Limestone Free Mars
Why are there no carbonated rocks which is also referred to as limestone, found on Mars? Even though there is much evidence that the early days of the red planet had a warm, wet climate which is ideal for limestone, none has been found. In the December 21, 2007, issue of Science by MIT’s Maria T. Zuber and Itay Halevy and Daniel Schrag of Harvard University may have come up with the answer to this question.
As on earth, the planet was being warmed by a greenhouse effect caused by carbon dioxide in the air. The early Mars could have had greenhouse gas sulfur dioxide in the air. This fact shows that the formation of carbonates may have interfered with the forming of carbonates. It could also explain another discovery—the twin Mars rovers, Spirit and Opportunity discovered sulfur-rich minerals that probably formed in bodies of water in the early Martian environment. This may give hints as to the earth’s history also.
The challenge was to interpret the planet’s history, based on the data gathered by the Mars rovers. This was expecially true of Opportunity’s discovery of sulfate minerals, from just tiny fractions of the surface, says Zuber, who is head of MIT’s Department of Earth, Atmospheric and Planetary Sciences and the E.A. Griswold Professor Geophysics. “How do you take very detailed measurements of chemical composition at one tiny place on Mars”, she says, “and put it into the context of the broad evolution of the planet?” The breakthrough, she said, was when she and her colleagues realized “we’d been after the wrong molecule.” After several years of exploring the role of carbon dioxide and the carbon cycle, she said, they realized “maybe the key is sulfur dioxide, not carbon dioxide.”
When Opportunity’s discovery of the mineral jarosite was made, which only forms in highly acidic water, they found that sulfur was the answer to the puzzle. New data proposes that on Mars sulfur went through a whole cycle through the atmosphere, bodies of water on the surface and burial in the soil and crust, comparable to the well-known carbon cycle on earth. The carbon dioxide has been released in volcanic eruptions through most of the earth’s history. The carbon dioxide has been absorbed into seawater where limestone is formed and gets buried in ocean sediments.
However, Mars may have had an analogous sulfur cycle. Researchers have provided evidence that says Mars may have had an ocean covering 1/3rd of the planet in its Northern hemisphere. Giant volcanoes of Mars’ Tharsis bulge may have spewed into the atmosphere where the sulfur dioxide (S02) dissolved into the water. It blocked the formation of carbonate minerals but formed silicates and sulfites, such as calcium sulfite. Since these minerals degrade quite rapidly, they aren’t found on the surface of the planet today. But, found on Mars, is clay formations which develop under the same conditions that produce carbonates. How the early Mars could have been warm enough to have liquid water is also proved by the sulfur cycle.
This analysis also gives us ideas about our planet in the past. Even though Mars and Earth’s environments may have been similar early on, earth’s climate and lectonics(?) have raised those ideas. “this might have been a phase that earth went through”, in its early years said Zuber. “It’s fascinating the think about whether this process may have played a role”, in the evolution of the early earth.
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