NASA Mars rover: What if we find signs of life? (+video)

In this weekly series, Life's Little Mysteries provides expert answers to challenging questions.

If all goes as planned, NASA's Curiosity rover will touch down on Mars late Sunday night. Then, after a few weeks' respite, it will begin probing the subsurface soils looking for organic molecules that could be the detritus of ancient Martian life.

A few billion years ago, vast oceans might have sloshed over the surface of the Red Planet, and a thick atmosphere probably enshrouded it. The liquids and gases have all but burned away by now, but any organisms Mars harbored in its ancient glory days would have left behind traces in the form of large, carbon-based molecules. "Organic molecules can last for billions of years," explained Alexander Pavlov, a planetary scientist at NASA's Goddard Space Flight Center.

Any simple organic matter Curiosity digs up could have biological origins, but it could also have been generated through more mundane chemical processes. However, if the rover detects complex organic structures — the kind we find in living things, and practically nowhere else but Earth — these would be "a very strong indicator" of ancient life on Mars, Pavlov told Life's Little Mysteries.

As Seth Shostak, senior scientist at the SETI Institute, put it, "It would be like finding 2-ton blocks of limestone in the desert in Egypt and saying, hmm, these might be leftover pieces of a structure around here somewhere."

Such a discovery would confirm for the first time that life has existed elsewhere in the universe. Big news, for sure — but what would life on Mars really mean for life here on Earth? [7 Theories on the Origin of Life]

Ethnically Martian

If life exists on Mars, then we might be ethnic Martians ourselves, scientists told Life's Little Mysteries. They explained that the small coincidence of having two life-bearing planets right next door to one another gets cleared up if one of the planets actually seeded life on the other — a concept called "panspermia." According to Pavlov, hundreds of thousands of Martian meteorites are strewn across Earth. These were hurled into space during past planetary collisions (such as the bash that left Mars with a crater covering nearly half its surface). One of these chunks of Mars could feasibly have contained spores that lay dormant during the interplanetary commute to Earth, and then blossomed upon arrival, some 3.8 billion years ago.

Alternatively, any Martian microbes we find could be ethnic Earthlings that made the trip from here to there. That's a little less likely, considering the relative locations and gravitational pulls of the planets, the scientists said.

Either way, we can tell if Martians and Earthlings have a common root by determining whether Martian life encodes itself the same way we do — with DNA. DNA breaks down on hundred-thousand-year time scales, so we would need to find living or freshly dead alien microbes in order to be sure that Mars' life arose independently of Earth's. Pavlov says it's very possible that living things are eeking out an existence in Mars' inhospitable modern landscape, if they were ever there in the first place. As attested to by the extremophiles inhabiting Earth's underground volcanoes and frozen tundras, life tends to adapt and persist once it gets started. Following this line of thinking, if Curiosity finds remains of ancient life, NASA's next Mars mission will go in search of extant microorganisms.

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