There may have once been life on Mars, but for now, it is beyond our technological reach to find it.
The soil chemistry at the site the rover Curiosity is exploring and radiation from space, combined with the limits of our tools, means it is impossible to detect molecules of organic carbon compounds that are the hallmark of living organisms on Earth, NASA scientists said Monday in San Francisco.
It is likely that it will take another mission, with more sophisticated test equipment, to surmount this challenge, said John Grotzinger of NASA's Jet Propulsion Laboratory in Pasadena.
Here is the good news: While trundling across an ancient and dusty lake bed called Gale Crater, the $2.5 billion Curiosity found conditions that would have enabled life 3.5 million years ago.
If living organisms were ever generated, they could have thrived in a mudstone that is rich in nitrogen and other key elements, according to new evidence the rover Curiosity collected.
"We found a habitat where, if life was present, it would have been sustained," said NASA Ames senior scientist David Blake of Moffett Field, designer of the kit analyzing the Red Planet's soil and rocks.
But finding proof of life itself? That's a different matter.
Two big problems complicate the hunt for life on Mars: unusual chemistry and very strong radiation from space.
The Martian site has perchlorate, a salt that causes a chemical reaction during testing that burns away organic carbon. So even if organic carbon were present, it wouldn't show up in the samples Curiosity scrapes from the surface and analyzes.
And the Martian atmosphere is too thin to block lethal solar radiation, which destroys organic carbon that might have been on the planet's surface.
The researchers said they plan to return to Mars with more sophisticated testing equipment, or perhaps bring samples back to Earth, and they need to drill for deeper samples shielded from radiation.
The news, summarized in six papers in the current issue of the journal Science, was discussed Monday at the annual meeting of the American Geophysical Union in San Francisco.
Today's Martian environment offers nothing of what we associate with life's finer settings -- say, palm trees, umbrella-topped drinks and warm turquoise water. Instead, Curiosity is probing an area similar to southern Utah, only far colder, more barren with deadly radiation streaming down.
In its prime, Gale Crater's ancient lake was 30 miles long and 3 miles wide, perhaps rimmed with snowcapped volcanic peaks. The planet would have resembled Earth at that time, Grotzinger said.
Except for the perchlorates, "It is pretty darn similar to Earth," he said.
"But finding a habitable environment is not the same thing as finding organics. That is a separate set of challenges," he added.
And there is another challenge: Even if life had existed, there likely wouldn't have been much of it. Even on Earth, ancestral carbon molecules or micro-fosssils are tough to find. So sampling has to be lucky and tests highly sensitive.
Scientists at the news conference described more hopeful discoveries on Mars of many of the needs on life's "Must Have" list, such as:
The scientists are thrilled by Curiosity's explorations as it continues its Martian trek 220 million miles from home.
"I am so amazed by what the rover can do and what we can see, from across the solar system," said UC Santa Cruz's Dave Rubin, a member of the team responsible for analyzing the sedimentary rocks the rover encounters and keeping it safe as it navigates the treacherous sands.
The 2012 landing of the car-size, nuclear-powered Curiosity robot vehicle was high drama as it hurtled through space to land safely at its precise destination.
Now researchers are embarking on a different drama -- more nuanced and incremental -- of scientific discovery. The Spirit and Opportunity missions found evidence of water. Curiosity's mission is to judge whether life was possible.
"Perhaps some future mission might try to find organic carbons," Rubin said.
Contact Lisa M. Krieger at 650-492-4098.
All six Science papers on the latest Mars research are available at http://mars.jpl.nasa.gov/msl/mission/science/researchpapers