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° 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 Earth's 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 μmol m-2 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.
Highlights
We present photophysical and chemical arguments for the potential of phototrophy in Venus’ clouds or the harnessing of light for metabolic purposes
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
With regard to cloud layer habitability, we present novel interpretations of the refractive index values obtained in situ and sulfuric acid vapor abundance profiles obtained through radio occultation that suggest, due to the presence of partly neutralized sulfuric acid, that acidity and water activity values in Venus’ aerosols may potentially be suitable for microbial growth
Summary
We present photophysical and chemical arguments for the potential of phototrophy in Venus’ clouds or the harnessing of light for metabolic purposes. We show that the wavelength-dependent photon fluxes calculated across Venus’ cloud layers are sufficient for terrestrial-like phototrophy. With regard to cloud layer habitability, we present novel interpretations of the refractive index values obtained in situ and sulfuric acid vapor abundance profiles obtained through radio occultation that suggest, due to the presence of partly neutralized sulfuric acid, that acidity and water activity values in Venus’ aerosols may potentially be suitable for microbial growth
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