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

M.R. Patel,L.R. Dartnell,J.P. Mason,T.A. Nordheim

doi:10.1016/j.icarus.2021.114796

Abstract

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.

Highlights

Venus and Earth share several similarities, yet the evolution of their planetary environments have followed distinctly different evolutionary paths(e.g. Walker, 1975; Kasting, 1988; Svedhem et al, 2007; Driscoll and Bercovici, 2013) leading to two very different environments in the context of potential habitability
We have modelled the penetration of solar ultraviolet and visible light through the venusian atmosphere to assess the habitability of the cloud layer
Based on thermal considerations alone, a potential aerial habitable zone can be defined across the altitude range set by the lower thermal limit for cell growth of − 20 ◦C at 62 km and the base of the lower cloud region at 48 km (100 ◦C), below which liquid droplets have evaporated

Summary

Introduction

Venus and Earth share several similarities, yet the evolution of their planetary environments have followed distinctly different evolutionary paths(e.g. Walker, 1975; Kasting, 1988; Svedhem et al, 2007; Driscoll and Bercovici, 2013) leading to two very different environments in the context of potential habitability. It has been proposed that the surface of early Venus may have been habitable and hosted a significant abundance of liquid water (Kasting, 1988; Donahue and Hodges, 1992; Kulikov et al, 2006; Bar abash et al, 2007) Over time, this water was lost from the surface as the rising temperatures lead to evaporation into the atmosphere. The possibility remains that the Venus provided an early environment sufficiently clement and long-lived (Kasting, 1988; Grins poon and Bullock, 2003; Way et al, 2016; Way and Del Genio, 2020), for life to have originated

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