Constraints on a potential aerial biosphere on Venus: II. Ultraviolet radiation

Despite the harsh conditions in the atmosphere of Venus, the possibility for an aerial habitable zone exists. A thermal habitable zone is predicted to exist at an altitude range of 62 to 48 km, above which temperatures drop below the lower thermal limit of cell growth and below which temperatures exceed the evaporation temperature. Many biocidal factors must be considered for the complete definition of an aerial habitable zone; in this study we consider the constraint specifically from the perspective of biocidal solar ultraviolet (UV) intensity in the atmosphere. We simulated the penetration of solar ultraviolet and visible light through the atmosphere using a radiative transfer model, to determine the spectral environment (and thus the UV biocidal effect) as a function of altitude in the atmosphere of Venus. At the top of the thermal aerial habitable zone (62 km) the incoming solar irradiance creates a severely challenging UV environment, with extremophiles such as Deinococcus radiodurans expected to be able to endure these UV conditions for approximately 80 s. At an altitude of around 59 km the biologically-weighted UV irradiance drops below that calculated for the Archean Earth, and continues to fall with decreasing altitude until at 54 km it is less than that found currently at the surface of Earth. Crucially, longer wavelength photosynthetically active light continues to penetrate to these altitudes and below, resulting in a solar radiation environment in the venusian atmosphere below around 54 km that screens biologically-damaging UV radiation yet permits the process of photosynthesis. Whilst not claiming to suggest the existence of an aerial habitable zone in general, by considering thermal conditions, ionising radiation and the UV flux environment of the venusian cloud deck alone, we define a potential habitable zone that extends from 59 km to 48 km. This region should form the focus of future remote and in situ astrobiological investigations of Venus.