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.