Evaluating Biosignatures for Life Detection

Abstract

Conceptual frameworks are developed for evaluating the ability of different biosignatures to provide evidence for the presence of life in planned missions or observational studies. The focus is on intrinsic characteristics of biosignatures in space environments rather than on their detection, which depends on technology. Evaluation procedures are drawn from extensive studies in decision theory on related problems in business, engineering, medical fields, and the social arena. Three approaches are particularly useful. Two of them, Signal Detection Theory and Bayesian hypothesis testing, are based on probabilities. The third approach is based on utility theory. In all the frameworks, knowledge about a subject matter has to be translated into probabilities and/or utilities in a multistep process called elicitation. We present the first attempt to cover all steps, from acquiring knowledge about biosignatures to assigning probabilities or utilities to global quantities, such as false positives and false negatives. Since elicitation involves human judgment that is always prone to perceptual and cognitive biases, the relevant biases are discussed and illustrated in examples. We further discuss at which stage of elicitation human judgment should be involved to ensure the most reliable outcomes. An example, how evaluating biosignatures might be implemented, is given in the Supplementary Information.

Chirality in Light–Matter Interaction

“…This homochirality is a striking chemical signature of all life on Earth. [348,349] Though its origin and significance are not completely understood, homochirality may be a prerequisite for any kind of aminoacid-based life: although artificial proteins involving a mixture of l-AAs and d-AAs can fold into structures resembling ordinary homochiral proteins, these structures have fewer internal hydrogen bonds and consequently are less thermodynamically stable. [350] Thus, natural selection for the more stable l-AA dominated proteins could have eventually led to the homo chirality seen in all modern organisms.…”

Technosignatures: Frameworks for Their Assessment

“…In astrobiology, Bayesian techniques are being increasingly employed in a diverse array of settings ranging from Earth to exoplanets (e.g., Waltham 2011;Spiegel & Turner 2012;Walker 2017;Chen & Kipping 2018;Balbi & Grimaldi 2020;Kipping 2020;Truitt et al 2020). For instance, they have been advocated and/or used to gauge the viability of potential biosignatures (Lorenz 2019a(Lorenz , 2019bPohorille & Sokolowska 2020;Affholder et al 2021), to wit, markers of extraterrestrial biology. In this context, the significance of Bayesian frameworks for assessing the plausibility of in situ and exoplanet biosignatures was underscored in impactful and wide-ranging explorations of the subject by Catling et al (2018), Walker et al (2018), and Meadows et al (2022).…”

Europan Molecular Indicators of Life Investigation (EMILI) for a Future Europa Lander Mission

“…Such samples would be investigated for any potential biosignatures, as direct evidence of organisms or indirectly indicating possible pre-biotic or biogenic processes in the ocean, that have survived transport to the surface and its associated damaging radiation conditions. While pre-defining what is or is not a Europan biosignature may appear as a daunting task, recent directions in astrobiology have begun to put potential evaluation criteria on a rigorous footing (National Academies of Sciences, Engineering, and Medicine (NASEM), 2019; Neveu et al, 2018;Pohorille and Sokolowska 2020).…”