美国国家公共电台 NPR NASA's Curiosity Rover Finds Chemical Building Blocks For Life On Mars（在线收听）

DAVID GREENE, HOST:

This seems big. Scientists say for the first time they have clear evidence that the chemical building blocks of life exist on Mars. So is this the moment we can say there is life on the red planet? Well, NPR's Joe Palca says not yet.

JOE PALCA, BYLINE: About three years ago, NASA's six-wheeled rover called Curiosity drilled into two rocks at the bottom of Gale Crater. Powder from the rocks went into an onboard analyzer to see what they were made of. But the analyzer results were garbled so NASA astrobiologist Jennifer Eigenbrode spent the intervening years removing the junk that was messing things up. Then she looked at the analyzer results again.

JENNIFER EIGENBRODE: And there was signals there that were telling us that we had detected different types of organic molecules.

PALCA: Organic molecules contain carbon, the chemical element central to life. That raises the obvious question, where was the carbon coming from?

EIGENBRODE: We don't know.

PALCA: Eigenbrode sees three possibilities.

EIGENBRODE: It could have been from meteorites.

PALCA: Meteorites are constantly pummeling Mars, and many contain organic molecules.

EIGENBRODE: It could be from rock processes.

PALCA: Processes that have been going on in the billions of years since Mars formed. And then there's the most intriguing possibility. Eigenbrode says the analyzed rocks came from the bottom of what was once a lake at a time when Mars was a much warmer, wetter place.

EIGENBRODE: Because this lake had everything the organisms needed to be happy, it could have supported life. Maybe there was life in the lake.

PALCA: And that life decayed, leaving behind the organic molecules the rover detected. Penn State astrobiologist Kate Freeman agrees the new evidence makes that interpretation possible, but...

KATE FREEMAN: It's not standing up and waving a flag saying, I'm life.

PALCA: Freeman isn't ruling out the possibility.

FREEMAN: I don't believe there's life on Mars at the present, although whether there was in the past or not is certainly an open question.

PALCA: In addition to finding organic molecules in the rocks in Gale Crater, rover scientists are reporting another intriguing finding. Chris Webster is a rover scientist at NASA's Jet Propulsion Laboratory. He says the rover has been seeing seasonal changes in the amount of methane in the Martian atmosphere. Methane is another organic molecule. Webster says he and his colleagues think the methane is coming from underground.

CHRIS WEBSTER: It's coming from subsurface reservoirs.

PALCA: So what's making the methane? Is it strictly chemical processes involving rocks alone, or could living or formerly living bacteria have generated the methane?

WEBSTER: We can't tell which one of those.

PALCA: Clearly, there are more questions about Mars that need answering. The rover results appear in the journal Science.

Joe Palca, NPR News.