Abstract EANA2025-170

Icy Worlds, such as Europa, Enceladus, Ganymede, Titan, and Ceres, are recognized as prime astrobiological targets due to the potential of hosting subsurface aquifers, active geology, and/or even (precursors) of microbial life. Protecting these environments from terrestrial contamination poses unique challenges, which led the COSPAR Planetary Protection Panel to propose an updated framework [1]. In support of scientific advancement and as a contribution to inform future policy updates, ESA tasked the European Science Foundation with evaluating the proposed revisions considering current scientific understanding and operational constraints.

The review, carried by a dedicated interdisciplinary expert group, assessed contamination risks, microbial survival thresholds, environmental transport mechanisms, and knowledge gaps. It supports extending the "Special Regions” concept to Icy Worlds. Special Regions are defined by environmental conditions exceeding thresholds of –28°C (lower limit temperature, LLT) and water activity of 0.5 (LLAw) [2]. The study also recognises that specific geomorphological features may indicate increased chance of connectivity to subsurface environments that meet this definition. While new studies suggest that metabolic activity may persist below these thresholds under certain conditions, the review recommends retaining current thresholds for replication but clearly distinguish it from metabolic activity or dormancy. The review calls for periodic re-evaluation as new empirical data emerge, especially from long-term survival studies under relevant stressors.

The report also proposes a refined definition of Icy Worlds as bodies that likely possess a water-ice rich global layer and with the potential for sufficient heat to support persistent liquid water during their planetary history. This proposed new definition would address concerns with ambiguous shape and surface composition criteria in the current definition [1] and ensures that bodies like Titan and Ceres are clearly included without having to rely on additional addendums to the definition.

Contamination risks must account for dynamic processes in icy shells. Updated fracture and transport models suggest terrestrial contamination could reach subsurface oceans within shorter timescales than the 1000-year period of biological exploration, especially through high-permeability zones (e.g., Enceladus’ tiger stripes or Europa’s chaos terrains). The review emphasizes organic contamination as a critical concern due to their potential to obscure detections of biosignatures or molecules relevant for prebiotic chemistry in regions targeted by future exploration. The review recommends that missions to Icy Worlds should default to Category III, requiring comprehensive organic and inorganic inventories using advanced spectroscopic methods, with data archived for future reference.

Significant knowledge gaps remain, particularly for Uranian moons and remote icy worlds. Therefore, missions to Icy Worlds should adopt probabilistic and adaptive landing site selection frameworks, precision landing technology and integrate geomorphological, thermal, and compositional data from ongoing and future missions to refine environmental models and contamination assessments.

References: [1] Doran et al. 2024. The COSPAR planetary protection policy for missions to Icy Worlds: A review of history, current scientific knowledge, and future directions. Life Sciences in Space Research 41, 86–99. [2] Rummel et al. 2014. A New Analysis of Mars “Special Regions”: Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2). Astrobiology 14, 887–968.