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D. S. Smith (Harvard University, UT-Austin), J. Scalo, J. C. Wheeler (UT-Austin)
Monte Carlo simulations of \gamma-ray and hard X-ray irradiation of planetary atmospheres are presented, with an emphasis on astrobiological implications involving atmospheric chemistry and direct surface mutational and sterilization affects. Possible radiation sources include flares from late-type parent stars, \gamma-ray bursts, and \gamma-ray lines from supernovae. We present spectra as a function of depth in the atmosphere and underlying oceans for various incident energy spectra, angles of incidence, and atmospheric column densities. Independent of composition, the fraction of photons reaching the ground and their spectrum are partly controlled by Compton downscattering high in the atmosphere to energies ~50 keV, below which the atmosphere becomes ``black" due to strong photoelectric absorption. The fraction of incident radiation that reaches the ground in the form of ionizing radiation for normal incidence and terrestrial surface gravity is found to depend on column density N as exp(-N/N0) where N0 is 16 gm cm-2. This suggests that Mars has been sterilized by \gamma-ray bursts many times during the past few eons. In addition, secondary electrons from these processes are capable of exciting UV spectral lines whose yield can be a significant fraction of the incident ionizing radiation. Depending on the presence of various UV atmospheric shielding components, a biologically significant dose of soft UV radiation can reach the ground even for atmospheres that are very optically thick to the incident ionizing radiation. Speculations concerning the formation of intense molecular ion emission lines due to secondary electron impact excitation and their implications for external detection of intense aurora from such planets and for photosynthesis on planets orbiting dMe stars are discussed.
This work was supported by NSF grant 9907582.
The author(s) of this abstract have provided an email address for comments about the abstract: dsmith@cfa.harvard.edu