Significance of O 2 and O 3 as Biomarkers on Extrasolar Planets James F. Kasting Dept. of Geosciences Penn State University Whats the best remote signature of life? Simultaneous presence of O 2 (or O 3 ) and a reduced gas such as CH 4 or N 2 O Refs: James Lovelock, Nature (1965) Joshua Lederberg, Nature (1965) Unfortunately, you cant see this for a planet like modern Earth! Not enough CH 4 (1.6 ppmv) Earthshine Spectrum (visible/ near-IR) Woolf et al. (2002) O2O2 H2OH2O O3O3 Cases where O 2 and CH 4 might both be visible 1.Proterozoic Earth Low, but finite, atmospheric O 2 level (~0.1 PAL?) Anoxic, sulfidic deep ocean (Canfield ocean) High production of CH 4 from marine sediments (Pavlov et al., Geology 31, 87, 2003) Cases where O 2 and CH 4 might both be visible 2.M-star planet Low near-UV flux leads to low tropospheric OH densities and long CH 4 lifetime (Segura et al., Astrobiology, in press) Earth around an M star Normalized stellar fluxesCalculated CH 4 mixing ratios (assuming constant flux) A. Segura et al., in press What about O 2 or O 3 by itself? Some authors have predicted high abiotic O 2 levels on Earth-like planets Berkner and Marshall ( ): to PAL Brinkman (JGR, 1969): 0.27 PAL Both of the above calculations were done before we understood hydrogen escape Maximum H escape rate limited by diffusion through the homopause (Hunten, J. Atmos. Sci., 1973) Still, similar predictions continue to appear Selsis et al. (Astron. Astrophys., 2002): Up to 0.1 PAL O 2, accompanied by very high O 3 Case B2 Humid CO 2 -rich atmosphere (1 bar CO 2 ) No volcanic outgas- sing O 3 column depth 0.7 atm cm (twice the terrestrial value) Selsis et al. argued that we would not be fooled by this signal because the 9.6- m band of O 3 would be obscured by the 9.4- and m hot bands of CO 2 However When we do the same type of calculation, we do not get these results! Early Earth-type atmosphere with and w/o outgassing (0.8 bar N 2 ; 0.2 bar CO 2 ) O 2 mixing ratioO 3 number density (Column depth < atm cm) Calculations by Antigona Segura What about planets orbiting young stars with high EUV fluxes? Could enhance high- altitude formation of O 2 by: CO 2 + h CO + O O + O + M O 2 + M Spectra from Martin Cohen and John Scalo O 2 and O 3 for high-CO 2 levels and high stellar UV fluxes O 2 mixing ratioO 3 number density (Column depth < 2 atm cm) Calculations by Antigona Segura Whats the difference in the two calculations? Our model includes a hydrologic cycle, i.e., rainout of reduced and oxidized gases Rainout of oxidized species, e.g., H 2 O 2, leads to production of hydrogen: 2 H 2 O H 2 O 2 + H 2 Oxidized species are assumed to react with reduced species, e.g., Fe +2, in the ocean The atmospheric hydrogen budget More specifically, our model keeps track of (and balances) the atmospheric hydrogen budget Start by defining neutral species: N 2 (for nitrogen) CO 2 (for carbon) H 2 O(for hydrogen) SO 2 (for sulfur) Calculate all redox changes relative to these species The atmospheric hydrogen budget Weight each outgassed or rained out species by its stoichiometric coefficient relative to H 2 e.g. For methane outgassing, write CH H 2 O CO H 2 The coefficent of CH 4 in the hydrogen budget is thus +4 The atmospheric hydrogen budget Similarly, for rainout of elemental sulfur (S 8 ) particles, write 8 SO H 2 S H 2 O The coefficient of S 8 is therefore +16 The atmospheric hydrogen budget With these definitions, the hydrogen budget can be written (in terms of H 2 ) as out (H 2 ) + rain (Ox) = esc (H 2 ) + rain (Red) To get an upper limit on O 2, set the escape rate equal to the diffusion-limited flux esc (H 2 ) 2.5 f tot (H 2 ) molec cm -2 s -1 where f tot (H 2 ) is the total hydrogen mixing ratio in the stratosphere Total hydrogen mixing ratio Homopause Tropopause So, we dont expect to get high abiotic O 2 or O 3 on Earth-like planets, regardless of CO 2 concentrations or stellar UV fluxes When can one get high abiotic O 2 or O 3 ? When can you get a false positive for O 2 or O 3 ? 1.When the stratosphere becomes wet, so that the diffusion limit for hydrogen escape is overcome This happens for runaway greenhouse planets like Venus J. F. Kasting, Icarus (1988) H 2 O-dominated stratosphere When can you get a false positive for O 2 or O 3 ? 2.When the planet is small (so no volcanism) and when its surface is frozen (so no rainout of oxidants) Both conditions are met on Mars today. However, Mars is so small that it loses O to space by nonthermal processes such as O e O + O Because the gravity is so low, these O atoms have enough energy to escape. This limits the O 2 mixing ratio to ~0.1%. If Mars was a little bigger, this wouldnt happen, and O 2 would build up to high concentrations When can you get a false positive for O 2 or O 3 ? We can summarize this by saying that we should beware of false positives when a planet is located outside the habitable zone around its star Corollary: O 2 and O 3 should be good biomarkers for planets located within the habitable zone, provided that liquid water is also present Conclusions O 2 and O 3 are often, but not always, good biomarkers, and we think we know where the pathological cases are to be found Researchers who work on this problem should be sure to balance the atmospheric hydrogen budget in their models!