for National Geographic News
Sulfur dioxide—not carbon dioxide as previously thought—may have helped heat up ancient Mars and sustained its liquid ocean, a new study has found.
Scientists have often proposed that the planet was enveloped in a carbon dioxide-rich atmosphere during its early days, which would have allowed for warm temperatures similar to those on Earth.
But the expected limestones and other carbonate rocks formed from carbon dioxide are apparently missing from Mars's surface, according to new research led by Itay Halevy, a Harvard planetary geochemist.
The contradiction makes sense if volcanoes on long-ago Mars released sulfur dioxide and hydrogen sulfide, Halevy and colleagues say.
Sulfur dioxide is a powerful greenhouse gas that could have also acidified the oceans enough to prevent the formation of carbonate minerals.
And if that was the major driver of Mars's climate, it could have yielded life that didn't depend on a carbon-based chemistry.
"These questions are especially relevant in the context of the search for habitable environments in the solar system [and] also for extrasolar planets," Halevy said.
The study appears in tomorrow's journal Science.
Sulfur-Dioxide Greenhouse Effect
The research team constructed a model of the sulfur and carbon cycles of early Mars roughly four billion years ago, when temperatures were just above freezing.
Today temperatures on Mars are well below freezing—the planet is so cold that most water on the poles occurs as ice.
Scientists have puzzled over how liquid water could have been sustained on the red planet in its first 500 million years, when the sun was much dimmer than it is today.
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