NASA’s robot explorer Curiosity rover has found organic molecules formed 3 billion years ago in sedimentary rocks near the surface of Mars, which suggest there might have been life on the Red Planet at that time. NASA announced on Thursday the rover also found seasonal variations in the levels of methane in the Mars atmosphere.

The new findings, which will appear in the June 8 edition of Science journal, are a good sign for future missions exploring the planet’s surface and subsurface, says NASA.

Curiosity rover landed in Mars in 2012.

NASA, however, clarifies organic molecules, though commonly associated with life, can also be created by non-biological processes, and are not necessarily indicators of life. Organic molecules contain carbon and hydrogen, and also may include oxygen, nitrogen and other elements.

“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. “I’m confident that our ongoing and planned missions will unlock even more breathtaking discoveries on the Red Planet.”

“Curiosity has not determined the source of the organic molecules,” said Jen Eigenbrode of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who is lead author of one of the two new Science papers. “Whether it holds a record of ancient life, was food for life, or has existed in the absence of life, organic matter in Martian materials holds chemical clues to planetary conditions and processes.”

Billions of years ago, data from Curiosity reveal, a water lake inside Gale Crater held all the ingredients necessary for life, including chemical building blocks and energy sources.

The surface of Mars is, however, inhospitable today.

“The Martian surface is exposed to radiation from space. Both radiation and harsh chemicals break down organic matter,” said Eigenbrode. “Finding ancient organic molecules in the top five centimeters of rock that was deposited when Mars may have been habitable, bodes well for us to learn the story of organic molecules on Mars with future missions that will drill deeper.”

To identify organic materials in the Martian soil, Curiosity drilled into sedimentary rocks known as mudstone from four areas in Gale Crater, NASA says in a statement.

Curiosity found the first indications of water on Mars in 2013 and it also determined that the concentration of methane in the planet’s thin atmosphere fluctuates regularly with the Martian seasons.

The paper on the second finding in Science journal will describe the discovery of seasonal variations in methane in the Martian atmosphere over the course of nearly three Mars years, which is almost six Earth years. This variation was detected by Curiosity’s Sample Analysis at Mars (SAM) instrument suite.

While methane had been previously detected in Mars’ atmosphere, the new result shows low levels of methane within Gale Crater repeatedly peak in warm, summer months and drop in the winter every year.

“This is the first time we’ve seen something repeatable in the methane story, so it offers us a handle in understanding it,” said Chris Webster of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, lead author of the second paper. “This is all possible because of Curiosity’s longevity. The long duration has allowed us to see the patterns in this seasonal ‘breathing.'”

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.