NASA's Viking probes were colossally disappointing for those looking for life on Mars - not only were there no signs of life, we didn't even find its essential building blocks. Or maybe we just didn't know what to look for.
NASA's current Martian explorer, the Phoenix Mars Lander, made a surprise discovery in 2008 that forced scientists to reevaluate the old Viking data. Phoenix found percholate, an oxygen/chlorine ion, in the soil around its landing sight. One of percholate's most important properties is that oxidizes materials around it when heated, creating compounds like chloromethane and dichloromethane. Those two materials were the only organic molecules Viking found when it carried out its tests back in 1976. And how did it do those tests? By heating the soil. If percholate was in the soil, it would have produced exactly those two molecules.
NASA scientist Chris McKay explains how Phoenix's finding changes our interpretation of the original Viking results:
"The lack of organics was a big surprise from the Viking. 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. 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."
The chlorine compounds we originally found were dismissed as contaminants from Viking's cleaning fluids, in part because the ratio of chlorine isotopes present in the sample matched the ratio of those isotopes on Earth. In the absence of any other evidence of organic molecules, NASA scientists figured that was just more proof the sample was contaminated. Still, there's no reason why the same isotope ratio couldn't exist on both Earth and Mars, and that's a question future probes can look into.
McKay and his colleagues stress that this doesn't mean Viking actually found life - or even that the organic molecules were native to Mars, as they could have been deposited by meteorite impacts. Still, however they got there, if there were organic molecules at the Viking site, then that means such compounds can survive on the Martian surface for millions, maybe even billions of years.
If that's the case, then the surface soil of Mars - which is unsurprisingly the easiest part of the planet for probes to examine - could be a great place to search for more complex signs of life, such as DNA. NASA will get a chance to test out a lot of these ideas when the new Curiosity rover reaches the red planet in 2012.