# The COSPAR Planetary Protection Policy for missions to Mars: ways forward based on current science and knowledge gaps

Karen Olsson-Francis, Peter Doran, Vyacheslav Ilyin, Francois Raulin, Petra Rettberg, Gerhard Kminek, María-Paz Zorzano Mier, Athena Coustenis, Omar Al Shehhi, Eleonora Ammannito, et al.

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## To cite this version:

Karen Olsson-Francis, Peter Doran, Vyacheslav Ilyin, Francois Raulin, Petra Rettberg, et al. The COSPAR Planetary Protection Policy for missions to Mars: ways forward based on current science and knowledge gaps. Life Sciences in Space Research, 2023, 36, pp.27-35. 10.1016/j.lssr.2022.12.001. hal-03855477

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Journal Pre-proof

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

Karen Olsson-Francis, Peter T. Doran, Vyacheslav Ilyin,
Francois Raulin, Petra Rettberg, Gerhard Kminek,
María Paz Zorzano Mier, Athena Coustenis, Niklas Hedman,
Omar Al Shehhi, Eleonora Ammannito, James Bernardini,
Masaki Fujimoto, Olivier Grasset, Frank Groen, Alex Hayes,
Sarah Gallagher, Praveen Kumar, Christian Mustin,
Akiko Nakamura, Elaine Seasly, Yohey Suzuki, Jing Peng,
Olga Prieto Ballesteros, Silvio Sinibaldi, Kanyan Xu,
Maxim Zaitsev

PII: S2214-5524(22)00101-8

DOI: https://doi.org/10.1016/j.lssr.2022.12.001

Reference: LSSR 427

To appear in: *Life Sciences in Space Research*

Received date: 3 October 2022

Revised date: 7 December 2022

Accepted date: 9 December 2022

Please cite this article as: Karen Olsson-Francis, Peter T. Doran, Vyacheslav Ilyin, Francois Raulin, Petra Rettberg, Gerhard Kminek, María Paz Zorzano Mier, Athena Coustenis, Niklas Hedman, Omar Al Shehhi, Eleonora Ammannito, James Bernardini, Masaki Fujimoto, Olivier Grasset, Frank Groen, Alex Hayes, Sarah Gallagher, Praveen Kumar, Christian Mustin, Akiko Nakamura, Elaine Seasly, Yohey Suzuki, Jing Peng, Olga Prieto Ballesteros, Silvio Sinibaldi, Kanyan Xu, Maxim Zaitsev, The COSPAR Planetary Protection Policy for robotic missions to Mars: a review of current scientific knowledge and future perspectives, *Life Sciences in Space Research* (2022), doi: https://doi.org/10.1016/j.lssr.2022.12.001

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2022 Published by Elsevier B.V. on behalf of The Committee on Space Research (COSPAR).

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

Karen Olsson-Francis<fnref n="1" />*, Peter T. Doran<fnref n="2" />, Vyacheslav Ilyin<fnref n="3" />, Francois Raulin<fnref n="4" />, Petra Rettberg<fnref n="5" />, Gerhard Kminek<fnref n="6" />, María Paz Zorzano Mier<fnref n="7" />, Athena Coustenis<fnref n="8" />, Niklas Hedman<fnref n="9" />, Omar Al Shehhi<fnref n="10" />, Eleonora Ammannito<fnref n="11" />, James Bernardini<fnref n="12" />, Masaki Fujimoto<fnref n="13" />, Olivier Grasset<fnref n="14" />, Frank Groen<fnref n="15" />, Alex Hayes<fnref n="16" />, Sarah Gallagher<fnref n="17" />, Praveen Kumar<fnref n="18" />, Christian Mustin<fnref n="19" />, Akiko Nakamura<fnref n="20" />, Elaine Seasly<fnref n="21" />, Yohey Suzuki<fnref n="22" />, Jing Peng<fnref n="23" />, Olga Prieto Ballesteros<fnref n="24" />, Silvio Sinibaldi<fnref n="25" />, Kanyan Xu<fnref n="26" />, Maxim Zaitsev<fnref n="27" />.

## Abstract

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

<footnote n="1">¹AstrobiologyOU, Faculty of Science, Technology, Engineering and Mathematics, The Open University, Milton Keynes, UK. k.olsson-francis@open.ac.uk</footnote>

<footnote n="2">²Department of Geology and Geophysics, Louisiana State, Baton Rouge, Louisiana, USA.</footnote>

<footnote n="3">³Institute for Biomedical Problems, Russian Academy of Sciences, Moscow, Russia.</footnote>

<footnote n="4">⁴Univ Paris Est Creteil and Université de Paris, CNRS, LISA, F-94010 Créteil, France.</footnote>

<footnote n="5">⁵German Aerospace Center (DLR), Institute of Aerospace Medicine, Radiation Biology Department, Research Group Astrobiology, 51147 Cologne, Germany.</footnote>

<footnote n="6">⁶European Space Agency, Noordwijk, The Netherlands.</footnote>

<footnote n="7">⁷Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain</footnote>

<footnote n="8">⁸LESIA, Paris Observatory, PSL University, CNRS, Paris University, 92195 Meudon Cedex, France.</footnote>

<footnote n="9">⁹Committee, Policy and Legal Affairs Section, Office for Outer Space Affairs, United Nations Office at Vienna, Austria.</footnote>

<footnote n="10">¹⁰UAESA.</footnote>

<footnote n="11">¹¹Italian Space Agency (ASI), Rome, Italy.</footnote>

<footnote n="12">¹²Office of Safety and Mission Assurance, NASA Headquarters, Washington, DC 20546, USA.</footnote>

<footnote n="13">¹³Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Science (ISAS), Kanagawa, Japan.</footnote>

<footnote n="14">¹⁴Nantes Université, Nantes, France.</footnote>

<footnote n="15">¹⁵Office of Safety and Mission Assurance, NASA Headquarters, Washington, DC 20546, USA.</footnote>

<footnote n="16">¹⁶Cornell University, Ithaca, NY 14853-6801, USA.</footnote>

<footnote n="17">¹⁷Institute of Earth and Space Exploration, Western University, London, Ontario, Canada.</footnote>

<footnote n="18">¹⁸Indian Space Research Organisation (ISRO), India.</footnote>

<footnote n="19">¹⁹Centre National des Etudes Spatiales (CNES), France.</footnote>

<footnote n="20">²⁰Dept. of Planetology, Graduate School of Science, Kobe Univ., Nada, Kobe, Japan.</footnote>

<footnote n="21">²¹Office of Safety and Mission Assurance, NASA Headquarters, Washington, DC 20546, USA.</footnote>

<footnote n="22">²²Dept. of Planetology, Graduate School of Science, Kobe Univ., Nada, Kobe, Japan.</footnote>

<footnote n="23">²³China National Space Administration, Beijing, China.</footnote>

<footnote n="24">²⁴Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain</footnote>

<footnote n="25">²⁵European Space Agency, Noordwijk, The Netherlands.</footnote>

<footnote n="26">²⁶Laboratory of Space Microbiology, Shenzhen Space Biotechnology Group, Chinese Academy of Space Technology, Beijing, China.</footnote>

<footnote n="27">²⁷Planetary Physics Dept., Space Research Inst. of Russian Acad. of Sciences, Moscow, Russia</footnote>

<footnote>* Corresponding Author</footnote>
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.

**1.0 Introduction**

The principal aim of the Committee on Space Research's (COSPAR) Planetary Protection Policy (hereafter referred to as the COSPAR Policy) is to: 1) ensure that scientific investigations of possible extra-terrestrial life forms, precursors, and remnants are not jeopardised and 2) Earth is protected from the potential hazard posed by extra-terrestrial matter carried by spacecraft returning from an interplanetary mission (COSPAR 2020).

Planetary protection measures relating to missions to Mars have received increased attention over recent years. Our understanding of Mars and terrestrial life has grown significantly, which has resulted in evolving views on the risk of harmful contamination associated with spaceflight missions that could confound search-for-life experiments and hence planetary protection guidelines (e.g., National Academies of Sciences, Engineering, and Medicine, 2021). Furthermore, the number of missions to Mars and the nations involved have increased considerably over the last forty years, with India, Japan, China and the United Arab Emirates joining Europe, USA and Russia. The USA and China have demonstrated capabilities to land and roam on the surface of Mars and more missions are in development to drill and return samples to Earth.

Commercial missions to Mars are also becoming more plausible; hence the landscape of Mars exploration is changing. Currently, the primary goals are scientific; however, endeavours are being planned that will allow experimentation with *in-situ* resource utilisation and human exploration of Mars (Liu et al., 2022), which will utilise smaller, affordable missions in performing high-priority science investigations to Mars (Barba et al., 2019). This would support ongoing plans from the National Aeronautics and Space Administration (NASA), the Chinese Academy of Sciences (CASA), the European Space agency (ESA) and some commercial companies, e.g., SpaceX or Blue Origin, to send humans to the planet by 2033, e.g., (Heldmann et al., 2021; National Academies of Sciences, Engineering, and Medicine, 2021; Xiaojun & Xiaowei, 2021), which will have extensive implications for planetary protection (see Spry et al., 2021 for detailed discussion).
The current scientific uncertainties mean that timely changes to the COSPAR Policy guidelines may be challenging in response to these missions. Further research and science-based tailoring of guidelines for specific missions would lead to a more nuanced approach to the guidelines that will enable and support future exploration.

## 1.1 COSPAR's Planetary Protection Policy

The international standards for planetary protection have been developed through consultation and discussion between the COSPAR Panel (hereafter referred to as the Panel), the scientific community and the national space agencies (COSPAR 2018). The resulting COSPAR Policy has a voluntary non-legally binding status, which allows for the Policy to be updated as scientific understanding develops. It is the only international standard on planetary protection for reference of space-faring nations to guide compliance with Article IX of the United Nations Outer Space Treaty of 1967 that stipulates against harmful contamination (Coustenis et al., 2019a, b). Under Article VI, States Parties to the Outer Space Treaty bear international responsibility for national activities (both governmental agencies and non-governmental entities) in outer space, including the moon and other celestial bodies. (UNOOSA, 2002). It is the prerogative of governments to determine how to implement the rights and obligations under the Treaty and how to authorize and supervise activities of non-governmental entities under its jurisdiction.

The technical aspects of the COSPAR Policy have been developed through discussions between the scientific community, the private sector and the national space agencies. To date, there are five categories of requirements, which are defined based on the mission's target, type, and scientific rationale. The categories outline the recommended measures to be applied to a mission (Coustenis et al., 2019a). While the Panel maintains the COSPAR Policy, compliance with Article IX is the responsibility of the State Parties authorising and overseeing the missions. The long-standing role of COSPAR in maintaining the COSPAR Policy as a reference for spacefaring nations and in guiding compliance with Article IX of the Outer Space Treaty was acknowledged by the United Nations Committee of the Peaceful Uses of Outer Space (UNCOPUOUS) in 2017 (A/72/20, para. 332, United Nations, 2017).

## 1.2 The COSPAR Panel on Planetary Protection

COSPAR established the Panel on Planetary Protection in 1999; the task statement of the Panel covers consolidation maintenance and updates to the COSPAR Policy, as well as dissemination to the appropriate audience (Kminek & Rummel, 2015). After restructuring in 2018, the Panel currently consists of 24 members, which include 12 representatives from national space agencies from various countries, including China, France, Germany, the
United Kingdom, India, Italy, Japan, the Russian Federation, Canada, the United Arab Emirates, the United States, the European Space Agency, and 9 thematic experts from the international scientific community. The Panel further has several ex-officio members from the National Academies of Science, Engineering and Medicine (NASEM), United Nations Office of Outer Space Affairs (UNOOSA), and COSPAR Committee on Industrial Relations. Other stakeholders, including the private sector and industry, are invited to attend and present at open meetings, and the Panel encourages such mutually beneficial dialogue with the private sector; hence the Panel seeks to increase discussion with private and commercial space actors (Coustenis et al., 2019 a,b). A complete list of members and documents related to the Panel's activities can be found on the website (COSPAR 2018).

To ensure that the COSPAR Policy is up to date, the Panel carries out regular reviews of the scientific data and community consensus that is underpinning the Policy through workshops, technical meetings, and discussions of the policy and implications at scientific and engineering congresses dedicated to spacecraft (Kminek & Rummel, 2015). As shown in Figure 1, the Panel evaluates the information, formulates an update to the Policy and makes recommendations to the COSPAR Bureau and Council regarding possible modifications to the policy and requirements. A recent example that resulted in an update of the Policy involved an issue that was highlighted by the managers of the Japan Aerospace Exploration Agency (JAXA)-led Martian Moons Exploration (MMX) mission. In this instance, the Panel identified the need for a category for unrestricted Earth return mission, which was based on scientific information that had demonstrated that samples returned from Phobos would not present a threat to Earth (Fujita et al., 2019; Kurosawa et al., 2019; Patel et al., 2019). The Panel also reviews outcomes from workshops and activities led by the community and national committees. For example, the COSPAR Policy requirements for the icy moons of the outer Solar System have been scrutinised as part of the Planetary Protection of the Outer Solar System (PPOSS) project, which was commissioned by the European Commission's H2020 Programme (Rettberg et al., 2019), and also as part a report: *Assessment of Planetary Protection Requirements for Spacecraft Missions to Icy Solar System Bodies* by the National Research Council (2012). Both reports made recommendations to the Panel, and the changes were incorporated in the COSPAR Policy published in 2020 (COSPAR 2020). Another example is the recent update of the COSPAR Policy requirements relating to the moon, which builds on the report: *Planetary Protection for Studying of Lunar Volatiles* (National Academies of Sciences, Engineering, and Medicine, 2020). The Panel conducted a dedicated community consultation, as previously described (COSPAR, 2021), that led to an updated COSPAR Policy specific to the moon with two new
sub-categories that reduced the reporting requirements in some cases while maintaining the necessary protection near the lunar poles (Fisk et al., 2021).

## 1.3 The COSPAR Policy Relating to Mars

The development of COSPAR Policy category-specific requirements for Mars have developed as our understanding of the planet has evolved. The current COSPAR guidelines reflect input from the scientific community, including several reports (e.g., Beaty et al., 2006; Kminek et al., 2010, National Academies of Sciences, Engineering, and Medicine, 2015; National Research Council, 1978, 1992, 2006, 2012, Rettberg et al., 2016, Rummel et al., 2014). The initial approach to planetary protection, employed in the 1970s, was the probabilistic approach to the likelihood of biological contamination, which aimed to limit the risk of contamination during the period of biological exploration to less than $1 \times 10^{-3}$ (Barengoltz, 2005). To address this requirement, the Viking mission employed a comprehensive planetary protection program that ensured that the probability of biological contamination of the spacecraft and launch vehicle was less than $1 \times 10^{-4}$. This required an extensive research program to establish further sub-allocations to flight hardware and mission phases, determine the impact probability, and develop procedures for the engineers, which carried programmatic risks (DeVincenzi et al., 1983).

Following the Viking mission, COSPAR changed its planetary protection approach, making the requirements more prescriptive. In 1984, the COSPAR Bureau and Council approved an update to the COSPAR Policy, decision no. 7/84 (COSPAR, 1984) established the criteria based on the type of mission (i.e., flyby, orbiter, probe or lander) and the target planet. This concept resulted in the definition of five categories based on our scientific knowledge at the time. For Mars only Category III and IV are relevant, and as scientific understanding of Mars and terrestrial life evolved, subsequent updates to the policy resulted in a definition of these categories, as defined below:

**Category III.** This applies to orbiter systems, including fly-by and gravity assist manoeuvres, and they must meet one of the following:

* The probability of impact on the surface of Mars by any aspect of the spacecraft (assembled in ISO Class 8 conditions) or is $<1 \times 10^{-2}$ for the first 20 years after launch or $<5 \times 10^{-2}$ for the period from 20 to 50 years after launch, for nominal and non-nominal flight conditions.

* The total bioburden of the spacecraft on Mars is less than $\le 5 \times 10^5$ bacterial spores.
**Category IV.** This is subdivided into three categories:

1) **Category IVa** is for lander systems not carrying instruments designed for searching for evidence of extant life on Mars.

* Bioburden levels on the spacecraft's surface are restricted to below $\le 3 \times 10^5$ spores with an average of $\le 300$ spores per square meter.

2) **Category IVb** is for lander systems that have a primary goal of searching for evidence of extant life on Mars. In addition to the above requirements:

* The total landed system is restricted to $\le 30$ spores or a sensitivity determined by the life-detection experiment.

* The sub-system involved in acquiring, delivering, and analysing the samples for the experiment must be sterilised to these levels. A method of preventing recontamination of the system and the material from being examined is in place.

3) **Category IVc** is for missions targeting Mars Special Regions. In addition to the above requirements, they must follow these additional requirements:

* Case 1: If the landing site is within the Special Region, there must be a bioburden level of $\le 30$ spores.

* Case 2: If the Special Region is accessed through horizontal or vertical mobility, either the entire lander has a bioburden level of $\le 30$ spores, or the subsystem that has direct contact with the Special Region shall be sterilised to these levels, and a method of preventing recontamination before accessing the site shall be provided.

The definition of Special Regions on Mars was introduced into the COSPAR Policy in 2002. The concept was developed during the Panel, and International Astronomical Union convened the workshop. The Panel defined a Special Region as an area within which terrestrial organisms are likely to proliferate or a region which is interpreted to have a high potential for extant martian life forms. They concluded that Category IV should be subdivided further to include Category IVc for missions to Special Regions (COSPAR 2003). NASA adopted the concept of Special Regions in 2005 and commissioned the NRC Preventing the Forward Contamination of Mars report to assess the requirements (National Research Council, 2006). The report recommended that all of Mars be considered a Special Region and hence have stringent Planetary protection requirements (similar to Viking bioburden levels) until additional data becomes available. However, due to the difficulties in employing
such measures, NASA requested that the Mars Exploration Program Analysis Group (MEPAG) explore Mars Special Regions to quantitatively clarify the definition and distinguish between special and non-special (Beaty et al., 2006).

Based on the NRC and MEPAG reports, in 2007, the Panel held a Colloquium on Mars Special Regions to evaluate the COSPAR Policy category-specific requirements. The Panel found that there was enough data to distinguish between special and non-special regions (contradicting the NRC report) and that lower limits for growth (as stated in the MEPAG report) were too high; for example, the water activity of 0.5 and a temperature of -25 °C, which was 5 °C lower than the limit recommended by the Special Region Science Analysis Group SR-SAG (Kminek et al., 2010). As part of this report, COSPAR declared that the definition should be evaluated every two years. To date, this is an ongoing COSPAR Panel agenda item (every two years), where new scientific data is presented and discussed during an open meeting. A series of workshops reviewed the definition of a Mars Special Region over the following years. A second report was prepared by MEPAG (Rummel et al., 2014) and reviewed by a NRC-ESF committee to assess the consistency with the current understanding of the martian environment and the physical and chemical limits for the survival and propagation of microbial life on Earth (National Academies of Sciences, Engineering, and Medicine (2015). Rettberg et al. (2016) detail the role of the COSPAR Panel and the scientific community in developing the guidance relating to Mars Special Regions.

Currently, a Special Region is defined as a region within which terrestrial organisms are likely to replicate. Any region interpreted to have a high potential for extant martian life forms. This is characterised by a temperature >-28°C, water activity >0.5 and a timescale of a geological or astronomical event that can affect the environment for 500 years (COSPAR, 2020; Kminek and Rummel, 2015).

In 2021, the National Academies of Science: Committee on Planetary Protection (CoPP) published the report Evaluation of Bioburden Requirements for Mars Missions (National Academies of Sciences, Engineering, and Medicine, 2021). This detailed study adds to the extensive literature that has been published to date. The report focused on microbial proliferation compared to survival, a similar approach previously used to define the Special Regions on Mars (e.g., Beaty et al. 2006, Kminek et al. 2010, Rummel et al. 2014). Caves were identified as a potential risk for planetary protection, as terrestrial microorganisms could be transported to potentially habitable sub-subsurface environments but remain largely
uncharacterised. The report outlined areas where knowledge was incomplete regarding the physiochemical conditions at the surface and subsurface of Mars.

Based on the sterilisation conditions at the surface of Mars, the CoPP focused on bioburden relaxation relating to potential access to sub-surface environments. It concluded that the bioburden requirements could be relaxed if the following criteria were met:

1a) Mission activities are restricted to the surface or

1b) For mission activities as deep as 1 m, the landing site must not show signs of ice in neutron or thermal data.

And

For either case above, the landing site should be a conservative distance from any subsurface access point. Distance to be determined considering wind conditions for the location and season and estimates of microorganism survival time in the surface environment.

*In-situ* bioburden control was considered as an alternative approach to pre-launch bioburden reduction methods. As mentioned in the paper, although this could be a more practical approach, there are also limitations to this approach, e.g., hidden surfaces, and verification of reduction. The report also outlined a flexible risk management approach that considers growing scientific knowledge and the specific type of mission to address the objectives of planetary protection, for example, taking into consideration the potential presence of extremophiles, the impact of post-landing bioburden reduction opportunities and the requirements of specific mission that can impact the risk of probable harmful contamination. This approach would allow for the utilisation of existing and new models, tools and methodologies; and incorporate the latest scientific data. However, to date, these suggestions have not been incorporated into the COSPAR guidelines, which will require broader community expert input and consensus.

**2.0 New considerations for Mars planetary protection**

Based on recent scientific data regarding Mars planetary protection requirements, a sub-committee of the Panel was formed to evaluate current scientific data regarding the bioburden control for Mars missions to not jeopardize search-for-life experiments while considering the need to support and enable future robotic Mars missions. The group focused on three key areas: 1) Biocidal effects of the martian environment, 2) water stability, and 3) transport of spacecraft bioburden. These areas were discussed in the context of survival of dormant cells (where cells are either dormant or in a state of maintenance) vs proliferation
(cells are actively defining) (National Academies of Sciences, Engineering, and Medicine. 2015; Rummel et al., 2014).

## 2.1) Biocidal effects of the martian environment

The surface has been considered biocidal (e.g., Pogoda de la Vega et al., 2007; Berry et al., 2010; Horneck et al., 2010; Schuerger et al., 2012; Wassmann et al., 2012), as conditions associated with the modern-day surface, including low temperature, desiccation, salinity and exposure to Ultra-Violet (UV) radiation have been deemed detrimental to life (Spry et al. 2021; Craven et al., 2021). UVC from 100 nm to 280 nm reaches the surface of Mars and is extremely biocidal with a maximal effect around 260 nm; however, UVB between 280 nm and 315 nm is also biocidal, but less than UVC ) on Earth, UVC is completely filtered by the atmosphere so it does not reach the Earth's surface). We note here that the nature of the surface oxidant on Mars is poorly understood and so its impact on microbiota (positive or negative) is not easily predicted until more is learned from *in situ* measurements.

Low temperature and water activity are known to impact microbial proliferation (discussed extensively in the context of Mars Special Regions (e.g., Rettberg et al., 2016; Rummel et al., 2014); yet also have a preservation effect (e.g., Rummel et al. 2014; Hallwsworth 2021). For example, microorganisms are routinely stored long-term under desiccated (lyophilisation) and freezing conditions in laboratory environments, e.g.-80°C (below the mean surface temperature on Mars). Furthermore, over geological timescales, microorganisms are known to survive in frozen and desiccated terrestrial environments (e.g., Santibáñez et al., 2018). Even the presence of perchlorates, first detected in the Martian regolith by NASA's Phoenix lander (Hecht et al., 2009), is not a stopper for the development of any microorganisms (Rzymski et al., 2022).

UV and ionising radiation are poised to be dominant stressors on the surface of Mars (Teodoro et al., 2018), as high levels are known to kill surface microorganisms effectively (e.g., Santos et al., 2013; Tapia-Guerrero, 2020). The UV intensity measured using the Rover Environmental Monitoring Station (REMS) on MSL has measured a maximum of UVA, UVB and UVC of 20 W/m² (Gómez-Elvira, 2014) with the UVC irradiance of between 2.5 and 3.5 W/m² (data generated from the NASA Planetary Data System: https://atmos.nmsu.edu/PDS/data/mslrem_1001/DATA_UV_CORRECTED/). This was obtained by downwelling UV irradiance on a flat surface facing the sky, in a Mars equatorial region, free from dust cover, and was dependent on the time of day and seasonal and orbital circumstances (Vicente-Retortillo et al. 2020). A UVC irradiance of between 2.5 and 3.5 W/m² effectively inactivates certain radiation-resistant microorganisms, e.g. *Deinococcus*
*phoenicis* (1P10ME), which were isolated from the Kennedy Centre (Vaishampayan, 2014). The average dosage of Galactic Cosmic Radiation (GCR) on the surface was measured to be 0.210 - 0.040 mGy/day (between the years 2012 and 2013), and most of the protons generate additional ionising energy within the top 10 cm (Hassler et al., 2014). Yet, within *mission-relevant* periods, the bactericidal effects of GCR are considered insignificant compared to the sterilising power of UVC at the surface, e.g., shown in *Deinococcus radiodurans* (Bauermeister et al., 2011).

The effect of UVC and ionisation radiation (similar to desiccation) causes similar damage directly by the absorption of the radiation by DNA. Furthermore, indirectly forming reactive oxygen species can damage DNA and other cellular components and lead to cellular death (Daly, 2009). The sterilisation effect is dependent on the type of microorganism, the metabolic state (vegetative, log or stationary growth phase, compared to dormant), or the physicochemical environment (e.g., nutrient availability, temperature, hydration state, aerobic or anaerobic conditions) (Craven, 2021). Previous work has shown that spores and radiation-resistant non-spore-forming bacteria can survive exposure to simulated solar UV radiation on the martian surface, with a few µm of dust, which emulates martian airborne particles, or dust can serve to reduce significantly, or absorb completely UV radiation (Cockell et al., 2005; Horneck et al., 2001, 2012; Llamas et al., 2022; Muñoz-Caro et al., 2006; Newcombe et al., 2005; Kawaguchi et al., 2020; Petters et al., 2010; Pogoda de la Vega et al., 2007; Rettberg et al., 2002; Schuerger et al., 2003; Smith et al., 2009; Tauscher et al., 2006).

The measurements made by the REMS/UV sensor showed that the dust deposition impacted the UV measurements and that only 70% of the UV flux at the surface was transmitted through the accumulated dust on the window of the sensor (Muñoz-Caro et al., 2006; Vicente-Retortillo et al. 2020). This is less intense than the UV irradiance experienced by spores during the EXPOSE-E mission outside the ISS that demonstrated in one of the microbiological experiments that approximately 50 % of *Bacillus subtilis* 168 and *Bacillus pumilus* SAFR-032 spores (in multilayers) were able to survive: 1) space vacuum, cosmic and extraterrestrial solar radiation, and temperature fluctuations (simulating travel to Mars) and: 2) a simulated martian environment that included atmospheric pressure and composition, and UV and cosmic radiation, for 1.5 years (Horneck et al., 2012). Desiccated vegetative cells of *Deinococcus radiodurans* (thickness 500 µm) exposed to low Earth conditions for three years during the Tanpopo mission showed similar survival levels to the ground controls (Kawaguchi et al. 2020).
Additional geological features could protect against microbial contamination, for example, subsurface cavities (National Academies of Sciences, Engineering, and Medicine, 2019). Mars is a geologically complex planet, and many environmental niches could protect against microbial contamination, e.g. cracks within the regolith or caves (e.g. Eppes et al., 2015; Sauro et al., 2020; Kearney et al., 2021). However, despite several decades of martian exploration with orbiters and landers, and a detailed characterisation of the landing site, there may not be enough knowledge at relevant scales to inform planetary protection decision making resulting in the need for a tailored process or an adaptive risk posture. For instance, images from the High-Resolution Imaging Science Experiment (HiRISE) and the MRO Context Camera (CTX) cover only 2-3% of the martian surface, and they are limited to the identification of horizontal cave openings >25 cm diameter (McEwen et al., 2007). The caves and subsurface openings on Mars remain primarily uncharacterised, although they are believed to be extensive (Sauro et al. 2020). However, the temperature is predicted to be approximately -25°C, which could support metastable water (Williams et al. 2010). On Earth, analysis has demonstrated that metabolically active microbial life is plausible (Carrier et al., 2020).

## 2.2) Circulation

Transport of terrestrial microorganisms to areas defined as Special Regions could lead to microbial proliferation and be a significant concern for planetary protection (Spry et al. 2021). Of particular concern is airborne transport since the martian atmosphere is not sufficiently dense to attenuate the UV radiation and protect suspended microbial cells. Though, cells could be transported through the air shielded from UVC radiation by local dust and/or by the total column of dust in the atmosphere (especially during dust storms, which are known to absorb UV efficiently).

On Earth, microbial cells associated with dust are known to travel for thousands of kilometres (Li et al. 2016; Mayol et al. 2014; Santibez et al. 2018), and aerial distribution of microorganisms has been demonstrated to occur in Mars analogue sites, e.g. the Atacama (Azúa-Bustos et al., 2019). On Mars, typical airborne dust aerosol size is on the order of 1 to 2 µm, except during the onset of global dust storms, when dust radii quickly exceed 3 µm, reaching maximal sizes of 8 µm (Lemmon et al., 2019). This is larger than the typical length of *Bacillus* spores, e.g., the mean value for *Bacillus subtilis* is 1.07 ± 0.09 µm (length) and 0.48 ± 0.03 µm (diameter) (Carrera et al., 2007). During a storm, the aerosol suspension can be greater than 3 µm for over 50 martian sols; whilst the atmosphere is clear of these large deposits, and only the smallest dust particles are present, the opacity of the atmosphere during such a storm can reduce UV fluences. For example, during the last
global dust storm monitored by REMS, the daily maximum UV radiation decreased by more
than 95% when the opacity reached 8.5 (Viúdez-Moreiras et al., 2019). The error of the UV
sensor does not allow quantifying the screening beyond this level. Still, generally, the
transmittance of the atmosphere is expected to decrease with exp(-tau), with tau being the
opacity. Hence, atmospheric processes may allow viable terrestrial contamination to be
transported from a vehicle to a Mars Special Region protected by a layer of a few grains of
protective dust or by the total column of dust.

To date, indirect dust accumulation rates have been inferred from several observations
acquired in-situ by every lander and rover that took measurements from the martian surface.
The *in-situ* studies of the MER rovers have estimated that airborne dust deposits at a rate of
0.004 ± 0.001 per unit of surface optical depth deposited per sol (martian solar day), which
means that after 30 sols, there is a layer on top of every flat, exposed surface that absorbs
12% of the incident UV radiation (Kinch et al., 2015). Furthermore, the dust deposition rate
on the spacecraft was studied at the Mars Pathfinder landing site, using indirect
measurements and estimated dust fall rates of about 20 - 45 µm per Earth year, consistent
with previous studies of dust deposition on Mars (Johnson et al., 2003).

Recent *in-situ* observations have demonstrated a continuous seasonal process of dust lifting and settling that affects the environment of the spacecraft. Dust at Gale Crater accumulates annually on the photodiodes of the UV sensor, absorbing up to 30% of the incident UV radiation (Vicente-Retortillo et al., 2018). This dust, which is in contact with the spacecraft, is lifted and transported elsewhere on the planet. During the onset of a dust storm, recent orbital and *in-situ* observations have shown that:

1) There is a rapid vertical transport of dust to high altitudes (up to 90 km) in the mid-to-high latitudes within just a few sols (Heavens et al., 2018; Vandaele et al., 2019).

2) Dust fronts slosh back and forth in a wide latitudinal range of up to 40° within one sol during major dust storms (Wu et al., 2020), which allows for a rapid interchange of materials (dust and water) between the polar region and the middle latitudes.

3) During one dust storm, the atmospheric haze blocking sunlight increased within ten sols from 1 to 8.5 at Gale Crater, and the daily maximum UV radiation decreased in that short period by more than 90% (Viúdez-Moreiras et al., 2019).

With the data-limited modelling done so far, dust circulation can be assumed to cover the
entire planet from the surface to 80 km altitude and from one hemisphere to the other,
including the polar caps (Heavens et al., 2018; Vandaele et al., 2019, Wu et al. 2020;
Broquet et al., 2021). This would allow for transport between all areas on Mars, including ice
deposits and recently discovered equatorial regions that contain 40% weight of water in the upper meter of the regolith, which could be associated with the presence of either water ice permafrost or large quantities of highly hydrated minerals (Mitrofanov et al. 2022). However, as previously discussed, metrological investigations are required to develop, test, and validate contamination transport models, which would require further missions or by adaptive risk posture and modelling (Race et al., 2019; Spry et al., 2021).

## 2.3) Stability of water on the surface of Mars

Observations from orbiting spacecraft and rovers provide evidence of an active hydrosphere on the surface of early Mars e.g. (Carr and Head 2010). Nowadays, liquid water is unstable, but brines may be present (Ramachandran et al., 2021). For example, REMS provides relative humidity, air temperature and ground temperature measurements showing that transient brines can exist within the uppermost 5 cm of the subsurface (Martin-Torres et al., 2015). Extensive studies have shown that deliquescent salts, e.g., sulfate, perchlorate and chloride within regolith, can absorb water from the atmosphere and form liquid brines through deliquescence and hydration (Ramachandran et al., 2021). These salts are abundant on Mars; for example, measurements from Phoenix and MSL showed that known abundances of deliquescent salts constitute up to ~0.5 wt % of the Martian regolith (Hecht et al., 2009; Bish et al., 2013; Glavin et al., 2013; Clark and Kounaves, 2016). For example, gypsum and bassanite have been detected, which can hydrate under present day conditions absorbing atmospheric water (Vaniman et al., 2018). A recent study has also suggested that in analogue environments on Earth, microorganisms are able to extract water from gypsum (Huang et al., 2020).

Experimental data have shown that meta(stable) brines may exist from the equator to high altitude for a small percentage of the year, up to six consecutive hours on the surface and in the shallow subsurface (Rivera-Valentin et al., 2020). Although, more recent work has demonstrated that; 1) metastable brines can also exist for shorter periods in open systems on the source (supported by salts or a salt/mix regolith); 2) pure liquid water can form spontaneously and persist during evaporation during short periods of a few hours when the near-surface is saturated (e.g. Vakkada-Ramachandran et al. 2021), there is still limited knowledge about the kinetics of phase transition (Rivera-Valentin et al., 2021). Although the conditions of these metastable brines have been deemed inhospitable to life (Rivera-Valentin et al., 2020), the potential for these brines to be habitable cannot be ruled out, as discussed (Carrier, 2020). On Earth, brines impact growth due to low water activity, high ionic strength, extreme chaotropicity, and/or multiple extremes (Hallsworth, 2021 and references within). Although this may hinder growth, there is no evidence that the conditions
would kill/eradicate microbial life in certain brines, e.g., sulfate salts. Experimental work has recently shown that cells of *Halomonas* sp. str. HL12 survived and grew after several cycles of desiccation (Cesur et al., 2022).

Further work is needed to understand the feasibility of terrestrial contamination to survive and proliferate under different simulated environmental settings on Mars. This is prudent, as our understanding of water distribution on Mars is evolving rapidly. For example, new research about the upper meter of the regolith, based on the latest observations from the ExoMars Trace Gas Orbiter (TGO), has shown the existence of large patches of up to 40.3 weight % water equivalent hydrogen in the central part of the equatorial canyon Valles Marineris (Mitrofanov et al., 2022). This demonstrates the existence of vast deposits of ice or hydrated salts near the equator within the upper meter of the regolith that has remained hidden until now.

## 3.0) Discussion

Our understanding of Mars as a planet and terrestrial microorganisms which has grown extensively since the initial COSPAR Policy on Planetary Protection was formulated in the 1970s. The current guidelines reflect input from the scientific community, including several reports (e.g., National Academies of Sciences, Engineering, and Medicine 2015, National Research Council 1978, 1992, 2006, 2012). Although the science underpinning the policy is advancing, as highlighted in more recent reports (e.g. National Academies of Sciences, Engineering, and Medicine 2021, Spry et al. 2021) and in this paper, there are still several knowledge gaps that need to be addressed before they can be directly applied to accommodate the interest of the user. In brief, these knowledge gaps fall within three main themes, all of which will benefit from more measurements by space missions and ground-based observations:

**a) Biocidal effects:** Further verification modelling and experimental work is required to understand the impact of multiple biocidal effects on terrestrial contamination, which is required for the consideration of *in-situ* bioburden reduction. A detailed discussion of the knowledge gaps regarding the biocidal effects was carried out in a COSPAR workshop on refining planetary protection requirements for human missions and published in a white paper, which highlights the importance of carrying out laboratory simulation experiments, analogue work, and studies onboard the international space station to understand microbial survival (Kminek et al., 2017; Race et al., 2019; Spry et al., 2021). Although the report was focused on human missions, understanding the impact of biocidal effects on terrestrial clean
room contamination will allow us to develop the models to predict the levels of forward contamination and the decay rates of terrestrial contamination, which is important for robotic missions. This will could further allow us to understand the spatial and temporal contamination potential of spacecraft in real “test-as-you-fly” conditions, which could potentially reduce initial bioburden levels of 300 spores/m² on spacecrafts to the 0.03 spores/m² levels required for exploration of special regions. We note that the nature of the surface oxidant on Mars is largely unknown and its influence on potential life on Mars can't be modelled without dedicated *in situ* observations.

**b) Contamination transport model:** Further meteorological investigations are required to develop a predictive model with a reasonable confidence level. As discussed in the 2018 COSPAR working Report, Refining Planetary Protection Requirements for Human Missions (Race et al., 2019), target meteorological studies are required to develop and validate contamination models for specific landing sites. These transport models will be useful to define forward contamination spheres of influence, for example, could directly feed into mission operation planning, e.g. closeness of the spacecraft to a special region without posing a contamination risk.

**c) Mars environmental conditions:** Further understanding of the distribution of habitable conditions on the surface/sub-surface of Mars. This would require continued mapping and meteorological modelling to assess the habitable conditions, e.g., water availability, temperature, and protection from radiation. Experimental work is also required to further understand the role of salts as a water sink and source, as previously discussed by Rivera-Valentin et al., 2020. Granularity on the resolution of microbial (habitability) potential on the surface and sub-surface is required to further assess the risk of forward contamination, in particular special regions. More detailed environmental modelling will define the forward contamination sphere of influence thereby helping to define operational limits of exploration zones.

**4.0) Future prospective**

The focus of this paper is specifically on the COSPAR Policy requirements for the next generation of robotic missions (the requirements for human missions have been extensively discussed elsewhere e.g. Spry et al., 2021). We do not address concerns that have been raised regarding the spore measurements and alternative bioburden monitoring techniques, as this is beyond the scope of this paper which focuses on Mars surface conditions and habitability. Below, we discuss requirements that will enable future robotic missions.
**Identification of knowledge gaps.** Extensive reports involving the COSPAR Panel, national space agencies and the planetary protection community have recognized that knowledge gaps exist in the underpinning science of the COSPAR Policy. For example, further understanding of the biocidal effects at the surface of Mars is required before *in-situ* bioburden reduction is considered as a plausible option for sterilization. A key step forward is to capture the knowledge gaps and allow the broader scientific community to evaluate and assess these gaps. Whilst it is not possible to close every knowledge gap, it is plausible to develop a responsible and transparent process to meet the COSPAR objectives whilst enabling missions. Future exploration will result in further scientific understanding of Mars and the Solar System, which may result in additional questions; hence, we may never have an ideal or “perfect” knowledge base. Building on this review, we will work with the space agencies, the scientific communities and other stakeholders to develop a planetary protection roadmap to address these gaps, as we did already for the Moon.

**b) A tailored approach.** This will allow proposed missions to be evaluated on a case-by-case basis based on a responsible and scientifically valid approach. COSPAR has leveraged a process for mission-specific tailoring based on a technical sound and responsible case being made by the mission on a case-by-case basis. A recent example, of tailoring the guidelines for a mission is the MMX mission (as discussed in Section 1.1), which identified the need for a category for unrestricted Earth return mission, based on scientific information that had demonstrated that samples returned from Phobos would not present a threat to Earth. This tailoring process could serve as a predictable, replicable, and transparent model to adopt in COSPAR Policy.

**5.0 Conclusions**

The COSPAR Panel on Planetary Protection will continue to work with the different national and international space agencies, the scientific community, and other stakeholders (e.g., the private sector and industry) to develop a roadmap for coordinating research activities addressing the identified knowledge gaps. This will include further characterisation of the biocidal effects at the surface of Mars, which needs to be addressed before *in-situ* reduction can be considered as an approach for bioburden control for robotic missions. Updates are dependent on various stakeholders acting on the research roadmap, and funding and conducting the research. Once information will become available either from ongoing and future missions or from the identified research activities, the COSPAR Panel will carefully review the new input and update the guidelines, if appropriate. Simultaneously, the Panel will review how the experience with the MMX mission can help define a transparent and
responsible process to enable specific mission concepts that advance planetary protection standards and practices. The process of a flexible risk management approach, as the Panel has applied to the MMX mission, that considers evolving scientific knowledge to formulate categorisation or any planetary protection requirements could be applied to a specific type of mission. In parallel, activities such as workshops, studies, community surveys, and meetings, co-sponsored by COSPAR will be an important role in updating the Policy.

## Acknowledgements

We thank John Rummel, Andy Spry and Bette Siegel for their discussions regarding the Policy and for activities related to enhancing our scientific knowledge as concerns planetary protection requirements. Karen Olsson-Francis was supported by a Research England, Expanding Excellence in England grant. Athena Coustenis was supported by the Centre National de Recherche Scientifique (CNRS) and the Centre National d'Etudes Spatiales (CNES). Francois Raulin was supported by a grant from CNES, the French Space Agency. María Paz Zorzano Mier was supported by grant PID2019-104205GB-C21 funded by MCIN/AEI/ 10.13039/501100011033. Petra Rettberg was funded by Deutsches Zentrum für Luft- und Raumfahrt German Aerospace Cente (DLR).

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Figures
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Figure 1: A schematic of the process required for updating the Policy (adapted Kminek and Rummel (2015) Rettberg et al., 2016). The process is initiated by a new consideration, this is either a peer-reviewed scientific paper, a request from a private or public entity, or a recommendation from an agency advisory group (this may lead to a national scientific institute or international scientific union study). The recommendation is discussed during a Panel business meeting at a COSPAR scientific assembly or a dedicated COSPAR Panel colloquium. Based on these discussions, the Panel makes a recommendation, which is voted on by the COSPAR Bureau and Council. If the recommendation is endorsed the Policy is updated.

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Figure 2: Image of openings (examples shown by arrows) to the subsurface on Mars which would not be detected from orbit (image from Curiosity, sol 712).
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