Constraints on the Early Terrestrial Surface UV Environment Relevant to Prebiotic Chemistry

Abstract

The UV environment is a key boundary condition to abiogenesis. However, considerable uncertainty exists as to planetary conditions and hence surface UV at abiogenesis. Here, we present two-stream multilayer clear-sky calculations of the UV surface radiance on Earth at 3.9 Ga to constrain the UV surface fluence as a function of albedo, solar zenith angle (SZA), and atmospheric composition. Variation in albedo and latitude (through SZA) can affect maximum photoreaction rates by a factor of >10.4; for the same atmosphere, photoreactions can proceed an order of magnitude faster at the equator of a snowball Earth than at the poles of a warmer world. Hence, surface conditions are important considerations when computing prebiotic UV fluences. For climatically reasonable levels of CO, fluence shortward of 189 nm is screened out, meaning that prebiotic chemistry is robustly shielded from variations in UV fluence due to solar flares or variability. Strong shielding from CO also means that the UV surface fluence is insensitive to plausible levels of CH, O, and O. At scattering wavelengths, UV fluence drops off comparatively slowly with increasing CO levels. However, if SO and/or HS can build up to the ≥1-100 ppm level as hypothesized by some workers, then they can dramatically suppress surface fluence and hence prebiotic photoprocesses. HO is a robust UV shield for λ < 198 nm. This means that regardless of the levels of other atmospheric gases, fluence ≲198 nm is only available for cold, dry atmospheres, meaning sources with emission ≲198 (e.g., ArF excimer lasers) can only be used in simulations of cold environments with low abundance of volcanogenic gases. On the other hand, fluence at 254 nm is unshielded by HO and is available across a broad range of [Formula: see text], meaning that mercury lamps are suitable for initial studies regardless of the uncertainty in primordial HO and CO levels. Key Words: Radiative transfer-Origin of life-Planetary environments-UV radiation-Prebiotic chemistry. Astrobiology 17, 169-204.

The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up

“…Our radiative transfer formalism is described in detail in Ranjan & Sasselov (2017). Briefly, we partition the atmosphere into 64homogenous layers of 1 km width and use a two-stream formalism with Gaussian (single) quadrature closure to compute propagation of UV light through the atmosphere (Toon et al 1989).…”

“…The absorption of most major atmospheric gases (e.g., N 2 , H 2 O, CH 4 ) is degenerate with this CO 2 absorption, meaning that surface UV is insensitive to their abundances (Ranjan & Sasselov 2017). Trace gases whose UV absorption is nondegenerate with CO 2 (e.g., O 3 , SO 2 ) do not build up to levels high enough to affect surface UV for atmospheric boundary conditions corresponding to the modern abiotic Earth, according to the photochemical calculations of Rugheimer et al (2015).…”

“…These parameter values correspond to global mean conditions; variations in surface albedo and SZA can generally be expected to drive changes in band-averaged fluence of approximately oneorder of magnitude (OOM) (Ranjan & Sasselov 2017). As in Ranjan & Sasselov (2017) and Ranjan et al (2017a), the fundamental quantity our code calculates is the surface radiance, i.e., the integral of the intensity field at the planet surface for elevations 0 > . This is the relevant radiative quantity for calculating photoreaction rates at planet surfaces, and in the two-stream formalism, this can be calculated as for Gaussian quadrature (Toon et al 1989).…”

“…We take the solar zenith angle (SZA) to be 48°.2, corresponding to the insolation-weighted global mean value (Cronin 2014). These parameter values correspond to global mean conditions; variations in surface albedo and SZA can generally be expected to drive changes in band-averaged fluence of approximately oneorder of magnitude (OOM) (Ranjan & Sasselov 2017). As in Ranjan & Sasselov (2017) and Ranjan et al (2017a), the fundamental quantity our code calculates is the surface radiance, i.e., the integral of the intensity field at the planet surface for elevations 0 > .…”

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

“…Moreover, the bioactive UV flux ∼ 4 Gya emitted by the Sun is predicted to have been a few times times higher than its present-day value (Rugheimer et al 2015;Rapf & Vaida 2016), which can enable independent UV-mediated prebiotic chemistry (Ranjan & Sasselov 2017;Ranjan et al 2017b). Thus, it appears plausible that prebiotic pathways driven by SEPs (and UV radiation) were functional on ancient Mars and Earth, consequently rendering both planets conducive to the origin of life in this particular scenario.…”