On Monday, NASA announced the best ever evidence for flowing water on Mars. This salty brine, which may periodically soak streaks of Martian dirt, is tantalizing because we assume that liquid water is important in forming and sustaining life.

Liquid water on Mars wouldn't mean that the planet necessarily harbored microbes. But it would make parts of the desolate world seem much more habitable, suggesting that something might once have survived there -- and might still persist today.

All of this new data comes from the Mars Reconnaissance Orbiter (MRO). In a perfect world, NASA could send one of its rovers over to where these supposed wet streaks appear and see if there are any microbes kicking around inside.

Unfortunately, it's not so simple.

“We know there’s life on Mars already because we sent it there," John M. Grunsfeld, Associate Administrator for the Science Mission Directorate, said at a news conference on Monday.

Grunsfeld is referring to the Earth microbes that are surely present on our Martian rovers. The spacecrafts undergo sterilization before they're sent off to Mars, but our local microbes are, as Grunsfeld puts it, "tenacious."

If Curiosity or Opportunity started digging around in the places where we hope we might find life, they wouldn't be able to tell whether the things they found were truly Martian. Scientists would need to take a very close look at the microbes in question to distinguish them from Earthling hitchhikers -- which would most likely require a mission, crewed or otherwise, that could bring samples home.

NASA actually has standards in place to maintain "planetary protection." Spacecraft have to meet a certain level of cleanliness -- even though true sterility is basically impossible -- and the standards are even stricter for visiting areas considered particularly hospitable to life. In essence, we want to protect ourselves from "discovering" alien microbes that turn out to be familiar critters.

Scientific American's Lee Billings reported Monday on a timely new review by the National Academy of Sciences and the European Science Foundation. In it, the groups conclude that it may be impossible to actually explore Mars without contaminating it. More and more spots on Mars have started to seem potentially friendly to either living microbes or ancient ones, and it's unlikely that upcoming NASA missions will be able to dedicate the time and funds needed to assure the proper level of cleanliness to visit them.

"Although inconvenient, the planetary protection issues associated with crewed missions to Mars are too severe to be dismissed, dodged or downplayed," Billings writes. "Now is the time to begin addressing them. Otherwise, human voyages there may at best prove to be nonstarters and at worst become fiascos that forever extinguish hopes of studying pristine examples of Martian life."

The New York Times reports that NASA officials have yet to decide whether or not the Curiosity rover can make some kind of approach toward the exciting wet streaks. But we can be certain that Curiosity won't be blundering into the area all willy-nilly.

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