The early evolution of Earth ’ s atmosphere and the origin of life took place at a time when physical conditions at the Earth where radically different from its present state .
The radiative input from the Sun was much enhanced in the high-energy spectral domain , and in order to model early planetary atmospheres in detail , a knowledge of the solar radiative input is needed .
We present an investigation of the atmospheric parameters , state of evolution and high-energy fluxes of the nearby star \kappa ^ { 1 } Cet , previously thought to have properties resembling those of the early Sun .
Atmospheric parameters were derived from the excitation/ionization equilibrium of Fe i and Fe ii , profile fitting of H \alpha and the spectral energy distribution .
The UV irradiance was derived from FUSE and HST data , and the absolute chromospheric flux from the H \alpha line core .
From careful spectral analysis and the comparison of different methods we propose for \kappa ^ { 1 } Cet the following atmospheric parameters : T _ { eff } = 5665 \pm 30 K ( H \alpha profile and energy distribution ) , \log g = 4.49 \pm 0.05 dex ( evolutionary and spectroscopic ) and [ Fe / H ] = +0.10 \pm 0.05 ( Fe ii lines ) .
The UV radiative properties of \kappa ^ { 1 } Cet indicate that its flux is some 35 % lower than the current Sun ’ s between 210 and 300 nm , it matches the Sun ’ s at 170 nm and increases to at least 2–7 times higher than the Sun ’ s between 110 and 140 nm .
The use of several indicators ascribes an age to \kappa ^ { 1 } Cet in the interval \sim 0.4–0.8 Gyr and the analysis of the theoretical HR diagram suggests a mass \sim 1.04 M _ { \odot } .
This star is thus a very close analog of the Sun when life arose on Earth and Mars is thought to have lost its surface bodies of liquid water .
Photochemical models indicate that the enhanced UV emission leads to a significant increase in photodissociation rates compared with those commonly assumed of the early Earth .
Our results show that reliable calculations of the chemical composition of early planetary atmospheres need to account for the stronger solar photodissociating UV irradiation .