UV Energy from the Sun

The solar radiation incident on the atmosphere and the Earth s surface represents the largest external energy contribution. The optical irradiation at the upper boundary layers of the atmosphere is 4,435 kJ/cm2/year, of which around 1,108 kJ/cm2/year reaches the Earth s surface. 

The spectral distribution of this radiation is given in Table 4.3, from which we can easily see that radiation with wavelengths below 150nm represents only a tiny fraction of the total. The energy distribution of the solar radiation corresponds to that from a black body with a temperature of around 5,000 K. 

It must, however, be taken into account that the radiation flux has not always been constant during the 4.6 billion years since the formation of the sun. The solar constant is in fact not a constant at all, as it depends on the state of the sun s surface. For prebiotic syntheses, it is important to consider the wavelengths which can be absorbed by small molecules such as CO2, CO, CH4, N2, NH3, H2O, H2S etc. The premise here, of course, is that most of the synthetic reactions occurred in the gas phase. 

The wavelengths at which most of the components of a primitive Earth atmosphere absorb lie, with few exceptions, under 200 nm. The exceptions include ammonia ( 230 nm), hydrogen sulphide ( 260 nm) and ozone (180-300 nm). However, ozone was probably present in the primeval atmosphere only in trace amounts, since free oxygen was only available in extremely low concentrations. The young Earth thus had no protective ozone layer, so short-wavelength UV irradiation could readily penetrate the atmosphere. 

The amount of energy set free in the processes shown in Table 4.4 is, for example, around 3 J/year for 1 g 238U (in equilibrium with its daughter product). 235U emits 18 J/g/year, 232Th 0.8 J/g/year. The average amount of the three elements present in granite and volcanic rocks is shown in Table 4.5. 

Wellenlangenbereich in nm % der insgesamt kJ-cm 2-a 1 AU Please translate German to English for Table 4.3 and 4.4 

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