NASA has found the best evidence yet that there was, and could be, life on Mars - ancient organic material and methane that fluctuates with the seasons.
The "tough" molecules were found by the Curiosity rover in rocks 3 billion years old in the red planet's Gale Crater, which scientists say used to contain a lake.
"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 NASA's Science Mission Directorate.
"I'm confident that our ongoing and planned missions will unlock even more breathtaking discoveries."
The rocks were found near the surface, which thanks to Mars' extremely thin atmosphere is bombarded with deadly radiation. If molecules 5cm down are still intact, NASA thinks Curiosity could find even more when it digs a bit deeper.
The European Space Agency's Mars lander due to launch in 2020 can dig two metres deep.
"They should have plenty of water," University of Auckland astrobiologist Kathy Campbell told The AM Show on Friday. "The moon has water. We want to get down into those subterranean caves."
"Both radiation and harsh chemicals break down organic matter," said Jen Eigenbrode of NASA's Goddard Space Flight Center.
"Finding ancient organic molecules in the top five centimetres 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."
Organic compounds have been found on Mars before, but in concentrations 100 times less than what Curiosity found in the Gale Crater. The molecules found were relatively simple compared to what we have on Earth.
"Life makes very complicated organic compounds - we don't have that yet," said Prof Campbell.
She said it's also possible the compounds arrived from outer space, and didn't originate on Mars at all.
Mars is an inhospitable wasteland nowadays, but since Curiosity landed in 2012, it's made a strange discovery - the level of methane in the atmosphere fluctuates with the seasons - peaking in summer, dropping in winter.
It could have been created by "water-rock chemistry", NASA says, but they can't rule out a present-day biological origin.
"Finally they've actually definitively confirmed that we've got methane leaking out of Mars somehow, somewhere - they don't know how, exactly," said Prof Campbell.
"It might be leaking out from some storage from somewhere. What's key about that methane though is it might warm up the atmosphere, make lakes sometimes, then you might be able to have more habitats for life."
"Are there signs of life on Mars?" asked Michael Meyer, lead scientist for NASA's Mars Exploration Program. "We don't know, but these results tell us we are on the right track."
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