The ultraviolet history of the terrestrial planets — implications for biological evolution

Abstract

A radiative transfer model is employed to investigate the comparative surface ultraviolet (UV) radiation histories of Earth, Mars and Venus from 4.5 Ga to the present and thus their comparative theoretical photobiological histories. Earth probably began with a period of higher ultraviolet radiation fluxes during the anoxic Archean. During the early Proterozoic UV fluxes declined as oxygen partial pressures and thus ozone column abundance rose, but the ozone column became subject to stochastic depletion events caused principally by impact events and possibly large-scale volcanism and less frequently, close cosmic events such as supernovae. In contrast Mars has been subject to a history dominated by a slow increase in solar luminosity and a reduction in partial pressures of CO 2, both of which have resulted in an increase in UV flux. The UV radiation history of Venus has been dominated by the greenhouse effect through which high partial pressures of CO 2 made the surface UV radiation environment clement. These distinct histories influence the potential comparative evolutionary photobiology of the three planets. On Earth, life transitioned from the Archean, when tolerance to UV radiation, particularly for exposed organisms, must have been high to a more photobiologically clement era. In this latter era the predominant evolutionary selection pressure is one that allows for tolerance of sudden and unpredictable increases in UVB radiation above seasonal and diurnal maxima caused by exogenous perturbation of the ozone column. In the case of Mars, the UV radiation flux has increased over time. Today the biologically effective irradiances to DNA are not considerably different from those that are calculated for Archean Earth. If the planet suffered an atmospheric collapse then it may have been subject to an ultraviolet crisis at some point in its past when DNA-weighted irradiance would have increased three to five-fold. Venus transitioned into a photobiologically clement era soon after late bombardment. The lifeless surfaces of Mars and Venus, when in the former case DNA-weighted irradiances are not much greater than Archean Earth and in the latter case, insignificant, are testament to the unimportance of UV radiation as an evolutionary selection pressure when other physical factors, particularly lack of liquid water, become limiting to life. Understanding the comparative evolutionary differences in surface UV flux of the terrestrial planets can help us understand the influence, and lack of influence, of UV radiation in determining their suitability as abodes for life at different stages in their past.