14 The Ultraviolet Radiation Environment of Earth and Mars: Past and Present

Exactly 130 years passed between the discovery by Isaac Newton that white light was composed of colors [1] and the discovery of ultraviolet radiation by Johann Wilhelm Ritter, a German electro chemist, in 1801. We now understand that ultraviolet radiation, although representing <2% of the total number of photons that reach the surface of present-day Earth, has had an important role in the evolution of life on Earth. This is because it has a high energy, energy being proportional to the frequency of the radiation. UV radiation is damaging to a number of key macromolecules, particularly DNA. On early Earth, the lack of an ozone column probably resulted in higher biologically weighted irradiance than the surface of present-day Earth as there were no other UV absorbers in the atmosphere. This is also the case for present-day Mars and probably was for Mars in its early history.