After a decades-long scientific quest, scientists analyzing data from the Thermal Emission Spectrometer (TES) on NASA's Mars Global Surveyor spacecraft have at last found critical evidence the instrument was built to search for – the presence of water-related carbonate minerals on the surface of Mars.
However, the discovery also potentially contradicts what scientists had hoped to prove: the past existence of large bodies of liquid water on Mars, such as oceans and seas.
In a report to be published in the August 22 issue of the journal Science, Arizona State University planetary geologists Joshua Bandfield, Timothy Glotch and Philip Christensen present an exhaustive analysis of TES observations made at scales as small as 3 km (2 miles) of dust-covered areas of Mars. While TES has found no detectable carbonate signature in mile-scale surface deposits at any point during its six-year Mars mapping mission, the instrument has detected the mineral's "ubiquitous" presence in martian dust in quantities between two and five percent.
Carbonate minerals have been an important target for Mars investigators because they form when carbon dioxide gas comes in contact with minerals and liquid water. Since Mars' atmosphere is largely carbon dioxide, scientists have theorized that any previous bodies of liquid water on the Red Planet could have left large carbonate deposits behind, as has been the case here on Earth.
"We have finally found carbonate, but we've only found trace amounts of it. This shows that TES can see carbonates -- if they are there – and carbonates can exist on the surface today," said Philip Christensen, Korrick Professor of Geological Sciences at ASU and TES Principal Investigator.
"However, we believe that the relatively small amounts that we see probably did not come from oceans, but from the atmosphere interacting directly with dust," Christensen said. "Tiny amounts of water in Mars' atmosphere can interact with the dust to form the small amounts of carbonate that we see. This seems to be the result of a thin atmosphere interacting with dust, not oceans interacting with the big, thick atmosphere, that many people have thought once existed there."
"What we don't see is massive regional concentrations of carbonates, like limestone," said Bandfield, who spent a year refining the techniques that allowed the group to separate carbonate's distinctive infrared signature from TES's extensive spectral database, despite the mineral's low concentrations and the masking effects of the martian atmosphere.
"We're not seeing the white cliffs of Dover or anything like that," he said. "We're not seeing high concentrations, we're just seeing ubiquitously low levels. Wherever we see the dust, we see the signature that is due to the carbonate."
Because there are known to be deposits of frozen water on Mars, the findings could have important implications for Mars' past climate history.
"This really points to a cold, frozen, icy Mars that has probably always been that way, as opposed to a warm, humid, ocean Mars sometime in the past," said Christensen. "People have argued that early in Mars history, maybe the climate was warmer and oceans may have formed and produced extensive carbonate rock layers. If that were the case, the rocks formed in those putative oceans should be somewhere."
Though ancient carbonate rocks might have been subsequently buried by later layers of dust, Christensen points out that TES's survey of the martian surface has found no strong carbonate signatures in any location on the planet, despite clear evidence of geological processes that have exposed ancient rocks.
"We see so much erosion, places where the surface has really been eroded down in canyons and valleys and plains that have been stripped bare, Christensen said. "It seems unlikely that the carbonate rocks could all be hiding out of view. When you look at the entire planet, you'd think that somewhere a little piece would be exposed".
Bandfield points out that carbonate deposits in dust could be partially responsible for Mars' atmosphere growing even colder, with an atmosphere as cold, thin and dry as it is today.
"If you store just a couple percent of carbonate in the upper crust, you can easily account for several times the earth's atmospheric pressure," Bandfield said. "You can store a lot of carbon dioxide in a little bit of rock.
"If you form enough carbonates, pretty soon your atmosphere goes away. If that happens, you can no longer have liquid water on the surface because you get to the point where liquid water is not stable."
The current martian atmosphere is so thin (six millibars of pressure as opposed to the average of about 1000 millibars on earth) that solid, frozen water evaporates directly into a gas rather than melting and water cannot stay in a liquid state. Though carbon dioxide is a "greenhouse" gas, the atmosphere is also too thin to trap much heat on the planet's surface.
Both Bandfield and Christensen say that the findings show the importance of carbon dioxide and its mineral interaction through water to the evolution of planets in the solar system, particularly Venus, Earth and Mars. Venus, which has a thick carbon dioxide atmosphere, is hot and dry; Mars has a thin carbon dioxide atmosphere and is cold and dry; Earth is wet and temperate, but has an atmosphere with only a minor amount of carbon dioxide left in it.
"Venus and Mars atmospheres are primarily carbon dioxide and Earth probably was too," Christensen notes. "The view now is that planets start off with an atmosphere that is mostly carbon dioxide. On Earth, the vast majority of that early, thick carbon dioxide atmosphere has been subsequently locked up in the carbonate rocks, which are everywhere thanks to the Earth's oceans. We went from a mostly carbon dioxide atmosphere to one where it is only a minor player. On Mars, it doesn't look like that happened."
"This gives us some feel for how planets are working," Bandfield said. "The planets are big huge science experiments that have proceeded for 4.5 billion years – if you have too much carbon dioxide you get too hot, if you have too little, you stay too cold.
"Mars appears to have locked up its atmosphere in minerals until it reached the point where the process largely stopped because liquid water ceased to exist at regional to global scales at the surface. Obviously from what we are seeing with the greenhouse effect, the level of carbon dioxide is also a very important issue in terms of everyday life on Earth."
The Mars Global Surveyor mission is managed by the Jet Propulsion Laboratory (JPL), Pasadena, Calif., for NASA's Office of Space Science, Washington, D.C. Arizona State University built and operates the Thermal Emission Spectrometer on Mars Global Surveyor. Lockheed Martin Astronautics, Denver, Colo., developed and operates the spacecraft.
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