Astrobiologists hunting for life in outer space may be looking for the wrong clues. While conventional wisdom suggests that evidence of water is the most important sign of past or present extraterrestrial life, an article published in last week's issue of Science suggests that organic nitrogen may actually be a better indicator.
"Water just provides the context for life as we know it," said Douglas Capone, an environmental biologist at USC and a co-author of the opinion piece, along with Earth scientist Kenneth Nealson. "If we wanted to look for really solid evidence that life had ever existed, organic nitrogen deposits would be good things to look for."
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Because of its ability to dissolve and recombine elementary material, water has long been considered the starting point for determining the feasibility of life on a certain planet. Research indicates underground water once existed on Mars and might still be there. Jupiter's moon Europa and Saturn's moon Enceladus may both hide oceans under a frozen surface. Each of these discoveries has provoked excitement and speculation about the possibility of finding water-borne life.
But water indicates only that an environment friendly to life may exist, Capone said. Finding water does not prove anything conclusively about the evolution and progress of life.
"They can go all over the place looking for evidence of past water formation, or even finding water deposits maybe down deeper in the geological strata," he said. "It's still not going to answer the fundamental question of whether there ever had been life on Mars."
Nitrogen, on the other hand, is an ingredient of life, one of the building blocks in the protein and nucleic acids from which all life on Earth is made. Its appearance on a planet is difficult to explain if life similar to ours is not also present, the authors said.
"At least on a body that has had a separation of continental and oceanic components, the existence of nitrogen on continents is not easy to explain without special life-supplied chemistry," reads the Science piece.
Currently, nitrogen has not been discovered in great quantities on Mars, but evidence suggests it may have been present in the ancient past of the red planet. While Capone said that he is unconvinced that life ever existed on Mars, in the article he and his colleagues argued for a greater emphasis on assessing nitrogen levels there, while still keeping track of the aquatic evidence.
"Our recommended approach might be to search for the nitrogen, characterize and quantify it," the authors suggest in their paper. If its abundance and chemistry cannot be explained by abiotic processes, do not leave until it is explained and when it comes to sample return, bring back anything that is enriched in nitrogen!"