PASADENA - NASA's twin Mars rovers are sending home the kind of geologic data that should answer the question of whether the two landing sites on the Red Planet were ever wet enough to have allowed life to develop, a top mission scientist said Thursday.

A confident but carefully worded assessment of the progress of the $820 million effort was offered at a Jet Propulsion Laboratory news conference by Ray Arvidson, the deputy principal scientist, who emphasized that the work of the Spirit and Opportunity rovers was a "work in progress."

"I think toward the end of the standard mission (90 martian days for each rover) we will have a lot of information, and I wouldn't be surprised but that we'll be able to say a lot about the role of water - or not - in terms of these two particular sites," said Arvidson, of Washington University in St. Louis.

"And water is the elixir of life," he added, "and if we come up with the conclusion that water has been involved in the surface or subsurface at some time in the past, then I think the probability that pre-biotic systems could have been generated, life could have gotten started, goes way up."

It will, however, take a few dozen more martian days "to finish the kind of critical measurements to come up with these definitive statements," he said.

"I'm pretty confident that we are getting the data that we need to answer the questions, for sure," Arvidson said.

Spirit, which landed Jan. 3, has been making its way toward a big crater dubbed "Bonneville" which scientists hope will give the rover a view of geology well below the surface. En route it is studying soil and rocks to provide a useful comparison to what may be found in the crater.

Opportunity, which landed in a little crater on the other side of the planet Jan. 24, has stayed there to microscopically examine an outcropping of layered rock, using a grinding tool to clear off the surface.

Asked to assess the Opportunity site, Arvidson likened the situation to "Christmas Eve, or a day before a holiday that includes gifts on the next morning."

"Layered rocks are interesting but the textures don't tell you how they formed, because they could have formed in a wind-blown environment … they could have formed in an explosive volcanic environment, they could have formed in a water laid environment," Arvidson said.

Cleaning off surface debris to allow multiple measurements of chemistry and mineralogy at the site is essential to learning the story of those rocks.

"Right now it's truly in the world of multiple working hypotheses; water is involved, but only on some of the hypotheses, and until we finish … the coordinated experiments on the outcrop we don't know what the right hypothesis is," he said.

Both rovers together have taken more than 11,000 images to Earth, said Jim Bell, lead scientist for panoramic cameras each rover carries. He showed a movie of a fast-fading martian sunset made by combining a sequence of photos from Opportunity's panoramic camera.

"As the sun sets it dims substantially," Bell said. "Those of you who live in Los Angeles are very familiar with this effect. … What's happening is the sun is sort of setting into the murky, dusty atmosphere of Mars."

Jennifer Trosper, the Spirit mission manager, said engineers plan to upload new software to both rovers to improve their ability to decide the best routes to travel toward target sites.

"We want to speed up the autonomous navigation so we can drive further in the time we have in a day," she said.

The new software will also disable a heater aboard Opportunity that turns on whenever its thermostat detects certain temperatures, using up battery power. The heater was supposed to operate only when rover managers wanted it to turn on. The fix should end the excess energy consumption, which has become more important because shortening winter days mean less sunlight on the rover's solar panels.