Here on Earth the only way to make carbonate rocks is with the aid of liquid water. Finding such rocks on Mars might prove, once and for all, that the barren Red Planet was once warm and wet.
A common substance found in ordinary classroom chalk could hold the key to a puzzle of planetary proportions: the mysterious whereabouts of water on Mars.
The brittle, white material in chalk - a form of carbonate - may seem rather ordinary, but finding carbonates on Mars would have some extraordinary implications. The discovery would provide strong evidence that liquid water once flowed on the Red Planet. Such carbonates might also harbor the fossils of ancient Martian bacteria.
Carbonates are rocks and minerals which contain a molecule made of both carbon and oxygen known as CO32-. Limestone is an example of a calcium carbonate, CaCO3, which means a combination of calcium (Ca2+) and carbonate (CO32-). Other examples of carbonates include calcite, dolomite, and marble.
Carbonate rocks on Earth are formed in two ways: through a purely chemical process or via the action of living things. Both means require liquid water.
The chemical pathway involves carbon dioxide gases that dissolve in surface waters. CO2 molecules combine with water to form carbonate ions, which in turn join with calcium or magnesium to create a solid that settles onto the sea floor. Limestone (CaCO3) is an example of such a carbonate. Geologic changes can later expose such deposits, revealing beautiful features such as the white cliff faces pictured above.
Because Mars's atmosphere contains mostly carbon dioxide, scientists would expect liquid surface waters (if they ever existed on Mars) to produce carbonate deposits in a similar fashion.
Another way carbonates are formed on Earth is by multicellular marine organisms that produce carbonates for shells or other hard parts. When these organisms die, the shells sink to the bottom, where they accumulate and eventually form a carbonate deposit. Blackboard chalk is one example of this type of carbonate, which comprises the majority of carbonates in our planet's crust.
Simple prokaryotic microbes in the ocean also collect carbonates on their bodies. This happens when they pull CO2 from the water for photosynthesis. Extracting CO2 causes the water to become more alkaline, which in turn causes carbonate to precipitate. This carbonate eventually weighs down the microbe and sinks it, leading to the formation of carbonate deposits on the seafloor.
"If you were lucky enough to find some carbonates in the layered terrains on Mars, scientists would get very excited about it," said Ken Nealson, director of the Center for Life Detection at NASA's Jet Propulsion Laboratory. "It would be just a zinger of a finding."
"Not only are carbonates often a product of life, they preserve the life that was in and around them very well," Nealson continued. "The whole notion of looking for certain mineral types that... tend to harbor life here on Earth is an important part of the search strategy [for signs of life on Mars]."
Roaming the entire surface of Mars searching for carbonate rocks would take a very long time. Fortunately, carbonates can be detected from orbit by looking at radiated heat.
Like all substances, carbonates emit heat as infrared (IR) radiation. Carbonate compounds have a distinctive infrared signature when viewed through an IR spectrometer. NASA's Mars Global Surveyor spacecraft, which is currently orbiting Mars, carries such an instrument - the Thermal Emission Spectrometer (TES) - which is able to read the infrared "fingerprints" of rocks on the Martian surface below. Scientists had hoped this sensor would find regions of exposed carbonate among the Martian landscape.
So far, the TES has not discovered any carbonate deposits.
"If they're really not there, it's very discouraging," Nealson said. "But we may not have seen them just because we haven't had the right instrument yet."
An improved version of the TES will be on its way to Mars soon. Called the Thermal Emission Imaging System (THEMIS), this new instrument will take more detailed infrared images of the Martian surface than the TES, enabling THEMIS to detect smaller carbonate deposits than TES can.
THEMIS will fly on board NASA's 2001 Mars Odyssey spacecraft, which is scheduled to launch in April.
While scientists wait for the results of THEMIS, it may be that evidence for carbonates on Mars has already been found here on Earth.
A rock from Mars, which was apparently ejected from the Red Planet by an asteroid impact millions of years ago, came to rest in Antarctica about 13,000 years ago, where it was found by scientists in 1984. The "Mars Rock," also known as the Allen Hills meteorite, caused a stir in 1996 when scientists announced that the rock contained signs of ancient Martian microbial life.
That conclusion has since been criticized by other scientists, but one of the pieces of evidence cited were small patches of carbonate mineral inside the rock. The location of the carbonate patches along with other clues suggested that the carbonate was there millions of years ago when the rock was still on Mars.
This Allen Hills meteorite isn't the only one to harbor carbonate.
"We know there are carbonates (on Mars), because we see them as weathering products in a variety of Martian meteorites," said Everett Gibson, an astrobiologist at NASA's Johnson Space Center in Houston, Texas.
"The big question is, Where are the carbonates on the surface of Mars? Shouldn't they be seen by some of the spectrometers that are looking at Mars now?"
Tiny patches of carbonate like those found in the "Mars rock" would not be detected by the thermal emission spectrometer currently in orbit around Mars, Gibson continued. Even THEMIS's 100-meter resolution isn't likely to reveal such diminutive deposits.
But if lakes or oceans once adorned the Martian landscape, scientists expect that sooner or later their instruments will reveal carbonate deposits. Such a discovery would prove, once and for all, that Mars was not always the barren desert it is today.
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