Their off-world exploits no longer dominate the headlines. Their mechanical joints are showing signs of age.
As officials at NASA's Jet Propulsion Laboratory in Pasadena, Calif., discuss how to celebrate the $820 million mission's first anniversary, scientists are taking stock of what the rovers and other scientific instruments have taught us about Mars in the last 12 months.
"Mars is practically a new world," says Jeffrey Kargel, a planetary geologist at the U.S. Geological Survey in Flagstaff, Ariz., who was not a member of the rover team but relies on its data for his research. "There are very few people whose work has not been fundamentally altered by the rover's findings." Many of these findings have trickled out in conferences and journals such as Science, which published a special section today on Opportunity's discoveries. (The journal devoted similar space to Spirit in August.)
The biggest lesson from 2004: Although Mars, biologically speaking, remains a dead world, the prospects for life are getting a whole lot better.
The credit goes mostly to the rovers, which for the first time proved that one of the key prerequisites for life - water - puddled on the surface at some point in the planet's ancient past.
Serious talk about water on Mars has been going on in scientific circles since 1971, when NASA's Mariner 9 became the first spacecraft to swing into orbit around the planet and snap clear pictures of its surface. The photos showed river beds and other geological features that could have only been sculpted by water.
Or at least that's how it seemed from space. But, says Kargel, "there have always been skeptics." Over the years, the doubters have proposed explanations ranging from high wind to liquid carbon dioxide to account for the planet's features.
Then the rovers arrived. Spirit settled into Gusev crater on Jan. 3, while Opportunity touched down 22 days later at Meridiani Planum, a vast plain near the Martian equator.
Opportunity was the first to strike paydirt, stumbling on BB-sized minerals the scientists dubbed "blueberries." Analysis showed the blueberries were made mostly of hematite, a mineral that forms primarily in water.
By boring into bedrock outcroppings with its robotic drill, the rover also turned up jarosite, another mineral linked closely to aquatic environments.
There was only one conclusion: Water, and lots of it, once soaked Meridiani. (Spirit also later turned up geological evidence that the Gusev crater had received a drenching at some point in its past.)
Three big questions - when the water was there, how much water there was, and where it all went - are still up in the air and may require the help of future robo-geologists to resolve. But Steven Squyres, the mission's science chief, says one thing is clear: "At least at one particular place, at one particular time, Mars was a habitable world."
"Of course," he quickly adds, "saying that a place was habitable and saying that a place was inhabited are two entirely different things."
The mineralogical evidence gathered by Opportunity shows the waters of Meridiani were hardly the bottling kind. They would have been salty and highly acidic. "Lemon juice sharp," says the USGS' Kargel. It's also possible the water would have been cold enough to freeze.
While Squyres says that would pose challenges to any life forms hoping to carve out an ecological niche, it doesn't automatically rule them out. On Earth, he points out, microbes have been found in the highly acidic mining tailings that drain into Rio Tinto in Spain. So has the mineral jarosite.
Hard evidence of water wasn't the only discovery causing a stir this year. The other was the discovery of methane in the Martian atmosphere. What makes the find so tantalizing is that on Earth methane comes primarily from one source: living organisms.
Three separate teams this year have reported spotting the gas' chemical signature. In March, a European team detected hints of the gas using a spectrometer aboard the Mars Express spacecraft.
A month later, Vladimir Krasnopolsky of Catholic University announced that he and his colleagues also saw signs of methane in the Martian air using a telescope on Mauna Kea, Hawaii.
Finally, last month Michael Mumma, of NASA's Goddard Space Flight Center in Greenbelt, summed up three years of Martian atmospheric observations using telescopes in Hawaii and Chile. His findings: Methane is not only present on the planet, but distributed unevenly around it. Mysteriously, the gas seems to be concentrated around the equator.
Because it's sensitive to sunlight, methane wouldn't have lasted on Mars more than a few hundred years, scientists say. That means the gas was manufactured more recently. The question is how.
The easiest explanation is that it's a mistake. The amount of methane in the Martian atmosphere is small - roughly one ten-thousandth of the amount present on Earth - and required enormously tricky measurements to detect. "If it's true, it's an amazing discovery," says Raymond Arvidson, a planetary geologist from Washington University in St. Louis. "But I think there's a high degree of skepticism."
Even if the discovery is real, the methane could be coming from a nonbiological source, scientists say.
One possibility is that it was made millions of years ago and has been slowly seeping from underground reservoirs, although many experts consider this unlikely.
The methane might also be the product of volcanic or geothermal activity. But thermal detectors aboard the Mars Odyssey spacecraft now orbiting the planet have failed to detect any hot spots on the planet's surface, as scientists would expect if there were geothermal activity.
All the other possibilities involve life. On Earth, most methane is produced by living sources, ranging from methane-producing bacteria to rotting plants.
Until scientists pinpoint the source of the gas, some researchers are cautioning that NASA may have to rethink its procedures for sterilizing Mars-bound spacecraft.
"I think we need to consider the possibility that Mars is a living world," says the USGS' Kargel. "We've already delivered hundreds of thousands of microbes to the planet. We may need to revisit our planetary protection strategies."
Back on Mars, the rovers are still far from finished. "New discoveries are literally being made daily," says Squyres.
Officially designed to last 90 days - the length of the original mission - Spirit has now spent 327 Martian days roaming the surface. Opportunity has clocked 306 days. Together, they have snapped more than 56,000 photos and covered 3 1/2 miles.
Rover project manager Jim Erickson says Opportunity is clawing its way out of a stadium-sized pit known as Endurance Crater. Once it gets out, perhaps by next week, Erickson's team wants to steer the rover to the charred remains of the spacecraft's heat shield, 300 feet away.
The shield, which protected the rover during its descent into the atmosphere, may hold valuable engineering lessons for future missions, says Erickson. When it slammed into the Martian soil, it also excavated a pit that may reveal scientifically valuable rock samples.
Meanwhile, on the opposite side of the planet, Spirit is nearing the crest of Husband Hill, a rise that promises views of a mysterious terrain that appears dark from space. "It's going to be like the beginning of a whole new mission," says Erickson.
Still, the rovers are showing signs of age. Spirit is having wheel troubles. Opportunity has stiffness in its robotic shoulder joint - "almost like arthritis," says Erickson.
NASA has funded the rover mission through April 1. Given how well they've held up, JPL engineers say it's anybody guess how much longer the rovers will last.
"I wouldn't be surprised if we have this phone call six months from now," says Arvidson.