title: Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs.
abstract: We show that terrestrial planets in the habitable zones of M dwarfs older than ∼1 Gyr could have been in runaway greenhouses for several hundred million years following their formation due to the star's extended pre-main sequence phase, provided they form with abundant surface water. Such prolonged runaway greenhouses can lead to planetary evolution divergent from that of Earth. During this early runaway phase, photolysis of water vapor and hydrogen/oxygen escape to space can lead to the loss of several Earth oceans of water from planets throughout the habitable zone, regardless of whether the escape is energy-limited or diffusion-limited. We find that the amount of water lost scales with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to the planet surface gravity. In addition to undergoing potential desiccation, planets with inefficient oxygen sinks at the surface may build up hundreds to thousands of bar of abiotically produced O2, resulting in potential false positives for life. The amount of O2 that builds up also scales with the planet mass; we find that O2 builds up at a constant rate that is controlled by diffusion: ∼5 bar/Myr on Earth-mass planets and up to ∼25 bar/Myr on super-Earths. As a result, some recently discovered super-Earths in the habitable zone such as GJ 667Cc could have built up as many as 2000 bar of O2 due to the loss of up to 10 Earth oceans of water. The fate of a given planet strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface. In general, we find that the initial phase of high luminosity may compromise the habitability of many terrestrial planets orbiting low-mass stars.
authors: 1 Astronomy Department, University of Washington , Seattle, Washington.
authors: R
authors: R
authors: Luger
authors: R
authors: R
authors: Barnes
mesh_terms: D001272:Atmosphere; D004058:Diffusion; D018534:Earth, Planet; D019419:Evolution, Planetary; D018559:Exobiology / Q000379:methods*; D005118:Extraterrestrial Environment; D008027:Light; D008962:Models, Theoretical; D009792:Oceans and Seas; D010100:Oxygen / Q000737:chemistry*; D016083:Planets; D055609:Stars, Celestial; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D010100:Oxygen
references: Astrobiology. 2014 Feb;14(2):67-86
references: 24432758
references: Nature. 2013 May 30;497(7451):607-10
references: 23719462
references: Science. 2014 Apr 18;344(6181):277-80
references: 24744370
references: Icarus. 2000 May;145(1):262-71
references: 11543302
references: Science. 2001 Aug 3;293(5531):839-43
references: 11486082
references: Science. 2007 Sep 28;317(5846):1903-6
references: 17901330
references: Astrobiology. 2013 Mar;13(3):279-91
references: 23537137
references: Astrobiology. 2013 Nov;13(11):1030-48
references: 24283926
references: Astrobiology. 2010 Sep;10(7):751-71
references: 20879863
references: Astrobiology. 2015 Jan;15(1):57-88
references: 25590532
references: Icarus. 1988;74:472-94
references: 11538226
references: Nature. 2011 Oct 05;478(7368):218-20
references: 21976024
references: Astrobiology. 2011 Jun;11(5):443-60
references: 21707386
references: Philos Trans A Math Phys Eng Sci. 2012 Sep 13;370(1974):4197-216
references: 22869797
references: Icarus. 1990;84:502-27
references: 11538474
references: Planet Space Sci. 1995 Jan-Feb;43(1-2):3-10
references: 11538435
references: Astrobiology. 2013 Nov;13(11):1011-29
references: 24251443
references: Nature. 2005 May 26;435(7041):466-9
references: 15917802
references: Astrobiology. 2013 Mar;13(3):225-50
references: 23537135
references: Orig Life Evol Biosph. 2011 Dec;41(6):503-22
references: 22314970
references: Science. 2014 Jul 25;345(6195):440-4
references: 24993348
references: Astrobiology. 2007 Feb;7(1):85-166
references: 17407405
references: Icarus. 1984;57:335-55
references: 11541985
references: Astrobiology. 2008 Jun;8(3):557-68
references: 18598142
references: Icarus. 1988;74:62-97
references: 11538227
references: Nature. 2013 Dec 12;504(7479):268-71
references: 24336285
references: Planet Space Sci. 2014 Aug;98:106-119
references: 25843981
references: Science. 2013 May 3;340(6132):587-90
references: 23599262
references: Icarus. 1993 Jan;101(1):108-28
references: 11536936