For the past two years, NASA's stalwart rovers, Spirit and Opportunity, have stolen most of the Mars headlines. In particular, the discovery by Opportunity of sulfate minerals on Mars confirmed what many scientists had suspected, that Mars, although now thoroughly dried out, had a watery past.
|Mars Express spots possible sites for life formation.
But another spacecraft, Mars Express, sent into orbit two years ago by the European Space Agency (ESA), has been expanding our understanding the history of water on Mars. Onboard Mars Express is an instrument known as OMEGA, which has been searching the planet's surface for signs of water-bearing minerals. Like Opportunity, OMEGA has found sulfates. It also has found phyllosilicates, or clay minerals.
OMEGA, says Jack Mustard, of Brown University, has uncovered, "a diverse mineralogy that we can tie in to a time record and get a picture of how the surface has evolved" on Mars. Mustard is a co-investigator on the OMEGA instrument. Jean-Pierre Bibring, of the Institut d'Astrophysique Spatiale, in Orsay, France, is the instrument's principal investigator.
Although both sulfates and clays form in the presence of water, they form under different conditions. Clays form when silicate rocks - quartz and feldspar, for example - are exposed to water, lots of water. They form in neutral or basic environments, not highly acidic ones. And they form under temperate conditions. The process is not a quick one. Clay formation can take hundreds of thousands, sometimes millions, of years. It can occur on the surface, but it "more typically occurs in the shallow subsurface, a few hundreds of meters below the surface," says Mustard.
Sulfates, in contrast, can form relatively quickly. And they are the product of evaporation. They form as water leaves an environment, not when it hangs around for millennia. Mars's sulfates are believed to have formed under highly acidic conditions.
The discovery of these two types of minerals on the martian surface, and more importantly, an analysis of where they were found, is helping geologists put together a picture of Mars's water history.
The clays are found in rocks that date from the earliest period of Mars's history, known as the Noachian, between about 3.5 and 4 billion years ago.
|Hydrated minerals as mapped by Mars Express' OMEGA
"The phyllosilicate deposits are all in the ancient uplands," says Mustard. "What it tells us is that 4 billion years ago, there was persistent liquid water, at least in the shallow subsurface, for long periods of time."
The sulfates are not found in these older rocks, however. They appear in regions believed to be of more recent origin. They date from the Hesperian era, which ranges from about 3.5 to 1.8 billion years ago.
Dating events on Mars is a bit of a fuzzy prospect. Geologists use crater counting - they literally count the number of craters of different sizes that are visible in images taken from orbit - to figure out approximate ages for different regions of the planet's surface. To obtain more precise dates would require bringing rocks back to Earth from the martian surface.
But rough dates, in combination with the mineral clues uncovered by OMEGA, are sufficient to enable OMEGA scientists to piece together the broad outlines of martian history. When Mars first formed, they believe, it was a soggy world. There may not have been oceans. Indeed, much of the water may have been underground. But there was a lot of water, and it stayed around for a long time. Long enough to form the clay minerals that OMEGA detected.
But about 3.5 billion years ago, Mars went through a tremendous change. Volcanoes erupted over much of the planet's surface. Huge floods were released that carved giant outflow channels. And then the water started to disappear. Exactly when this change took place, or how long it lasted, no-one can yet say. But by the time the process was finished, Mars was a desiccated world, a global desert, with only empty canyons and valleys - and some clays and sulfates - left behind as a reminder the planet's wetter past.
|Hydrated minerals in Marwth Vallis on Mars
"There was a very distinctive change in the nature of the type of minerals formed in the presence of water between the Noachian and the Hesperian," says Mustard. "There had to be a major transformation in the martian environment to take you from a phyllosilicate [clay] world to a sulfate world."
And what do OMEGA's discoveries say about martian habitability? Could life have emerged and taken hold during Mars's early years? Possibly. The presence of the clay minerals make a clear that there was plenty of water around, in liquid form. Moreover, many biologists believe that clays played an important role in the emergence of life on Earth. Perhaps they played a similar role on Mars. Not only are these likely sites for life to have taken hold - if it ever did - but they are also good sites for preserving that evidence.
That is why OMEGA scientists propose that future lander missions to Mars be sent to regions where the clays are found.
"Clay materials seem to be the most favorable to have hosted life emergence," says Mustard. "They're good at preserving organic matter. So let's go to those clay sites and let's see a different type of system on Mars. Cause we've seen the sulfate systems."
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