Planetary Protection Knowledge Gap Closure Enabling Crewed Missions to Mars

3.6.1. How do we systematically provide for microbial monitoring of the environment (KG 1A)?

Environmental microbial monitoring for the purpose of planetary protection, in transit to Mars and both inside and outside the crewed surface habitat, is necessary to evaluate how microbial populations change over time and to understand the microbial load (in type and quantity) that is present at any particular location, addressing planetary protection concerns around both “forward” (from Earth to Mars) and “backward” (from Mars to Earth, or, to the astronauts) contamination. For spacecraft environments, the overall objective is to understand how microbial populations behave over time, with the goal of distinguishing adaptations to spaceflight and isolated closed human support environments, versus perturbations resulting from potential effects of the introduction of planetary materials (e.g., lunar or Mars dust) into the habitats and human support systems. Collecting these data sets, starting with current Low Earth Orbit (LEO) and planned lunar spaceflight systems, is a high near-term priority required to provide baseline data on environments and enable changes to be monitored as future Mars systems are being developed.

Although the MinION metagenomic DNA sequencing technology currently being developed for crew health monitoring (see Section 3.1) was repeatedly assessed to be the best available technology to meet this need, enhancement is needed to establish generation of WGS data sets to allow increased depth of understanding of the microbiomes present in these environments. Other improved technologies may be available for flight by the time of the first crewed mission launch to Mars, but it cannot be presumed so. Independent of the technology used, the metagenomics approach results in a DNA-based survey of the microorganisms present, with proof of concept already demonstrated under the ISSMO study (Checinska Sielaff et al., 2019; Urbaniak et al., 2022). Finalized protocols, data interpretation, and decision-making processes for a crewed Mars mission are still to be determined based on findings from anticipated future ground, ISS, and cislunar data sets.

3.6.2. How do we systematically do microbial monitoring of humans (KG 1B)?

Monitoring selected microorganisms using classical (cultivation-based) microbiological assays to assess potential risks to astronaut health has a long history as standard medical practice during human space exploration. These are mass, volume, and labor-intensive assays. For ISS and other LEO missions, where the samples are returned to the ground for analysis, this is less important. However, for in situ microbial analyses, as would be required on a crewed Mars mission, an alternative is sought. Ongoing microbiome initiatives using the MinION platform are greatly expanding the depth of the analysis, beyond assessing a few marker organisms of crew health and disease, to facilitate efforts to identify a broader range of potential human health indicators and infection markers that would establish baseline crew microbiome profiles and might better inform crew health status.

The primary objective for systematic monitoring of human-associated microorganisms is to distinguish potential causes of disease by providing a baseline to identify changes that would inform decisions on treatment. A substantial body of work already exists, from the ISS and other spaceflight experiments, describing changes in selected microorganisms and small multicellular organisms that suggest there are changes to host–pathogen interactions due to spaceflight, although those changes are not observed to be large (Blaustein et al., 2019). As this research continues, it is a high near-term and ongoing priority to ensure that the potential to address questions relevant to backward planetary protection is retained as standard protocols and capabilities for assessing astronaut health are developed: parameters such as sampling frequency and depth of analysis may be different for understanding the implications of changes observed after exposure to planetary environments, compared with regular health assessments. Again, finalized protocols, data interpretation, and decision-making processes for a Mars mission are still to be determined, based on findings from anticipated future ground analog, ISS, and cislunar data sets.

3.6.3. How do we design spaceflight systems to mitigate microbial growth (KG 1C)?

This question is intended to address all aspects of spaceflight hardware, including the pressurized rover/habitats, unpressurized vehicles, space suits, and other support systems, as described in the HEOMD-415 concept. Materials selection is a critical aspect of reducing microbial growth and considering how materials degrade over time from both chemical and structural standpoints. Smooth surfaces without pits and crannies are easier to clean, since potential microhabitats where microorganisms can adhere and accumulate or form biofilms are eliminated. Some microorganisms can derive nutrients or energy from reactive materials, releasing volatiles or breaking down over time, so careful materials selection, in combination with effective cleaning practices, should reduce, for example, microorganism-associated corrosion and biofouling. For components or locations where significant microbial accumulation is expected, repeated sterilization or sanitization is a common method to control contamination, but this has to be balanced with toxicity or damage from the sterilization/sanitization process.

With the advent of additive manufacturing and in situ manufacturing capabilities, innovative solutions could be explored, such as regular re-manufacture and replacement of contaminated parts, where the remanufacturing process itself will “resterilize” the microbial contaminants present in the stock material. Microbial monitoring can be used for microorganisms classified as “problematic,” ones that persist or recur through time at sensitive locations. Metagenome samples collected over a period of time from the same sampling location can be examined for the presence of such problematic microorganisms. Using filtered metagenome reads, “problematic” microorganisms that persist over multiple sampling periods from a single location can be identifiable (as has already been done for organisms detected on three consecutive flights on the ISS). Tools such as SourceTracker (Brown et al., 2019) can be used to determine potential sources of contamination and to track successional changes at a location before implementing appropriate mitigations. Each of these solutions needs to be adopted as appropriate by the engineering teams responsible for hardware development, re-evaluating preferred or standard go-to heritage material choices in the context of controlling or reducing microbial burden. This will be a topic for case-by-case implementation early in the design stages of relevant spacecraft hardware.

3.6.4. What operational guidelines are needed to address planetary protection concerns and crew health (KG 1D)?

Standardized safe operating procedures that are implemented correctly by astronauts during both nominal and off-nominal scenarios are essential for effective planetary protection implementation. However, detailed operational procedures can only be developed in the context of specific hardware systems and operational concepts. Although HEOMD-415 is the concept under which this study was developed, this KG 1D will be addressable at a later time, for example, through Earth analog studies. Early (lunar surface/Gateway) exploration target missions can be used to establish necessary draft guidelines. These will include the development of operations and hardware usage that enable clean and efficient activities, while minimizing exposure to potentially harmful planetary materials. In the near term, this KG should be addressed by training and knowledge capture from previous robotic missions, the Apollo experience, and terrestrial analog activities such as submarine, Antarctic, and Arctic exploration, so that the experience accumulated over decades to date is retained and supplemented as approaches for clean operations are established. Subsequently, final operational guidelines applicable to the mission design and final hardware performance would be established based on these earlier analog and data capture activities.

Even during this first human Mars surface mission, there is a presumed significant reliance on EVAs to accomplish both exploration activities (e.g., scientific experiment deployment and selective sample collection) as well as mission support-related activities (e.g., maintenance/repair and logistics replenishment). Although details for Mars EVA equipment are still conceptual, experience with past and current EVA systems indicates that there will be leakage at joints and seals in the pressure garment and from the portable life-support system (LSS). Filtration technologies (e.g., HEPA filtration) have been proposed to mitigate this forward contamination source. More analysis is needed to refine what planetary protection requirements will be needed compared with the currently existing guidelines for human missions. The analysis will determine both the acceptability of the approach and the required level of filtration.

HEOMD-415 analysis assumes use of a rapid cabin ingress/egress mechanism such as a suitport, which would require suitport-compatible EVA suits. A suitport is a concept technology that may accomplish the “surface mating and pressurized transfer of crew/cargo” capability for the interface between a Mars surface exploration suit and the pressurized cabin of the Pressurized Rover. This interface concept allows the majority of suit hardware to remain outside the cabin, with the crew entering and exiting the suit through a hatch built into the back of it that can be mated to an interfacing hatch in the Pressurized Rover hull. Use of suitports could reduce dust migration into habitable crew cabins and/or improve the safety of rapid and frequent EVA excursions. It remains forward work to develop technologies, concepts, and operations for both the Mars surface exploration suit hardware and the surface mating and pressurized transfer of crew/cargo capability to address compatibility with the chemically reactive soil, as well as forward (planetary protection) and backward (crew health) contamination during crew ingress/egress operations.

Although the suitport concept interface allows Mars exploration suit hardware to remain outside the Pressurized Rover cabin, suit components known to wear out with repeated use may need maintenance or replacement during a mission of even this short 30-sol duration. Examples include gloves and boots. There are also items, such as joints, rotating bearings, and seals that are likely to wear out under Mars surface conditions, but the replacement frequency is unknown (and may be unknowable until testing experience is obtained). This implies that, until enough observational data are available from which a prediction for deterioration or failure of these items can be made, a program of periodic inspection and repair will need to be incorporated into surface mission timelines. Currently, an assumption of no more than 24 cumulative EVA hours will pass before replacement of gloves is needed, and possibly boots, filters, and batteries as well. Other wear-susceptible items will be inspected and then repaired or replaced if necessary. For short duration surface missions, where only a pressurized rover is available to the crew, this type of maintenance may need to be carried out inside the rover cabin.

In the current concept, the cabin would be depressurized to allow the crew to enter through a hatch (not the suitport), and EVA equipment doffed in the repressurized cabin while maintenance tasks are accomplished. When maintenance is complete, the crew don their suits before a second depressurization of the cabin is made to allow the crew to exit through the hatch and dock their suits to the suitports. An entire sol is anticipated to accomplish all these activities. Dust mitigation and planetary protection will be concerns when suits are brought inside for maintenance and will need to be factored into an integrated mitigation protocol between all martian surface assets. This means, as the Pressurized Rover design matures, a functional capability must be incorporated that mitigates the intrusion of dust and martian regolith into the cabin and prevents cabin contaminant material from being transported to the external environment when the hatch is used for EVA ingress or egress. This functional capability will be vital for crew health and/or planetary protection reasons and is an area of forward work. It should also be noted that, although the HEOMD-415 analysis assumed suitports, functional equivalents and alternative approaches that would mitigate dust intrusion into the cabin and minimize forward contamination from the cabin may also be considered.

As one important principle in breaking the chain ^‡ of contact for backward planetary protection, current operational concepts assume the Mars surface exploration suits are left behind on Mars, and crew return to Mars orbit in separate intravehicular activity (IVA) suits. The Mars IVA Suit System concept of operation is expected to be substantially similar to crew Earth launch/landing, lunar transit, and Gateway operations. IVA Suit Systems for Mars descent and ascent operations are expected to be similar enough to crew Earth launch/landing that a common IVA suit can be used for both. Because the IVA suits are stowed after arrival and remain unused until the time of departure, they can be considered relatively clean from a planetary protection perspective; certainly cleaner than the Mars surface exploration suits. A minimum-functionality Pressurized Tunnel is a hypothetical “one job, one time” device that permits IVA crew transfer between the Pressurized Rover and the Mars Ascent Vehicle. Such a device would facilitate planetary protection compliance by allowing crew to access the Mars Ascent Vehicle without going outside; this also minimizes Mars Ascent Vehicle cabin volume by eliminating the need for Mars surface exploration suit don/doff and stowage.

In addition to these EVA-associated activities and their planetary protection implications, the Pressurized Rover is also directly involved in another activity with planetary protection concerns, that is, trash/waste disposal. One possible concept for trash/waste management on the martian surface is described in HEOMD-415. In this concept, the Pressurized Rover has a limit for onboard logistics and trash. Although this limit can be changed as the Pressurized Rover design matures and as choices are made regarding its operation, a limit of 14 sols is assumed in this example.

Part of the payload delivered with the crew and their Pressurized Rover are a number of small logistics containers that are used to replenish consumable items on the Pressurized Rover during the surface mission. These small logistics containers are also assumed to be used for long-term trash disposal (Section 4.4). For example, on Sol 12 of the mission described in HEOMD-415, crew conduct their first logistics restocking and trash removal operation. During a 3-h EVA the morning of Sol 12, crew offload a logistics container from the lander deck for repositioning onto a Pressurized Rover suitport. Fresh logistics will be transferred from the small logistics container into the Pressurized Rover, then the empty logistics container will be filled with trash. The now trash filled logistics container will be placed at a location on the surface next to the lander as its permanent disposal location. (Note: this disposal location reflects the current best guidance available, including forward planetary protection considerations from the COSPAR meeting series. The approach to disposal will be revisited as new guidance becomes available.)

With these two assigned capabilities—delivery of crew consumables to the martian surface and long-term storage of trash/waste on the surface—the small logistics containers (or their functional equivalents) will incorporate planetary protection concerns into their design. Specifically, concerns related to mitigating forward contamination that could originate from certain delivered crew consumables (e.g., food) as well as concerns related to trash/waste containment suggest that evolving protocols could require viability for decades of Earth years (a specific period of containment has yet to be determined and will require additional analyses and consideration of the risks involved). The surface power system and rovers will likely be fully functional at the time of crew departure, with significant remaining service life anticipated. Science and other utilization (including technology demonstrations) plans will be in place to take advantage of this capability without crew being present. Consequently, the Pressurized Rover closeout activities will focus on gathering material that will be returned to the Deep Space Transport (such as transferring returned samples and logistics) and configuring this vehicle for uncrewed operations.

Because no decision has been made at this time regarding subsequent crews returning to any particular Mars landing site, uncrewed operations could mean permanently decommissioning those subsystems needed for crew support and configuring these subsystems for long-term containment of biological material, consistent with planetary protection guidelines. If crew reuse of these elements is anticipated during a subsequent mission, then uncrewed operations mean placing crew support subsystems and other infrastructure in a dormant state, but in a manner that also provides for biological containment during this inactive period.

5.1.1. Temperature limits on growth

Efforts to assess the extremes of life on Earth have been made to guide decisions on planetary protection on Mars and are integral to the establishment of Special Regions on Mars (Beaty et al., 2006; Rummel et al., 2014). Based on these reviews, the lower temperature limit for replication likely lies around −15°C to −18°C. Mykytczuk et al. (2013) showed growth of the permafrost isolate Planococcus halocryophilus (aerobic gram-positive bacterium) in brine at −15°C with a doubling time of 50 days. Collins and Buick (1989) reported replication at −18°C in Rhodotorula glutinis (yeast) with a doubling time of 34 days. Based on these studies, COSPAR's planetary protection policy has adopted a lower temperature threshold for replication of −25°C, which provides sufficient safety for new discoveries (COSPAR, 2021).

5.1.2. Extreme desiccation (i.e., low a_w) limits on growth

The atmospheric environment on Mars is deemed highly desiccating (Banfield et al., 2020). On Earth, it has been shown that as soil loses water during desiccation overall respiration decreases (Liu et al., 2022); for example, Moyano et al. (2012) showed that carbon dioxide production in bulk soil ceased at a water activity (a_w, a measure of the amount of unbound biologically available water) value of 0.89. To date, the water limit for microbial activity is 0.585 (24°C and pH 6.1) (Stevenson et al., 2017). However, although not actively growing, certain microorganisms, for example, strains of Deinococcus radiodurans, have been shown to be resistant to desiccation (Saffary et al., 2002). Under highly desiccating conditions, certain microorganisms can enter a dormant state, forming spores, which are low in water content, high in dipicolinic acid and divalent metals, and protect against desiccation effects, as well as dry and moist heat exposure (see Nicholson and Schuerger, 2005, for details). On Earth, spores have been shown to remain viable for extended periods of time (Leishman et al., 2010; Wood et al., 2015). In space, spores survived for 6 years, if shielded against solar UV radiation (Horneck et al., 1994).

5.1.3. Lethality of solar UVC and UVB radiation

Solar UV irradiation on Mars is attenuated below 190 nm by the column abundance of CO₂ in the atmosphere (Kuhn and Atreya, 1979). Of the UV bands that strike the surface, UVC (200–280 nm) contributes ∼98% to the biocidal effect compared with ∼2% by UVB (280–320 nm) and <0.1% by UVA (320–400 nm) (Keller and Horneck, 1992; Setlow, 2001; Santos et al., 2013). Rapid inactivation of common spacecraft microorganisms can occur when cells or spores are exposed as monolayers to UVC irradiation similar to the surface of Mars (Nicholson and Galeano, 2003; Schuerger et al., 2003, 2006, 2019; Link et al., 2004; Moeller et al., 2007; Moeller et al., 2009; Beblo et al., 2011; Taylor et al., 2020). The inactivation is fast enough that fully exposed horizontal spacecraft surfaces will likely experience greater than six orders of magnitude bioburden reduction in under one sol under clear sky conditions (Schuerger et al., 2006).

Based on UV flux models derived from the Rover Environmental Monitoring Station (REMS) instrument on the Mars Science Laboratory (MSL) rover (Vicente-Retortillo et al., 2015; Moores et al., 2017), if the UVC flux at the surface were 3 W/s ( = 10.8 kJ/m²/h), a 6-log reduction for the UV-resistant bacterium Bacillus pumilus SAFR-032 (derived from Schuerger et al., 2006) would be achieved in ∼360 min. Six hours is significantly less than 1 sol on equatorial Mars at L_s  = 180 (autumnal equinox) and tau = 0.5 (clear sky). As direct UVC exposure is attenuated by shadowing, sun orientation, dust accumulation on spacecraft, dispersal into the local terrain, or cell pigments, the time to achieve this degree of bioburden reduction will increase, perhaps significantly (Mancinelli and Klovstad, 2000; Horneck et al., 2012).

5.1.4. Lethality of solar particle events and galactic cosmic radiation

Solar particle events (SPEs) are composed of outflowing protons—and to a lesser extent helium atoms and high atomic number and energy (HZE) particles—from the Sun's surface that are accelerated to high energies by its atmosphere or corona. The flux at 1 AU on the Moon is adequate to deliver up to 3 Gy/year on average (Kim et al., 2009). At Mars (∼1.52 AU) this will be reduced by a factor of 3.3-fold due to the volumetric dilution effects of the greater distance, as well as being further attenuated by the (although tenuous) martian atmosphere. However, this low flux suggests that ∼5 × 10³ years might be required at 1 Gy/year to accumulate a bioburden reduction of −10 logs assuming 0.5 kGy/log reduction (Schuerger et al., 2019). Because SPE fluence rates are relatively low—and the SPE occurrences intermittent—the microbial reduction effect on Mars spacecraft during a mission is expected to be very low.

By contrast, galactic cosmic radiation (GCR) is effective at a constant background level. Over time, for both organisms and other biomolecules, radiation-based degradation can occur, particularly from the HZE component of the exposure. The GCR absorbed radiation dose on the surface of Mars as measured by the Radiation Assessment Detector (RAD) instrument is 0.21 mGy/day, translating to ∼0.08 Gy/year (Hassler et al., 2014). This means to accumulate a bioburden reduction of −10 logs assuming 0.5 kGy/log reduction (per Schuerger et al., 2019) would take ∼6.25 × 10⁴ years, again resulting in insignificant microbial reduction effect on a crewed Mars spacecraft in the time frame of a mission.

However, the effect of GCRs on biomolecules, down to significant depth, over time, is a key element of the justification (extensively reviewed, and accepted by the COSPAR PPP) by the MMX project for unrestricted Earth return from Phobos (NAS-ESF, 2019). Here, the basis that any martian material (that could have originally had viable organisms) that is present in samples from Phobos would have been sterilized due to GCR exposure. Similarly, for old terrain on Mars not exposed to hydration events, the martian surface and shallow subsurface down to a depth of 0.7–2.5 m is predicted to to be sterile, absent some martian biological process we are not aware of (Pavlov et al., 2002; Cheptsov et al., 2021)

5.1.5. Lethality of oxidants

Oxidants remain one of the major unconstrained factors for microbial lethality on Mars. Oxidants may be produced through solar UV interactions with the atmosphere (e.g., H₂O₂) or regolith (e.g., perchlorates), present in the regolith (e.g., Fe-bearing minerals), produced through water–mineral interactions (e.g., reactive oxygen species), or a combination of these processes (Lasne et al., 2016). Oxidation of biomolecules renders them nonfunctional; and the extent of oxidation will determine whether cells are inhibited or killed. To date, H₂O₂ has been detected in the martian atmosphere, has seasonal variability, an average atmospheric concentration of about 30 ppb (Clancy et al., 2004; Encrenaz et al., 2008), and is postulated to be present in the regolith. Perchlorates (ClO₄ ⁻) have been detected at several locations on the martian surface (e.g., 0.4–0.6 wt % at the Phoenix landing site) and are postulated to be globally distributed at concentrations of 0.5–1 wt % (Hecht et al., 2009; Kounaves et al., 2014).

Despite an abundance (10–20 wt %) of iron-bearing species in martian regolith, only hematite (Fe₂O₃) and goethite have been conclusively identified (Christensen et al., 2000, 2004). Clays may also be important catalysts of oxidizing reactions on Mars given their likely global distribution at concentrations of 4–5 wt % (Carter et al., 2010). The presence of oxidants has been proposed as an explanation for the low level of detectable organics in martian soils despite the likely habitability of early Mars (Benner et al., 2000). Although more recently, the SAM experiment on NASA's Curiosity rover has demonstrated that complex organics are present in the martian regolith (Eigenbrode et al., 2018), much remains unknown concerning the distribution, heterogeneity, and effect of oxidants in the martian environment with regard to habitability and as a planetary protection concern (Kminek et al., 2017; Spry et al., 2021; Olsson-Francis et al., 2023).

Critical parameters for models of contamination control that account for the effect of oxidants remain undefined and include oxidant species identity, distributions (area and depth), concentrations, lifetimes, rates of production, water availability, synergy between oxidants, and, perhaps most importantly, rate of organic degradation by oxidants under martian conditions (atmosphere, radiation, temperature, etc.). These parameters have only been examined in a few limited studies to date (McDonald et al., 1998; Shkrob et al., 2010; Wadsworth and Cockell, 2017).

5.1.6. High salt concentrations in some regolith (e.g., MgCl₂, NaCl, FeSO₄, and MgSO₄)

Evaporite deposits, for example, chlorides, sulfates, and (per)chlorates on the surface of Mars, have been interpreted as evidence of a water-rich evaporitic past where putative life may have existed (Wänke et al., 2001; Hecht et al., 2009). Extensive studies have suggested that some of these salts are highly hygroscopic, absorbing moisture from the atmosphere through deliquescence and hydration for short periods of time (dependent on the location, season, and atmospheric conditions) (Davila et al., 2010; Gough et al., 2011; Fischer et al., 2016; Rivera-Valentín et al., 2020).

On Earth, salts are known to impact microbial growth due to low water activity, high ionic strength, and in some cases extreme chaotropicity (Crisler et al., 2012; Schneegurt, 2012; Hallsworth, 2021). Although this may hinder growth, no evidence exists that highly saline conditions on Mars would kill or eradicate microbial life.

A recent study has demonstrated that microorganisms can survive periods of desiccation and deliquesce in a Mars-simulated brine (Cesur et al., 2022). During the drying process, microorganisms can be trapped within fluid inclusions allowing them to survive for long periods of time (Rothschild et al., 1994; Grant et al., 1998). When dissolved, some of these salts may also be highly acidic (e.g., ferric sulfate). At low pH biological molecules will be protonated, negatively impacting their function. Although inhibitory, low pH may not be immediately lethal. Furthermore, current martian conditions are not conducive to the formation of acidic solutions from these minerals. Notably, high salts in Mars analogs had a direct biocidal effect on Bacillus subtilis spores under both dehydrated and hydrated conditions, whereas acidic Mars analog soils were biocidal only under hydrated conditions (Schuerger et al., 2012, 2017). Furthermore, it has been shown that the acidophilic iron–sulfur bacterium Acidithiobacillus ferrooxidans is able to survive in a desiccated state for several days at Mars surface conditions and to grow solely on the nutrients provided by minerals in (analog) Mars regolith under aerobic and anaerobic acidic conditions, participating in the redox cycling of iron (Bauermeister et al., 2014).

5.1.7. Lethality of low (Mars ambient) pressure (7–12 mbar) and space vacuum

There are few published reports that describe direct testing of microbial survival at low pressures similar to the martian surface. Much of what is known must be extracted from experiments on Mars surface simulations in which nontreatment laboratory controls exist. For example, when comparing low-pressure versus laboratory controls in a series of Mars simulations, Schuerger et al. (2003) reported that low pressure appeared to induce ∼20% reduction in spore survival of the bacterium B. subtilis HA 101 over short-term exposures of tens-of-minutes to a few hours. Other literature also supports this conclusion. For example, Schuerger et al. (2019) (Fig. 1) identified that low-pressure experiments with B. subtilis demonstrated spore survival losses of ∼50% over the course of 200 days in vacuum, with losses of approximately ‒2 logs observed over the course of nearly 6 years in LEO during experiments by Horneck et al. (1994) in the Long Duration Exposure Facility.

A series of experiments were performed on free-flying satellites in Earth orbit (in the Biopan facility on Foton missions with a duration of ∼2 weeks), and on the ISS in the Expose missions where several different microorganisms were exposed to space vacuum for 1.5–2 years. It was demonstrated that most of them could survive exposure to vacuum, if shielded against solar UV radiation (Horneck et al., 2001; Cottin and Rettberg, 2019).

5.1.8. Microbial growth under Mars-relevant conditions

Significant literature presents empirical evidence that extremophiles and mesophiles on Earth can metabolize organics, grow, and potentially evolve under a diversity of conditions relevant to the surface or shallow subsurface of Mars. It is beyond the scope here to review this full body of literature; however, a few broad conclusions can be drawn.

First, at least 30 bacteria—but no archaea or fungi—have been identified that are capable of metabolism and growth under simulated Mars surface conditions of low pressure (7 hPa), low temperature (0°C), and CO₂-enriched anoxic atmospheres (called low-pressure temperature atmosphere [PTA] conditions) (Nicholson et al., 2013; Schuerger et al., 2013; Schuerger and Nicholson, 2016; Schwendner et al., 2020). Other studies have characterized microbial activity under low-pressure environments that are much higher than the surface pressure on Mars but significantly lower than Earth's sea-level pressure of 1013 hPa (Kanervo et al., 2005; Pokorny et al., 2005; Schirmack et al., 2014; Waters et al., 2014; Mickol and Kral, 2018). These later studies suggest that many subsurface niches on Mars with pressure down to 50 hPa may support a wide diversity of terrestrial microbiota.

Second, numerous studies have demonstrated that terrestrial algal, bacterial, and archaeal species are capable of growth in diverse Mars analog soils doped with organics (Nicholson et al., 2012; Bauermeister et al., 2014; Al Soudi et al., 2017; Kölbl et al., 2017; Schuerger et al., 2020) and carbonaceous chondritic materials (Mautner, 2002a, 2002b). However, most of these studies failed to evaluate growth under simulated martian low-PTA conditions. One notable exception was the study by Schuerger et al. (2020) in which the hypopiezotolerant (defined as being capable of growth at 7–10 hPa) Serratia liquefaciens was capable of growth in diverse Mars analog soils under laboratory conditions of 1013 hPa, 30°C, and Earth-normal gas composition of pH₂/pO₂ (78%/21%) but failed to grow under low-PTA conditions. The added geochemical and physical stressors in the analogs appeared to be enough to inhibit growth under low-PTA conditions, even when a known hypopiezotolerant bacterium was used. This last study calls attention to the need to conduct a wider diversity of biological growth experiments under regolith geochemical and low-PTA conditions similar to the surface or shallow subsurface of Mars. As numbers of stressors are added to metabolic and growth assays that approach actual conditions on Mars, we may discover that few, if any, terrestrial microbiota are capable of growth under conditions found at the surface or shallow subsurface of current-day Mars.

5.1.9. Synergism among biocidal factors on Mars

Approximately 20 biocidal or inhibitory factors are likely to be present in martian surface or subsurface environments (Beaty et al., 2006; Schuerger et al., 2013; Rummel et al., 2014). Most studies into microbial survival, metabolism, growth, and evolution under Mars-relevant conditions explore single parameters. However, synergistic interactions among the 20 biocidal factors are likely to significantly increase the lethality of the martian environment. Numerous articles have described synergistic biocidal effects between vacuum (VAC) and high temperatures (Dose and Klein, 1996; Schubert and Beaudet, 2011), VAC and low temperatures (Ashwood-Smith and Horne, 1972), VAC and UV irradiation (Keller and Horneck, 1992; Horneck, 1993; Saffary et al., 2002), and VAC and ionizing radiation (Silverman et al., 1967). The overarching results of these—and other studies on synergism in the space environment—suggest that synergistic interactions can often add 1–3 logs of additional biocidal effects for a given time-step between a few hours to a few days.

In contrast, few studies have examined multifactorial combinations of the biocidal factors listed for the surface or subsurface of Mars. The difficulties in doing so are twofold. First, complex Mars simulation chambers (Schuerger et al., 2008; Martin and Cockell, 2015; dos Santos et al., 2016; Rabbow et al., 2016) are difficult to build and maintain. Second, multifactorial experiments increase exponentially in complexity as the numbers of factors being tested jump from three (3! = 6 combinations) to four (4! = 24 combinations) biocidal factors. However, it is imperative that such multifactorial experiments be developed to probe the actual suite of biocidal or inhibitory conditions likely to be encountered on the martian surface. It is plausible, perhaps even very likely, that synergistic interactions among the ∼20 biocidal factors on Mars will make metabolism and growth of Earth microbiota untenable for the martian surface.

5.1.10. KG closure

In this study, we summarize the parameters to be addressed to close the KG 3C in microbial survival and KG 3E on protective mechanisms for microorganisms at Mars, identified across the COSPAR meeting series and companion white paper (Spry et al., 2021), and identify immediate research needs.

5.2.1. Martian dust transport

In parallel with the microbial survival characteristics of Mars, consideration also needs to be given to the potential mobility of microorganisms through aeolian transport. Olsson-Francis et al. (2023) state that “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).” The basis of this is that, on Earth, microbial cells associated with dust are known to travel for thousands of kilometers (Mayol et al., 2014; Li et al., 2016), and aerial distribution of microorganisms has been demonstrated to occur at Mars analog sites, for example, the Atacama (Azua-Bustos et al., 2019).

The typical airborne dust aerosol size of Mars is on the order of 1–2 μm, with the notable exception of global dust storms, when suspended particle sizes have been observed to reach maximal sizes of 8 μm (Lemmon et al., 2019). This is much larger than the typical size of Bacillus spores (the mean values for B. subtilis, e.g., are 1.07 ± 0.09 μm [length] and 0.48 ± 0.03 μm [diameter]) (Carrera et al., 2007). Conceivably, a ∼1 μm diameter spore encapsulated within a suspended 8 μm diameter particle could have a 3.5 μm layer of biomaterial protecting it from UVC exposure.

5.2.2. Atmospheric attenuation of solar irradiation

In addition, the atmosphere itself could attenuate the UVC, with Olsson-Francis et al. (2023) again observing: “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) … generally, the transmittance of the atmosphere is expected to decrease with exp(-tau), with tau being the opacity.” Although dust lifting events predominantly occur during the martian day, any particle suspended during dark hours during a dust storm will not be exposed to solar UVC. Hence, atmospheric processes may allow viable terrestrial contamination to be transported from a spacecraft to a distant Mars location (potentially a Special Region) and be protected by a layer of a few grains of protective dust or by the total column of dust, over time. Mars Exploration Rover (MER)-era measurements have estimated that airborne dust deposits at a rate of 0.004 ± 0.001 per unit of surface optical depth deposited per martian sol. This would mean that, for that location, 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). Of course, at the microscopic level, under some particles, this could potentially result in close to 100% UV obscuration. The dust deposition rate was also studied at the Mars Pathfinder landing site, which estimated dust fall rates of about 20–45 μm per Earth year at that location, consistent with previous studies of dust deposition on Mars (Johnson et al., 2003).

5.2.3. The global dust storm phenomenon

Olsson-Francis et al. (2023) summarized that, during the course of a dust storm, orbital and in situ observations show the following:

Based on the analysis so far, adopting a conservative planetary protection stance, data support that 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 of terrestrial contaminants between all areas of 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). Although not instantaneous, and so exposing the transported contaminants to solar UVC, this process is occurring during a period of solar UV obscuration.

5.2.4. Surface–atmosphere processes that move sand and raise dust

The mechanisms by which sand is moved and dust is lifted on Mars are not well quantified. Observations provided by the Mars Environmental Dynamics Analyzer onboard Perseverance at Jezero Crater have shown that, during the first 216 sols, four convective vortices raised dust locally. At the same time, on average, four passed the rover daily, >25% of which were significantly dusty (“dust devils”) (Newman et al., 2022). As for gusts, all imaged dust lifting events occurred during strong convective activity from ∼10:30 to 16:00 Local True Solar Time, which manifests as large temporal variability in wind speed. However, dust lifting by wind gusts appears relatively rare inside the Jezero crater, whereas dust lifting by convective vortices is very common. Dustier vortices typically have larger pressure drops and maximum wind speeds, which are expected to be correlated. No local dust lifting by vortices was found for tangential wind speeds below 15 m/s, or central pressure drops below 2.6 Pa. This was also the minimum wind speed reported at which surface darkening (inferred to be dust lifting by a passing vortex) was observed at InSight (Baker et al., 2021).

5.2.5. Dust in the atmosphere of Mars: “Clear” sky and localized, regional, and global dust storms

The omnipresent dust haze, a regular feature of the Mars atmosphere even for “clear sky” or dust storm free conditions, is responsible for the permanent reddish color of the Mars sky. Over its 14-year history, from 2004 to 2017 of measurements of the atmospheric column dust opacity or “tau,” the MER Opportunity found that the opacity varied from tau = 0.5 for “clear sky” conditions to tau >2.0 for dust storm conditions (Zurek et al., 2018). Surface dust enters the atmosphere by the lofting action of surface winds. When these winds are strong enough, surface dust is actively lofted into the atmosphere generating a dust storm. Depending on the strength and duration of the surface winds, dust storms on Mars may be local, regional, or global in scale. Global dust storms can cover the entire planet. Regional dust storms (covering ∼10⁶ km²) and planet-encircling global dust storms occur in the southern hemisphere during spring and summer. Local dust storms can occur in any season and can impact almost any geographical region on the planet, but there are preferred storm tracks (Zurek et al., 2018). The exact mechanism for local dust storms evolving into regional and planetary-encircling dust storms is not known but may be related to the vertical height of the localized dust storm. There is a pattern during the southern spring and summer seasons, when Mars is near perihelion and solar heating is greatest that regional dust storms are generated (Zurek et al., 2018). Some regional dust storms will evolve into planet-encircling hemispheric or even global dust storms (Zurek et al., 2018). Historically, the planetary-scale dust events appeared to occur every 3–4 years (Zurek et al., 2018). The clearing of the atmosphere after a global dust storm can take several months, whereas regional and local dust storms may last a few weeks or from one to a few days.

Estimates of the mass of dust suspended in the atmosphere of Mars have been derived from the Viking Infrared Thermal Mapper by Martin (1995), who found that during the peak of the 1977b regional dust storm, a total dust mass of ∼430 million metric tons (4.3 × 10¹⁴ g) was suspended in the atmosphere, equivalent to a global layer 1.4 μm thick. During a local dust storm near Solis Planum, ∼13 million metric tons (1.3 × 10¹³ g) of dust were lofted, equal to about a 6-μm layer of dust in that vicinity (Martin, 1995).

5.2.6. Geographical distribution of incident solar radiation at the surface of Mars for different atmospheric dust conditions

The distribution of incoming solar radiation at the surface of Mars varies for different atmospheric dust opacities and has been calculated (Levine et al., 1977). Figure 4 (left side) shows the distribution of solar radiation at the surface of Mars for “clear” sky conditions, corresponding to an atmospheric opacity of tau = 0.10. In general, this distribution follows the distribution at the top of the atmosphere, except for magnitude, which is decreased somewhat because of clear sky absorption and scattering. A significant hemispheric asymmetry exists in that there is considerably more insolation over the southern polar regions than the northern polar regions during the local summer solstice. Since the southern winter season occurs at aphelion, it is colder and of longer duration than the northern winter season. This results in a more extensive southern polar cap. The rapid near-complete melting of the southern polar cap results from the hotter southern hemisphere. The northern polar cap remnant is a permanent surface feature that results from the cooler summer in the northern hemisphere.

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FIG. 4.

Solar radiation incident on the surface of Mars. Left: for “clear” atmospheric conditions (atmospheric opacity, tau = 0.10) [in calories cm-2/(planetary day)]. Right: for dust storm conditions (atmospheric opacity, tau = 2.0) [in calories cm⁻²/(planetary day)] (from Levine et al., 1977). Used with permission.

In contrast, Fig. 4b (right side) shows the distribution of solar radiation at the surface of Mars during the 1971 global dust storm observed by Mariner 9, with an atmospheric opacity, tau = 2.0 (Masursky et al., 1972). The insolation distribution closely parallels the seasonal march of the Sun, with maximum insolation in the tropics and only small amounts of solar radiation reaching the polar regions. For example, in the polar latitudes during the southern hemisphere summer, the insolation is only about 5% of the clear sky values.

With the increasing focus on a crewed mission concept for Mars in the 2039 time frame, many Mars-specific environmental factors are now being considered by NASA and other engineering teams. Learning from NASA's Apollo Missions to the Moon, where lunar dust turned out to be a significant challenge to both crew safety and mission success, attention is now turning to the dust in Mars' atmosphere and regolith.

To start the process of identifying possible dust-caused challenges to human presence on Mars, and thus aid early engineering and mission design efforts, the NASA Engineering and Safety Center organized and conducted a workshop entitled “Dust in the Atmosphere of Mars and Its Impact on Human Exploration” that was held in Houston, TX, during June 2017. This workshop was held independently of the planetary protection meeting series being discussed in this article, but addressed topics of common interest. The workshop addressed the topics of knowledge of Mars dust physical and chemical properties and abundance, composition, and impact on human health, as well as its impact on surface mechanical systems such as spacesuits, habitats, and mobility systems. Approximately 70 participants from NASA centers, universities, and industry participated, resulting in a report by Winterhalter et al. (2018). In summary, the assembled experts concluded that dust in the atmosphere of Mars is an issue to be addressed well before spacecraft are built to carry humans to the red planet.

In particular, measurements and experiments need to be taken and conducted on the surface of Mars by precursor landers to ascertain dust characteristics that will influence hardware design as well as provide toxicology data to safeguard crew health. In addition, it was considered that dust samples need to be collected and examined for possible extant life, perhaps through an MSR mission, with contemporary findings by the Curiosity rover team regarding the presence of complex organics and seasonal methane being considered important steps in that direction.

5.2.8. Measuring and modeling atmospheric transport on Mars

A key KG identified in the 2018 COSPAR Refining Planetary Protection Requirements for Human Missions was KG 3A, described as “Measurements/models for Mars atmospheric transport of contaminants.” The highest priority in understanding the natural transport of contaminants on Mars is to understand how they are dispersed by the wind, as this drives the requirements for equipment design and operations in the field. The first step in addressing KG 3A is to develop and apply atmospheric dispersion models to one or more high-priority candidate sites for a future human exploration zone. These preliminary dispersion models can be used to conduct sensitivity analyses with a variety of input conditions to determine both the degree and transport range of contamination. The modeling results will inform preliminary recommendations for reducing the risk of forward contamination during robotic and human operations within an exploration zone. Ultimately, the nature of boundary layer physics requires empirical measurement of the boundary layer.

In particular, measurements of the atmospheric state (e.g., pressure, temperature, and humidity) and direct measurement of atmospheric forcing (e.g., turbulent fluxes) are needed to adequately characterize the lower atmosphere (MEPAG, 2015). Moreover, the properties of the planetary boundary layer are season-, daily-, and location-specific. In situ measurements are critical to accurately describing the natural transport of contaminants. Long-term high-frequency meteorological measurements are needed at multiple fixed concurrent locations within the specific exploration zone to enable assimilation into atmospheric dispersion models. The minimum number of concurrent locations will be dictated by the size and topographical variations of the areas within an exploration where humans would have their base and be exploring with rovers. Ideally, all future surface assets (irrespective of their primary purpose) should incorporate the capability to make high-fidelity meteorological measurements to begin building this data set.

Refinement of the preliminary aeolian dispersion models will require in situ measurements at multiple key sites within an exploration zone of the processes that control the local climate near the surface, particularly those relating to the entrainment, transport, and deposition of airborne particulates and saltating grains. Although details of requirements will require further study, especially after selection of an exploration zone, it is anticipated that, at a minimum, they will include measurement of the turbulent fluxes of heat and momentum; high precision measurements of air temperature, pressure, humidity, and 3D wind velocity; the concentration and atmospheric column abundance, deposition, and erosion rates; and finally the physical and chemical properties of mobilized grains. The latter measurements can be used to establish the biocidal properties of the dust, sand, and regolith, which will feed into KG 3C to help determine microbial survival rates. In addition to making these measurements by one or more precursor missions, it will be critical that the weather stations are long-lived or replaced/reactivated once humans arrive and begin to operate within the exploration zone. Because martian weather patterns are not strictly repetitious year-to-year, these measurements must be made over one or more annual cycles, preferably during both intense dust storms and relatively dust-free quiet conditions (i.e., over a broad range of L_s ).

Some of the in situ measurements listed earlier have not been made to date by any landed spacecraft on Mars (e.g., turbulent fluxes or 3D wind velocity), but because they have been listed as high-priority investigations by MEPAG (Mischna et al., 2009; Rafkin et al., 2009; MEPAG, 2015), Goal II, Objective A.1, Investigations 1, 2, and 3 (GII: A1.1–3); Goal II, Objective A.4, Investigation 1 (GII: A4.1), they are of significant interest to the scientific community and would undoubtedly result in benefits beyond those required for the development of a planetary protection implementation solution.

5.3.1. Study the potential for particle-bounce forward contamination (into a special region)

Subsurface transport can be considered on multiple scales from shallow (0–2.5 m) to km depths, and transport of contaminants is different for each. Shallow drilling, such as the proposed Icebreaker mission (McKay et al., 2013), could collect ice-saturated samples at 0–1 m depth (Glass et al., 2014) and assess their habitability and assay for martian life biosignatures protected from surface chemistries. This near surface environment is also at the greatest risk from forward contamination as a result of human activities but is of the most interest in the near term for reasons of accessibility and potential use as a resource (Sanders et al., 2015; Kleinhenz et al., 2018). The ice table beneath tens of centimeters of soil could host perchlorate brines but may not be habitable at modeled temperatures (Rivera-Valentín et al., 2020).

Lander or rover missions that are specifically investigating potential martian life will be COSPAR Category IVb missions. As was done for the Robotic Arm assembly on Mars Phoenix (Arvidson et al., 2009), planetary protection guidance requires that elements of a life detection mission contacting the sample (e.g., part of a drill extending below the surface) be appropriately sterilized (NASA STD 8719.27). One acceptable approach is to heat-sterilize the components that will either be placed below the ground or in contact with the components that will penetrate below the ground and place these inside a biobarrier that is removed after a Mars landing (an example is shown in Fig. 5). To prevent terrestrial contaminant organisms from traveling onto the drill auger/bit through sample transfer, there must be an air gap between the sterilized drill and a sample delivery subsystem that contacts “dirty” instruments (which cannot be sterilized by heat or other methods).

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FIG. 5.

Icebreaker drill concept: (A) Drill biobarrier shown closed lying across the lander deck (B) when opened the biobarrier drapes over the deck (C) arm/scoop biobarrier closed (D) arm/scoop biobarrier open. Note drill and arm biobarriers are not shown at the same scale (from Glass et al., 2021, used with permission).

Other forward contamination vectors include the venting of materials by the spacecraft or instruments (which can be managed with knowledge of emissions and wind direction) and “particle-bounce” of transported material. Both windblown and mechanically transported material may fall into contact at times with spacecraft or instrument components that are not appropriately clean. Contaminants acquired through contact may then be carried along in this surface dust, which can be blown or fall onto the drill (or directly into the open borehole) and, therefore, reach the shallow subsurface. At moderate depths (up to 100s meters), subsurface temperatures should block penetration of fluid-borne physical/chemical/biological contaminants by freezing. As long as these contaminants are not themselves generating heat, then the underlying potential martian aquifer where fluid transport could transmit contamination on a regional scale will not be accessed.

As seen in the flows of contaminants in terrestrial sites, it is not unreasonable to consider the transport of contaminating terrestrial organisms from surface operations into a deep subsurface aquifer, which perhaps could result in alteration or destruction of a hypothetical deep subsurface martian ecosystem. However, there is currently no identified mechanism on Mars for direct contaminant transport to these deep environments, or any indication of extant ecosystems at depth. The current lack of evidence for these, and the difficulty of accessing 100 m–km depths on Mars, will relegate these deeper-subsurface gap investigations to be a lower priority for inputs for the initial crewed human missions (Heldmann et al., 2022). A possible gap is our lack of knowledge regarding the extent of particle-bounce contamination between a drill and robotic sample transfer devices during drilling and sampling operations.

One school of thought regarding the discovery of biosignatures or fossil or extant subsurface life on Mars is that it would be such a profound discovery that understanding its local provenance or stratifigrafic position might be deemed less important. However, verification of those results and learning more about the subsurface environments that preserved them requires some control and knowledge of the back-contamination spread of Mars subsurface materials brought up and scattered across the spacecraft and the mission site.

Uncontrolled mixing of sample materials would destroy some potentially valuable data. Characterizing the external contamination of Mars-drilled sample return tubes or the site exposure of human crew members (Bussey and Hoffman, 2016; Hoffman, 2022) is a strong motivation to study the provenance of locally scattered subsurface materials in Mars-like laboratory and field conditions. Similarly, might scattered subsurface sample find its way into warmed environments on spacecraft or other facilities that could be habitable to any extant local organisms brought up?

The entire sample acquisition chain (drills and scoops, robotic transfer mechanisms, and instrument ports) is a potential vector for subsurface materials, scattered by wind and/or spillage (Davé et al., 2013). Another sampling gap or uncertainty regard the extent of sample-scatter during full-scale drilling, sample acquisition, and handling.

5.3.3. Technologies for remediation/alleviation of subsurface forward contamination

Since the Viking missions of the 1970s, the technology has existed for heat, and later chemical, microbial reduction on spacecraft systems before launch. Biobarriers, as demonstrated on Phoenix (Arvidson et al., 2009), are a means of preserving that cleaned state until Mars arrival (see also Fig. 5). To prevent spores from traveling onto the drill auger/bit through mechanical sample transfer, there must be a gap maintained between a sterilized drill and the sample delivery subsystem that itself touches the “dirty” instruments (which cannot be heat sterilized to Viking standards). At the same time, clumping and sticking experiences with samples within the Phoenix scoop mean that future Mars sampling missions must have positive actuation, not relying solely on gravity. This complicates the gap issue, effectively requiring samples to be actively propelled by some means (such as mechanical or pneumatic contact) across the gap.

However, we currently have no means to reclean or recover from inadvertent contamination, once the biobarriers are discarded after arrival. If particle-bounce or operational errors (unplanned contacts) cause a drill to become contaminated, NASA could be faced with a choice of an end-of-mission versus violating international agreements regarding Special Regions. Furthermore, if material is blown from a (less cleaned) lander or rover deck down into an adjacent open borehole, a Special Region could be violated. These issues are already a serious scientific (and political) obstacle to drilling on Mars and/or exploring the subsurface locales where Special Regions and, hence, biosignatures or life might exist.

We lack a demonstrated technology (using heat, UV, and chemicals) that will enable re-sterilizing (in the sense of heat microbial reduction to Viking levels) a contaminated drill string. In situ inadvertent “contamination accidents” may require a local drill cauterization device to mitigate potential borehole contamination.

5.3.4. We lack sufficient knowledge of contamination mitigation performance in relevant field analog Mars-like environments

Field testing in Mars-relevant environments, together with Mars environmental chamber testing, is necessary to create enough confidence in technologies and results to propose them for flight application. Laboratory testing alone is not sufficiently rigorous, as a priori knowledge and control of test materials and environments allow their anticipation (whereas nature is unpredictable and unforgiving). Selected field test sites should strive to apply the highest-fidelity terrestrial analogs for drilling in the martian icy regions, including ice-cemented soils, massive ice, and other soils with periglacial characteristics. Past NASA-developed planetary drill prototypes have been tested at Arctic (Glass et al., 2008) and Antarctic (Zacny et al., 2013) analog sites, establishing performance and operational baselines.

6.1.1. Technology studies in analog environments

The Desert Research and Technology Studies (DRATS) is a multiyear annual NASA campaign conducted in the high desert area of northern Arizona at Black Point Lava Flow near Flagstaff since 1997. DRATS objectives include testing RV-like pressurized rovers for human crews in desert (analog) terrains, while testing science instruments, communication systems, and operational planning for human and robotic surface exploration (Eppler and Bleacher, 2013). For instance, 2010 DRATS integrated a purpose designed geological laboratory (GeoLab) that consisted of a pressurized glovebox for astromaterials handling and geological characterization analysis into NASA's Habitat Demonstration study (Evans et al., 2013). A 2022 reboot of the DRAT program involved moonwalking-roving-operations and geology traverses for upcoming Artemis missions (Artemis 3 and beyond). There remains potential for incorporation of planetary protection KG studies in future DRATS campaigns.

6.1.2. Astrobiology-focused technology studies

As described in Section 4, without appropriate mitigation, human crewed systems would release their microbiome into the martian environment. Furthermore, because mission operations timelines are constrained, bioburden decontamination tasks must be time-effective. Thus, an essential requirement for human astronauts is the development and use of nonaggressive (safe), simple, quick, and effective cleaning and microbial reduction protocols (procedures here referred to as “decontamination”), and rapid bioburden monitoring of viable terrestrial bulk microbiota and wherever necessary. Crews can test efficacy of decontamination approaches inside habitats and pressurized rovers, as well as for cleaning suits inside airlocks outside the habitat before EVAs in the martian environment. In addition, biological recontamination mishaps could occur immediately after EVA primary decontamination protocols for rapid decontamination cycles of hardware tools in the field are specifically needed.

Contamination prevention procedures applied to laboratories and cleanrooms are not transferable to terrestrial fields or spacecraft environments. Life detection technology experiments in Analog Environments test concepts and pivotal aspects of planetary protection implementation for human exploration missions. These campaigns involve a combination of high-fidelity science, exploration technology, and mission operations and can address KGs, including (1) mission's operations-integrated “decontamination and verification of space hardware,” (2) contamination prevention/mitigation by refinement of engineering design of spacecraft hardware, (3) understanding of the fate of source to sink microbial contaminants, including “transport and survival of viable terrestrial microorganisms/bioburden in Mars-like soil and habitable settings. Examples of Astrobiology Technology campaigns follow.

6.1.3. Polar analog environments

Analog mission trials in terrestrial polar environments have broad testing goals. Examples involve (1) human crewed science traverses and human factor performance evaluations; (2) science and operations for aseptic sampling of ice-cemented ground, ice cores, and subsurface water sampling; and (3) long-term ecology and contamination studies. Similar studies are relevant to closing planetary protection KGs for crewed Mars missions.

6.4.1. Artemis

The Human Landing System (HLS) Program is developing demonstration landing missions and sustaining lunar landing systems, largely by, but not limited to, leveraging commercial systems development and services. In addition to requirements and approaches captured in the HLS Planetary Protection Plan (HLS-PLAN-013), there are additional potential opportunities to address planetary protection KGs. One of the most relevant activities in which HLS can engage to help close planetary protection KGs is with the highly dynamic landing of relatively large crewed vehicles.

The landing dynamics will cause complex plume surface interactions that can give rise to (1) contaminants adhering to the outside of the lander (e.g., potentially by electrostatic dust adhesion) and (2) that can cause the distribution of contaminants well beyond the lander. Plume surface interactions and transport dynamics on Mars are anticipated to be different from those on the Moon; however, monitoring, and potentially testing mitigation strategies, can still be conducted during these landed missions to help inform how to address potential contamination challenges from human Mars landings. Examples of KGs that can be informed from human lunar landings include 1A, 2A, 2B, and 2C.

As occurred with Apollo, even early HLS landing missions will presumably leave waste behind, including vented materials. Such waste materials could be monitored, and potential mitigation and containment strategies can be tested, to help inform how to handle waste products during human Mars missions. This can help address KGs 2B (microbial/organic releases from humans and support systems) and 2G (acceptable contamination level from wastes left behind).

Unpressurized and pressurized lunar rovers are presently being pursued under the EVA and Human Mobility Program. Similar to how we can address KGs from crewed lunar landings, we can also learn from unpressurized and pressurized surface vehicles. All the KGs noted earlier also apply to using lunar rovers to help inform similar Mars vehicles, including the venting from spacesuits during unpressurized roving, and perhaps more importantly, what presumably could be a very different quantity and quality of venting from pressurized vehicles. The operation of such vehicles should be able to help specifically address how contaminants are vented and spread as a result of surface mobility. In addition, the design of such surface mobility assets could conceivably include potential contamination mitigation approaches that could be tested on the moon to inform their potential utility at Mars.

Lunar base camp planning and design efforts are underway and present numerous opportunities to help address KGs. In addition to the KGs noted earlier, base camp design can also help address KGs 1D (Operational guidelines), 2F (ISRU compatibility), and 2I (Approaches to “Break the chain”). For example, the location of ISRU assets could be important for addressing and mitigating contamination impacts on human assets such as habitats and mobility vehicles, although this is more particularly useful for subsequent missions beyond that described in HEOMD-415.

Conducting teleoperations on the Moon, particularly low-latency teleoperations (LLT), either from lunar orbit from the Gateway or completely on the lunar surface, has the potential to inform how LLT could be used at Mars for planetary protection-related reasons. KGs 1D, 2C, 2D, 2G, and 2I could potentially be informed by the use of LLT. LLT can be used to develop and test operational guidelines and protocols that could decrease crew health and safety risk, and help execute decontamination protocols and verifications methods, depending on the availability of suitable robotic assets. LLT could also be used as a potential strategy to help “break the chain,” by allowing orbiting or surface crew to manipulate samples without having direct contact with them. More generally, LLT will likely be a planetary protection risk management strategy suitable for use at Mars, including possibly from Mars orbit before crew landings (Lupisella and Race, 2018). The Moon offers a good opportunity to test these kinds of LLT methods in advance of using them at Mars (Bobskill et al., 2015). It is even possible (even though it is not currently planned) that LLT from the Gateway (or possibly the lunar surface base) could be used to examine contamination at the Apollo sites without having to introduce additional contamination so that we can baseline the contamination implications relative to the original Apollo missions for which we have a reasonable inventory of contamination sources (Glavin et al., 2004, 2010; Lupisella et al., 2018). LLT operations of this kind can also help address KG 3F by conducting detailed assessments of the degradation of materials at the Apollo sites.

6.4.2. Gateway

Given one of the present operations concepts that the crew landing vehicle (presently being developed under HLS noted earlier) will mate with the Gateway to transfer crew to the human lander before landing on the Moon, there is an opportunity to monitor and precisely measure the transfer of potential contaminants from the Gateway to the HLS vehicle and vice versa, both internally and even externally, as well as performing baseline monitoring of crew and systems. This kind of contaminant transfer could be important for potential martian architectures that may include crew transfers in Mars orbit. This monitoring and measuring, as well as testing of potential mitigation strategies, are particularly important for KGs 1A and 1B, but could also help address KGs 1D, 2A, 2B, 2C, 2D, and 2G.