News Notes
Water clues from martian carbonates

For the first time, scientists have convincingly detected small yet possibly widespread amounts of carbonate minerals in the dust on Mars’ surface. The findings provide new hints about water on Mars, as well as the history of the planet’s atmosphere.

A portrait of Mars on Aug. 26, in the Hubble Space Telescope’s closest view ever, and the red planet’s closest approach to Earth in 60,000 years. New evidence provides hints about Mars’ hydrologic past. Image courtesy of NASA.

Carbonate deposits can sequester carbon dioxide from the atmosphere. On Earth, hundreds of meters of the deposits are common, from the thick chalk Dover Cliffs to China’s karst topography. Earth’s carbonates tend to be made of microscopic life-forms with calcium carbonate shells, formed in the presence of liquid water. If Mars once had a thick carbon dioxide-greenhouse atmosphere, it too could have maintained warm enough temperatures to keep water liquid — pulling carbon dioxide from the atmosphere to form thick layers of carbonates, in the presence or absence of life.

In order to document carbonates on Mars, researchers from Arizona State University in Tempe turned to the dust that makes the planet red. The team scoured infrared spectra data captured by the Thermal Emission Spectrometer onboard NASA’s Mars Global Surveyor spacecraft. They found that the spectra best matched a type of carbonate containing magnesium, known as magnesite. According to the researchers’ results, published in the Aug. 22 Science, the spectra indicate that magnesite particles compose between about 2 and 5 percent by weight of the dust on Mars’ surface.

“The lack of serious amounts of carbonates is consistent with Mars being cold and dry for a long time,” says Joshua Bandfield, lead author on the paper. “But it certainly isn’t a nail in the coffin.” A thick carbon dioxide greenhouse could still have surrounded early Mars.

Other theories could account for the missing atmospheric carbon dioxide through removal by solar wind or meteorite impacts. But traces of carbonate were found in the Allen Hills meteorite, dislodged from deeper in Mars’ crust, which confirms that carbonate deposits could be present somewhere on the planet. If carbonate minerals were distributed in Mars’ crust at the same concentrations as in its dust, at about 2 percent by weight, Bandfield and his co-workers calculated that surface deposits about 1 kilometer thick could store enough carbon dioxide to account for several bars of atmospheric pressure, a significant carbon dioxide greenhouse. “We found at least part of the past martian atmosphere, and it’s stored in the rocks,” Bandfield says.

To support the geomorphological evidence of Mars’ seemingly water-carved surface features, “carbonates are expected to be ubiquitous on Mars,” says Diana Blaney, a planetary scientist at the Jet Propulsion Lab in Pasadena, Calif. She says that the team’s spectral analysis is convincing, but that such a thick layer of carbonate-containing rocks at depth “is much more than [what is] probably geologically reasonable.”

The actual mass of martian dust is so small that “it’s premature to say that these carbonates are the [carbon dioxide] sink,” says Bruce Jakosky, a planetary geologist at the University of Colorado in Boulder. A conservative estimate from that mass might allow for deposits at closer to 10 meters thick, he suggests. The answer will require a more exacting global inventory of carbonates on the ground and in the upper atmosphere, as well as further research into how much carbon dioxide solar wind or meteor impacts could strip from the planet’s atmosphere, Jakosky says. “We still don’t know the relative importance of each of these processes.” Rovers that will land on Mars over the next several months may help answer those questions, Jakosky adds, but a more accurate inventory will require an intensive global survey.

Naomi Lubick

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