NASA's Curiosity rover has discovered more building blocks of life on Mars after carrying out a chemistry experiment never before conducted on another planet, scientists said Tuesday.

The organic molecules are not definitive evidence of past life, the NASA-led team emphasized, because they could also have formed on the red planet or crash-landed on meteorites.

But it proves that these important clues to Martian history have been preserved on the surface for more than 3 billion years, they added.

Back then, the surface of Mars was thought to have been dotted with huge lakes and rivers full of liquid water, a key ingredient for life as we know it.

"The ongoing characterization of organic matter on Mars is a pillar of modern robotic exploration, as space agencies send rovers and landers to explore Mars' past and present habitability and to search for signs of life," the authors wrote in the study published on Tuesday in the journal Nature Communications.

Curiosity landed in a former lake bed called the Gale crater in 2012, and has been searching for signs of possible past life since.

The car-sized rover carried two tubes of a chemical called TMAH, which can break apart organic matter to see what it is made out of.

"This experiment's never been run before on another world," Amy Williams, an astrobiologist working on the Curiosity mission, told AFP.

The team was under pressure because they only had "two shots to get it right," added Williams, the lead author of a new study describing the results.

The experiment, conducted in 2020, detected more than 20 organic molecules, including several that had never before been confirmed on Mars.

These included a molecule called benzothiophene, which has also been found in meteorites and asteroids.

"The same stuff that rained down on Mars from meteorites is what rained down on Earth, and it probably provided the building blocks for life as we know it on our planet," Williams said.

Another molecule containing nitrogen "is a precursor to how DNA is eventually built," she added.

"We cannot yet say that Mars ever harbored life, but our findings further support the evidence that Mars was a habitable world around the time that life on Earth originated," Williams told Reuters.

Future missions

But none of this can prove that life — even tiny, microbial organisms — once flourished on Mars.

One way to potentially make such an "extraordinary claim" would be to bring some Martian rocks back to Earth so scientists can study them more closely, Williams said.

NASA's Perseverance rover has already collected a bunch of rocks for such a mission, called Mars Sample Return.

However the mission has effectively been canceled by the administration of President Trump following a vote in Congress in January.

Last year, Perseverance uncovered rocks in a dry river channel that may hold potential signs of ancient microscopic life on Mars.

Future missions could still benefit from Curiosity's demonstration that experiments using the TMAH chemical work on other worlds.

The European Space Agency's Rosalind Franklin rover, which has a much longer drill than Curiosity, will take the chemical to Mars.

After years of delays, NASA announced last week the ESA's rover is now scheduled to blast off toward the red planet in late 2028.

The chemical will also be on board the Dragon rotorcraft, which is planned to launch in 2028 on a mission to explore Saturn's moon Titan.

The latest discovery by Curiosity follows a series of recent images taken by the rover.

Last June, Curiosity captured the first close-up images of a part of Mars scientists say provide evidence of how water once flowed on the red planet. The ridges look like spiderweb patterns from space and had previously only been observed from orbit, NASA said.

Five months prior, the rover captured new images showing colorful clouds in the sky over Mars.