For months, Spirit and Opportunity have been pouring forth unprecedented images and data that show, nearly beyond a doubt, the red planet once supported liquid water. Where only a year ago the possibility of a wet Mars remained just that, a possibility, now scientists are building a fairly strong -- if unconfirmed -- case that living Martian organisms just might be active.
At first glance, it seems a natural progression: The twin rovers, which landed in January 2004, got rolling across the Martian soil and, using their formidable arrays of instruments, promptly began transmitting data that established the solid case for a once-wet Mars.
Not quite. Stephen Squyres of Cornell University, principal scientist for the rover missions, disclosed Lady Luck's contribution to the process.
"Our best science occurred after 90 sols," Squyres said, referring to the designed operational lifetime of the rovers and the term used for the Martian day -- 24 hours and 37 minutes.
Speaking first at a news briefing and then to hundreds of attendees at the American Association for the Advancement of Science annual meeting, Squyres related how the latest mission could have turned up the same negative results as the three previous vehicles that managed to land safely on Mars -- the twin Viking landers in 1976 and the Sojourner rover in 1997 -- if not for some serendipitous occurrences.
When Spirit landed in the Martian equatorial region called Gusev crater and began grinding away at nearby rocks, the readings it sent back for weeks all showed basaltic rock.
"We landed on lava," Squyres said, adding the disappointment among the mission team at NASA's Jet Propulsion Laboratory in Pasadena, Calif., was palpable.
It also was understandable. Squyres showed the audience an aerial image of Gusev taken by NASA's Mars Global Surveyor spacecraft. It resembles a sperm, with a 90-mile-wide circular head and a long, winding tail. The tail, however, also looks like a channel carved by water -- a channel running into the crater. The rover team chose Gusev as Spirit's landing site because it looked like a dry lake bed.
Dry continued to be the operative word for Spirit for a long time. At sol 100 -- or 10 past the rover's expected lifetime -- the mission team decided to change strategy, Squyres explained. They looked at an elevated formation named Columbia Hills, in honor of the lost crew of the space shuttle, that lay some 60 sols' driving distance away.
"We decided we could trust the vehicle (to get there) and hit the gas," he said.
The gamble paid off. After sol 156, at the base of Columbia Hills, Spirit "crossed a geological boundary," Squyres said. The basaltic content was replaced with increasing values of magnesium and sulfates, which is the unmistakable signature "of some kind of aqueous process," he said.
The RAT, or rock abrasion tool, aboard the rover found a much easier time grinding the rocks because they were softer. Spirit's instruments showed increasing amounts of phosphorus, sulfur, chlorine and bromine, and minerals called goethite and jarosite -- all related to water chemistry.
Since that time -- Spirit recently passed 400 sols on Mars -- the golf-cart-sized rover has transmitted data showing one water-associated rock after another, including the fine layering and scalloping that indicate wave action. The conclusion: Gusev crater, sometime during its history, was wet. Had Spirit lasted only 155 sols, it would not have transmitted any water evidence.
Spirit's good fortune continued most recently when the rover inadvertently dug into the red Martian soil -- and revealed a bright, salty layer underneath.
"It was a big surprise," Squyres said. While attempting to climb a steep grade on Husband Hill -- named for Rick Husband, shuttle Columbia's commander -- Spirit's wheels churned up the soil, which Squyres described as containing a rich concentration of salts, the kind that usually occurs when seawater evaporates.
Not just Spirit's discoveries have been confounding its builders for some time now. The rover itself is beginning to exhibit heroic qualities. Recently, the mission team ordered both rovers to take self-portraits. Spirit's resulting image showed its solar-power panels have suffered considerable degradation from the assault of Martian dust.
"She's a very dirty rover," Squyres said. "She's kind of dinged up."
That is not the case with Opportunity, however, whose portrait reveals a gleaming solar array.
"Opportunity looks like it just came off the showroom floor," Squyres said, adding the later-landing rover represents an even bigger good-luck story.
Take its landing site, another flat equatorial area called Meridiani Planum. When Opportunity's protective airbags deployed and the rover bounced across the landscape, it came to rest inside a small crater named Eagle, which contains exposed bedrock that revealed water-affected rock almost immediately.
Squyres showed his audience a Global Surveyor image of the site overlain with a representation of Opportunity's trajectory and bounce pattern. It resembles a fine golf shot ending up in the cup -- hence the name Eagle.
"Tiger Woods on his best day ... " he said, leaving the thought hanging.
Since its touchdown on Jan. 24, 2004, Opportunity has been sending a constant stream of data linking the local rocks with the action and chemistry of water.
"It was once wet at Meridiani Planum -- no question about it," Squyres said.
Asked how much longer he thought the rovers could continue operating, he responded, "My wife asks me that every day."
Maybe the biggest piece of luck of all consisted of a rock that has nothing to do with Martian water.
The rock, named Bounce because it was hit by Opportunity's airbag on the way to Eagle crater, was found by the rover's instruments to match perfectly another rock called EETA79001. The latter is the catalog number for a Martian meteorite found on Earth.
The chemistry and composition of Bounce and EETA79001 are identical. They must have been produced by the same impact event -- except one ended up on the ice in Antarctica, where it was discovered in 1979, and the other landed back on the Martian surface, where it got bumped by a NASA spacecraft.
What would be the odds on that?