NASA's Viking landers may have detected the ingredients for life on Mars after all, according to a new study.

Back in the 1970s, the two Viking probes scooped up and heated Martian dirt, then looked for organic molecules — the carbon-based building blocks of life as we know it — in the samples. The landers found little, aside from two strange chlorine compounds that researchers at the time attributed to contamination from cleaning fluids.

But the new study suggests that the soil did indeed contain organics, which can have biological or nonbiological origins. They were just destroyed before Viking could detect them.

"This result is saying that there are organic molecules on Mars," study co-author Chris McKay, of NASA's Ames Research Center in Moffett Field, Calif., told "It doesn't say anything about life now, or life in the past. But it does open up the possibility of searching for organic molecules produced by life, and that's very exciting."

Accounting for perchlorate

A 2008 find by NASA's Phoenix Mars lander helped motivate McKay and his colleagues, led by Rafael Navarro-Gonzalez of the National Autonomous University of Mexico, to take another look at the Viking results.

Phoenix detected a chlorine-containing chemical called perchlorate at its landing site, near the Martian north pole. The researchers suspected that perchlorate may have produced what Viking found, destroying original soil organics and leaving behind the two chlorinated compounds, chloromethane and dichloromethane.

So the scientists performed a lab experiment. They grabbed some dirt from Chile's Atacama Desert — widely considered to be a Martian analog environment — and spiked it with perchlorate. Then they heated the mixture up in the lab, just as the Viking landers did on Mars.

Just as with Viking, the researchers found chloromethane and dichloromethane.

"The simplest, most reasonable explanation of the Viking results is that there were organics in the soil, and they were consumed by the perchlorate," McKay said. "I think it's pretty convincing."

Navarro-Gonzalez, McKay and their colleagues reported their findings last month in the Journal of Geophysical Research - Planets, though the results were first announced last September.

Not proof of life

The results don't prove that life exists — or ever existed — on Mars. While organics are associated with life here on Earth, that's not necessarily the case elsewhere in the solar system, McKay said.