The rocks became much more interesting. Spirit's instruments showed many of the minerals one finds in stream beds on Earth.
There was an added benefit to driving through mountain passes -- they're windy. Spirit's solar panels had became covered with fine Martian dust, blocking the sunlight they needed to produce electricity and keep the rover going. By sheer luck, gusts of wind cleaned them off.
On the other side of Mars, Opportunity got into a bigger fix. It accidentally drove into fine sand, where it got stuck for weeks.
"We were all pretty confident that we'd be able to get out," Squyres said. "But the first rule in a situation like that is, 'Do no harm.' Don't do anything dumb until you've analyzed the situation. We have rovers on the ground, here on Earth, that are duplicates of what we have up on Mars. And we use them all the time.
"We tried everything that you could imagine, different ways of operating the wheels and so forth. Turns out that, after lots and lots of research, the optimum way is basically put it in reverse and gun it."
Squyres, who has written a book called "Roving Mars," said the planet interests him because it could help humans learn about our place in the universe.
"Life might have originated on Mars. Did it? We don't know," he said. "But if you can show that life arose independently on two different worlds, just in this one solar system, it takes no great leap of imagination or faith, or anything else, to begin to believe that life might be common throughout the universe."
"And the reverse might also be important?" a visitor asked.
"The reverse might also be true," he said. "You might get to Mars and find that the conditions were once just right for life -- it was warm, it was wet, there were pools of water -- and you could search for years and find no evidence of life.
"That would be important too," Squyres said. "It means that life is pretty special."
ABC News' Ned Potter originally reported this story for "Nightline" on July 25, 2005.