Planetary Protection requirements should address pollution from chemicals and materials

Governments and private entities are currently developing plans to send humans to the surface of Mars (1, 2). Transporting humans to Mars, sustaining them, and enabling their activities on the surface will require landing systems, housing units, vehicles, operational gear, and protection suits, among other equipment. Along with unavoidable microbial contamination (3), human support and exploration systems will deliver a plethora of novel chemicals and materials to the planet (4). Humans on Mars will thus inevitably pollute the environment with anthropogenic chemicals and novel materials in a range of sizes, shapes, and composition, as well as via the degradation products of these chemicals and materials (5, 6). However, thus far, there has been little research on the impacts of anthropogenic chemicals and materials in the context of Planetary Protection, nor has there been an effort to compose regulations or agreements that consider these impacts. This needs to change.

NASA’s Curiosity rover, which arrived on Mars in August 2012, is among the many Mars missions that contribute chemical and material pollution to the Red Planet. Image credit: NASA/Jet Propulsion Laboratory (JPL)–California Institute of Technology (Caltech)/Malin Space Science Systems.

On Earth, human activities result in both intentional and unintended release of chemicals and materials into the environment, and such pollution has caused substantial harm to living organisms and disrupted abiotic processes (7–9). In response, various international agreements have been established to limit or stop the use of certain chemicals and materials and thereby curb their environmental release; e.g., ref. 10. On Mars, there is already evidence that humanity’s robotic explorers of the past decades have contaminated the environment with chemicals and engineered materials in an uncontrolled manner (Fig. 1).

Fig. 1.

Anthropogenic debris from various robotic rover missions on Mars (A–E) and plausible emission routes of chemicals and materials from a crewed Martian base (F). (A) Remnants of the Perseverance rover’s backshell and parachute in Jezero Crater (2022). (B) Smoke plume rising from the crash site of the “Skycrane” used to lower the Perseverance rover onto the Martian surface (2021). (C) String-like material from the Perseverance rover’s landing system (2022). (D) Debris from the Curiosity rover (potentially associated with its landing; 2012). (E) Heatshield from the Opportunity rover (2004). (F) Some emissions pathways were originally discussed in refs. 5, 6, and 14. Image credits: NASA/JPL–Caltech (A–C); NASA/JPL–Caltech/Malin Space Science Systems (D); NASA/JPL–Caltech/Cornell University (E); and NASA (F).

The legally binding Outer Space Treaty (OST), initiated in 1967 and now ratified by 114 nations, establishes rules for how nations and entities under their purview use and explore outer space (11). Article IX requires that parties to the treaty avoid activities causing “harmful contamination” that may interfere with the exploration or use of outer space (including celestial bodies) by other State Parties to the treaty. While not explicitly defined in the OST (12), agents that cause “harmful contamination” have traditionally been limited to biological organisms or their organic chemical derivatives (13–15). Specifically, the Committee on Space Research (COSPAR) develops and maintains guidelines for countries to follow when planning and conducting activities on celestial bodies to fulfill the requirements of the treaty’s “harmful contamination” clause (16). The COSPAR guidelines aim to prevent introducing Earth-based biological material into the environments of celestial bodies and avoid disrupting the search for signs of past or present indigenous life by other State Parties to the OST. Such disruptions could be a false-positive identification of signs of indigenous life by discovery of biological material brought from Earth, or Earth-based microorganisms negatively interacting with potential indigenous life (e.g., acting as an invasive species) (13, 16).

A key concern motivating the guidelines is contamination by terrestrial microbes in environments where they may be capable of replicating. In principle, a small amount of microbial contamination could lead to self-amplifying biological contamination that is difficult or impossible to reverse. COSPAR defines categories of missions based on whether the target body is of interest to the chemical evolution or origin of life. If the target body is not of interest for life (e.g., Jupiter’s moon Io), then COSPAR does not impose any Planetary Protection requirements, whereas missions to some parts of Mars, where conditions for microbial contamination might be favorable, would involve the strictest microbial decontamination requirements (13).

As currently implemented, the OST fails to address the potential harmful contamination of celestial bodies by novel chemicals and materials (5, 17). We call on the astrobiology, Planetary Protection, and environmental chemistry communities to come together to investigate how anthropogenic pollution, via chemicals and materials associated with human exploration, may behave in the environments of celestial bodies. The goal: understanding the threat such pollution could pose to potential extant biological activity or abiotic processes in off-Earth environments. A better understanding of these threats would enable COSPAR to implement additional Planetary Protection regulations. These hazards related to chemicals and materials, knowledge gaps, and regulatory needs apply not only to Mars, but to other celestial bodies as well.

We call on the astrobiology, Planetary Protection, and environmental chemistry communities to come together to investigate how anthropogenic pollution, via chemicals and materials associated with human exploration, may behave in the environments of celestial bodies.

Anthropogenic Chemicals and Materials

Many chemicals and materials produced for use in consumer and industrial products have unintended consequences (7). Chemicals that persist in the environment and undergo long-range transport from their emission source to other regions of Earth can accumulate in the environment and cause unexpected, adverse biotic or abiotic effects. One high-profile example: perfluoroalkyl and polyfluoroalkyl substances (PFAS), used extensively in a myriad of applications, including waterproof clothing, nonstick coatings, electronics, fire-fighting, and manufacturing worldwide (18). PFAS are now prevalent in oceans, lakes, and even rainwater worldwide, where humans and other species are exposed to them, causing toxic effects (18). Similarly, the widespread use of chlorofluorocarbons (CFCs) in aerosol sprays and refrigerants in the mid-1900s led to their environmental release, accumulation in the stratosphere, and catalytic degradation of the ozone layer (19). The high persistence of CFCs and their role as catalysts amplified their ozone-depleting effect over time (19). Other anthropogenic chemicals and materials with a global impact include polychlorinated biphenyls (PCBs) (previously used in transformers, paints, and sealants) (20), chemicals used as flame retardants (in, e.g., electronics, furniture, and clothing) (21), and plastics (8).

Fig. 1 outlines possible emission routes of chemicals and materials from a hypothetical human base on the Martian surface. Chemicals present in the housing, suits, vehicles, and electronics of human explorers could volatilize from their source articles, particularly under warmer daytime surface temperatures that can reach 280 Kelvin (22) or when exposed to the harsh ultraviolet (UV) radiation (23) on Mars. Possible leaks from housing or vehicles could emit lubricants or other fluids to the environment (5). Waste disposal, aside from the biological concern (4), could also be a source of chemical emissions, whether through burial or some other imperfect containment mechanism. The harsh dust environment (24) of Mars could scour the outside of housing and other equipment, generating small pieces of material (e.g., microplastics). Once emitted into the air or regolith, these chemicals and materials may persist in the Martian environment and potentially spread from the human exploration site. For example, dust storms are a common occurrence on Mars; during such events, the “dust front” can traverse a distance of 40 degrees longitude per day (24), highlighting the potential for regional or global transport of microplastics or chemicals that have sorbed to dust particles.

While no conclusive evidence for past or present life on Mars has been found to date, there is extensive evidence that Mars may have been habitable billions of years ago (25). Furthermore, a number of locations could potentially support life (13). Such areas are defined by the COSPAR Planetary Protection guidelines as “special regions”; access to them would require enhanced levels of biological cleanliness for robotic exploration (13). Such guidelines do not consider the possibility of chemicals liberating directly from robotic rovers that may explore these regions in situ or the potential long-range transport of chemicals to these regions.

Theoretically, anthropogenic chemicals in air or subsurface water could be taken up by extant Martian life present in these media, whether through passive diffusion across the outer boundary of the organism (26) or through active air/water exchange with its environment (27). Microscale particles from engineered materials (e.g., microplastics, paint fragments, or tire-wear particles) could be taken up into an extant organism, where they could cause physical blockage (28). Furthermore, as with the unintended accumulation of CFCs in Earth’s stratosphere that catalyzed damage to the ozone layer (19), truly persistent chemicals released into the Martian environment could conceivably damage poorly understood abiotic processes, with the destructive effects being amplified over time due to their persistence. Thus, a small amount of chemical contamination could cause a sizeable disruptive effect that is difficult or impossible to reverse. This mirrors concerns about contamination from Earthly microorganisms (i.e., the current focus of the COSPAR guidelines).

Interplanetary Chemical Management

Human exploration of the solar system is advancing rapidly—witness India’s recent achievements near the lunar south pole. Chemical and material contamination of celestial bodies could interfere with these fledgling efforts in exploration and would thus, in effect, be a violation of the “harmful contamination” clause of the OST. But in its current implementation through the Planetary Protection guidelines of COSPAR (focused on biological contamination), the OST is not managing potential chemical and material risks. Conceptual frameworks for assessing and managing off-Earth environmental impacts that go beyond threats from biological material have been proposed before (5, 6, 12, 17). None, however, has been elaborated or inspired purpose-driven research.

We therefore call for scientific research to support the interplanetary management of anthropogenic chemicals and engineered materials—research that could inform an extension of the existing Planetary Protection guidelines promulgated by COSPAR under the OST. Furthermore, others have called for a transition of the existing Planetary Protection framework (focusing on biological contamination) from nonbinding guidelines to legally enforceable rules with clear sanctions for noncompliance (29). In the long-term, chemicals and materials should be regulated alongside microorganisms in a legally binding international Planetary Protection treaty, whether it be through modification of the current OST or through additional treaties.

Members of the astrobiology, Planetary Protection, and environmental chemistry communities (across government, academia, and the private sector) should work together to assess chemicals and engineered materials for their potential for emissions, long-range transport, and accumulation into potentially sensitive regions of the environments of celestial bodies. Furthermore, there should be ample opportunity for synergies between investigations into microbial contamination and possible chemical contamination of other planets. For example, current COSPAR guidelines do not address the transport of microbes from regions of human exploration to Mars special regions. Already, there have been calls to better understand and model meteorological processes on Mars in order to manage such microbial transport (see discussion in ref. 30). Enhanced understanding of Martian meteorology would offer important insights into chemical fate and transport management as well.

Researchers and policymakers should focus on two of the key characteristics used for management of chemicals and materials on Earth: persistence and the potential for long-range transport (10). Chemical persistence on Earth is strongly influenced by microbial biodegradation, reaction with atmospheric hydroxyl radicals (or other oxidation/reduction reactions), and breakdown in water or through sunlight (31). Chemicals and materials are more mobile when they reside in the air, water, or aerosols, allowing them to be entrained in regional or planet-wide transport processes (32). Working from the known environmental differences between Earth and Mars (e.g., Mars’ lack of liquid water or an active microbiosphere at the surface and harsher UV radiation), researchers could estimate the persistence and long-range transport potential of chemicals and materials on the Red Planet. Such preliminary findings could inform additional modeling and lab-scale experiments. The ultimate goal: informing the chemical and material engineering constraints and regulations necessary for Planetary Protection.

Humanity is reaching farther into the solar system. If we are to explore our celestial environment in a sustainable and ethical way, we must understand the potential impacts of chemical and material pollution on other planets before such pollution occurs. It is therefore crucial that we learn more about pollution prevention on other worlds and that we incorporate that knowledge into existing Planetary Protection guidelines.