Satellite observations: A three-dimensional
image of the Nili Fossae region of Mars showing the presence of abundant
clay minerals (in magenta and blue hues)
A study of rocks at an
old A-bomb test site in the Pacific has led a team of scientists to
conclude that early Mars was not so warm and wet as many argue.
The rocks at Mururoa Atoll in French Polynesia contain clay minerals that look like those seen on the Red Planet.
But whereas the Martian clays are taken to be the products of
weathering of rocks by liquid water, the atoll's clays have a very
different origin.
These were precipitated directly from water-rich molten rock as it cooled.
The research is published in Nature Geoscience by Prof Alain Meunier from the University of Poitiers, France, and colleagues.
It is interesting because it strikes at the heart of the
notion that the Red Planet was awash with water, perhaps at its surface,
more than 3.75 billion years ago - an idea that has been put forward to
explain the great abundance of some clay deposits observed from orbit
by satellites.
However, the process of clay production at Mururoa, if
replicated and widespread on early Mars, would remove the need for such
large volumes of water, and with it possibly a more benign environment
for life to establish itself on the planet.
"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," Prof
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."
Free space
The atoll was the site of French nuclear testing from the 1960s to the 1990s.
A lot of rock was drilled from the island as part of that programme and is now available for study.
Prof Meunier's team shows that clays in these volcanic samples
were formed directly in place, in the spaces that sometimes arise
between cooling rock crystals. They were not the product of later
alteration of the rock through long-term contact with water - the more
familiar route to these minerals.
"Inside the basaltic rock as the lava is cooling, the
crystals are separated sometimes by free spaces in which the residual
fluids are concentrated," Prof Meunier explained.
"These fluids contain all the components that have not been
consumed in the high-temperature crystals like pyroxene, olivine and
plagioclase; and among these components, of course, there is water.
"As the temperature decreases, these fluids are
supersaturated with a lot of phases that consume water and all the
remaining elements. And this favours the formation of clay minerals."
What is more, when the team examined the infrared reflectance
of the Mururoan lavas, they found the signal to be very similar to the
observational data obtained by the Mars orbiters that have mapped the
Red Planet's clay deposits.
Below the surface
Prof John Mustard, of Brown University, Rhodes Island, US, 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.
"The question is: how do you generate thick sequences of this
stuff? Their model cannot, I don't think, explain a Mawrth Vallis and
other thick sections where we can quite clearly demonstrate many
hundreds of metres, if not more, of clay formation. Mawrth Vallis has
far too much clay to be produced by this process. The amount of clays
produced by this degassing process is a relatively small amount."
The Curiosity rover will investigate clays at the foot of Mount Sharp in Gale Crater
Prof Mustard himself prefers the idea that many of the clays
were produced sub-surface at Mars, where warm water could interact with
rocks for long periods - such as in hydrothermal systems. Only later
were these buried clays excavated into view by impacts or through the
erosion of overlying ground by short-lived bursts of flowing surface
water.
To maintain stable water on the surface of the planet for
extended durations would have needed a thick atmosphere - something
which is quite hard to reproduce in the climate models of early Mars,
explained Prof Mustard.
"We make the clear argument that a good chunk of the clays were subsurface," he told BBC News.
"I think it's abundantly clear that surface hydrologic
systems were probably responsible for a subset of the clay occurrences
that we see - but not the dominance. There's very good evidence for
there having been interconnected rivers and lakes, but they're very
immature. This fluvial renaissance of Mars was very short-lived."
The debate is about to get lot more interesting thanks to the imminent first direct measurements of clay minerals on Mars.
Nasa has two operational rovers on the planet currently, and both should encounter the deposits.
The Opportunity vehicle is driving around the rim of a big crater known as Endeavour where clays have been sighted from orbit.
And the Curiosity robot, which has just landed in Gale
Crater, will be commanded to drive to the base of a mountain where,
again, clays have been detected by satellite.