Solar X-ray flare hazards on the surface of Mars

摘要

Putative organisms on the Martian surface would be exposed to potentially high doses of ionizing radiation during strong solar X-ray flares. We extrapolate the observed flare frequency-energy release scaling relation to total X-ray energies much larger than seen so far for the Sun, an assumption supported by observations of flares on other solar- and subsolar-mass main sequence stars. Flare spectra are taken as power laws, with the logarithmic slope a parameter based on the observed statistics of the most energetic hard X-ray flare spectra. We calculate surface spectra using a Monte Carlo code we developed for the transport of X-rays and gamma rays, including photoabsorption and detailed Compton scattering. Biological doses from indirect genome damage are calculated for each parameterized flare spectrum by integration over the X-ray opacity of water. The resulting doses depend sensitively on spectral slope, which varies greatly and unsystematically for solar flares. Using the roughly uniform observed distribution of spectral slopes, we estimate the mean waiting time for solar flares producing a given biological dose of ionizing radiation on Mars and compare with lethal dose data for a wide range of terrestrial organisms. These timescales range from decades for significant human health risk to 0.5 Myr for D. radiodurans lethality. Such doses require total flare energies of 10~(33)-10~(38) erg, the lower range of which has been observed for other stars. Flares are intermittent bursts, so acute lethality will only occur on the sunward hemisphere during a sufficiently energetic flare, unlike low-dose-rate, extended damage by cosmic rays. We estimate the soil and CO_2 ice columns required to provide 1/e shielding as 4-9g cm~(-2), depending on flare mean energy and atmospheric column density. Topographic altitude variations give a factor of two variation in dose for a given flare. Life in ice layers that may exist ～100g cm~(-2) below the surface would be well protected. Finally, we point out that designing spacesuits to sufficiently block this radiation on Mars missions may be difficult, given the conflict between solutions for lightweight protection from energetic particles and those from X-rays.