HABITABILITY METRICS for ASTROBIOLOGY. Abel Méndez1

Astrobiology Science Conference 2017 (LPI Contrib. No. 1965) 3217.pdf

HABITABILITY METRICS FOR ASTROBIOLOGY. Abel Méndez1, Dirk Schulze-Makuch2, and Guillermo Nery3, 1Planetary Habitability Laboratory, University of Puerto Rico at Arecibo ([email protected]), 3Technical University Berlin, Germany ([email protected]), 3University of Puerto Rico at Arecibo ([email protected]).

Introduction: On Earth, habitability is generally izing a habitat and describing its complexity. None of correlated with the presence of life, but this will not these indices alone intend to summarize all the habita- necessarily be the case for all habitable planets. There bility details of a system as sometimes assumed. might be many planets on which habitable conditions Habitability and Biosignatures: How habitability for Earth-like life prevail, but where the origin of life, and biosignatures are correlated is a standing problem for which the conditions are expected to be much more in astrobiology. For example, planets with high ESI constrained, never took place. Also, a biosignature on a (i.e. more similar in size and insolation as Earth) or non-habitable planet could be interpreted as a false- PHI should be more likely to show the presence of positive or suggest a type of life with which we are not gases in their atmosphere (i.e. oxygen and methane familiar, among other explanations. Therefore, it is together), which can be interpreted to have a biological necessary to include quantitative measures of habita- cause. It will take many exoplanet observations in the bility to properly assess not only the distribution of following decades to test this and similar hypothesis. potentially habitable worlds, but also the significance Meanwhile, it is necessary to develop the mathematical of any biosignature detections. framework to explore potential correlations between Habitability metrics is an emerging field within as- habitability and biosignatures (Fig. 1). trobiology and exoplanet science [1]. One of the most frequent questions is how to measure habitability. Since 2007 astrobiologists have been proposing practi- cal definitions of habitability, but there is no consensus yet [2,3,4]. However, the basis for defining and meas- uring habitability was established more than three dec- ades ago in the early 80’s by ecologists [5,6,7]. They are formally called habitat suitability and not habitabil- ity in ecology. However, the astrobiology community has not implemented them in previous studies.

The First Habitability Index: The Habitat Suita- Fig. 1. Hypothetical correlation between a biosignature bility Index (HSI) was developed by the US Fish and and habitability proxy. Each proxy consists of one or a Wildlife Service in 1980 as part of their Habitat Evalu- combination of many environmental variables. ation Procedures (HEP) [5]. The index involves using The Planetary Habitability Laboratory (PHL) of the the same key habitat components to compare the ratio University of Puerto Rico at Arecibo organized the 2nd of existing with optimum habitat conditions for a spe- Earth-like Worlds Workshop: Habitability Metrics cies or community of interest. The HSI value obtained (phl.upr.edu/ew2) to continue the development of hab- by this ratio becomes an index of relative carrying ca- itability frameworks extensible to the astrobiology pacity, a range between zero and one. Therefore, the community, from microenvironments to exoplanets. HSI is intended to be linearly related with carrying This presentation will report the final results of this capacity. workshop. The HSI is not extendable to the astrobiology field References: [1] Schulze-Makuch, D., Méndez, A., in its current form. The spatial and temporal scale (e.g. Fairén, A. G., et al. (2011) Astrobiology, 11, 1041. microenvironments to planetary scales) together with [2] Shock, E. L., & Holland, M. E. (2007) Astrobiolo- the life forms under consideration (e.g. simple or com- gy, 7, 839. [3] Hoehler, T. M. (2007) Astrobiology, 7, plex life) makes the application difficult. Previous ef- 824. [4] Cockell, C. S., Bush, T., Bryce, C., et al. forts, like the Earth Similarity Index (ESI) and the (2016) Astrobiology, 16, 89. [5] US Fish and Wildlife Planetary Habitability Index (PHI), were inspired by Service (1980) www.fws.gov/policy/ESMindex.html. the HSI [1]. However, the ESI and PHI are comple- [6] Roloff, G. J., & Kernohan, B. J. (1999) Wildlife menting indices (i.e. similarity and habitability) that Society Bulletin (1973-2006), 27(4), 973–985. [7] Hir- are only applicable to planetary scales, particularly zel, A. H., & Le Lay, G. (2008) Journal of Applied exoplanet for which little information is available. In Ecology, 45(5), 1372–1381. practice, following the experience with the HSI, we Acknowledgments: This work is supported by the should end up with a library of many indices, each PHL and the NASA Astrobiology Institute (NAI). addressing a different scale and objective of character-