John Grotzinger, a professor of geology at the California Institute of Technology who is the project scientist for the Curiosity mission, said that if certain microbes such as those on present-day earth had plopped into that ancient Martian lake, they would most likely have found a pleasant place to call home.

"The environment would have existed long enough that they could have been sustained, prospered, grown, multiplied," he said. "All the essential ingredients for life were present.

"Potentially the aqueous stream, lake, groundwater system could have existed for millions to tens of millions of years."

The interpretation comes from detailed analysis of two mudstones drilled by Curiosity earlier this year. The structure, chemistry and mineralogy of the sedimentary rocks were not alien.

"The whole thing just seems extremely earth-like," Grotzinger said.

The scientists presented their latest findings on Monday at a meeting of the American Geophysical Union in San Francisco and in a set of six articles published in the journal .

The surface of Mars today is frigid and arid, bombarded by sterilising radiation, but after it formed and cooled with the rest of the solar system about 4.5 billion years ago, it was initially a warmer and wetter place during its first billion years. Over the past decade, scientists have identified several sites on Mars that they think were once habitable.

In 2004, after the US space agency's rover Opportunity discovered evidence that the Martian places it was traversing had once been soaking wet, Steven Squyres, the mission's principal investigator, declared: "This is the kind of place that would have been suitable for life."

But that location would have been an extremely challenging environment for life to take hold - very salty and highly acidic. Later, the scientists said the soils had been soaked not so much by water as by sulphuric acid.

Nasa chose the 160 kilometre-wide Gale Crater as Curiosity's landing site because readings from orbit identified the presence of clay minerals, which form in waters with a neutral pH. Curiosity's instruments indeed detected clays in the two mudstones. The clays appear to have formed at the lake bottom, not swept down from the walls of Gale Crater, backing the idea that the water was not acidic.

Curiosity also measured carbon, hydrogen, oxygen, sulphur, nitrogen and phosphorus, elements that are critical for life on Earth, as well as iron and sulphur minerals that could have served as food for microbes.