Habitat of early life: Solar X-ray and UV radiation at Earth's surface 4-3.5 billion years ago

Solar X-ray and UV radiation (0.1-320 nm) received at Earth's surface is an important aspect of the circumstances under which life formed on Earth. The quantity that is received depends on two main variables: the emission of radiation by the young Sun and its extinction through absorption and scattering by the Earth's early atmosphere. The spectrum emitted by the Sun when life formed, between 4 and 3.5 Ga, was modeled here, including the effects of flares and activity cycles, using a solar-like star that has the same age now as the Sun had 4-3.5 Ga. Atmospheric extinction was calculated using the Beer-Lambert law, assuming several density profiles for the atmosphere of the Archean Earth. We found that almost all radiation with a wavelength shorter than 200 nm is attenuated effectively, even by very tenuous atmospheres. Longer-wavelength radiation is progressively less well attenuated, and its extinction is more sensitive to atmospheric composition. Minor atmospheric components, such as methane, ozone, water vapor, etc., have only negligible effects, but changes in CO₂ concentration can cause large differences in surface flux. Differences due to variability in solar emission are small compared to this. In all cases surface radiation levels on the Archean Earth were several orders of magnitude higher in the 200-300 nm wavelength range than current levels in this range. That means that any form of life that might have been present at Earth's surface 4-3.5 Ga must have been exposed to much higher quantities of damaging radiation than at present.