The Effect of Cryoconite on the Deglaciation Thresholds of Snowball Planets

A possible material that may form on the cold surfaces of extrasolar planets is cryoconite — a dark, powdery, windblown dust that accumulates on snow, glaciers, and ice caps. Because of its low albedo, the presence of cryoconite may have altered Earth’s energy budget and deglaciation threshold during globally ice-covered, “snowball” episodes. Using a one-dimensional energy balance climate model, we simulated the equilibrium climate response of an airless planet with varying surface percentages of cryoconite to a range of instellations from F-, G-, K-, and M-dwarf stars. Assuming an Earth-like land/ocean distribution, we find that the effect of cryoconite is greatest for planets orbiting stars with more visible and near-UV output. This is because cryoconite has a much lower albedo at these shorter wavelengths, compared with pure water ice and snow. For an F-dwarf planet with the land surface covered with a 75%/25% water ice/cryoconite mixture, full deglaciation occurs with ~33% less instellation than a similar planet with no cryoconite on its surface. For planets orbiting M-dwarf stars, whose spectral output peaks at infrared wavelengths, the albedos of ice and snow at these longer wavelengths are comparable to those of cryoconite, resulting in relatively small differences in the deglaciation thresholds between cryoconite concentrations on these planets. These results have implications for the climate stability of planets with substantial land fractions, for which cryoconite could be a significant contributor to the overall planetary albedo. This work is the first to explore the radiative effects on climate and habitability of the formation of cryoconite onto the cold surfaces of exoplanets.