Mars research teams are ecstatic about the progress of the Spirit and Opportunity rovers, each now in position to open new chapters in exploring the enigmatic red planet.

The dual rovers each about the size of a golf-cart have reached their "next port of call," said Firouz Naderi, manager of the NASA Mars Exploration Rover (MER) program at JPL. In doing so, the robotic duo will continue their hunt for more clues as to Mars past, he said.

For Spirit, the hills that looked so distant and seemingly unreachable on landing day in January are today a stone throw away, Naderi said. For its part, Opportunity has started to perform a "toe tipping" maneuver into Endurance crater, a feature laden with exposed rock outcrops that serve as geological time capsules.

Engineers are cautiously preparing to wheel Opportunity ever deeper into the giant hole in the martian surface with the knowledge that the robot might not be able to climb back out.

"If you want to learn about history, there are worse places to get stuck than in a library," Naderi said.

"Weve got absolutely unprecedented events about to unfold at both of our landing sites," said Steve Squyres of Cornell University in Ithaca, New York. He is principal investigator for the science payload carried on each of the Mars rovers.

Squyres reported that a recent trench dug by Spirit at Gusev Crater has uncovered a surprising find. "Its not the story that we expected. Its not the one that we initially went hunting for," he added.

Squyres, along with Johannes Brueckner, science member of the Mars rover team from Germanys Max Planck Institute for Chemistry, reported that an enriched compound likely magnesium sulfate has been found at Gusev Crater in high concentration.

A more common and earthly term for magnesium sulfate is Epsom salt. Its medicinal uses were discovered from mineral waters at Epsom, England from where Epsom salt got its name.

The chemical compound was detected in the wall of the rover-created trench by Spirits Alpha Particle X-ray Spectrometer (APXS). The concentration is very high, some 15 percent, Squyres said. "If you went and you tasted it, you would taste the salt," he said.

This latest finding from Spirits APXS is the most compelling and exciting part of the story of water at Gusev Crater, Squyres said. He speculated that water had percolated through the subsurface, dissolving minerals out of rocks such as magnesium sulfate in the process. As the solution of salty water ascends towards the surface, it evaporates away. The result: a leftover deposit of magnesium sulfate salt.

Squyres noted that the salt has been detected in higher concentration as Spirit has wheeled closer toward the Columbia Hills.

Spirit is within the length of a football field from the base of the Columbia Hills. "Getting up into the hills and digging some trenches there, I think, will be very revealing as well," Squyres told SPACE.com via telephone. "The bottom line is that this is clear evidence that water has played a role" at Gusev Crater, he said.

Jim Erickson, deputy project manager at JPL, said both Spirit and Opportunity are in great shape.

Opportunity at Endurance crater has started maneuvering itself down into the feature, Erickson said. The robot will first make observations of how well its wheels are doing rolling over soil and rock at the sloping crater site.

"Well be incrementally driving in and examining our results as we go," Erickson told reporters. Extensive testing at JPL has convinced rover builders, project managers, and Mars scientists that Opportunity can drive down the deep crater to a designated target point a band of rock expected to reveal a great deal about the Mars of long ago.

Erickson said rover operators expect to reach the science target inside Endurance crater early next week.

"In addition to being robots, the Mars rovers are interplanetary all-terrain vehicles. And this week, were going to prove it one more time on Mars at Meridiani," said a confident Randy Lindemann, rover mobility engineer at JPL.

Squyres said the intent now is only to drive to a band of rock within Endurance, conduct detailed science measurements, then climb back out and have Opportunity continue wheeling across the Meridiani Planum flatlands.

What Opportunity will find inside Endurance is anybodys guess.

"Is it a record of deep standing water? Is it a record of sand dunesor volcanic activity? We simply dont know. But the record is there to be read," Squyres explained.

Driving Opportunity down into Endurance, while risky, is a "pure win-win situation," Squyres suggested.

"The biggest risk weve got here is the risk that were going to get stuck in a candy store for the rest of the mission. Its a good place to be," Squyres pointed out. "This really is a good move."