A NASA robot has detected complex organic matter on Mars in a "significant breakthrough" in the hunt for life on the planet.
The unmanned Curiosity rover discovered the matter from 3.5 billion-year-old rocks on the surface of the Red Planet, scientists have revealed.
The samples were drilled from the base of Mount Sharp, inside a basin called Gale Crater.
The discovery is not a direct evidence of life.
But the compounds are the most diverse array ever found on the surface of the planet since the robotic vehicle landed six years ago, experts claim.
Curiosity has also found increasing evidence for seasonal variation of methane on Mars – indicating the source of the gas is likely the planet itself, or possibly its subsurface water.
"This is a significant breakthrough because it means there are organic materials preserved in some of the harshest environments on Mars," said lead author of one of two studies in Science, Jennifer Eigenbrode.
"And maybe we can find something better preserved than that, that has signatures of life in it," added Ms Eigenbrode, an astrobiologist at NASA Goddard Spaceflight Center.
A smaller discovery of organic matter by NASA's Curiosity rover was announced in 2012.
The new study reveals the discovery of complex and diverse organic compounds in more detail, however.
"This is the first really trusted detection," said 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," he added.
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"This is the first time we have detected such a diverse array of these sorts of things."
The compounds may have come from a meteorite or geological formations similar to coal and black shale on Earth, Ms Eigenbrode said.