Sulphur the key to unlocking Martian carbonate mystery

2019-02-26 09:06:01

By Jeff Hecht The mystery of why the carbonate rocks abundant on Earth are missing from Mars may have been cracked. Sulphurous fumes from primordial Martian volcanoes appear to have blocked their formation, though it’s not entirely clear what molecular form the sulphur took. On Earth, oceans absorb carbon dioxide from the atmosphere and deposit it as carbonate rock. Scientists had thought the same could happen on Mars, since the apparent flow of water there 3.5 billion years ago suggested the planet may have been warmed up by a thick carbon-dioxide greenhouse. Yet recent probes have found that the Martian surface is rich in sulphates, which form in the presence of water, but virtually devoid of carbonates. Previous research suggested a possible explanation: any bodies of water on Mars were acidic, possibly due to sulphuric acid, which would have prevented carbonates from forming. Mark Bullock of the Southwest Research Institute in Boulder, Colorado, and Jeffrey Moore of NASA, both in the US, recently suggested an explanation for where the sulphuric acid came from (see Geophysical Research Letters (doi:10.1029/2007/GL030688)). They say photochemical reactions converted volcanic sulphur dioxide to sulphuric acid clouds like those on modern Venus. Because rain from these clouds would have made surface waters too acidic for carbonates to form, that allowed more CO2 to remain in the atmosphere, warming the planet. As volcanic emissions waned, the solar wind would have blown away most of that CO2, leaving Mars with only a tenuous atmosphere. Yet that much sulphuric acid also would have stopped the formation of clays, which are found on Mars, says Itay Halevy, a graduate student at Harvard University in Cambridge, Massachusetts, US. Instead, he and colleagues argue that the sulphur dioxide directly pumped out of volcanoes shaped early Martian geochemistry and climate cycles. They believe the chemistry of the early Martian atmosphere allowed SO2 to be stable in air, where as a greenhouse gas it would have helped warm the planet. Dissolved in water, SO2 forms sulphites, which remove calcium and magnesium from water before they can form carbonates but are not acidic enough to prevent clay formation. Exposed to water and oxidants, the sulphite compounds would have reacted to become sulphates. As volcanic emissions stopped, Mars cooled and dried, and some of the remaining CO2 would have seeped into the soil, forming carbonate veins found in Martian meteorites. Bullock says he and Halevy’s team both agree that sulphur from the Tharsis volcanic plateau could have acidified surface waters enough to prevent the formation of carbonates. But he isn’t convinced that SO2 could have accumulated to high enough levels in the atmosphere to control what minerals were deposited on the surface. That’s because SO2 rapidly reacts with hydroxyl ions – which are abundant on a moist planet – to form sulphate. “That’s one of the remaining scientific issues – how long-lived was SO2 in an early Mars atmosphere?” he asks. He hopes that data being collected by the CRISM spectrometer on the Mars Reconnaissance Orbiter will be able to resolve the differences between the models, which predict that different minerals should be found on the surface. Journal reference: Science (vol 318,