A 4 billion-year-old meteorite from Mars that caused a splash here on Earth decades ago, after all, contains no evidence of ancient, primitive life from Mars, scientists reported Thursday.
In 1996, a NASA-led team announced that organic compounds in the rock appeared to have been left behind by living creatures. Other scientists were skeptical, and scientists tossed that premise over the decades, most recently by a team led by the Carnegie Institution of Sciences Andrew Steele.
Small samples from the meteorite show that the carbon-rich compounds are actually the result of water - most likely salt or salt water - floating over the rock for an extended period of time, Steele said. The results appear in the journal Science.
During Mars' wet and early past, at least two impacts occurred near the rock that warmed the planet's surrounding surface before a third impact reversed it from the red planet and into space millions of years ago. The 4-pound (2-kilogram) stone was found in Antarctica in 1984.
Groundwater that moved through the cracks in the rock while still on Mars, according to the researchers, formed the small spheres of carbon present. The same can happen on Earth and may help explain the presence of methane in Mars' atmosphere, they said.
However, two scientists who participated in the original study took issue with these recent findings, calling them "disappointing." In a shared email, they said they stand by their observations from 1996.
"While the data presented gradually increase our knowledge of (the meteorite), the interpretation is hardly new, nor is it supported by research," wrote Kathie Thomas-Keprta and Simon Clemett, astronomers at NASA's Johnson Space Center in Houston.
"Unsupported speculation does nothing to solve the riddle surrounding the origin of organic matter" in the meteorite, they added.
According to Steele, advances in technology made his team's new achievements possible.
He praised the measurements of the original researchers and noted that their life-consuming hypothesis "was a reasonable interpretation" at the time. He said he and his team - which includes NASA, German and British scientists - made sure to present their findings "for what they are, which is a very intriguing discovery about Mars and not a study to disprove" the original premise.
This finding "is huge for our understanding of how life started on this planet and helps refine the techniques we need to find life elsewhere on Mars or Enceladus and Europe," Steele said in an email, referring to the moons of Saturn and Jupiter with subterranean oceans.
The only way to prove whether Mars has ever had or still has microbial life is, according to Steele, to bring samples to Earth for analysis. NASA's Perseverance Mars rover has already collected six samples for return to Earth in a decade or so; three dozen samples are desired.
Millions of years after drifting through space, the meteorite landed on an ice field in Antarctica thousands of years ago. The small gray-green fragment got its name - Allan Hills 84001 - from the hills where it was found.
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