Martian soil may contain carbon-based organic molecules, the building blocks of life.
A re-examination of material examined by NASA's Viking Mars landers in 1976 show it may have contained carbon compounds after all.
"This doesn't say anything about the question of whether or not life has existed on Mars, but it could make a big difference in how we look for evidence to answer that question," said Chris McKay of NASA's Ames Research Center.
All that was found at the time were the chlorine compounds chloromethane and dichloromethane - and at the time researchers put this down to contamination from cleaning fluids.
But when the NASA team added a little perchlorate - found in Martian soil by the Phoenix Mars Lander in 2008 - to desert soil from Chile containing organics, these compounds are exactly what they found.
"Our results suggest that not only organics, but also perchlorate, may have been present in the soil at both Viking landing sites," said the study's lead author, Rafael Navarro-González of the National Autonomous University of Mexico.
Of course, the presence of organics doesn't necessarily indicate a biological origin. Many meteorites contain them, and scientists have always expected to find them on Mars.
"The lack of organics was a big surprise from the Vikings," McKay said. "But for 30 years we were looking at a jigsaw puzzle with a piece missing. Phoenix has provided the missing piece: perchlorate. The perchlorate discovery by Phoenix was one of the most important results from Mars since Viking."
Perchlorate, an ion of chlorine and oxygen, becomes a strong oxidant when heated. "It could sit there in the Martian soil with organics around it for billions of years and not break them down, but when you heat the soil to check for organics, the perchlorate destroys them rapidly," McKay said.
If organic compounds really can persist in the surface soil of Mars, one way to search for evidence of life could be to check for types of large, complex organic molecules, such as DNA, that imply biological activity, says the team.