When NASA’s Mars Exploration Rover Spirit arrived at a site in the Gusev Crater nicknamed “Home Plate,” what it found left scientists puzzled and sparked a healthy debate among the team members running the mission.

“It’s the most spectacular layered rock we’ve ever seen at Gusev,” said Steve Squyres, lead Mars Rover Exploration scientist at Cornell University in Ithaca, N.Y. “Many of us were pretty much reduced to incoherent babble … like ‘wow !’, ‘Holy Toledo!’ … in our e-mails to one another as the first good pictures were coming down.”

But, excitement aside, the real task ahead is attempting to figure out the true nature of Home Plate. “And that’s going to take a little while yet,” Squyres said.

No one is quite sure just what it is Spirit has found.

Squyres said there are a bunch of possibilities, including impact deposits, volcanic deposits or possibly sediment layers caused by wind or water.

As Spirit wheeled ever closer to Home Plate, and as the rover science team learned more about what is likely and unlikely in that area of Mars, ” the hypotheses for Home Plate have gotten more diverse,” Squyres said.

Currently, the theories include such possibilities as playa (a desert basin that was once a shallow lake), exhumed crater floor filled with typical Columbia Hills-layered deposits, a tuff cone (stratified porous rock formed by the combination of volcanic ash with other surface material) , and a hydrothermal vent area, said Larry Crumpler, a research curator for volcanology and space science at the New Mexico Museum of Natural History and Science in Albuquerque, N.M .

“In fact, it is a healthy debate within the team right now,” Crumpler said.

As a Mars rover science team member, Crumpler said deliberations within the team about what they are viewing “have been the closest thing to passionate debate that I have seen yet.”

For his part, Crumpler said that he refuses to accept one spectacular interpretation: “Namely, that it is a volcanic vent structure.”

Crumpler said more detailed rover images are needed.

Home Plate has been a target for Spirit since shortly after the robot landed on the red planet in January 2004. The feature stood out in overhead images taken by Mars Global Surveyor’s Mars Orbiter Camera. It stood out as a bright, nearly circular spot in the Columbia Hills region, Crumpler said.

“It has a shape when seen from above that is reminiscent of a playa or evaporite basin … so that has made it a point of possible interest in a mission seeking evidence for past water on Mars,” Crumpler said.

But based on previous experience with Mars so far, Crumpler said it is not wise to place any bets on initial interpretations. The debate is never fully over until Spirit makes use of its science instrument-tipped robot arm, he said.

Tasking Spirit’s robot arm to intensive scrutiny of the area is underway, said Jim Rice, a Mars Exploration Rover Project science team member at Arizona State University in Tempe.

Given Spirit’s navigation camera images now in hand, many members of the Mars rover science team sense that the rocks are possibly explosive volcanic deposits.

“But that’s purely conjecture at this point, … a working hypothesis,” Squyres said. “Everything is on the table until we’ve gotten more data down.”

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...