NASA's Curiosity rover has discovered the most complex organic matter yet on the surface of Mars, the US space agency announced on Thursday.

The compounds were found in 3.5-billion-year-old sediments in the lowest point of the Red Planet's Gale Crater.

NASA has widened the search for organic molecules that could indicate past life on the surface of Mars.

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"This is a significant breakthrough because it means there are organic materials preserved in some of the harshest environments on Mars," said Jennifer Eigenbrode, astrobiologist at NASA Goddard Spaceflight Center and lead author of one of two studies in Science journal.

"And maybe we can find something better preserved than that, that has signatures of life in it," Eigenbrode added.

"This is the first really trusted detection," said study co-author Sanjeev Gupta, a professor of Earth science at Imperial College London.

"What this new study is showing in some detail is the discovery of complex and diverse organic compounds in the sediments. That doesn't mean life, but organic compounds are the building blocks of life," Gupta continued.

Detection of 'seasonal patterns'

The rover also collected data that confirms the detection of "seasonal patterns" in methane levels, NASA geophysicist Ashwin Vasvada said in the live-streamed announcement.

NASA scientist Chris Webster said the detection of a "repeatable identifiable methane cycle" could be a sign of active biological processes.

Webster confirmed water has been found on Mars and has been present for "a very long time," which points strongly toward a "habitable environment."

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Eigenbrode said Thursday's discoveries were some of the most important findings yet from NASA's search for organic molecules on Mars, which started in 1996.

NASA's first announced it had discovered the presence of organic compounds on Mars in 2014 when small levels of chlorinated methane compounds were detected.

But chlorinated methane is less indicative of biological activity than the new findings, Eigenbrode said.

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NASA intends to launch a new rover as early as July 2020 to comprehensively determine whether life ever existed on Mars,

It will also characterize the climate and geology of Mars and prepare for human exploration.

The probe is scheduled to land on the planet in February 2021.

law/bw (AFP, dpa)