Organic matter has been found on Mars in soil samples taken from 3 billion-year-old mudstone in the Gale crater by the Curiosity rover, NASA announced Thursday.
Finding methane in the atmosphere and ancient carbon preserved on the surface gives scientists confidence that NASA's Mars 2020 rover and ESA's (European Space Agency's) ExoMars rover will find even more organics, both on the surface and in the shallow subsurface. That leaves open the possibility that microorganisms once populated the red planet - and still might.
The rover has also detected methane in the Martian atmosphere.
Water-rock chemistry might have generated the methane, but scientists can not rule out the possibility of biological origins.
The new findings are also detailed in two studies published Thursday in the journal Science. While commonly associated with life, organic molecules also can be created by non-biological processes and are not necessarily indicators of life.
Kirsten Siebach, a Rice University geologist who also was not involved in the studies, is equally excited.
"The detection of organic molecules and methane on Mars has far-ranging implications in light of potential past life on Mars", she said.
"And maybe we can find something better preserved than that, that has signatures of life in it", she told AFP.
"We've been able to rule out some of the more simple or accepted ideas of Mars's methane", Dr Webster said.
The surface of Mars may be inhospitable today, but there is strong evidence that the Martian climate once accommodated liquid water - which, as most of you will know, is one of the key components to life - to pool at the surface.
Curiosity's methane measurements occurred over four-and-a-half Earth years, covering parts of three Martian years.
The amount of methane peaked at the end of summer in the northern hemisphere at about 2.7 times the level of the lowest seasonal amount.
Christopher Webster, an atmospheric science research fellow at NASA's Jet Propulsion Laboratory in California, said it is possible existing microbes are contributing to the Martian atmospheric methane.
"We have detected the bits and pieces of something bigger", said Eigenbrode. "It's tripling ... that's a huge, huge difference". The diameter is slightly smaller than a USA dime.
Dr Webster said the data pointed to methane trapped in water-based crystals deep under the planet's surface, which slowly seep to the surface when temperatures rise.
There is a seasonal variation to the methane that repeats, which means the methane is being released from the Martian surface or from reservoirs beneath the surface.
"We can not rule out its creation from biological activity ..." The twin Vikings came up pretty much empty.
Curiosity sampled sites by drilling five centimeters below the surface in the Gale crater, which is where the rover landed in 2012. So they looked elsewhere.
"Curiosity has not determined the source of the organic molecules", Jen Eigenbrode of NASA's Goddard Space Flight Center in Greenbelt, Maryland, said. "We need to go to places that we think are the most likely places to find it".