September 10, 2012
Martian Clay Suggests Planet May Have Been Hotter, Less Wet
Lawrence LeBlond for redOrbit.com - Your Universe Online
Evidence that the surface of the Red Planet had a watery past is being challenged by scientists after studies of rock samples from an old atom bomb test site in the Pacific. The new finding suggests that Mars was not as warm and wet as many scientists have predicted.
The Caltech team determined the atoll clays were made up from water-rich molten rock as it cooled--a finding that could debunk the long-held belief that Mars had free-flowing water on its surface.
NASA surveys of the Martian planet shows ice-locked water at the poles and visual evidence of valleys thought to be carved by water millions of years ago. Even clay formations discovered on the surface appear to have been formed in free-flowing water. Scientists had been searching Earth´s distant neighbor for signs that could lead to evidence of life--or more appropriately the building blocks of life--and they believed the clays would give them the evidence they were hunting for. It was reasonable to believe that water must have flowed on or just below the surface to enrich these clays.
However, the new theory, published in Nature Geoscience, suggests these clays were formed by water-rich hot magma and would have been much less hospitable for life to exist.
Study coauthor Bethany Ehlmann, a planetary geologist at Caltech, said her team´s theory is in sharp contrast of two other high-held theories. One of which suggests liquid water flowing on the Martian surface would have interacted with surrounding minerals, forming the clays that we see today. Another popular theory is that underground water warmed by the planet´s internal heat provided a comfy living habitat before being bound up by the clays.
Clays are known to be remarkable trappers of organic material, such as water. So what better place on Mars to study evidence of the building blocks of life than the abundance of clay structures found there? If either of the prevailing theories about water is true, then Mars would have been hospitable for life, said Ehlmann.
But Caltech scientists could likely find ways to poke holes in those theories all day.
“The clays would form as the lava cools from 1,500 degrees Celsius,” she said. “That would not be a good habitat.”
Ehlmann and colleagues examined clay minerals found in the Mururoa Atoll that were visibly similar to the clay formations observed on Mars. These atoll clays had been formed by water vapor escaping the Earth´s interior, forming bubbles in the magma, which hardened into pockets of clay. The light signatures found in the earthly clays are very similar to the Martian deposits. And some of the Martian meteorites collected on Earth support the evidence, Ehlmann and colleagues wrote.
Ralph Milliken, a planetary scientist at Brown University who was not involved in the study, said it is possible that all three theories are accurate.
“It's certainly a different take on trying to explain the origin of some clay minerals on Mars,” he told the Los Angeles Times℠ Amina Khan. “It does have some merit, and alternative hypotheses need to be considered fully.”
However, this new theory doesn´t really explain why the Martian surface appears to have tracks cut out by flowing liquid, said Milliken. Nor does it account for blueberry-shaped mineral deposits of hematite that many scientists believe formed when water ran past them.
But if the Caltech theory holds water, then it is likely the chances of finding evidence of micro-bacterial life on Mars is slim to none.
Professor Alain Meunier of University of Poitiers, France, a coauthor of the study, said this finding is interesting because it stands in the face of other notions that Mars had a watery past, and forces a rethink of the Red Planet´s early history.
“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 Jonathan Amos of BBC News.
However, the Mururoa process only explains the earliest generation of clays on Mars, during the Noachian Period. “In later periods, liquid water has existed on Mars' surface; that is undoubtedly the case,” he acknowledged.
The Mururoa Atoll was the site of French nuclear testing from the 1960s to the 90s. A huge amount of rock was drilled from the island as part of that program and is now available for study.
Meunier and colleagues found that clays in the volcanic samples taken from the atoll 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,” 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 favors the formation of clay minerals,” he noted.
When the team examined the infrared reflectance of the atoll lavas, they found the signal to be extremely similar to the observational data obtained by Mars orbiters that have mapped the clay deposits on Mars´ surface.
The satellite clay data from Mars has been extensively studied by Professor John Mustard of Brown University since the data was made available in 2005. He welcomed the new research, adding that it would give another look on the early environmental conditions that existed on the Red Planet.
While intrigued by the findings, Mustard said he is not yet convinced the Mururoa lavas could explain great abundance of clays seen in some regions of the Martian landscape.
“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,” Mustard told the BBC.
Mustard is more attuned to the notion that many of the Martian clays were formed sub-surface, where warm water could interact with rocks for long periods--such as in hydrothermal vents. Only later were these buried clays exposed through erosion of overlying ground by short-lived bursts of flowing surface water.
To maintain stable water on the surface of the planet for extended time there would need be a thick atmosphere--something that is extremely difficult to reproduce in the climate models of early Mars, explained Mustard.
“We make the clear argument that a good chunk of the clays were subsurface," he said in the BBC News interview. “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 will surely thicken as two rovers on the Martian surface are set to begin taking direct measurements of clay minerals on the planet. Opportunity, which has been blazing along on the Red Planet since 2004, has accomplished a marathon feat. NASA has also recently landed the Curiosity rover, which will be the agency´s boldest and most sincere project to find evidence of past life on Mars.
Opportunity is heading to the rim of a big crater called Endeavour where clays have been sighted from orbit.
Curiosity, which landed in Gale Crater, will take a scenic drive to Mount Sharp where it will study clay samples buried in the lower layers of the mountainside. It is expected to arrive there in about a year.