JEFFREY KAYE: For two years a pair of NASA robots have been exploring the inhospitable surface of Mars, surviving sub-zero temperatures, punishing terrain and the vehicles' own expected demise. The rovers -- really mini science labs -- arrived on the red planet in January of 2004.
When they rolled onto the Martian surface, scientists and engineers had scheduled them for only a three month mission. But the rovers -- one named Spirit, the other Opportunity -- have left their predicted life spans in the dust and kept on rolling.
JIM ERICKSON: The vehicles have really surprised everybody.
JEFFREY KAYE: Jim Erickson is project manager for the $900 million Mars rover expedition.
JIM ERICKSON: They have been doing a great job and proved to be far more capable and long lasting than any of us had ever dreamed of. I mean, I was one of the optimistic ones who thought, "Gee, we might get six months out of these things."
JEFFREY KAYE: NASA engineers expected the rovers, which rely on solar panels to generate on-board electricity, would eventually sputter to a stop and die as they received less sunlight during the short, cold days of Martian winters.
But back on Earth, the rovers' operators learned how to adapt and keep the power flowing, as Erickson explained to NewsHour producer Saul Gonzalez.
JIM ERICKSON: Once we lasted long enough to actually understand how these vehicles worked, we were able to find new ways of keeping them in the position of having more power.
We would park the rovers every day sort of on the most northerly tilt that we could find to face the solar panels closer to an upright position where they would get the maximum efficiency from the sun that they could.
JEFFREY KAYE: Spirit, the first of the two vehicles to arrive on the red planet, has journeyed more than three miles from its landing site in the Gusev crater.
Opportunity, which landed in a vast flatlands area called Meridiani Planum, has racked up over four miles on its trip. Together, the rovers' cameras have sent back nearly 140,000 pictures.
Working on opposite sides of Mars, the twin robots' instruments have probed and analyzed rocks and soil, all with one primary mission: to look for signs that water, an essential building block of life, once existed on the planet's surface.
MATT GOLEMBEK: We've found compelling evidence that Mars was warm and wet at a time when life started here on Earth.
JEFFREY KAYE: Matt Golembek heads the rover project's science operations team. He says his claim of a once warm and wet Mars is based on the geology the rovers have encountered, particularly Opportunity.
MATT GOLEMBEK: The evidence from Opportunity is unambiguous, I would argue. It shows rocks that are evaporites; effectively they form when seawater evaporates away typically in hot and dry climates and leaves behind the minerals that are in solution in the water.
And the rocks that we found at Meridiani are tell-tale signs that liquid water pooled and sat on the surface for significant periods of time at about 3.5 billion years ago.
JEFFREY KAYE: Scientists are uncertain about how much water Mars had in its past and where it was.
MATT GOLEMBEK: One possibility is that you had a groundwater table that fluctuated locally, and that created the environment in which the materials were deposited. There as no ocean elsewhere and it may have been intermittently wet and dry.
Another interpretation is that you filled up the northern plains and that you had an ocean that was kilometers deep on Mars at that time.
JEFFREY KAYE: Many scientists believe Mars still holds much water in the form of ice below the surface. Beyond the hunt for clues to water, the rovers are also sending back valuable information about present day Martian weather patterns and the planet's more recent geological history.
As Opportunity and Spirit continue their journeys, mission personnel are increasingly adventurous about where they send the rovers.
JIM ERICKSON: We've expanded the envelope of what we would consider the rovers actually even do. Instead the nice flat gentle perch with some rocks on the terrain, we now fully expect these things to go up slopes, into sand dunes, you name it.
JEFFREY KAYE: But the robotic explorers are starting to show their age, and ailments, from a bad wheel on Opportunity to a worn out rock cutting tool aboard Spirit.
JIM ERICKSON: So it's sort of like we're into middle age. We're looking forward to old age, and we're trying to make it a nice, graceful old age but at the same time we really want to wear these things out. Our whole goal is to get as much for the -- as much bang for the buck as we can.
JEFFREY KAYE: Erickson acknowledges the rovers could go dead at any moment, but he believes they still have many more Martian miles to travel.
JIM ERICKSON: And at the end of their days, years from now, they are going to be creaking down to try to get that one last glimpse into a new crater. And that is the right way for them to go.
SAUL GONZALES: Years from now?
JIM ERICKSON: Well, we see what happens. We've gone two years. They always say the best prediction of the future is the past. I would not be surprised to see these things a year or two from now still moving around.
JEFFREY KAYE: In the days ahead, Spirit is on its way to investigate a geological feature dubbed "home plate." Opportunity, meanwhile is steering a course to the edge of a large crater named Victoria.