Sulfur Dioxide Kept Ancient Mars Ocean Flowing

Anne Minard
for National Geographic News
December 20, 2007
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.

Most agree that some sort of greenhouse gas helped maintain the bodies of water that formed Mars's channels and other water-carved geology. (Related news: "Greenhouse Effect May Expand Habitable Planet Roster" [November 7, 2007].)

Greenhouse gases, such as carbon dioxide, trap heat in a planet's atmosphere, which warms the surface. (Learn about the greenhouse effect on Earth.)

Sulfur dioxide "provides a potential explanation of early Martian warmth," Halevy said.

It also explains "... the absence of carbonates, the existence of clays on ancient Martian surfaces, [and] the abundance of sulfates and the acidic conditions later in Martian history."

On Earth sulfur dioxide rapidly oxidizes and then leaves the atmosphere—often as acid rain.

But on an early, oxygen-free Mars, the gas would remain longer, the authors say.

Volcanically produced sulfur dioxide could have even played the same role on Earth about to 2.5 to 4 billion years ago, when no carbonate rocks were deposited.

(Related news: "Mars Melt Hints at Solar, Not Human, Cause for Warming, Scientist Says" [February 28, 2007].)

Not a Perfect Fit?

Jeffrey Moore, a research scientist at the NASA Ames Research Center in Moffett Field, California, was not involved in the study.

In his previous work, he has outlined a similar scenario: "vigorous belching of sulfur dioxide [in Mars] from volcanoes that might have kept the carbon dioxide from being taken up by rocks."

But Moore and Mark Bullock of the Southwest Research Institute in Boulder, Colorado, say sulfur dioxide released from volcanoes would have turned very quickly to sulfuric acid.

According to the new paper, the sulfur dioxide should have left its evidence in the form of sulfites.

The Bullock and Moore scenario would lead to the deposition of sulfates, a different byproduct.

Several of Earth's envoys to Mars—including the rovers Spirit and Opportunity and the orbiting Mars Express and Mars Observer—have picked up evidence of sulfates on the Martian surface.

(See pictures of the red planet.)

"People are looking for sulfites all over Mars," Moore says, "but no one sees [them] anywhere."

With exposure to even small amounts of water, sulfite will transform to a combination of sulfate and elemental sulfur, Halevy said.

That could partly explain why Mars missions are finding sulfates, not sulfites, on the surface.

A Job for Robots

François Poulet, an astronomer at the Université Paris-Sud in Orsay, France, called the new work "a great study," but more research is needed, he said.

"First, this greenhouse gas [sulfur dioxide] has been already proposed by previous works," he said.

"Other greenhouse gases may also have played a part in sustaining a warm and wet climate on the early Mars: ammonia and methane."

Poulet doesn't take issue with the missing sulfites.

"I like the scenario [that] explain[s] the formation of sulfates from large deposits of sulfites," he said.

"In any case, their hypothesis should be tested by further in situ robotic explorations."

Halevy and colleagues are working on more detailed atmospheric modeling of Mars to better explain the chemistry that would allow for their sulfur dioxide greenhouse model.

They are also investigating further implications of their hypothesis for conditions on early Earth.

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