In findings that are as scientifically significant as they are crushing to the popular imagination, NASA reported yesterday that its Mars rover Curiosity, which has been trundling across the red planet for a little more than a year, has deflated hopes that life could be thriving on Mars today.
The conclusion, published in the journal Science, comes from the fact that Curiosity has been looking for methane, a gas that is considered a possible calling card of microbes, and so far has found none of it.
While the absence of methane does not entirely preclude the possibility of present-day life on Mars, it does return the idea to the realm of pure speculation without any hopeful data to back it up.
“You don’t have direct evidence that there is microbial process going on,” as Sushil K. Atreya, a professor of atmospheric and space science at the University of Michigan and a member of the science team, put it.
A decade ago, observations from telescopes on Earth and a spacecraft orbiting Mars suggested that vast plumes of methane were rising from certain regions, but Curiosity’s readings now bring the earlier claims into question.
“It just isn’t there,” said Atreya, referring to methane.
Michael J. Mumma, a senior scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and the leader of one of the teams that reported methane in 2003, said in an interview this week that he was certain that his earlier measurements were still valid. He said he now believed that methane on Mars was episodic — released in large plumes and then quickly destroyed.
Mumma said he could not identify any phenomena that would explain why methane plumes spurted out that year but not more recently, or how methane could be destroyed much more quickly on Mars than on Earth.
“Mars may not be operating the same way,” he said. “It’s a puzzle.”
To pursue the methane mystery, Curiosity was outfitted with an instrument that can measure minute quantities of methane and other gases. The first measurements by Curiosity last fall indeed showed a definite signal from methane.
But that turned out to be from residual air from Earth carried all the way to Mars. Once the Earth air was pumped away, the methane readings disappeared, too.
Atreya said he originally thought that highly reactive chemicals on Mars could be destroying methane, as Mumma envisioned. But with subsequent work, “That’s not panning out,” he said.