Importance of Biologically Active Aurora-like Ultraviolet Emission: Stochastic Irradiation of Earth and Mars by Flares and Explosions

Abstract

Habitable planets will be subject to intense sources of ionizing radiation and fast particles from a variety of sources-from the host star to distant explosions-on a variety of timescales. Monte Carlo calculations of high-energy irradiation suggest that the surfaces of terrestrial-like planets with thick atmospheres (column densities greater than about 100 g cm -2 ) are well protected from directly incident X-rays and g-rays, but we find that sizeable fractions of incident ionizing radiation from astrophysical sources can be redistributed to biologically and chemically important ultraviolet wavelengths, a significant fraction of which can reach the surface. This redistribution is mediated by secondary electrons, resulting from Compton scattering and X-ray photoabsorption, the energies of which are low enough to excite and ionize atmospheric molecules and atoms, resulting in a rich aurora-like spectrum. We calculate the fraction of energy redistributed into biologically and chemically important wavelength regions for spectra characteristic of stellar flares and supernovae using a Monte-Carlo transport code and then estimate the fraction of this energy that is transmitted from the atmospheric altitudes of redistribution to the surface for a few illustrative cases. For atmospheric models corresponding to the Archean Earth, we assume no significant ultraviolet absorbers, only Rayleigh scattering, and find that the fraction of incident ionizing radiation that is received at the surface in the form of redistributed ultraviolet in the biologically relevant 200-320 nm region (UV-C and UV-B bands) can be up to 4%. On the present-day Earth with its ultraviolet ozone shield, this fraction is found to be 0.2%. Both values are many orders of magnitude higher than the fraction of direct ionizing radiation reaching the surface. This result implies that planetary organisms will be subject to mutationally significant, if intermittent, fluences of UV-B and harder radiation even in the presence of a narrowband ultraviolet shield like ozone. We also calculate the surficial transmitted fraction of ionizing radiation and redistributed ultraviolet radiation for two illustrative evolving Mars atmospheres whose initial surface pressures were 1 bar. We discuss the frequency with which redistributed ultraviolet flux from parent star flares exceeds the parent star ultraviolet flux at the planetary surface. We find that the redistributed ultraviolet from parent star flares is probably a fairly rare intermittent event for habitable zone planets orbiting solar-type stars except when they are young, but should completely dominate the 2 direct steady ultraviolet radiation from the parent star for planets orbiting all stars less massive than about 0.5 solar masses. Our results suggest that coding organisms on such planets (and on the early Earth) may evolve very differently than on contemporary Earth, with diversity and evolutionary rate controlled by a stochastically varying mutation rate and frequent hypermutation episodes.

The Propitious Role of Solar Energetic Particles in the Origin of Life

“…Hence, SEPs might serve as the most significant mechanism for the synthesis of amino acids and RNA/DNA nucleobases on exoplanets around Mdwarfs; we find that ∼ 10 6 − 10 7 kg/yr of nucleobases might be produced via SEPs and this value is comparable to, or slightly higher than, the upper bound of ∼ 10 6 kg/yr estimated from Table 1 • The above point is rendered even more important by the fact that UV-mediated prebiotic pathways are potentially inefficient on M-dwarf exoplanets, owing to the lower bioactive UV fluence (by a factor of ∼ 1000) reaching the surface (Buccino et al 2007;Rugheimer et al 2015). It is, however, quite possible that flares may ameliorate this UV deficiency to some degree (Smith et al 2004;Buccino et al 2007;Ranjan et al 2017a). …”

Ionizing Radiation and Life

“…The forcing of atmospheric chemistry and global climate by CR ionization of the atmosphere is likely to be particularly significant for planets orbiting stars less massive than the Sun, due to the closer stellar habitable zone and tidal locking Grießmeier et al, 2009), and especially for planets around the subset of M-class dwarves that exhibit frequent flares (Smith et al, 2004b).…”

“…These longer wavelengths are more effectively transmitted through the atmosphere (although dependent on atmospheric thickness and UV absorbers), and for a planet with an atmosphere of around 100 g/cm 2 up to 10% of the incident energy could reach the planetary surface as UV reemission from the atmosphere, which would subject exposed surface organisms to a harsh UV insult. Smith et al (2004b) expanded on this analysis for Archean Earth (3.9-2.5 Ga), considering the photon spectrum from nearby supernovae as well as solar flares. Assuming only Rayleigh scattering of UV photons by the *1000 g/cm 2 atmosphere and no UV filters such as O 3 , Smith et al (2004b) found that up to 4% of the incident ionizing radiation reaches the surface as biologically effective UV radiation (200-320 nm; UVC and UVB bands).…”

“…Smith et al (2004b) expanded on this analysis for Archean Earth (3.9-2.5 Ga), considering the photon spectrum from nearby supernovae as well as solar flares. Assuming only Rayleigh scattering of UV photons by the *1000 g/cm 2 atmosphere and no UV filters such as O 3 , Smith et al (2004b) found that up to 4% of the incident ionizing radiation reaches the surface as biologically effective UV radiation (200-320 nm; UVC and UVB bands). For the current terrestrial situation, including an ozone shield, only 0.2% of the incident energy reaches the surface as biologically active UV reemission, although this is still many orders of magnitude higher than the energy of the ionizing radiation that penetrates to the surface.…”

“…Considering just the solar flare influence on planets orbiting within the habitable zone of a Sun-type star, significant fluxes of hazardous UV would occasionally penetrate to the surface, even on evolved terrestrial planets with a photosynthetic biosphere and ozone shield, but would not exceed the direct UV from the sun. For terrestrial planets orbiting within the habitable zone of lower-mass stars ( < 0.5 solar mass), such as M-class dwarves, however, the energy from solar flare X-rays and gamma rays redistributed into short wavelength UV by the atmospheric shield would completely dominate the direct UV flux from the parent star (Smith et al, 2004b). Organisms evolving on such a planetary surface may therefore experience frequent pulses of elevated UV flux and, if not adequately protected, consequent episodes of cellular damage or hypermutation.…”