembered all too well that they'd blown it last time, that over the course of 10 weeks in 1999 they'd lost two spacecraft and in the process nearly scuttled NASA's Mars exploration program. So when the identical twin rovers, Spirit and Opportunity, rolled to a stop inside their protective airbags on opposite sides of the planet on January 4 and January 25, then sent back panoramic photos confirming their safe landing, half the battle was already won.

The other half was finding evidence that water once existed on Mars. Life requires water, biologists tell us, and planetary scientists have made water the central preoccupation of Martian research. Spirit landed in the vast, nearly flat Gusev Crater, which is thought to have long ago been a lake (see "Next Stop: Gusev Crater," Dec. 2003/Jan. 2004). Opportunity was sent to a plain named Meridiani to search for hematite, a mineral that on Earth typically forms in the presence of water. With their collection of high-resolution cameras, microscopic imagers, drills, and spectrometers, the rovers were well equipped for what project scientist Steve Squyres of Cornell University called "the coolest geologic field trip in human history."

By March, Squyres and his colleagues had what they were looking for. Spirit found no lake deposits at its flat, rocky landing site, but by sheer luck, Opportunity landed in a crater gouged out of the dry landscape eons earlier by a meteorite impact. Photos of the crater's inner rim showed an outcrop of exposed rock, the first ever seen on Mars. And a closer look at the texture and chemistry of the layered rocks revealed that long ago, they had been underwater for an extended period of time.

Continuing on, the rovers found evidence of ancient water in other locations, to the scientists' delight and no one's great surprise. In May, Opportunity rolled up to the edge of a deep, stadium-size crater called Endurance (above, with rippled sand dunes at the bottom), then ventured inside for a closer look, while Spirit set new distance records for a Mars rover. A geologist undoubtedly could have done the job much faster, but nobody seemed to mind. The robots accomplished everything they were asked to do, and more.

Setting a spacecraft down on Mars is a tricky business. All that mission planners could do ahead of time was pick an area that, from their scrutiny of orbital photography, altimetry, and other data, looked relatively safe, then hope for the best. Both Spirit and Opportunity came down within six and 15 miles, respectively, of their target points, in the center of predetermined elliptical landing zones. Their parachute descent was monitored by radio signal while onboard cameras took pictures of the approaching surface, making these Mars landings the most closely tracked ever. The Mars Orbiter Camera, which has been circling the planet since 1997, even took pictures of the landers after they touched down, tiny pinpoints of white against a dark surface.

Spirit landed on a rolling plain—now renamed the Columbia Memorial Station—that looked similar to the landing sites for the Viking and Mars Pathfinder spacecraft, but with fewer large boulders. Three hundred fifty yards to the northeast, orbital photos showed a 230-yard-wide crater, dubbed "Bonneville" after a prehistoric lake in Utah. One and a half miles to the southeast were the Columbia Hills, named for the astronauts who died in that shuttle. These would become Spirit's primary destinations.

Three weeks later and 6,000 miles away, Opportunity came to a bouncing stop in a shallow, 70-foot-wide crater called Eagle. Dismay at landing in a hole, which obstructed the scientists' view of the horizon, soon gave way to joy when Opportunity's panoramic views showed exposed bedrock in Eagle's rim. The rover would spend nearly two months poking around those rocks, examining the layers of geologic history on display and nailing down the evidence for past water. Few scientists would have guessed it beforehand, but craters now appear to be the perfect place for a Martian explorer to land. In fact, after Opportunity left Eagle, JPL engineers sent it into an even deeper hole, after ground tests showed that the rover could climb back out again.

The rovers were the first spacecraft sent to Mars with tools powerful enough to answer fundamental questions about local geology—for example, whether the rocks are sedimentary or volcanic in origin. Their panoramic cameras have about the same resolution, or ability to see detail, as the human eye. A miniature thermal emission spectrometer (Mini-TES) can make infrared heat maps and identify minerals in the surrounding terrain. And unlike the stationary Viking landers of the 1970s or the smaller and far more limited Mars Pathfinder of 1997, Spirit and Opportunity can wander far from their landing sites.

Images and spectral data systematically gathered from the ground and from Mars orbit in the days following landing helped scientists plan a detailed campaign of exploration for the two rovers. If either came upon an oddly textured rock or a patch of discolored soil, it could be added to the list for future study.

In contrast to orbital missions, "study" in this case meant detailed, close-up examination. At the end of each rover's extendable arm, more instruments, including a microscopic imager, a drill for getting at fresh material beneath the weathered surface of a rock, and more spectrometers, determined the chemical and mineral makeup. In keeping with NASA's tradition of acronyms, the drill had a geeky name: the rock abrasion tool, or RAT. It was common to hear project scientists talk about "ratting," or drilling into, a rock two "Sols" in the future—a Sol being a 24-hour, 39-minute Martian day.

Soon after Opportunity's landing, the rover was busily ratting into rock outcrops, where it found BB-size spheres, formed in the distant past by minerals dissolving out of Martian rock. The scientists nicknamed these "blueberries." An instrument called a Mössbauer spectrometer, tuned to discriminate among various kinds of iron, identified a mineral called jarosite—further evidence of water. Yet another spectrometer revealed sulfur-bearing salts that typically form in wet environments. The ripples and layering in the rocks, along with other clues, gave the researchers more confidence in their conclusion that the rocks in Eagle Crater had once been drenched with water.

Weeks later, inside a larger crater called Endurance, Opportunity found additional signs that the planet had once been wet. When it looked as though Spirit would come up empty, in April scientists announced that it, too, had found traces of past water seepage from a crack in a half-buried rock called Mazatzal. Not as dramatic as Opportunity's salty sea, maybe, but evidence just the same.

The travels of Spirit and Opportunity mark a new age of Mars exploration. Not only are two rovers operating simultaneously on the planet's surface for the first time, but the expedition uses data and radio relays from three more satellites in orbit—Mars Global Surveyor, Mars Odyssey, and Europe's Mars Express. The last arrived in December for its own extended study of the planet, and immediately made the first direct measurement of water in the planet's polar cap. In 2004, Mars was a busy place.

High-resolution pictures from the Mars Global Surveyor were key to mapping the rovers' route (see below). Spirit landed not far from Bonneville crater, which project scientists picked as an early destination. When Bonneville proved a disappointment (no rock outcrops), the rover set off on a long journey to the 330-foot-high Columbia Hills, nearly two miles away. The attraction for scientists was that these rocks are higher, and likely to be of a different age, than those near the landing site. Spirit reached the base of the hills in mid-June, and was still exploring them in late summer.

The Mars rovers were built to last at least 90 Martian days, but project officials always knew this was just "the warranty period," as JPL engineers put it, and that the machines were likely to last longer. By the time the nominal 90-Sol mission was over, the odometer on the golf-cart-size Spirit read 637 meters—about 4/10 of a mile. By late summer the rover had traveled several times that distance, and NASA had decided to extend the mission—as if shutting down two healthy Mars rovers was even an option.

Meanwhile, engineers back on Earth were preparing more Mars missions, which will be launched in 2005, 2007, and 2009. With about as much land mass on this dry planet as Earth has on its watery surface, Mars still offers plenty of territory to explore.

—The editors