Ozone concentrations and ultraviolet fluxes on Earth-like planets around other stars

Coupled radiative-convective/photochemical modeling was performed for Earth-like planets orbiting different types of stars (the Sun as a G2V, an F2V, and a K2V star). O-2 concentrations between 1 and 10(-5) times the present atmospheric level (PAL) were simulated. The results were used to calculate visible/near-IR and thermal-IR spectra, along with surface UV fluxes and relative dose rates for erythema and DNA damage. For the spectral resolution and sensitivity currently planned for the first generation of terrestrial planet detection and characterization missions, we find that O-2 should be observable remotely in the visible for atmospheres containing at least 10(-2) PAL of O-2. O-3 should be visible in the thermal-IR for atmospheres containing at least 10(-3) PAL of O-2. CH4 is not expected to be observable in 1 PAL O-2 atmospheres like that of modern Earth, but it might be observable at thermal-IR wavelengths in "mid-Proterozoic-type" atmospheres containing similar to10(-1) PAL of O-2. Thus, the simultaneous detection of both O-3 and CH4-considered to be a reliable indication of life-is within the realm of possibility. High-O-2 planets orbiting K2V and F2V stars are both better protected from surface UV radiation than is modern Earth. For the F2V case the high intrinsic UV luminosity of the star is more than offset by the much thicker ozone layer. At O-2 levels below similar to10(-2) PAL, planets around all three types of stars are subject to high surface UV fluxes, with the F2V planet exhibiting the most biologically dangerous radiation environment. Thus, while advanced life is theoretically possible on high-O-2 planets around F stars, it is not obvious that it would evolve as it did on Earth.