15 Ultraviolet Radiation in Planetary Atmospheres and Biological Implications Petra Rettberg and Lynn J. Rothschild 15.1 Solar UV Radiation The extraterrestrial solar spectrum extends far into short wavelengths of UV-C (190- 280 nm) and vacuum UV ( <190 nm), wavelengths that no longer reach the surface of the Earth. The intensity of solar radiation reaching the Earth's atmosphere would probably be lethal to most living organisms without the shielding afforded by the at- mosphere. Solar UV undergoes absorption and scattering as it passes through the Earth's at- mosphere with the absorption by carbon dioxide, molecular oxygen and ozone being the most important processes. Carbon dioxide has a peak absorbance at 190 nm, and so attenuates radiation below 200 nm. Ozone forms a layer in the stratosphere, thinnest in the tropics (around the equator) and denser towards the poles. The amount of ozone above a point on the Earth's surface is measured in Dobson u nits (DU) - typically ~260 DU near the tropics and higher elsewhere, though there are large seasonal fluctuations. It is created when ultraviolet radiation strikes the stratosphere, dissociating (or "split- ting") oxygen molecules (O 2 ) to atomic oxygen (O). The atomic oxygen quickly com- bines with further oxygen molecules to form ozone (O 3 ). Figure 15.1-A shows the absorption cross section of ozone as a function of wavelength and Fig. 15.1-B a part of the extraterrestrial solar spectrum compared to terrestrial spectra calculated for differ- ent ozone concentrations. Increasing ozone concentrations result in lower irradiances in the UV-B range of the spectrum. Surface UV-B radiation levels are highly variable because of sun angle, cloud cover, and also because of local effects including pollut- ants and surface reflections. Solar UV radiation affects life on Earth today, and probably even has had a stronger impact on early evolution [1]. The composition of the Earth's atmosphere at that time differed from that of today. Although its exact composition is not known, from model calculations it can be assumed that during the Archaean era, during which the diversifi- cation of early anaerobes took place and the first anaerobic photosynthetic bacteria appeared (about 3.5 Ga ago), the amount of free oxygen in the atmosphere was signifi- cantly lower than today (see Chap. 14, Cockell)]. There was very little or no absorption of solar UV radiation by ozone. The situation on the early Mars might have been com- parable (see Chap. 13, Lammer et al. and Chap. 14, Cockell). Taking