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.