The ultraviolet environment of Mars: Biological implications past, present, and future

A radiative transfer model is used to quantitatively investigate aspects of the martian ultraviolet radiation environment, past and present, Biologica l action spectra for DNA inactivation and chloroplast (photosystem) inhibit ion are used to estimate biologically effective irradiances for the martian surface under cloudless skies, Over time Mars has probably experienced an increasingly inhospitable photobiological environment, with present instant aneous DNA weighted irradiances 3.5-fold higher than they may have been on early Mars, This is in contrast to the surface of Earth, which experienced an ozone amelioration of the photobiological environment during the Protero zoic and now has DNA weighted irradiances almost three orders of magnitude lower than early Earth, Although the present-day martian UV flux is similar to that of early Earth and thus may not be a critical limitation to life i n the evolutionary context, it is a constraint to an unadapted biota and wi ll rapidly kill spacecraft-borne microbes not covered by a martian dust lay er, Microbial strategies for protection against UV radiation are considered in the light of martian photobiological calculations, past and present, Da ta are also presented for the effects of hypothetical planetary atmospheric manipulations on the martian UV radiation environment with estimates of th e biological consequences of such manipulations. (C) 2000 Academic Press.