Conceptual challenges in astrobiological analog environments and paths toward resolution

Juliana Campos Meurer, Jorge Quillfeldt, Milton de Souza Mendonça

Analog environments are terrestrial environments that resemble extraterrestrial sites; this concept originated in space sciences for defining methods to study planetary geology, training for missions and testing research instruments. As biological research became integrated into these investigations - through studies of limits for life and possible biosignatures - and astrobiology grew as a discipline, the term expanded in scope and nowadays analogy underlies much of the epistemic foundation of astrobiology. This expansion, however, also introduced conceptual inconsistencies: criteria for defining and evaluating analogs remain unclear, the term “analog” is increasingly misused, and analog environments are often treated as equivalent to extreme environments. Here we examine these conceptual issues and analyze them historically, highlighting the relevance of analogical-reasoning frameworks to resolve some of the indicated problems: it is possible to distinguish correct, incorrect, strong, weak and fruitful analogies and also apply principles such as transparency, adaptability and systematicity. We argue that a target must always be explicit; therefore, terrestrial organisms cannot be considered analogs for hypothetical extraterrestrial life, although they may serve as analogs for confirmed past terrestrial life. We clarify the differences between AAEs and extreme environments: AAEs require a defined target and occur only on Earth, while extreme environments are defined by obligatory extreme conditions and may occur anywhere in the Universe. Finally, we propose a refined and slightly broader definition of astrobiological analog environments (AAEs) as Earth-based environments that exhibit characteristics comparable to those of a target locale - whether another planetary body or Precambrian Earth conditions - whose selected features influence extant or fossilized biological components and allow researchers to develop an astrobiological understanding.