Potential for Phototrophy in Venus' Clouds

Abstract

We show that solar irradiances calculated across Venus clouds support the potential for Earth-like phototrophy and that treatment of Venus aerosols containing neutralized sulfuric acid favor a habitable zone. The phototrophic potential of Venus atmosphere was assessed by calculating irradiances (200-2000 nm, 15 deg solar zenith angle, local noon) using a radiative transfer model that accounted for absorption and scattering by the major and minor atmospheric constituents. Comparisons to Earths surface (46 W m-2, 280-400 nm) suggest that Venus middle and lower clouds receive 87% less normalized UV flux (6-7 W m-2) across 200-400 nm, yet similar normalized photon flux densities (4400-6200 micromol m2 s-1) across 350-1200 nm. Further, Venus signature phototrophic windows and subwindows overlap with the absorption profiles of several photosynthetic pigments, especially bacteriochlorophyll b from intact cells and phycocyanin. Therefore, Venus light, with limited UV flux in the middle and lower clouds, is likely quite favorable for phototrophy. We additionally present interpretations to refractive index and radio occultation measures for Venus aerosols that suggest the presence of lower sulfuric abundances and/or neutralized forms of sulfuric acid, such as ammonium bisulfate. Under these considerations, the aerosols in Venus middle clouds could harbor water activities (0.6) and buffered acidities (Hammett acidity factor, H0 -0.1 to -1.5) that lie within the limits of acidic cultivation (H0 -0.4) and are tantalizingly close to the limits of oxygenic photosynthesis (H0 0.1). Together, these photophysical and chemical considerations support a potential for phototrophy in Venus clouds.