DALLAS — The Mars rovers, Spirit and Opportunity, continue to make tracks across two diverse locales on Mars – far outstripping their original warranties of 90 days of lifetime and a target of roughly 600 meters of driving range when they landed on the planet in January 2004.

Both rovers remain hard at work, said Steven Squyres, principal investigator for the NASA Mars Exploration Rover Project and a professor of astronomy from Cornell University in Ithaca, N.Y.

Squyres and other experts discussed past, present and future Mars exploration plans here during the National Space Society’s 26th annual International Space Development Conference (ISDC).

After 1,200 sols, or solar days on Mars, Spirit now is surveying a site called Home Plate, a plateau within Gusev Crater. The robot recently wheeled itself into a remarkable discovery, Squyres reported.

Due to an out-of-commission right front wheel that no longer turns, Spirit is driving backwards dragging that mechanical appendage.

“That’s rough … it’s a tough way to drive,” Squyres said. “But what we have discovered is that as you drive it, it digs a wonderful trench as you move along and sometimes interesting things will pop up in that trench.”

A few weeks ago as Spirit used a spectrometer on board the rover to investigate a self-dug trench that exposed bright white soil, scientists found that the uncovered material was composed of 90 percent pure silica.

Rover scientists since have dubbed the little valley that Spirit explored Silica Valley.

“This is the kind of stuff that you need to have water to make that kind of concentration of silica,” Squyres said.

The possible origins of the pure silica concentration has spurred thought about volcanic fumerals and hot springs on Mars, Squyres added. “Something very, very interesting happened here. And we discovered this after 1,200 days on the martian surface. It really makes me wonder what else is out there.”

Spirit is presently investigating a rock outcrop, “one of the most beautiful outcrops I have ever laid eyes on,” Squyres said.

The rover also is engaged in an experiment that combines the overhead imagery from NASA’s Mars Reconnaissance Orbiter with Spirit’s on-the-surface set of cameras in an effort to simultaneously catch dust devils in their whirlwind act from two different angles, Squyres said.

On the other side of the planet, the Opportunity rover at MeridianiPlanum is studying the “geologic promised land” of Victoria Crater, a large impact feature some 800 meters in diameter and 70 meters deep, Squyers said. “It’s just a geologic history book.”

Opportunity has completed a partial traverse around Victoria and is now headed back to an alcove dubbed Duck Baya site seen as an entrance way down into the crater.

“If a careful safety review indicates that it’s safe to go in, we’re going to go in,” Squyres added. “We’re going to do a lot of good science … and then we’re going to come out again and keep exploring.”

Squyres told ISDC attendees that, in looking into the future, he takes some comfort in the fact that the same agency — NASA — that placed the first humans on the Moon nearly 40 years ago also put Spirit and Opportunity on Mars less than four years ago. “And that gives me a lot of hope for the future,” he said.

Another Earth-launched probe is headed to Mars next year. NASA’s Phoenix Mars lander is being prepped for an early August liftoff, cruise through interplanetary space and then touch down on the red planet in May 2008.

“It’s the truth of Mars … that’s what we’re after,” said Peter Smith, Phoenix principal investigator at the University of Arizona in Tucson. “What are the treasures that are hidden within Mars.”

Phoenix will use a set of thrusters to slow down and plant itself down upon the red planet — this time, no airbags and no bouncing across the planet. Once firmly footed at its arctic landing spot, the craft will use a robotic arm to dig down and scoop up icy soil for detailed, scientific scrutiny.

Smith said his goal with Phoenix is to get instruments on the surface and obtain “the inner truth of the planet” – to try and make progress on the search for life on planets beyond Earth.

Phoenix also will be carrying a microphone as part of its descent imager equipment.

“We have the opportunity to be listening during descent,” Smith said.

Moreover, once on the surface, the device will be able hear the robotic arm plowing into the icy landscape, adding an “extra sense to the touch and feel aspects” of the mission, Smith said.

Smith said both Mars orbiters and landers will further a deeper understanding of that distant world.

“There are definitely caves on Mars,” Smith said. But that raises a key question, he added: Could there be fractures inside those caves that come up from deep underground? “Maybe water vapor is trapped into the cave and you have the kind of environment where, perhaps, biology could exist,” Smith said. This is certainly seen on the Earth, he continued, and many of those caves — even ones that are sealed from the atmosphere and receive no sunlight – are stocked with forms of life.

The armada of prior Mars missions have narrowed down how best to search for life on the red planet, said Donna Shirley, president of Seattle-based Managing Creativity and a retired manager of NASA’s Mars Exploration Program.

NASA’s ongoing Mars exploration program strategy is founded upon the build-up of knowledge from successive missions to the planet, Shirley said.

Shirley also said that understanding Mars’ past – how it changed and why – may well provide clues as to Earth’s own future.

“I think the driving force for people exploring Mars is going to be the question, can we expand to other planets? Are there places to live? Can we actually live there? Can we afford to go to Mars and live there for a reasonable amount of money?” she said.

Leonard David has been reporting on space activities for nearly 50 years. He is the 2010 winner of the prestigious National Space Club Press Award and recently co-authored with Apollo 11’s Buzz Aldrin the book “Mission to Mars — My Vision for Space...