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19 Apr 2017
— Edited @ @
16 May 2017
Contact: David Catling, dcatling *at* u.washington.edu
Summary: We present a general scheme for observing potential exoplanet biosignatures and gaining and expressing confidence levels for positive detection of signs of life. An appropriate framework uses models with data (in the form of exoplanetary system properties and spectral or photometric data) to find the Bayesian likelihoods of those data occurring if the exoplanet has or does not have life. The latter includes the case of false positives, i.e., where abiotic sources mimic biosignatures. Prior knowledge (including all factors that influence habitability and previous exoplanet observations) would be combined with the likelihoods to arrive at the probability of life existing on a given exoplanet given the observations.
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09 Jun 2017
You might also want to include a more general expression for the average equilibrium temperature (Equation 20 from Mendez & Rivera-Valentin, 2017) that includes eccentricity in equation 4 (Line 433). It is not obvious at first, but the average equilibrium temperature slowly decreases with eccentricity while the average stellar flux increases. The albedo and greenhouse effect are also stellar flux dependent (Kasting et al., 1993).
Line 382 might include additional references that suggest that climate variations from extreme eccentricity may not compromise habitability. For example (Dressing et al. 2010; Bolmont et al. 2016; Way & Georgakarakos, 2017; Mendez & Rivera-Valentin, 2017).
Dressing, C. D., Spiegel, D. S., Scharf, C. A., Menou, K., & Raymond, S. N. (2010). Habitable climates: the influence of eccentricity. The Astrophysical Journal, 721(2), 1295.
Bolmont, E., Libert, A. S., Leconte, J., & Selsis, F. (2016). Habitability of planets on eccentric orbits: Limits of the mean flux approximation. Astronomy & Astrophysics, 591, A106.
Way, M. J. & Georgakarakos, N. (2017) Effects of Variable Eccentricity on the Climate of an Earth-like World. The Astrophysical Journal Letters, 835(1), L1.
Méndez, A., & Rivera-Valentín, E. G. (2017). The Equilibrium Temperature of Planets in Elliptical Orbits. The Astrophysical Journal Letters, 837(1), L1.
09 Jun 2017
David et al., great draft! Regarding the eccentricity comment above, the effect of eccentricity on equilibrium temperature and climate has been dealt with for some time. Dawn and I published a paper about that back in 2012 and demonstrated the effect on equilibrium temperature of planets moving in and out of the HZ (Kane & Gelino, 2012, AsBio, 12, 940). However, I think eccentricity will end up being relatively unimportant for two reasons. First, the results from Kepler indicate that small (terrestrial) planets have low eccentricities (Kane et al. 2012, MNRAS, 425, 757), and second because substantial liquid water oceans tend to have large thermal inertia. The effect of this is that reasonable eccentricities can have a large effect on climate forcing (essentially taking over from obliquity) but a low effect on habitability.
On an unrelated note, I think Table 1 should include stellar luminosity as an intrinsic property and distinct from spectral type. Obviously this is crucial for calculating the incident flux. Also, in Table 2, SO2 and H2S are mentioned as sourced from volcanic outgassing, but these should be listed as mere examples since the possible range of gases from volcanos can be quite broad depending on composition. Additionally, depending on the mass and age of the planet, the overall contribution of outgassing to the bulk atmosphere could end up being substantial for prolonged periods.
09 Jun 2017
Great paper overall! I love the Bayesian framework and particularly how you summarize it all in table 6. Here are some comments and suggestions. Please contact me for any clarification or more detailed references etc.
Page 5 – re: there being two planets in the HZ of the Sun and then use this to get a prior of P(life) = 0.5. It could be argued that Venus was habitable in the past, as certainly if you were to switch Mars and Venus to each other's orbits a Venus at Mars' orbit might be habitable much longer and a Mars at Venus' orbit might still be habitable, especially with some of the dry inner edge estimates by Zsom et al. and others. I think something to this effect should be touched on in the manuscript and possibly one could put P(life) = 0.33 or put some range on it.
Page 7 - In table 1 under the stellar properties, you have "pan-chromatic spectrum including the UV flux. Most of the SED (apart from the UV) is fairly well determined from models, with perhaps some caveats for lower mass stars. So perhaps just knowing the spectral type and getting UV observations would be sufficient.
Page 8 – For the sentence starting (near the middle of the page) “The spectral energy distribution is needed….” With the citation to Segura et al., 2003. One could also add (my) Rugheimer et al. 2013 paper to that since it is the primary point of that paper to tease out specifically the interplay between the SED vs the UV flux. Also I think Grenfell et al., 2007 should be cited here and also probably some of his later papers (such as on N2O) throughout the manuscript as his papers are notably missing.
Page 9 - re: elemental composition of the parent star. It might be worth adding a sentence about the C/O ratio in particular. Since a C/O ratio > 1 could mean that plate tectonics would be unlikely due to SiC rather than silicates forming (Kuchner & Seager 2005). https://arxiv.org/abs/astro-ph/0504214
Page 14 - When describing anti-biosignatures, you specifically refer to H2 and CO2 in combination, but I thought I’ve heard that H2 alone might be an anti-biosignature. Also you mention that H2 and CO2 are a redox couple for primitive metabolisms known on earth, but isn’t there an energetics reason too just relating to H2 in that the bond is easy to break? Not really my area, but just wondering if there is some reason that could be argued that is more general than relying on Earth life.
Page 14 – re: reducing gases, why and when is CO a reducing gas? Could a footnote be added in explanation for the reader.
Page 15 – the manuscript highlights being able to see Rayleigh scattering in reflected light spectra, but it’s also present in transmission spectra, right? And therefore could indicate some atmospheric pressure is present to create that signal? Re: “For reflected-light observations, Rayleigh scattering can indicate pressure….”
Page 15 – re: organic smog on Titan, probably a reference to Sarah Hörst’s research on smog and haze formation is necessary in addition to the Marley reference.
Such as: Yoon, Y.H., Hörst, S.M., Hicks, R.K., Li, R., J.A. deGouw, and Tolbert, M.A. "The Role of Benzene Photolysis in Titan Haze Formation." Icarus, 233, 233-241, doi:10.1016/j.icarus.2014.02.006, 2014.
And this review - Hörst, S.M. "Titan's atmosphere and climate " JGR Planets, 122, doi:10.1002/2016JE005240, 2017. (Invited review for the 25th anniversary issue of JGR Planets)
Page 18 – Seager, Bains and Hu 2013 – Biomass based model paper should be cited in the first paragraph of the section of the candidate biogenic gases.
Page 19 – Table 4 – For the VIS-NIR band, this is the only place where only cm^-1 is used. Maybe use microns instead? To match the two columns better on either side. Same in Table 5 for the VIS-NIR column
In the abiogenic false positive column of table 4, it is stated for N2O “some abiotic sources that don’t build up on Earth” yet the same could be said for O2, the abiotic sources from water photo-dissociation don’t build up on Earth. I’d recommend re-wording. As well I’d just state the abiotic sources rather than only including it in the footnote. So state chemodenitrification, but keep the asterisk to explain why this might still indirectly indicate a biosphere as you have done.
Page 20 – when introducing equation 8, I think it should include the phrase “ideal gas law” ie) given by the ideal gas law:
Page 23 – Kiang’s two papers in 2007 (especially the second paper in the series) on photosynthetic pigments should be referenced here in the paragraph about red-edges, along with a sentence about how these VREs might be increased reflectance in different wavebands due to a different star type.
Page 24 – There is again a sentence of an anti-biosignature of H2 and CO2 not being present on an inhabited planet. Would it be possible though if there was a large starting abundance on say a super-Earth or mini-Neptune? If you start with so much of both gases that microbes would take a long time to pull down those concentrations?
Page 25 – near the bottom of the page, first sentence of last paragraph there is a small typo. “an additional corroborating biosignature would the red-edge…” after would insert “be”.
Page 26 – last paragraph – I’d change the sentence starting “Specifically, oceans….” To “In one such case, oceans are….” Because there are lots of cases and here you are only highlighting one, not the only one. Also I think Schaefer et al., 2016 on GJ 1132b has a calculation on how large amounts of O2 would likely have huge sinks in a hot mantle such as might be the case on these super-luminous M dwarfs, thus taking 100 bars to 1bar. I think this paper also needs to be mentioned as a caveat along with yours.
Page 27 – In this section on the abiotic sources of O2, I think you need to add citations for Hu et al., 2012, Rameriz & Kaltenegger 2014, and Wordsworth and Pierrehumbert 2014 (you mention Wordsworth and Pierrehumbert later but I think it should be here too). I’d also add a reference to Guzmán-Marmolejo and Segura 2013 on abiotic CH4 production and estimates in the Geothermal paragraph.
Page 28 – You need to cite Levi et al. 2014 in the sentence on clathrates releasing methane: “Structure and Dynamics of Cold Water Super-Earths: The Case of Occluded CH4 and its Outgassing” by Amit Levi, Dimitar Sasselov, Morris Podolak (2014)
Page 28 – When it is again discussed about the large build-up of O2 being 10 to 100s of bars and therefore having a strong dimer signal to rule out the false positive, it might be worth giving the caveat from Schaefer et al., that perhaps there will be much less abiotic O2 stable in the atmosphere, leaving the potential to be mistaken about it being biotic.
In this same paragraph, I’d also include a sentence that Wordsworth and Pierrehumbert’s mechanism would be much harder to mitigate since it doesn’t necessarily say that the O2 buildup would be high enough to get dimmers and N4 will be difficult to detect. And this mechanism can happen for any planet around any stellar type, and within the middle of the HZ.
I love your Table 6 – really cool idea!
General comment. It seems like the other reviews reference each other just by the year (Meadows et al., 2017) rather than “this issue” which would be more difficult for future readers to find.
09 Jun 2017
Thanks everyone for your comments: Abel M., Stephen K., Sarah R., and people who emailed me separately. I'll be revising the paper next week (hopefully) and will be sure to take account of your various helpful suggestions. -David
09 Jun 2017
I would like to comment on your comment to David's paper "Most of the SED (apart from the UV) is fairly well determined from models, with perhaps some caveats for lower mass stars. So perhaps just knowing the spectral type and getting UV observations would be sufficient.". There is no evidence that most of the SED (what is the most?) is determined from models? Astrophysics of magnetically driven eruptive processes that produces X ray and EUV emission is now well understood theoretically, and I do not know any of the models that can offer such explanation. This is why we need to use observations to reconstruct SED from X-rays to UV.
09 Jun 2017
I have a comment to Table 1 that states that the stellar rotation rate of star is correlated with UV flux which is not exactly correct. It correlates with X-ray flu, which is in turn correlates with some of strongest UV spectral lines forming at high temperatures in processes physically related to the formation of X-ray stellar corona.
10 Jun 2017
Sarah, great comment about two planets in the HZ. It's an interesting thought experiment to swap Mars and Venus. In that case I suspect the following: Mars would become tidally locked and have essentially no atmosphere due to the dramatic increase in stellar flux, whilst Venus would never have lost it's liquid water and would probably have a rotation period close to its primordial rotation rate (assuming its current rotational angular momentum is due to a combination of solar tidal forces and interaction with its thick atmosphere, NOT impact related). The point is that P(life) for N planets in the HZ where N > 1 will be heavily weighted by critical planetary properties, such as planet mass.