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.

This article was originally published with the title Dropping Acid.

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