For instance although we cannot definitively determine what type of life may

For instance although we cannot definitively

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For instance, although we cannot definitively determine what type of life may evolve in dense multi-bar CO 2 atmospheres (see Section 15.1 ), previous work has inferred the likely spectral signatures of Earth over time [ 108 , 261 ]. This is a great first step, although much higher (and lower) pressures and different star types should be considered for future work. Nevertheless, assuming life exactly like Earth’s can evolve elsewhere, future work could similarly assess how volcanically-outgassed H 2 may influence early Earth spectral signatures. Indeed, volcanically outgassed H 2 had been suggested as a significant constituent in the early Earth’s atmosphere [ 46 ]. Alternatively, conditions dissimilar to Earth’s can be assessed for likely spectral signatures, using appropriate theoretical criteria [ 19 , 152 , 283 ]. Regardless, our understanding of the expected biosignatures in atmospheres markedly different from Earth’s (e.g., multi-bar CO 2 , dense CO 2 -CH 4 , dense CO 2 -H 2 , primordial H 2 ) is very poor and more work is greatly needed in these areas. In this review, I hope to not have only accurately surveyed the recent literature on the topic, but more importantly, I hope that the summary here could serve as a framework from which future missions could build upon. I have also strived to provide additional recommendations that can complement existing classical definitions and observational approaches. Criticisms against the HZ concept are popular (e.g., [ 270 , 271 , 284 ]), and it is this author’s opinion that the underlying criticism is rooted in skepticism against classical HZ assumptions that are perceived to be too geocentric (whether
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Geosciences 2018 , 8 , 280 37 of 48 or not they are) or rooted in unproven assumptions. Such critiques have some merit and partially motivate the writing of this work. Some argue that being clearer about HZ limitations using more precise language would solve some problems [ 271 , 284 ]. However, such problems are resolved once we realize that life may or may not be like what we see on our own pale blue dot. Funding: This research received no external funding. Acknowledgments: I acknowledge funding from the Earth-Life Science Institute (ELSI). I also enjoyed discussions with Aki Roberge, Shawn-Domagal Goldman, and James F. Kasting. I also thank the four anonymous reviewers for their many comments and suggestions. Finally, I thank Jacob Haqq-Misra for kindly providing Figure 4 . This work also benefited from enlightening discussions about planetary habitability and the origin of life at the recent ELSI “Puzzles and Solutions in Astrobiology” workshop. Conflicts of Interest: The author declares no conflict of interest. References 1. Kasting, J.; Whitmire, D.; Raynolds, R. Habitable Zones around Main Sequence Stars. Icarus 1993 , 101 , 108–128. [ CrossRef ] [ PubMed ] 2. Ramirez, R.M.; Kaltenegger, L. A Methane Extension to the Classical Habitable Zone. Astrophys. J. 2018 , 858 , 72. [ CrossRef ] 3. Heng, K. The Imprecise Search for Extraterrestrial Habitability. Am. Sci. 2016 , 104 , 146–149. 4. Benner, S.A.; Ricardo, A.; Carrigan, M.A. Is there a common chemical model for life in the universe?
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