Organism-substrate interactions and astrobiology: Potential, models and methods
Organism-substrate interactions and their products - biogenic structures - are important biosignatures on Earth. This study discusses the application of ichnology - the study of organism-substrate interactions - to the search for present and past life beyond Earth. Three main questions are addressed: (1) Why to look for biogenic structures (i.e. traces and ichnofabrics) beyond Earth? (2) What biogenic structures to expect on other planets, moons and asteroids? (3) How to study extraterrestrial biogenic structures? Review of terrestrial evidence highlights a set of properties that make traces and ichnofabrics important for the search of potential extraterrestrial life: trace fossils preserve the activity of soft-bodied organisms; biogenic structures are resilient to processes that obliterate other biosignatures (i.e. mechanical and chemical degradation, diagenesis, tectonism and metamorphism); traces are very visible biosignatures; traces indicate environment and behaviour; traces can be universal biosignatures, i.e., biosignatures ideally suited for detecting any type of life. A model of organism-substrate interactions beyond Earth is here proposed. Expected extraterrestrial traces are those that manifest behaviours that allow to maintain homeostasis: excavations, meandering traces and biodeposition structures. Most of the existing rovers and orbiters provide basic instruments for searching these traces. It is here suggested that the search for extraterrestrial biogenic structures by rovers would also benefit from artificial adjustable lighting, GPR, LiDAR, and drilling equipment with optical televiewer. In this study, open-access databases of rover and orbiter imagery have been searched for traces and ichnofabrics, but no unquestionable evidence of biogenic structures beyond Earth has been found besides those produced by humans. This sounds along the lines of the famous Fermi Paradox: if the universe is teeming with aliens, where are their traces? Results of this search show that habitable environments are not the only place to look for biogenic structures; non-habitable environments such as moons without atmosphere can favour the preservation of shallow-tier traces. The better preservation potential of traces compared to other biosignatures greatly widens the issue of planetary protection, including the interaction between astronauts or vehicles and the substrate may produce disturbances. Although this study highlights a new direction of study with the tools and concepts of ichnology, dialogue between the astrobiological and ichnological communities is needed to use its full potential and possibly answer one of the major questions of science: Does life exist beyond Earth?