Prebiotic Chemistry Insights for Dragonfly. II. Thermodynamic Favorability of Nucleobases, Ribose, and Fatty Acids in Selk Crater on Titan

Abstract Saturn’s moon Titan is a prime destination for investigating prebiotic chemistry beyond Earth, particularly at impact crater sites where transient liquid water may have enabled aqueous reactions among organic molecules. Selk crater represents one such environment and is a primary target of NASA’s Dragonfly mission. Here, we present a thermodynamic assessment of nucleobases, ribose, and fatty acids formed from simple atmospheric precursors (HCN and C 2 H 2 ) within a Selk-sized aqueous melt pool across varying ammonia (NH 3 ) abundances. We find that ammonia acts as a chemical gatekeeper for molecular accessibility. In NH 3 -free systems, accessibility is restricted to adenine and butanoic acid. Once ≥1% NH 3 is introduced, all investigated molecular classes become thermodynamically accessible. Distinct molecular classes exhibit different NH 3 sensitivities: nucleobases, ribose, and C 2 –C 6 fatty acids peak at 1% NH 3 , whereas C 7 –C 12 fatty acids peak at 2% NH 3 . The modeled preference for pyrimidines versus purines and monotonic decline of fatty acid abundance with chain length qualitatively mirror patterns observed in carbonaceous meteorites and returned asteroid samples. We show how molecular distributions and cross-class correlations may provide indirect constraints on Selk’s past aqueous environment, help constrain past ammonia availability, and distinguish abiotic production from potential anomalies. By coupling thermodynamic predictions with an assessment of Dragonfly’s mass spectrometer capabilities, we posit concrete, testable predictions for evaluating Selk’s prebiotic potential in situ.