The carbon compounds are the first definitive organics detected on the Martian surface, scientists said on Tuesday
The rover also found spurts of methane gas in the atmosphere, a chemical that on Earth is strongly tied to life. Additional studies, which may be beyond the rover's capabilities, are needed to determine if either phenomenon was produced by past or present life on Mars or if they stem from geochemical processes.
"We have had a major discovery. We have found organics on Mars," Curiosity lead scientist John Grotzinger, with the California Institute of Technology in Pasadena, Calif., said during a webcast news conference at a science meeting in San Francisco.
"The probability of any of these things being sources [from life] … we just have to respect that it is a possibility," he added.
Curiosity picked up hints of organics in its earliest chemical analysis of rocks in Gale Crater, a 154-kilometre wide impact basin where the rover made a sky-crane landing in August 2012.
Last week, scientists published research showing the crater was once filled with water, with sediments building up over time to form five-kilometre high Mount Sharp, which rises from the basin's floor.
The presence of methane in the Martian atmosphere has been reported and seemingly refuted before. Earth-based observatories reported seeing large "plumes" of methane in 2003 but, following its arrival in 2012, Curiosity turned up nothing.
Now, according to a paper published Tuesday in the journal Science, scientists say there is a faint hint of methane — about one part per billion — near the Gale Crater.
The strange thing is, that reading unexpectedly jumped, at least once, to almost 10 times that amount. The methane levels then returned to normal.
"We've really been racking our brains," about the jump, said Prof. John Moores of York University in Toronto, one of the team members that co-authored the study. "What does it all mean?"
Curiosity sends back data about once a month, Moores said. Over the last two years, the readings were providing a clearer picture of the Martian atmosphere and its methane content — until one set revealed a spike to about seven parts per billion.
The leading theories about why the level spiked have some significant problems, Moores told CBC News.
A carbon-rich meteorite might have struck the surface near Curiosity, and that carbon could have broken down into methane. But there are no fresh craters in the neighbourhood.
Methane could also be created by a chemical interaction with the Martian surface, but lab work suggests this is "not very likely."
The methane could also be produced underground and occasionally vented to the surface.
"That makes you wonder why it happened at that time," said Moores. "But we certainly can't rule it out.
"Maybe there's something about the atmosphere on Mars we don't understand and this is our hint."
Methane can be produced geologically, though on Earth it is also made by a class of bacteria known as methanogens. Its presence on Mars could be a sign of life.
But, if so, that does not necessarily mean there is life on Mars now, Moores said. Today's methane could be left over from biological activity that took place billions of years ago, when the Red Planet is thought to have been more hospitable. Or it could be purely geological.
"It's a bit of a puzzle," Moores said.