The material was discovered by the Mars Curiosity rover, which has been collecting data on the Red Planet since August 2012. Hints have been found before, but this is the best evidence yet.
In four locations, including the spot nicknamed Mojave (pictured here), the Curiosity rover discovered thiophenes (molecules that include a ring of carbon and sulphur atoms) and other substances that on Earth can be linked to biological activity. About 3.5 billion years ago, research suggests, this pockmark on the Martian surface was brimming with water.
Experts have hailed the two new studies as milestones for astrobiology.
The US space agency presented evidence of ancient organic material and atmospheric methane - two major clues in the search for extraterrestrial life.
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Scientists hope to further the search for signs of life on Mars with the European and Russian rover, ExoMars, scheduled to land in 2021. The term "organic" is ambiguous - we often take it to mean "life-related" but it doesn't have to mean that. The Martian surface is bombarded with radiation that can degrade organic compounds, explains Eigenbrode.
"It's incredibly exciting, because it shows that Mars is an active planet today", says Caltech planetary scientist Bethany Ehlmann, a Mars expert who wasn't involved with the studies.
The methane observations provide "one of the most compelling" cases for present-day life, she said.
"With these new findings, Mars is telling us to stay the course and keep searching for evidence of life", said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters, in Washington.
"They could be changed from something like we've observed at the base of the mountain into methane that eventually makes its way back in to the atmosphere", she said.
Now, with years of Curiosity's atmospheric readings at their disposal, Webster and his colleagues were able to analyze 55 Earth months (or roughly three Martian years) of data, finding that there were indeed low levels of background radiation - and that it seemed to experience seasonal surges, almost tripling at its peak near summer's end in the northern hemisphere (and winter's end in the south).
"This is the first time we've seen something repeatable in the methane story, so it offers us a handle in understanding it", Chris Webster, the lead author of the second paper, said in the NASA statement. On Mars, where we only have a few molecules from a remote probe, this stuff is light years away from being conclusive. "It's tripling ... that's a huge, huge difference". The space agency already has its eyes on the future as the Mars 2020 Rover has been called a "souped-up science machine" that will expand upon Curiosity's findings. The diameter is slightly smaller than a USA dime. In the winter, the gas could get trapped underground in icy crystals called clathrates, which may melt in the summer and free the gas. The gas creeps from below the surface up to be released into the Mars atmosphere via riverbeds, cracks, and crevices in the surface of the planet.