Hopes of discovering life on Mars has suffered a setback after new findings from NASA's Curiosity rover detected only trace amounts of methane gas in the Red Planet's atmosphere.
In the past decade, scientists have reported large "plumes" of methane in the Martian atmosphere, findings that have remained controversial because they were made on the basis of observations from Earth or an orbiting satellite.
Researchers said in March 2003 that they had found a cloud near the Martian equator containing some 19,000 (17,240 tonnes) methane, considered a key indicator of microbial life.
However, analysis of data from Curiosity's onboard instruments shows only trace amounts of methane in Mars's atmosphere.
Scientists say Curiosity's findings indicate that the maximum level of methane is 1.3 parts per billion by volume - about six times lower than previous estimates.
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The low atmospheric methane level greatly reduces chances that Martian soil contains living microbes or organic fossil materials that would produce the gas, scientists say.
Michael Meyer, NASA's lead scientist for Mars exploration, did not definitively rule out the possibility of finding life in the planet's soil following the methane findings.
"This important result will help direct our efforts to examine the possibility of life on Mars," he said.
