THE COSPAR PLANETARY PROTECTION POLICY FOR MISSIONS TO ICY WORLDS

Abstract

Recent discoveries related to the habitability and astrobiological relevance of the outer Solar System have expanded our understanding of where and how life may have originated. As a result, the Icy Worlds of the outer Solar System have become among the highest priority targets for future spacecraft missions dedicated to astrobiology-focused and/or direct life detection objectives. Therefore, it is both important and timely to review and update planetary protection policies for exploring Icy Worlds. Herein, we propose an update to the COSPAR Planetary Protection policy, focusing on the low-temperature limit for life as known on Earth. Established in 1999, the COSPAR Panel on Planetary Protection (PPP) consolidates, maintains, and updates the COSPAR Policy, ensuring dissemination to relevant stakeholders. With 24 members, including representatives from national space agencies and thematic experts, thePanel conducts regular reviews of scientific data through studies, consultations, workshops, and discus-sions at dedicated space exploration events. In 2022, a PPP subcommittee was formed to review guidelines for Icy Worlds and identify areas where the policy could be updated and improved. The outcome of the committee’s work was discussed and refined at several PPP meetings. Here we summarize the results of those discussions, reviewing the current knowledge and the history of planetary protection considerations for Icy Worlds as well as suggesting ways forward. We suggest to 1. Establish a new definition for Icy Worlds for Planetary Protection that captures the outer Solar System moons and dwarf planets like Pluto, but excludes more primitive bodies such as comets, centaurs, and asteroids: Icy Worlds in our Solar System are defined as all bodies with an outermost layer that is believed to be greater than 50% water ice by volume and have enough mass to assume a nearly round shape. 2. Establish indices for the lower limits of Earth life with regards to water activity (LLAw) and temperature (LLT) and apply them into all areas of the COSPAR Planetary Protection Policy. These values are currently set at 0.5 and -28◦C and were originally established for defining Mars Special Regions; 3. Establish LLT as a parameter to assign categorization for Icy Worlds missions. The suggested categorization will have a 1000-year period of biological exploration, to be applied to all Icy Worlds and not just Europa and Enceladus as is currently the case. 4. Have all missions consider the possibility of impact. Transient thermal anomalies caused by impact would be acceptable so long as there is less than 10−4 probability of a single microbe reaching deeper environments where temperature is >LLT in the period of biological exploration. 5. Restructure or remove Category II* from the policy as it becomes largely redundant with this new approach, 6) Give sample return from Icy Worlds careful consideration due to unknown extant life survivability and the uncertain duration of viable preservation in ice. A conservative approach suggests categorizing any sample return as restricted Earth return, aligning with current policy questions. These are recommendations and findings. Specific policy changes will be developed for validation by the COSPAR Bureau. The Panel continues its work on developing rigorous guidelines in consultation with the scientific community, national and international space agencies, and various stakeholders. Knowledge gaps identified during the study of Icy Worlds require further investigation and scientific input.