On Mars, signs of wetness keep pouring in: deeply carved river valleys, vast deltas and widespread remnants of evaporating seas have convinced many experts that liquid water may have covered large parts of the Red Planet for a billion years or more. But most efforts to explain how Martian climate ever permitted such clement conditions come up dry. Bitterly cold and parched today, Mars needed a potent greenhouse atmosphere to sustain its watery past. A thick layer of heat-trapping carbon dioxide from volcanoes probably shrouded the young planet, but climate models indicate time and again that CO₂ alone could not have kept the surface above freezing.

Now, inspired by the surprising discovery that sulfur minerals are pervasive in the Martian soil, scientists are beginning to suspect that CO₂ had a warm-up partner: sulfur dioxide (SO₂).

Like CO₂, SO₂ is a common gas emitted when volcanoes erupt, a frequent occurrence on Mars when it was still young. A hundredth or even a thousandth of a percent SO₂ in Mars’s early atmosphere could have provided the extra boost of greenhouse warming that the Red Planet needed to stay wet, explains geochemist Daniel P. Schrag of Harvard University.

That may not sound like much, but for many gases, even minuscule concentrations are hard to maintain. On our home planet, SO₂ provides no significant long-term warmth because it combines almost instantly with oxygen in the atmosphere to form sulfate, a type of salt. Early Mars would have been virtually free of atmospheric oxygen, though, so SO₂ would have stuck around much longer.

“When you take away oxygen, it’s a profound change, and the atmosphere works really differently,” Schrag remarks. According to Schrag and his colleagues, that difference also implies that SO₂ would have played a starring role in the Martian water cycle—thus resolving another climate conundrum, namely, a lack of certain rocks.

Schrag’s team contends that on early Mars, much of the SO₂ would have combined with airborne water droplets and fallen as sulfurous acid rain, rather than transforming into a salt as on Earth. The resulting acidity would have inhibited the formation of thick layers of limestone and other carbonate rocks.

Researchers assumed Mars would be chock-full of carbonate rocks because their formation is such a fundamental consequence of the humid, CO₂-rich atmosphere on Earth. Over millions of years, this rock-forming process has sequestered enough of the carbon dioxide spewed from earthly volcanoes to limit the buildup of the gas in the atmosphere. Stifling this CO₂-sequestration step on early Mars would have forced more of the gas to accumulate in the atmosphere—another way SO₂ could have boosted greenhouse warming, Schrag suggests.

Some scientists doubt that SO₂ was really up to these climatic tasks. Even in an oxygen-free atmosphere, SO₂ is still extremely fragile; the sun’s ultraviolet radiation splits apart SO₂ molecules quite readily, points out James F. Kasting, an atmospheric chemist at Pennsylvania State University. In Kasting’s computer models of Earth’s early climate, which is often compared with that of early Mars, this photochemical destruction capped SO₂ concentrations at one thousandth as much as Schrag and his colleagues describe. “There may be ways to make this idea work,” Kasting says. “But it would take some detailed modeling to convince skeptics, including me, that it is actually feasible.”

Schrag admits that the details are uncertain, but he cites estimates by other researchers who suggest that early Martian volcanoes could have spewed enough SO₂ to keep pace with the SO₂ destroyed photochemically. Previous findings also indicate that a thick CO₂ atmosphere would have effectively scattered the most destructive wavelengths of ultraviolet radiation—yet another example of an apparently mutually beneficial partnership between CO₂ and SO₂ on early Mars.

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