The air above the soil was then monitored for signs that the nutrients had been metabolized. The instrument detected tracer gases the first time the experiment was done, but subsequent runs did not. The results were considered inconclusive and remain contested.
New evidence for organics on Mars does not mean Viking found life, cautions McKay.
SEE ALSO: Travel to Mars in 3 Hours
"Finding organics is not evidence of life or evidence of past life. It's just evidence for organics," he said.
But if NASA had realized there were organics on Mars, there might not have been a 20-year hiatus in sending landers for follow-up studies, said Rafael Navarro-González, with the Institute of Nuclear Science at the National Autonomous University in Mexico.
"We might have had continuing missions," Navarro-González told Discovery News.
NASA plans to launch a follow-up mission to look for organics on Mars in November.
SEE ALSO: A Blue Sunset On Mars
The research appears in last month's Journal of Geophysical Research.