What about running into a totally one-of-a-kind, made-on-Mars form of life?

"There are those who say if we find an independent species on Mars, this means that we cannot colonize Mars. To that I say why not?" Zubrin said.

"The assertion that humans should abort the existence of a new branch of civilization on Mars to preserve the habitat of native bacteria is not an ethical statement. It’s an aesthetic statement. Ethics needs to be based on human needs and what is good for human beings," he said.

"The statement that there is some ethical imperative to leave native bacteria alone is dubious. Furthermore, if there were an independent origin of life on Mars -- and it’s actually independent from Earth -- then that means that independent origin of microbial life is a common event in the Universe. There is not some huge cosmic importance to that life other than its value for scientific study," Zubrin said.

"Ultimately, it is up to the people of the future. I believe that humans, once they become established on Mars they will terraform Mars because it’s the natural thing to do," Zubrin said.

Even if Mars is dead, be on the lookout for dead cells.

So advises Chris McKay, a research scientist at the NASA Ames Research Center in Mountain View, California. "We would be able to pull out from dead cells their genetic material, paste it back together, and recreate Martian life. People are already doing that on Earth with extinct species like the mammoth. We could certainly do it on Mars," he said.

McKay said it’s a dilemma.

"You can’t go to Mars because of the possibility of life there. But the reason we’re going, and the only way we’ll find out about life, is to go there. How do you resolve that dilemma?" he asked.

A solution offered by the NASA scientist is to go to Mars in a manner that is "retreatable".

"We have already contaminated Mars. Let’s face it," McKay said. As for past Mars robotic spacecraft launched in a purported sterile mode, "we have not even achieved that ... even if we try to kid ourselves that we have."

However, the Mars environment itself is sterilizing. Intense ultraviolet light baking down on previously landed probes eradicates any organisms and genetic material that may have gone along for the ride. But lurking inside spacecraft equipment, organisms from Earth do reside, McKay said.

McKay said that collecting all the past hardware that now rests on Mars is possible. Such a clean-up campaign he tags as "reversible exploration".

"We can explore Mars in a way that, if we choose to, we can untrace all the traces we’ve made and leave no trace," he said.

Until there is more certainty about the status of life on Mars, conducting reversible exploration on Mars is a logical step, McKay said.

"Based on the meager knowledge we have, I bet that Mars is dead. But I would also bet that the fragments of Mars life are preserved and that we can recover and reconstruct that life," McKay said.