One way to study distant Mars is to look for analogues here on Earth.
After all, the two worlds are similar in many ways — they both are what planetary astronomers called "rocky" (as opposed to gaseous Jupiter or Saturn), and they both have water.
In recent decades, scientists have begun to think of the Red Planet as having once been reddish-blue: Caverns and other formations indicate that liquid water flowed on the surface 3.5 billion years ago, they believe. Massive extinct volcanoes on Mars (including the largest in the solar system) are also testament to a geologically active past.
One of the clues that Mars was warm and awash in liquid water (and possibly life at one time) is clay deposits thought gradually precipitated by vast Martian oceans.
But a new paper, published in Nature Geoscience, throws some doubt on that picture.
Similar clay minerals found in the rocks of Mururoa Atoll in French Polynesia were left there by water-infused molten rock as it cooled — not gradually deposited by flowing water. That means the storied ancient oceans of Mars may be pure fiction, says Alain Meunier of the University of Poitiers, France.
"Mars was not as warm and wet in its earliest time as some have suggested. I do not believe in an early ocean on Mars," Meunier told BBC News.
" 'But [The Mururoa process] explains only the earliest generation of clays on Mars, in the early Noachian period. In later periods, liquid water has existed on Mars' surface; that is undoubtedly the case.'
"But John Mustard, a professor at Brown University, has studied the satellite clay data extensively since its first acquisition in 2005.
He said the new research was a welcome addition to the debate about the early environmental conditions on the planet but that he was not convinced the Mururoan lavas could explain the great abundance of clays seen in some regions of the Red Planet."
It turns out that NASA is well-situated to investigate further on the surface of Mars. The Opportunity rover is heading for the rim of a big crater known as Endeavour, where the clays in question have been spotted from orbit. Likewise for Opportunity's larger cousin, Curiosity, which will drive to the base of its landing site — Gale Crater — to look at another specimen of the Martian clay, the BBC says.