Planetary Protection Research Articles

Astrovirology: how viruses enhance our understanding of life in the Universe.

Viruses are the most numerically abundant biological entities on Earth. As ubiquitous replicators of molecular information and agents of community change, viruses have potent effects on the life on Earth, and may play a critical role in human spaceflight, for life-detection missions to other planetary bodies and planetary protection. However, major knowledge gaps constrain our understanding of the Earth's virosphere: (1) the role viruses play in biogeochemical cycles, (2) the origin(s) of viruses and (3) the involvement of viruses in the evolution, distribution and persistence of life. As viruses are the only replicators that span all known types of nucleic acids, an expanded experimental and theoretical toolbox built for Earth's viruses will be pivotal for detecting and understanding life on Earth and beyond. Only by filling in these knowledge and technical gaps we will obtain an inclusive assessment of how to distinguish and detect life on other planetary surfaces. Meanwhile, space exploration requires life-support systems for the needs of humans, plants and their microbial inhabitants. Viral effects on microbes and plants are essential for Earth's biosphere and human health, but virus-host interactions in spaceflight are poorly understood. Viral relationships with their hosts respond to environmental changes in complex ways which are difficult to predict by extrapolating from Earth-based proxies. These relationships should be studied in space to fully understand how spaceflight will modulate viral impacts on human health and life-support systems, including microbiomes. In this review, we address key questions that must be examined to incorporate viruses into Earth system models, life-support systems and life detection. Tackling these questions will benefit our efforts to develop planetary protection protocols and further our understanding of viruses in astrobiology.

Development of a NASA roadmap for planetary protection to prepare for the first human missions to Mars

As part of planning for future space exploration, COSPAR (The Committee on Space Research) together with participating space agencies, organized and held interdisciplinary meetings to consider next steps in addressing knowledge gaps for planetary protection for future human missions to Mars. Beginning with the results of these meetings and earlier work by NASA, ESA, and COSPAR (e.g., Criswell et al., 2005; Hogan et al., 2006; Rummel et al., 2008) as a base the authors of this paper carried out a follow-on NASA planning activity to identify the necessary steps to be accomplished to close knowledge gaps. We identified significant overlap between the planetary protection needs and other sets of Mars preparation roadmaps (1) microbial monitoring requirements for crew health and medical systems, (2) studies of the microbiome of the built environment, (3) environmental control and life support systems (ECLSS), (4) waste management, and (5) planetary surface operations. In many cases, efforts to mature exploration class systems for Mars that are occurring in other domains can be leveraged with minor changes to address planetary protection gaps as well. In other cases, work planned for testing on the International Space Station (ISS) as an analog for crew Mars transit, or on the lunar surface as an analog for Mars surface operations can be used to close planetary protection technology and knowledge gaps. An overall strategic framework that combines these domains has the advantage of being more comprehensive, efficient, and timely for closing gaps.This approach has led to the development of a NASA roadmap for addressing planetary protection integrated with other related roadmaps. NASA's development and execution of the planetary protection is now viewed in an integrated way with related technology development and testing. Key features of the integrated capabilities roadmap include:•A focus on microbial monitoring needs for crewed systems overall, and the additional work needed to develop and demonstrate baseline vs. abnormal changes in microbial communities over time.•Integrated consideration of planetary protection and crew health requirements relevant to crew quarantine and science sample transportation.•Specific and targeted studies of microbial growth and biofilms in microgravity and lunar surface ECLSS systems.•Development of sterilization, cleaning, and waste handling technologies that meet habitation needs as well as planetary protection needs.

Multi-faceted metagenomic analysis of spacecraft associated surfaces reveal planetary protection relevant microbial composition.

The National Aeronautics and Space Administration (NASA) has been monitoring the microbial burden of spacecraft since the 1970's Viking missions. Originally culture-based and then focused 16S sequencing techniques were used, but we have now applied whole metagenomic sequencing to a variety of cleanroom samples at the Jet Propulsion Lab (JPL), including the Spacecraft Assembly Facility (SAF) with the goals of taxonomic identification and for functional assignment. Our samples included facility pre-filters, cleanroom vacuum debris, and surface wipes. The taxonomic composition was carried out by three different analysis tools to contrast marker, k-mer, and true alignment approaches. Hierarchical clustering analysis of the data separated vacuum particles from other SAF DNA samples. Vacuum particle samples were the most diverse while DNA samples from the ISO (International Standards Organization) compliant facilities and the SAF were the least diverse; all three were dominated by Proteobacteria. Wipe samples had higher diversity and were predominated by Actinobacteria, including human commensals Cutibacterium acnes and Corynebacterium spp. Taxa identified by the three methods were not identical, supporting the use of multiple methods for metagenome characterization. Likewise, functional annotation was performed using multiple methods. Vacuum particles and SAF samples contained strong signals of the tricarboxylic acid cycle and of amino acid biosynthesis, suggesting that many of the identified microorganisms have the ability to grow in nutrient-limited environments. In total, 18 samples generated high quality metagenome assembled genomes (MAG), which were dominated by Moraxella osloensis or Malassezia restricta. One M. osloensis MAG was assembled into a single circular scaffold and gene annotated. This study includes a rigorous quantitative determination of microbial loads and a qualitative dissection of microbial composition. Assembly of multiple specimens led to greater confidence for the identification of particular species and their predicted functional roles.

Microbial Protocols for Spacecraft: 3. Spore Monolayer Preparation Methods for Ultraviolet Irradiation Exposures.

Developing robust microbial survival models for interplanetary and planetary spacecraft requires precise inactivation kinetics for vehicle bioburdens. To generate such data, reliable protocols are required for preparing, testing, and assaying microbial cells or spores on simulated spacecraft materials. New data are presented on the utility of the liquid droplet protocol for applying Bacillus subtilis spores to aluminum coupons. Results indicate that low-density spore monolayers should be created between 2 and 5 × 106 spores per cm2 on individual coupons to prevent the formation of aggregates or multilayers of spores. Such aggregation or multilayers will interfere with the precision of characterizing the effects of UV irradiation on spore survival. Optimum spore monolayers are defined as spore monolayers without overlapping or clustered cells and in which all spores will receive UV photons during assays. The best spore monolayers were created with sterile deionized water (SDIW) on uncoated aluminum coupons, or with SDIW + Triton X-100 (at 0.5 × of the critical micellar concentration) on either uncoated Al-coupons or on Chemfilm Class 1A-coated coupons. The Triton X-100 surfactant improved the uniformity of the monolayers without affecting the sensitivity of the spores to UV irradiation. Furthermore, spore layers created at either 2 × 107 or 2 × 108 spores/cm2 created multi-stacking effects that clearly reduced the precision of the UV irradiation assays. A set of standardized protocols is suggested for spacecraft processing and planetary protection communities to permit directly comparing results from divergent labs.

Planetary protection: Updates and challenges for a sustainable space exploration

Planetary protection enables scientific return from solar system bodies investigations and at the same time protects life on Earth. As we continue to explore our solar system by landing machines and humans on other planets, we need to ascertain that we do not bring potentially dangerous material home to Earth or carry anything from Earth that may contaminate another planetary body and prevent scientific investigations.A Planetary Protection Policy has been developed by the Committee on Space Research (COSPAR), which provides a forum for international consultation in the area of space research. The COSPAR Planetary Protection Policy, and its associated requirements, is not legally binding under international law but is an agreed standard with implementation guidelines for compliance with Article IX of the Outer Space Treaty. States Parties to the Outer Space Treaty are responsible for national space activities under Article VI, including the activities of governmental and non-governmental entities.The current members of the COSPAR Panel on Planetary Protection are representatives from national space agencies and thematic experts from the science community of different countries (https://cosparhq.cnes.fr/scientific-structure/ppp). Other stakeholders, including the private sector, are invited to attend and present at the PPP meetings.The COSPAR PPP maintains and updates the COSPAR Planetary Protection Policy regularly, always reviewing all available scientific knowledge leading to updates to the policy, in particular as concerns the outer solar system and lunar exploration. Such updates are performed in a careful and balanced way to ensure that the right measures are envisaged to fulfil the rationales for planetary protection.

Draft Genome Sequences of Spacecraft-Associated Microbes Isolated from Six NASA Missions.

Whole-genome sequencing can be used to better understand and assess the functional abilities of microorganisms isolated from spacecraft hardware and associated surfaces for planetary protection (PP) purposes. We sequenced 191 isolates from 6 spaceflight missions with PP requirements and identified them using Illumina-based sequencing methods and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry.

The COSPAR planetary protection requirements for space missions to Venus

The Committee on Space Research's (COSPAR) Planetary Protection Policy states that all types of missions to Venus are classified as Category II, as the planet has significant research interest relative to the processes of chemical evolution and the origin of life, but there is only a remote chance that terrestrial contamination can proliferate and compromise future investigations. "Remote chance" essentially implies the absence of environments where terrestrial organisms could survive and replicate. Hence, Category II missions only require simplified planetary protection documentation, including a planetary protection plan that outlines the intended or potential impact targets, brief Pre- and Post-launch analyses detailing impact strategies, and a Post-encounter and End-of-Mission Report. These requirements were applied in previous missions and are foreseen for the numerous new international missions planned for the exploration of Venus, which include NASA's VERITAS and DAVINCI missions, and ESA's EnVision mission. There are also several proposed missions including India's Shukrayaan-1, and Russia's Venera-D. These multiple plans for spacecraft coincide with a recent interest within the scientific community regarding the cloud layers of Venus, which have been suggested by some to be habitable environments. The proposed, privately funded, MIT/Rocket Lab Venus Life Finder mission is specifically designed to assess the habitability of the Venusian clouds and to search for signs of life. It includes up to three atmospheric probes, the first one targeting a launch in 2023.The COSPAR Panel on Planetary Protection evaluated scientific data that underpins the planetary protection requirements for Venus and the implications of this on the current policy. The Panel has done a thorough review of the current knowledge of the planet's conditions prevailing in the clouds. Based on the existing literature, we conclude that the environmental conditions within the Venusian clouds are orders of magnitude drier and more acidic than the tolerated survival limits of any known terrestrial extremophile organism. Because of this future orbital, landed or entry probe missions to Venus do not require extra planetary protection measures. This recommendation may be revised in the future if new observations or reanalysis of past data show any significant increment, of orders of magnitude, in the water content and the pH of the cloud layer.

Distributed compact plasma reactor decontamination for planetary protection in space missions

This paper presents a proof-of-concept study establishing effectiveness of the Active Plasma Sterilizer (APS) for decontamination in planetary protection. The APS uses Compact Portable Plasma Reactors (CPPRs) to produce surface dielectric barrier discharge, a type of cold plasma, using ambient air to generate and distribute reactive species like ozone used for decontamination. Decontamination tests were performed with pathogenic bacteria (Escherichia coli and Bacillus subtilis) on materials (Aluminum, Polycarbonate, Kevlar and Orthofabric) relevant to space missions. Results show that the APS can achieve 4 to 5 log reductions of pathogenic bacteria on four selected materials, simultaneously at 11 points within 30 min, using power of 13.2 ± 2.22 W. Spatial decontamination data shows the APS can uniformly sterilize several areas of a contaminated surface within 30 min. Ozone penetration through Kevlar and Orthofabric layers was achieved using the CPPR with no external agent assisting penetration. Preliminary material compatibility tests with SEM analysis of the APS exposed materials showed no significant material damage. Thus, this study shows the potential of the APS as a light-weight sustainable decontamination technology for planetary protection with advantages of uniform spatial decontamination, low processing temperatures, low exposure times, material compatibility and the ability to disinfect porous surfaces.

GPU-based high-precision orbital propagation of large sets of initial conditions through Picard–Chebyshev augmentation

The orbital propagation of large sets of initial conditions under high accuracy requirements is currently a bottleneck in the development of space missions, e.g. for planetary protection compliance analyses. The proposed approach can include any force source in the dynamical model through efficient Picard–Chebyshev (PC) numerical simulations. A two-level augmentation of the integration scheme is proposed, to run an arbitrary number of simulations within the same algorithm call, fully exploiting high performance and GPU (Graphics Processing Units) computing facilities. The performances obtained with implementation in C and NVIDIA® CUDA® programming languages are shown, on a test case taken from the optimization of a Solar Orbiter-like first resonant phase with Venus.

Snow Surface Microbial Diversity at the Detection Limit within the Vicinity of the Concordia Station, Antarctica.

The Concordia Research Station provides a unique location for preparatory activities for future human journey to Mars, to explore microbial diversity at subzero temperatures, and monitor the dissemination of human-associated microorganisms within the pristine surrounding environment. Amplicon sequencing was leveraged to investigate the microbial diversity of surface snow samples collected monthly over a two-year period, at three distances from the Station (10, 500, and 1000 m). Even when the extracted total DNA was below the detection limit, 16S rRNA gene sequencing was successfully performed on all samples, while 18S rRNA was amplified on 19 samples out of 51. No significant relationships were observed between microbial diversity and seasonality (summer or winter) or distance from the Concordia base. This suggested that if present, the anthropogenic impact should have been below the detectable limit. While harboring low microbial diversity, the surface snow samples were characterized by heterogeneous microbiomes. Ultimately, our study corroborated the use of DNA sequencing-based techniques for revealing microbial presence in remote and hostile environments, with implications for Planetary Protection during space missions and for life-detection in astrobiology relevant targets.

The COSPAR Planetary Protection Policy for robotic missions to Mars: A review of current scientific knowledge and future perspectives

Planetary protection guidance for martian exploration has become a notable point of discussion over the last decade. This is due to increased scientific interest in the habitability of the red planet with updated techniques, missions becoming more attainable by smaller space agencies, and both the private sector and governments engaging in activities to facilitate commercial opportunities and human-crewed missions. The international standards for planetary protection have been developed through consultation with the scientific community and the space agencies by the Committee on Space Research's (COSPAR) Panel on Planetary Protection, which provides guidance for compliance with the Outer Space Treaty of 1967. In 2021, the Panel evaluated recent scientific data and literature regarding the planetary protection requirements for Mars and the implications of this on the guidelines. In this paper, we discuss the COSPAR Planetary Protection Policy for Mars, review the new scientific findings and discuss the next steps required to enable the next generation of robotic missions to Mars.

A Short History of Panspermia from Antiquity Through the Mid-1970s.

Panspermia is the philosophical proposition that the precursors of life are present in space and able to initiate life on reaching a suitable environment (especially the Earth). Only in the past century has the subject advanced from the lowly status of a dreamy hypothesis to a vibrant new science that is testable and observable. This history of panspermia presents the major figures associated with such hypotheses. It examines their motives, methods, and arguments, situated when possible in their historical context and culture rather than their impact on the present. From antiquity to the early modern period (1500-1800), the debates on the plurality of worlds and panspermia overlapped considerably. In the later modern period (1800 to the present), the narrative thread interweaves panspermia with the origin of life and the theory of evolution, and we can see authentic inputs from scientists rather than philosophers. In the interests of concision, I have omitted topics such as the Search for Extraterrestrial Intelligence, the exploration of the solar system by probes and landers, planetary protection (the inverse of panspermia), and the discovery of exoplanets. The historical literature is sometimes confusing: to correct for that, I have personally examined the original sources of every work cited rather than simply accepting the published findings of other scholars at face value.

Line Sampling Procedure for Extensive Planetary Protection Analysis

Exploration missions to other planets have to satisfy planetary protection requirements to limit the probability of impacts between mission-related objects and celestial bodies, with the goal of reducing the risk of contaminating them with biological material coming from Earth. The verification of these requirements can become a lengthy and computationally expensive task when addressed with common methods such as Monte Carlo simulations, as they involve analysing the interplanetary trajectories and the uncertainties associated with them for time spans up to 100 years, and estimating small probabilities with strict confidence levels. This paper presents novel improvements of the line sampling method, already introduced for the verification of planetary protection requirements as a way to estimate the impact probabilities more efficiently and with greater accuracy than achieved with standard Monte Carlo. These newly developed techniques are presented, with the aim of making the analysis with Line Sampling more effective, and providing more information about the distribution of impacts in the initial uncertainty distribution: an algorithm to identify the time intervals where most close approaches are clustered, and an algorithm to improve the determination of the main sampling direction and increase the accuracy of the probability estimation.

Centring Environmentalism in Space Governance: Interrogating Dominance and Authority Through a Critical Legal Geography of Outer Space

This article brings into conversation scholarship in law and the social studies of outer space around the question of the “uses” of outer space, the shortcomings of existing legal instruments, and the possibility to reform them to place environmental concerns at their core. Starting from the Outer Space Treaty (OST), and using legal geography and an environmental lens, we highlight the interconnections between de-territorializing outer space, the image of the province of (hu)mankind, and the seemingly consequent egalitarian principle of benefit sharing. By reading this language of the OST against its historical context, we note that these seemingly egalitarian clauses mask the persisting hegemony of older space powers in access to and benefits from outer space. In this context, environmental protection clauses originally proposed by Japan, which were marginalized in the drafting of the OST, remain marginal to many governance mechanisms. Even when we decentre the OST and look at multiple legal and governance frameworks of outer space, from Planetary Protection to the International Telecommunications Union, these different approaches reiterate a utilitarian view of space environments that ties them to their usefulness to exploration and exploitation. These mechanisms, while useful for de-homogenizing outer space, do not go far enough in proposing that environmental protection underpins all principles of space governance. One of the effects of this failure is that more technically able nations keep crowding the orbits with megaconstellations. We propose an interdisciplinary approach to understanding the complex interrelation between space governance, geopolitics, and concerns about the future of outer space environments. A critical legal geography of outer space provides us with a possibility to examine the role of the law in relation to both geographical imaginaries and historical contexts and advance discussions on the role and responsibilities of humans beyond the planet we inhabit.

Dry heat sterilization modelling for spacecraft applications.

Inactivation processes using heat are widely used for disinfection and sterilization. Dry heat sterilization of spacecraft equipment has been the preferred microbial inactivation method as part of interplanetary travel protection strategies. An antimicrobial model, based on temperature and exposure time based on experimental data, was developed to provide reliable sterilization processes to be used for interplanetary applications. Bacillus atrophaeus spores, traditionally used to challenge dry heat sterilization processes, were tested over a range of temperatures in comparison with spores of Bacillus canaveralius that have been shown to have a higher heat resistance profile. D-value and Z-values were determined and used to develop a mathematical model for parametric sterilization applications. The impact of the presence of a contaminating soil, representative of Mars dust, was also tested to verify the practical application of the model to reduce the risk of microbial contamination in such environments. The sterilization model developed can be used as an intrinsic part of risk reduction strategies for interplanetary protection. Forward and backward planetary protection strategies to reduce the risks of microbial contamination during interplanetary exploration and research is an important consideration. The development of a modern sterilization model, with consideration of microorganisms identified with higher levels of heat resistance than traditionally deployed in terrestrial applications, allows for the consideration of optimal inactivation processes to define minimum criteria for engineering design. The ability to inactivate living microorganisms, as well as to degrade biomolecules, provides a reliable method to reduce the risk of known and potentially unknown contaminants in future applications.

Effects of Desiccation and Freezing on Microbial Ionizing Radiation Survivability: Considerations for Mars Sample Return.

Increasingly, national space agencies are expanding their goals to include Mars exploration with sample return. To better protect Earth and its biosphere from potential extraterrestrial sources of contamination, as set forth in the Outer Space Treaty of 1967, international efforts to develop planetary protection measures strive to understand the danger of cross-contamination processes in Mars sample return missions. We aim to better understand the impact of the martian surface on microbial dormancy and survivability. Radiation resistance of microbes is a key parameter in considering survivability of microbes over geologic times on the frigid, arid surface of Mars that is bombarded by solar and galactic cosmic radiation. We tested the influence of desiccation and freezing on the ionizing radiation survival of six model microorganisms: vegetative cells of two bacteria (Deinococcus radiodurans, Escherichia coli) and a strain of budding yeast (Saccharomyces cerevisiae); and vegetative cells and endospores of three Bacillus bacteria (B. subtilis, B. megaterium, B. thuringiensis). Desiccation and freezing greatly increased radiation survival of vegetative polyploid microorganisms when applied separately, and when combined, desiccation and freezing increased radiation survival even more so. Thus, the radiation survival threshold of polyploid D. radiodurans cells can be extended from the already high value of 25 kGy in liquid culture to an astonishing 140 kGy when the cells are both desiccated and frozen. However, such synergistic radioprotective effects of desiccation and freezing were not observed in monogenomic or digenomic Bacillus cells and endospores, which are generally sterilized by 12 kGy. This difference is associated with a critical requirement for survivability under radiation, that is, repair of genome damage caused by radiation. Deinococcus radiodurans and S. cerevisiae accumulate similarly high levels of the Mn antioxidants that are required for extreme radiation resistance, as do endospores, though they greatly exceed spores in radioresistance because they contain multiple identical genome copies, which in D. radiodurans are joined by persistent Holliday junctions. We estimate ionizing radiation survival limits of polyploid DNA-based life-forms to be hundreds of millions of years of background radiation while buried in the martian subsurface. Our findings imply that forward contamination of Mars will essentially be permanent, and backward contamination is a possibility if life ever existed on Mars.

11.B. Round table: From Science to Policy Making: Lessons learned from ASPHER’s COVID-19 Task Force

Abstract ASPHER, the Association of Public Health Schools in the European Region, convened a COVID-19 Task Force (TF) at the start of the pandemic. The TF has involved over 60 experts, 30 member schools, more than 20 countries across four continents, supported by young professionals (YPs). The COVID-19 TF became a unique expert forum for mutual support, sharing, reviewing, and presenting evidence on epidemiological, technical, societal, and political dimensions of the COVID-19 pandemic across Europe. Working with European and national health authorities and non-governmental organisations (NGOs), it also prompted and supported the coordination of policy responses across the WHO European Region. Drawing on members’ collective knowledge and expertise, the TF produced a significant body of work that speaks to different public health aspects of the pandemic and gaps in responses. Since its inception in early 2020, the TF has produced more than 30 peer-reviewed publications, regular position statements and reports on topics from personal to planetary protection including: face masks, testing, tracking, vaccination, health inequalities, protecting vulnerable groups, safe schools, advocacy for wider social protection and global vaccine equity. The panel will reflect on key lessons learned over two years of TF work. First, that effective cross-country comparative work was made possible by the group's independence, interdisciplinarity, and high trust between its members. This mix of characteristics allowed for unencumbered weekly sharing of ideas, utilisation of data, insights available in local languages, and unique access to the front-line experience of members, those with positions in health authorities or advising national or regional governments. The second lesson is the importance of a flexible, bottom-up organisation, which allows the members to pursue individual research, education, and advocacy agendas while acting in concert. Cooperation between ASPHER Schools of Public Health existed before the pandemic, but the TF strengthened these relationships and made inter-school collaborations more frequent. Lesson three is a successful combination of policy advocacy, with shaping public health education and providing training opportunities for YPs. Thirteen YPs have been involved in the work of the COVID-19 TF since its inception, playing a critical role by preparing weekly situation reports, a horizon scanning exercise, surveys of ASPHER members, and contributing an early-career perspective to the groups’ outputs. Involving young public health professionals effectively in expert forums provides crucial training opportunities in knowledge transfer and leadership that should be more widely available. The panel will also reflect on how to set-up, build, scale-up, and sustain collaborations with wide geographic, cultural, linguistic, and political-administrative coverage to support the capacity and preparedness of public health institutions during future challenges. Key messages • The panel will recount key lessons from ASPHER’s COVID-19 Task Force: fostering independence, interdisciplinarity, and trust; a flexible, bottom-up organisation; and involving young professionals. • The panel will reflect on setting up collaborations across cultural and political-administrative boundaries to strengthen the advisory capacity of public health institutions during future challenges. Speakers/Panellists Alison Mc Callum University of Edinburgh, Edinburgh, UK John Reid Chester University, Chester, UK Rok Hrzic Maastricht University, Maastricht, Netherlands

Methods of planetary protection requirements fulfillment at the stages of assembly and testing of the SC with Martian landing module

Methods of planetary protection requirements fulfillment at the stages of assembly and testing of the SC with Martian landing module

Planetary Protection Obligations in Space Exploration Law and Ethics Regarding the Biological Contamination of Outer Space

Planetary Protection Obligations in Space Exploration Law and Ethics Regarding the Biological Contamination of Outer Space

Planetary Protection Assessment of Radioisotope Thermoelectric Generator (RTG)-Powered Landed Missions to Ocean Worlds: Application to Enceladus.

Landed missions to icy worlds with a subsurface liquid water ocean must meet planetary protection requirements and ensure a sufficiently small likelihood of any microorganism-bearing part of the landed element reaching the ocean. A higher bound on this likelihood is set by the potential for radioisotope thermoelectric generator (RTG) power sources, the hottest possible landed element, to melt through the ice shell and reach the ocean. In this study, we quantify this potential as a function of three key parameters: surface temperature, ice shell thickness (i.e., heat flux through the shell), and thickness of a porous (insulating) snow or regolith cover. Although the model we describe can be applied to any ocean world, we present results in the context of a landed mission concept to the south polar terrain of Saturn's moon Enceladus. In this particular context, we discuss planetary protection considerations for landing site selection. The likelihood of forward microbial contamination of Enceladus' ocean by an RTG-powered landed mission can be made sufficiently low to not undermine compliance with the planetary protection policy.