But recently, some have revisited the results from one of Viking's other instruments. The so-called Labeled Release life-detection tool mixed Martian soil with a nutrient "soup" to see if any living organism would digest the solution and then release labeled gases. Experiments on both landers signaled the release of the labeled gas — suggesting that something had possibly eaten the "soup."
Scientists had originally dismissed the results, chalking them up to quirky contaminants in the Martian soil, but some are now having second thoughts.
"There are a lot of factors to consider," said Dworkin. "The Viking spacecraft were just looking at the top bit of soil in a volcanic area. Also the detectors have come a long way since the 1970s — they're much more sensitive now."
Part of the reason scientists like Dworkin are interested in reassessing the nearly 30-year-old results is a growing theory that dormant life may have piggybacked on comets and meteorites to travel between planets in the solar system.
The idea has been bolstered by the discovery of primitive life in extreme locations on Earth, such as around smoldering deep ocean vents and within frigid ice in barren Antarctica.
"We know there are terrestrial bacteria that could survive in space for years," said Bill Irvine, a professor of astronomy at the University of Massachusetts in Amherst. "And calculations indicate if you blast something off from Mars, some of it could get here pretty quickly."
During the planets' early history, it's believed that Mars was once warmer and wetter and both Mars and Earth were bombarded by comets, which may have delivered water, carbon, nitrogen and other key components of life. Once life sprung up on one of the planets, it may have then spread (in primitive form) to the other planet on board an asteroid.
Learning where life may have first originated could also shed light on another, deeper question: how life arose in the first place.
Astronomers have shown that organic materials and water are prevalent in the universe but are still stumped about what trigger might have caused these components to form life.
"Were we extremely lucky and Earth provided the special conditions for life?" said Irvine. "Or is it possible that Mars once held that special ingredient? It's a big jump to go from proteins to DNA — just how that happened is a step we don't completely understand."