In 2016, the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) issued parallel requests to the National Academies and the European Science Foundation (ESF), respectively, to undertake a study to determine the planetary protection classification of robotic sample return missions to the martian moons. In response to these requests to their parent organizations, the Space Studies Board (SSB) and the European Space Science Committee (ESSC) established a joint committee to address the requested tasks. (See “Statement of Task” in the Preface.)
Chapter 1 provides background to the statement of task and is organized in five sections: planetary protection policies, current understanding of the martian moons, background on martian meteorites, the Japan Aerospace Exploration Agency (JAXA) planned Martian Moons Exploration (MMX) mission, and a brief overview of research in support of MMX conducted by ESA (via the so-called SterLim team) and JAXA.
Chapter 2 contains a detailed overview of the work conducted in support of the planetary protection aspects of MMX by JAXA and the SterLim team sponsored by ESA. Chapter 2 also includes the committee’s detailed critique and assessment of the research activities undertaken by the JAXA and SterLim teams.
Chapter 3 summarizes the committee’s assessment of the JAXA and SterLim methodology, assumptions, and findings. This final chapter also investigates some additional arguments regarding planetary protection requirements for a sample return mission from the martian moons, and contains the committee’s recommendations.
The first item in the committee’s statement of task was as follows (see Preface):
- Review, in the context of current understanding of conditions relevant to inactivation of carbon-based life, recent theoretical, experimental, and modeling research on the environments and physical conditions encountered by Mars ejecta during the following processes:
- Excavation from the martian surface via crater-forming events;
- While in transit through cismartian space;
- During deposition on Phobos or Deimos; and
- After deposition on Phobos or Deimos.
In this context, the committee reviewed the work of the SterLim and JAXA teams and issued the following findings:
- Even if life exists on Mars, the cell density and even its biochemical nature is unknown. Therefore, the value employed by the SterLim and JAXA teams is, as appropriate for a planetary protection calculation, a very conservative estimated based on current understanding of life as it exists in Mars-like extreme environments on Earth. (See “Potential Microbial Density on the Martian Surface” in Chapter 2.)
- The reason for the significant discrepancy in the amount of material transported to the martian moons as determined by the SterLim and JAXA teams could not be identified. Nevertheless, these uncertainties represent, in some sense, the current state of the art. (See “Mars Ejecta Formation and Transportation from the Martian Surface” in Chapter 2.)
- Shock heating during impacts is a highly localized process. When trying to resolve this adequately in numerical simulations, very high spatial resolutions are required. (See “Sterilization During Mars Ejecta Formation” in Chapter 2.)
- The survival rate during hypervelocity impacts cannot be determined based on the information available. However, the proposed survival rate of 10 percent is a reasonable estimate; albeit one lacking significant experimental evidence. (See “Sterilization During Mars Ejecta Formation” in Chapter 2.)
- The JAXA team’s conclusion that particles smaller than 10 cm do not escape the martian atmosphere is not well supported. Therefore, subsequent analyses relying on this limit should be treated with care. (See “Sterilization by Aerodynamic Heating of Mars Ejecta” in Chapter 2.)
- The JAXA team’s conclusion that aerodynamic heating of ejecta during passage through the martian atmosphere does not cause any significant sterilization is valid. (See “Sterilization by Aerodynamic Heating of Mars Ejecta” in Chapter 2.)
- The experimental hypervelocity impact data generated during the SterLim study is limited with respect to the large spectrum of possible impact conditions on the martian moons, could be biased, and is not conclusive. Given the small footprint of the data within the vast parameter space, extrapolations drawn from the experimental data currently available seemed ill-advised. SterLim’s impact data was used to calibrate the exponential function used by the JAXA group to estimate and extrapolate the likely sterilization due to impact. (See Sterilization During Hypervelocity Impact on Phobos/Deimos Surfaces” in Chapter 2.)
- The estimations of the two teams as to the distribution and fate of Mars ejecta fragments deposited on the martian moons were based on different and limited experimental data. Therefore, a factor of uncertainty remains in the fraction deposited at the first impact. (See “Distribution of Mars Ejecta Fragments by Impacts, Recirculation, and Reimpact” in Chapter 2.)
- The SterLim team’s use of aluminum, rather than a chemically inert surface, as a simulant environment for irradiation on Phobos/Deimos is problematic. In addition, the samples were irradiated in a frozen state, whereas the surface temperatures on the surfaces of the martian moons is frequently above the freezing point of water. (See “Sterilization by Radiation on Phobos/Deimos Surfaces” in Chapter 2.)
- Diurnal temperature cycling is an extremely significant factor in determining the survival of martian organisms deposited on the surfaces of Phobos or Deimos. Desiccation is bactericidal to even the most radiation-resistant microbes in a matter of months. (See “Sterilization by Radiation on Phobos/Deimos Surfaces” in Chapter 2.”)
- The effect of meteoroid impacts following deposition of martian material on the surface of Phobos and Deimos has a minimal sterilizing effect due to the low flux of impactors. However, the fragmentation of ejecta due to the effects of thermal fatigue could significantly enhance the rate at which any organic matter present is degraded by exposure to the radiation. (See “Phobos/Deimos Surface Reformation by Natural Meteoroid Impacts” in Chapter 2.)
The second item in the committee’s statement of task was as follows (see Preface):
- Recommend whether missions returning samples from Phobos and/or Deimos should be classified as “restricted” or “unrestricted” Earth return in the framework of the planetary protection policy maintained by the ICSU Committee on Space Research (COSPAR).
A key factor in answering this question focused on whether or not an unidentified large (>10 km), young (<<1 million years) crater might exist on Mars. The committee finds that it is highly unlikely that such a large, young crater exists and has somehow escaped detection. (See Task 2 in Chapter 3.)
In determining whether samples returned from Phobos or Deimos should be classified as restricted or unrestricted Earth return, the committee considered the following factors:
- The work of the SterLim and JAXA teams can be considered as state of the art in regard to the modeling of the process of deposition of martian material on the surface of the martian moons. However, significant deficiencies exist in understanding, and there remain experimental and computational challenges associated with the quantitative estimation of ejecta mass and temperature distributions. Nevertheless, their work is convincing in showing that there is significant sterilization introduced throughout the chain of events. (See Task 2 in Chapter 3.)
- The issue of desiccation—as a result of diurnal thermal cycling on the surface of the martian moons—on any martian microbes was not considered by the SterLim and JAXA teams. At temperatures above the freezing point of water, desiccation is bactericidal to even the most radiation-resistant microbes in a matter of months. (See Task 2 in Chapter 3.)
- The relative influx of martian microbes from a Phobos/Deimos sample return mission versus the natural influx of direct Mars-to-Earth transfer can be shown to be smaller by several orders of magnitude. (See Task 2 in Chapter 3.)
Recommendation: After considering the body of work conducted by the SterLim and JAXA teams, the effect of desiccation on the surfaces of the martian moons, and the relative flux of meteorite- to spacecraft-mediated transfer to Earth, the committee recommends that samples returned from the martian moons be designated unrestricted Earth return.
The third item in the committee’s statement of task was as follows (see Preface):
- In what specific ways is classification of sample return from Deimos a different case than sample return from Phobos?
The different orbits and cross-sectional areas of Phobos and Deimos result in differences in the velocities associated with impacts of martian ejecta to their surfaces and in the total mass of material delivered to each moon. Both factors affect the total likelihood that microbes could survive delivery to the moons from Mars, and therefore raise the important question of whether Phobos and Deimos should be treated differently with respect to planetary protection requirements. While the studies conducted by the JAXA team did suggest that more martian material was likely to be present on Phobos than on Deimos, they also suggested that more organisms could theoretically survive transfer from Mars to Deimos. However, the latter conclusion was strongly dependent on the specific ejection geometries and velocities associated with modeling of a particular impact on Mars. (See Task 3 in Chapter 3.)
Recommendation: Given the uncertainty associated with impact sterilization assumptions, the committee recommends that Phobos and Deimos should not currently be treated differently in their planetary protection requirements.
The fourth item in the committee’s statement of task was as follows (see Preface):
- What relevant information for classification of sample return is available from published studies of martian meteorites on Earth?
An overview of the literature is included in Chapter 1 (see “Earth Inventory of Martian Meteorites”). The committee finds that the study of martian meteorites provides important context for studies of Mars and its moons
and limited information (e.g., mass and flux to Earth) of relevance to planetary protection considerations. The unambiguous detection of an indigenous martian organism in a meteorite would be of great scientific and societal significance. (See Task 4 in Chapter 3.)
The fifth item in the committee’s statement of task was as follows (see Preface):
- What are the planetary protection consequences of taking a surface sample at depths of 0-2 cm versus taking a sample extending down to depths of 2-10 cm or deeper?
The committee identified two factors that could cause microbial survival probabilities to be different in these two depth ranges: ultraviolet irradiation and diurnal temperature cycling. Irradiation decreases microbe survival rates at the surface of Phobos or Deimos, but such radiation is attenuated within the top few millimeters of surface material. Therefore, this effect has no impact on sampling depth. Diurnal temperature changes are a significant factor in the top few cm. Therefore, samples from shallower depths on Phobos or Deimos have a lower risk for microbial contamination that those at a greater depth due to sterilization by thermal cycling. However, this additional factor is not needed to give confidence that samples from 2-10 cm depth will be below the established planetary protection limits for expected microbial contamination. (See Task 5 in Chapter 3.)
Recommendation: The committee recommends that no differences need to be made in planetary protection requirements for samples collected on the martian moons from depths 0-2 cm versus samples from 2-10 cm.
The sixth item in the committee’s statement of task was as follows (see Preface):
- Suggest any other refinements in planetary protection requirements that might be required to accommodate spacecraft missions to and samples returned from Phobos or Deimos.
With respect to this last task, the committee limits its response to comments on three specific topics: uncertainty quantification, implications of the present work for Mars sample return missions, and the need to publish the work undertaken by the SterLim and JAXA teams.
Uncertainty quantification—The work of the SterLim and JAXA teams are prime examples of attempts to reach a specific conclusion about real-world activities based on combining the results from multiple numerical simulations and laboratory experiments. Each individual calculation or experiment is subject to various degrees of uncertainty. The science of uncertainty quantification seeks to determine the likelihood of specific outcomes for a system given that specific aspects of it are unknown or only weakly constrained. (See Task 6 in Chapter 3.)
Recommendation: The committee recommends that a significant effort be made by the planetary protection community to formally develop an uncertainty quantification protocol that can be used to estimate the cascading uncertainties that result from the integration of multiple computational models or other factors relevant to the quantitative aspects of planetary protection. Specific attention should be given to consideration of the significant uncertainties in the model inputs that exist because of limited available experimental or observational data.
Implications for Mars sample return—What implications for a Mars sample return (MSR) mission can be drawn from this study and the work of the JAXA and SterLim teams? The main differences between MSR and Phobos/Deimos sample return missions are as follows:
- MSR sampling sites will be specifically selected to maximize sampling of evidence of extinct or extant life, whereas materials deposited on the martian moons originates from randomly distributed crater impact sites.
- Martian material present in a Phobos/Deimos sample would have undergone several physical sterilization processes (e.g., excavation by impact, collision with Phobos, and exposure to radiation), before it is actually sampled. Material collected on the surface of Mars will not have undergone such processes.
- MSR material might come from sites that mechanically cannot survive ejection from Mars and thus any putative life-forms would de facto not be able to survive impact ejection and transport to space. Such mechanical limitations do not apply for material collected on Mars.
Therefore, the committee finds that the content of this report and, specifically, the recommendations presented in it do not apply to future sample return missions from Mars itself. (See Task 6 in Chapter 3.)
Publication of the work of the SterLim and JAXA teams—The planetary protection, astrobiology, and planetary science communities would greatly benefit from the publication of the work undertaken by the SterLim and JAXA teams if for no other reason than to demonstrate the care and attention given to the investigation of planetary protection issues. (See Task 6 in Chapter 3.)
Recommendation: The committee recommends that the SterLim and JAXA teams formally publish the details of and results from their studies or make them readily available in some publicly accessible form.