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.

Kasting maintains that an SO2 climate feedback could not have made early Mars as warm as Earth, but he does allow for the possibility that SO2 concentrations may have remained high enough to keep the planet partly defrosted, with perhaps enough rainfall to form river valleys.

Over that point, Schrag does not quibble. “Our hypothesis doesn’t depend at all on whether there was a big ocean, a few lakes or just a few little puddles,” he says. “Warm doesn’t mean warm like the Amazon. It could mean warm like Iceland—just warm enough to create those river valleys.” With SO2, it only takes a little.