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Uv Surface Environment of Earth-Like Planets Orbiting Fgkm Stars Through Geological Evolution S View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by St Andrews Research Repository The Astrophysical Journal, 806:137 (10pp), 2015 June 10 doi:10.1088/0004-637X/806/1/137 © 2015. The American Astronomical Society. All rights reserved. UV SURFACE ENVIRONMENT OF EARTH-LIKE PLANETS ORBITING FGKM STARS THROUGH GEOLOGICAL EVOLUTION S. Rugheimer1,3, A. Segura2, L. Kaltenegger3, and D. Sasselov1 1 Harvard Smithsonian Center for Astrophysics, 60 Garden st., 02138 MA Cambridge, USA; [email protected] 2 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, México 3 Carl Sagan Institute, Cornell University, Ithaca, NY 14853 USA Received 2014 October 27; accepted 2015 May 1; published 2015 June 12 ABSTRACT The UV environment of a host star affects the photochemistry in the atmosphere, and ultimately the surface UV environment for terrestrial planets and therefore the conditions for the origin and evolution of life. We model the surface UV radiation environment for Earth-sized planets orbiting FGKM stars in the circumstellar Habitable Zone for Earth through its geological evolution. We explore four different types of atmospheres corresponding to an early-Earth atmosphere at 3.9 Gyr ago and three atmospheres covering the rise of oxygen to present-day levels at 2.0 Gyr ago, 0.8 Gyr ago, and modern Earth. In addition to calculating the UV flux on the surface of the planet, we model the biologically effective irradiance, using DNA damage as a proxy for biological damage. We find that a pre-biotic Earth (3.9 Gyr ago) orbiting an F0V star receives 6 times the biologically effective radiation as around the early Sun and 3520 times the modern Earth–Sun levels. A pre-biotic Earth orbiting GJ 581 (M3.5 V) receives 300 times less biologically effective radiation, about 2 times modern Earth–Sun levels. The UV fluxes calculated here provide a grid of model UV environments during the evolution of an Earth-like planet orbiting a range of stars. These models can be used as inputs into photo-biological experiments and for pre-biotic chemistry and early life evolution experiments. Key words: astrobiology – Earth – planets and satellites: surfaces – planets and satellites: terrestrial planets 1. INTRODUCTION UV surface radiation environments for both pre-biotic and post-biotic planets orbiting other stars at the 1 AU equivalent Thousands of extrasolar planets have been found to date with distance based on Earth’s evolution. thousands more awaiting confirmation from space and ground- Previous studies used an analytic atmospheric attenuation based searches. Several of these planets have been found in or model to model UV environment for Archean Earth (Cockell near the circumstellar Habitable Zone (see, e.g., Udry et al. 1998, 1999, 2000; Cnossen et al. 2007) as well as F stars (Sato 2007; Borucki et al. 2011, 2013; Kaltenegger & Sasselov 2011; et al. 2014). An earlier study (Segura et al. 2003) examined the Batalha et al. 2013; Kaltenegger et al. 2013; Quintana et al. amount of UV radiation reaching the surface of an Earth-like 2014) with masses and radii consistent with rocky planet fi planet with varying oxygen atmospheric concentration orbiting models. The quest for nding habitable planets focuses on an F2V, G2V (the Sun), and K2V host star. Our paper expands main-sequence stars with lifetimes conducive for the origin and the grid of host stars as well as models of the UV surface evolution of life, namely, the F, G, K, and M spectral types environments for atmospheres that correspond to geological with stellar main-sequence lifetimes at least 2 Gyr or more. The epochs throughout Earth’s evolution (following Kaltenegger UV environment of a host star will affect the atmosphere, and et al. 2007). We focus on four geological epochs corresponding ultimately the surface UV environment for terrestrial planets. to a 3.9 Ga (assumed prebiotic)4, 2.0 Ga, 0.8 Ga, and the (see e.g., Segura et al. 2003; Rugheimer et al. 2013, 2014; Sato ) modern Earth atmosphere for a grid of FGKM host stars. et al. 2014 . In Section 2, we describe our model, Section 3 presents the Future mission concepts to characterize Earth-like planets calculated UV fluxes at the surface and top of the atmosphere are designed to take spectra of extrasolar planets with the of an Earth-like planet for 12 stellar types and the four different ultimate goal of remotely detecting atmospheric signatures that atmosphere models through geological time. In Section 4 we ( can indicate habitability and life e.g., Beichman et al. conclude by summarizing the results and discussing their 1999, 2006; Cash 2006; Kaltenegger et al. 2006; Traub et al. implications. 2006; Seager & Bains 2015).The UV surface environment for planets is an important component in providing boundary conditions for biological models exploring the origin of life on 2. MODEL DESCRIPTION Earth as well as planets orbiting other types of stars. 2.1. Stellar and Planetary Model Depending on the intensity, UV radiation can be both useful and harmful to life as we know it. UV radiation from We use a grid of host stars from F0V to M8V (Teff = 7000 to 180–300 nm can inhibit photosynthesis and cause damage to 2400 K; see Rugheimer et al. 2013, 2014). All stellar models DNA and other macromolecule damage (Voet et al. 1963; use observations in the UV by the IUE5 except for 3 M dwarfs Matsunaga et al. 1991; Tevini 1993; Kerwin & Remmele with Hubble Space Telescope HST data and reconstructed Lyα 2007). However, these same wavelengths also drive several fluxes (France et al. 2013) up to 3000 Å combined with reactions thought necessary for the origin of life (e.g., Senanayake & Idriss 2006; Barks et al. 2010; Ritson & 4 Ga—billion years ago. Sutherland 2012; Patel et al. 2015). In this paper we model the 5 http://archive.stsci.edu/iue 1 The Astrophysical Journal, 806:137 (10pp), 2015 June 10 Rugheimer et al. Figure 1. Stellar flux (black lines), TOA fluxes (gray lines), and surface UV fluxes (colored lines) for each of the four geological epochs for an F0V (top left), the Sun (top right), a K7V (bottom left), and M8V (bottom right) host star. PHOENIX (Allard et al. 2000; Allard 2014) or ATLAS used in the visible/near-IR to calculate the shortwave fluxes. (Kurucz 1979) stellar models6 for larger wavelengths (F0V, Four-term, correlated-k coefficients parameterize the absorption 7 F7V, Sun (G2V), G8V, K2V, K7V, M1V, M3V, M8V ). The by O3,H2O, O2, and CH4 (Pavlov et al. 2000). Clouds are not three M dwarfs with updated UV data are GJ 581 (M3V, Teff = explicitly calculated. Clouds can either reduce or enhance UV ( 3498 K), GJ 832 (M1.5 V, Teff = 3620 K), and GJ 1214 radiation reaching the surface of an Earth-like planet Grant & ) (M4.5 V, Teff = 3250 K; France et al. 2013). Heisler 1997; Parisi & Downs 2004 . The climatic effects of To model the planetary atmospheres, we use a coupled 1D clouds on the temperature versus pressure profile are included radiative-convective atmosphere code developed for rocky by adjusting the planet’s surface albedo to the value that for the exoplanets. It iterates between a 1D climate (Kasting & modern Earth–Sun system yields a surface temperature of ( Ackerman 1986; Pavlov et al. 2000; Haqq-Misra et al. 2008) 288 K following Kasting et al. 1984; Pavlov et al. 2000; ) and a 1D photochemistry code (Pavlov & Kasting 2002; Segura et al. 2003, 2005 . The photochemistry code, originally ( ) Segura et al. 2005, 2007) to calculate the atmosphere developed by Kasting et al. 1985 , solves for 55 chemical transmission of UV fluxes to the ground of Earth-sized planets. species linked by 220 reactions using a reverse-Euler method ( ) We simulate the effects of stellar radiation on a planetary see Segura et al. 2010, and references therein . environment with an altitude range that extends upwards to For the geological epoch at 3.9 Ga, we use a 1D 60 km, corresponding to a pressure of 1mbar, with 100 height photochemical model for high-CO2/high-CH4 terrestrial atmo- spheres (see Pavlov et al. 2001; Kharecha et al. 2005; Segura layers. A two-stream approximation (see Toon et al. 1989), et al. 2007, and references therein). This model simulates an which includes multiple scattering by atmospheric gases, is anoxic atmosphere composed of 0.9 bar of N2 and fixed amounts of CO2 and CH4. We run the radiative-convective 6 BT-Settl PHOENIX models are used for the M stars and ATLAS models are used for the FGK stars. model to convergence and then use the resulting temperature 7 The M1V, M3V, and M8V are the active stellar models defined in profile to run the photochemical model that contains 73 Rugheimer et al. (2014). chemical species involved in 359 reactions. The model spans 2 The Astrophysical Journal, 806:137 (10pp), 2015 June 10 Rugheimer et al. − the region from the planetary surface up to 64 km in 1 km steps. of 2.4× 10−4 and 1.2× 10−4 cm s 1, respectively, correspond- All of the simulated planets at 3.9 Ga are assumed to be devoid ing to the maximum rate of transfer into an ocean (Domagal- of life; hence, none of the compounds in the atmosphere are Goldman et al. 2014). For the later stellar types, K7V, M1V, considered to have a biological source. M3V, and the three observed MUSCLES stars, the CH4 boundary condition was changed to a fixed mixing ratio (see 2.2. Simulation Set-up also Segura et al. 2005; Rugheimer et al.
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