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Planetary Protection Considerations for Missions to Solar System Small Bodies: Report Series—Committee on Planetary Protection (2023)

Chapter: 3 Criteria for Planetary Protection Categorization of Small Body Missions

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Suggested Citation:"3 Criteria for Planetary Protection Categorization of Small Body Missions." National Academies of Sciences, Engineering, and Medicine. 2023. Planetary Protection Considerations for Missions to Solar System Small Bodies: Report Series—Committee on Planetary Protection. Washington, DC: The National Academies Press. doi: 10.17226/26714.
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3

Criteria for Planetary Protection Categorization of Small Body Missions

Small bodies in the solar system have preserved information from different eras of the evolution of our solar system, from the composition of the presolar nebula that is preserved in cometary nuclei to the more evolved compositions found in some asteroids. As shown in Chapter 2, the small bodies include a wide variety of different asteroid types, the comets, Trojans, Centaurs, and Kuiper Belt objects (KBOs). Box 2-1 discusses the specific case of Ceres.

CRITERIA CONSIDERED FOR CATEGORIZATION

In considering planetary protection categorization for missions to the different types of small bodies, in particular between Category I and Category II, the committee considered the following criteria.1

Size of Population

The size of the population of any one subclass of bodies could be considered as a criterion for determining a mission’s planetary protection categorization. If a group of some type of small body is sufficiently large, one could postulate that missions to some percentage of that type of small body could possibly be allowed to be categorized as Category I given that there are plenty of other bodies of that group that could remain at Category II. However, the committee concluded that not enough is known about any one subclass to use the size of a population as a criterion for planetary protection categorization.

Status of Knowledge

The status of knowledge of any subclasses of small bodies might, in theory, be used to argue that missions to some small bodies can be classed as Category I rather than Category II because they are well-understood and therefore it might be considered acceptable to allow a mission to visit with no requirements for planetary protection documentation (Category I).

The issue here is that current knowledge of small bodies in the solar system is limited. Although some small bodies have been imaged with high resolution radar or adaptive optics data, currently most information about small solar system bodies is derived from ground-based observations that show the objects merely as points of light. Very little is known about Centaurs, Trojans, and even outer Main Belt asteroids (MBAs); these types of bodies never been visited. At the same time, each mission that has visited a small body has found it to be somehow unique and scientifically important; furthermore, taxonomic classes cannot be used with certainty to understand individual targets using Earth-based information alone. Thus, too little is known about any one class of small bodies to use current knowledge as the only criterion for planetary protection mission categorization. However, the committee notes that it

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1 Simultaneously, the committee considered the subtle differences between Category I and Category II, as discussed in Chapter 4.

Suggested Citation:"3 Criteria for Planetary Protection Categorization of Small Body Missions." National Academies of Sciences, Engineering, and Medicine. 2023. Planetary Protection Considerations for Missions to Solar System Small Bodies: Report Series—Committee on Planetary Protection. Washington, DC: The National Academies Press. doi: 10.17226/26714.
×

is important that the status of knowledge of small bodies be considered periodically and that planetary protection categorization be reconsidered as needed. For instance, nearly all missions to small bodies are currently classified as Category I or Category II. Some future observations of some body or class of bodies could make missions to those bodies appropriately classed as Category IV; for example, information from measurements on the presence of geologic activity. Alternately, future observations of some type of small body could indicate that follow-on missions to those types of bodies could be changed from Category II to Category I.

Likelihood of Revisiting

The foundational concept of planetary protection is the preservation of the scientific integrity of a body or region for future astrobiological studies of that body or region. If there is no likelihood that a target will be revisited for astrobiological study, there is no compelling reason to not categorize a mission to that target as Category I. However, the committee concluded, along the lines of the above criterion on status of knowledge, that one cannot assume, prior to ever visiting a target, that there will be no scientific reason to revisit it.

Geological Activity and Resurfacing

Currently, with some possible exceptions, the only known Main Belt small body demonstrating evidence of potential geologic activity (based on a very limited suite of spacecraft visits) is Ceres. Ceres is special because of the Dawn discovery of organics on the surface of Ceres, and because Dawn found Ceres to be a water-rich body with evidence of brine-driven cryovolcanism occurring very recently in history and perhaps even currently (see Box 2-1). Most of Ceres’s surface is more than a few hundred million years old, and a large fraction of it is >1 Gyr,2 based on crater counts. Occator crater, however, has a fresh morphology, and could experience transfer of material from the deep interior via impact-produced fractures, currently or in the recent past. More robust evidence for ongoing material exposure3 is in the form of hydrated salt (hydrohalite, NaCl·2H2O) found at the top of a dome in the center of one of Occator’s faculae.4 Because hydrated salts are not stable in vacuum at the temperature on Ceres’s surface and dehydrate over a short timescale (months or years),5 this material is likely currently being emplaced.

Comets are active planetary bodies because they display evidence for nearly all fundamental geological processes, which include impact cratering, tectonism, and erosion. Comets also display sublimation-driven outgassing, which is comparable to volcanism on larger planetary bodies in that it provides a conduit for delivering materials from the interior to the surface. However, in the domain of active geological bodies, comets occupy a special niche because their geologic activity is almost exclusively driven by externally supplied energy (i.e., solar energy) as opposed to an internal heat source, which makes them “seasonally active” bodies. During their active phase approaching the Sun, comets also develop a transient atmosphere that interacts with the surface and contributes to its evolution, particularly

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2 S.C. Mest, D.C. Berman, D.L. Buczkowski, et al., 2021, “The Global LAMO-Based Geologic Map of Ceres,” 2021 Annual Meeting of Planetary Geologic Mappers, 2610(7035), https://www.hou.usra.edu/meetings/pgm2021/eposter/7035.pdf.

3 J.C. Castillo-Rogez, M. Neveu, V. Vinogradoff, et al., 2022, “Science Drivers for the Future Exploration of Ceres: From Solar System Evolution to Ocean World Science,” The Planetary Science Journal 3(64), https://doi.org/10.3847/PSJ/ac502b.

4 M.C. De Sanctis, E. Ammannito, A. Raponi, et al., 2020, “Fresh Emplacement of Hydrated Sodium Chloride on Ceres from Ascending Salty Fluids,” Nature Astronomy 4:786–793, https://doi.org/10.1038/s41550-020-1138-8.

5 C. Bu, G. Rodriguez Lopez, C.A. Dukes, L.A. McFadden, J.-Y. Li, and O. Ruesch, 2017, “Instability of Magnesium Sulfate Hexahydrate (MgSO4.6H2O) on Ceres: Laboratory Measurements,” LPI Contributions 2996, 48th Annual Lunar and Planetary Science Conference, https://www.hou.usra.edu/meetings/lpsc2017/pdf/2996.pdf.

Suggested Citation:"3 Criteria for Planetary Protection Categorization of Small Body Missions." National Academies of Sciences, Engineering, and Medicine. 2023. Planetary Protection Considerations for Missions to Solar System Small Bodies: Report Series—Committee on Planetary Protection. Washington, DC: The National Academies Press. doi: 10.17226/26714.
×

by transporting materials across the surface. Variations in solar energy input on diurnal and seasonal scale cause buildup of thermal stresses within consolidated materials that lead to weathering through fracturing, and eventually mass-wasting. The commonly irregular shapes of comets also play a major role in their evolution by leading to

  1. Non-uniform gravitational forces that affect material movement across the surface; and
  2. Spatially heterogeneous outgassing patterns that affect the comet’s orbital dynamics and lead to tidal stresses that can further fracture the nucleus.6

The literature discusses the seasonal mass transfer on the nucleus of 67P/C-G, which shows the complexity of the problem. To date, data taken by Rosetta provide the only set of observations of a comet nucleus along its orbit with the onset of activity, through perihelion passage until the next “dormant” phase at large heliocentric distance. An average erosion of the surface of about 0.4 m was determined,7 depending on the model used; however, this value for surface erosion can reach even 20 m during a single perihelion passage due to the strong insolation around the comet’s southern summer solstice. Taking a surface erosion or ablation of about 1 m for a comet nucleus as an average per orbit around the Sun is a conservative estimate. This means that the surface of a comet nucleus changes continuously along its orbit when it is inside 3 AU and a revisiting spacecraft will thus see a totally different surface morphology, and perhaps composition.

For some geologically active bodies, resurfacing can imply a “freshening” of the surface between spacecraft visits. This is likely the case for comets and could be the case for some active asteroids. One can consider that a body or region that is frequently resurfaced could be a prime candidate for Category I missions, because there may be no need for contamination control. Alternately, such resurfacing could, perhaps, be a mechanism for burial of terrestrial microbes, possibly to a subsurface location where they can survive.

Size of Target Body

Larger asteroids (≳100 km diameter) are generally considered to be primordial whereas smaller bodies are likely collisional fragments, as discussed in Chapter 2. Thus, in investigations of original, unaltered primitive material that is left over from the early solar system, these larger small bodies are more likely to hold unprocessed or unaltered evidence of the early solar system environment. Larger targets are thus considered to hold more astrobiological interest for potential future science missions than small fragmentary bodies.

Composition

As noted in Finding 1 of this report, small bodies throughout the solar system are primarily astrobiologically important not because they could harbor life but because they hold compositional clues to the early solar system and to the distribution of organics and other volatiles that are the building blocks of life and that could have brought water and life to Earth. Consequently, the committee considers those small bodies that are volatile-rich and/or organic-rich to be more astrobiologically important than the rocky types. The volatile- and/or organic-rich small bodies are generally of the C-complex group (C-,

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6 M.R. El-Maarry, O. Groussin, H.U. Keller, et al., 2019, “Surface Morphology of Comets and Associated Evolutionary Processes: A Review of Rosetta’s Observations of 67P/Churyumov-Gerasimenko,” Space Science Reviews 215(36), https://doi.org/10.1007/s11214-019-0602-1.

7 H.U. Keller, S. Mottola, S.F. Hviid, et al., 2017, “Seasonal Mass Transfer on the Nucleus of Comet 67P/Chuyumov-Gerasimenko,” Monthly Notices of the Royal Astronomical Society 469:S357–S371, https://doi.org/10.1093/mnras/stx1726.

Suggested Citation:"3 Criteria for Planetary Protection Categorization of Small Body Missions." National Academies of Sciences, Engineering, and Medicine. 2023. Planetary Protection Considerations for Missions to Solar System Small Bodies: Report Series—Committee on Planetary Protection. Washington, DC: The National Academies Press. doi: 10.17226/26714.
×

Cb-, Ch-, Cg-, Cgh-, and B-types) or the P- and D- types, primarily with semi-major axes of >2.5 AU (Figure 2-5), while the rocky types are generally S-types; numerous other taxonomic types have been identified, including the metallic M-types. Comets are likely even more primordial than asteroids, given their origin in the Kuiper Belt or Oort Cloud; thus, despite (or because of) their active resurfacing, the primitive nature of comets makes them astrobiologically important. Ceres has been found to have local surface deposits of organic matter,8 adding to the astrobiological interest of this special world.

The committee concludes that target composition (namely, taxonomic type for asteroids) is the primary factor to use in categorization decisions. The committee does not consider Trans-Neptunian objects (TNOs) beyond 50 AU in this assessment. Furthermore, the committee concludes that planetary protection assessment for missions to Pluto and other KBOs dwarf planets will no doubt need to be considered once such a mission is prioritized; the committee anticipates that any such mission will be a scientific mission and thus scientific requirements will likely outweigh planetary protection cleanliness requirements.

Finding 3: The committee does not find a need to change current categorization of missions to small bodies. Category II is an appropriate planetary protection category for missions to relatively primitive, volatile-rich, and organic-bearing small bodies that have astrobiological importance—including C-complex (C-, Cb-, Ch-, Cg-, Cgh-, and B-types), P-type, and D-type MBAs and near-Earth objects (NEOs), Trojans, comets, KBOs, and Centaurs. These objects have the potential to provide insights about prebiotic chemistry. Category II requires the provision of information that is important for future missions to the same targets, such as spacecraft impact or landing sites. The chemistry of other small bodies is likely not of astrobiological interest, and Category I is an appropriate category for missions to these objects, including rocky, metamorphosed, and metallic NEOs and MBAs.

Finding 4: Current scientific knowledge regarding some large asteroids (e.g., low-albedo objects ≳100 km in diameter and having an orbital semi-major axis greater than ~2.5 AU) is not sufficient to support well-informed categorization of missions to those objects, but Category II is acceptable until future reassessment. Ceres is a notable example of a large object with recently discovered importance to astrobiology and thus future missions to Ceres merit reassessing in terms of planetary protection categorization. Future missions to Ceres will likely require more rigorous planetary protection protocols than Category II.

Finding 5: The committee endorses the periodic reassessment of the planetary protection categorization scheme for all small bodies on a regular cadence, allowing for the most recent science information to be taken into account.

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8 M.C. De Sanctis, E. Ammannito, H.Y. McSween, et al., 2017, “Localized Aliphatic Organic Material on the Surface of Ceres,” Science 355:719–722, https://doi.org/10.1126/science.aaj2305.

Suggested Citation:"3 Criteria for Planetary Protection Categorization of Small Body Missions." National Academies of Sciences, Engineering, and Medicine. 2023. Planetary Protection Considerations for Missions to Solar System Small Bodies: Report Series—Committee on Planetary Protection. Washington, DC: The National Academies Press. doi: 10.17226/26714.
×
Page 27
Suggested Citation:"3 Criteria for Planetary Protection Categorization of Small Body Missions." National Academies of Sciences, Engineering, and Medicine. 2023. Planetary Protection Considerations for Missions to Solar System Small Bodies: Report Series—Committee on Planetary Protection. Washington, DC: The National Academies Press. doi: 10.17226/26714.
×
Page 28
Suggested Citation:"3 Criteria for Planetary Protection Categorization of Small Body Missions." National Academies of Sciences, Engineering, and Medicine. 2023. Planetary Protection Considerations for Missions to Solar System Small Bodies: Report Series—Committee on Planetary Protection. Washington, DC: The National Academies Press. doi: 10.17226/26714.
×
Page 29
Suggested Citation:"3 Criteria for Planetary Protection Categorization of Small Body Missions." National Academies of Sciences, Engineering, and Medicine. 2023. Planetary Protection Considerations for Missions to Solar System Small Bodies: Report Series—Committee on Planetary Protection. Washington, DC: The National Academies Press. doi: 10.17226/26714.
×
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The ultimate goal of planetary protection for outbound missions is to prevent harmful contamination that would inhibit future measurements designed to search for evidence of the existence or evolution of extraterrestrial life. Preventing harmful contamination is achieved by following specific guidelines based on existing scientific knowledge about the destination and the type of mission. This report responds to NASA's request for a study on planetary protection categorization of missions to small bodies, including whether there are particular populations of small bodies for which contamination of one object in the population would not be likely to have a tangible effect on the opportunities for scientific investigation using other objects in the population. In addressing NASA's request, the authoring committee considered surface composition of target bodies and their importance for prebiotic chemistry, along with size of the small-body populations, the current state of knowledge on the types of objects, the likelihood of a future scientific mission returning to any specific object, active object surface processes, and the size.

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