Study of the History of the Solar System and Its Prebiotic Chemistry: Permanently Shadowed Regions on the Moon
The Moon’s polar axis is nearly perpendicular to the ecliptic plane (axial tilt = 1.5°) and as a result, some cratered polar regions are continually in shadow. Some of these regions have been in shadow for as much as two billion years or more1 and are referred to as permanently shadowed regions (PSRs). Consequently, PSRs may have stored the scientific record of volatiles from many sources, including early lunar outgassing events, asteroids and comets encountering the Earth-Moon system, and ongoing solar wind interactions with the lunar regolith. Cometary and meteoroid impacts on the Moon over the history of the solar system brought in a range of volatiles, including water and organics. Among these volatiles could be prebiotic chemical components that also seeded Earth. Material from comets and chondritic or carbonaceous meteorites that impacted the Moon may be protected in PSRs or the subsurface and serve as inventories of volatiles, including some organics, that penetrated the early solar system.2 Thus, PSRs in the lunar polar regions are of considerable astrobiological interest for their potential role in storing prebiotic chemicals and serving as a window to the early evolution of Earth and the other terrestrial planets and to the emergence of life.
CURRENT SCIENTIFIC UNDERSTANDING
Knowledge of Volatiles Likely to be Present on the Moon
Knowledge of the lunar polar regions has grown substantially since the potential for retention of water and other volatiles within PSRs was first proposed by Urey in 1952 and detailed by Watson et al. in 1961.3,4 The areal extent of PSRs in the polar regions is substantial—in excess of 104 km2.5 Prior research has provided estimates of vacuum evaporation rates for representative organic and inorganic compounds, including molecular compounds and elements that could be present in the polar regions of the Moon.6 With the exception of sulfur, most inorganic compounds are more volatile than water. Aromatic hydrocarbons, linear amides, and carboxylic acids are also less volatile than water. Most simple organics
1 M.A. Siegler, R.S. Miller, J.T. Keane, M. Laneuville, D.A. Paige, I. Matsuyama, D.J. Lawrence, A. Crotts, and M.J. Poston, 2016, Lunar true polar wander inferred from polar hydrogen, Nature 531: 480-484, doi:10.1038/nature17166.
2 I.A. Crawford, M. Anand, C.S. Cockell, H. Falcke, D.A. Green, R. Jaumann, and M.A. Wieczorek, 2012, Back to the Moon: The scientific rationale for resuming lunar surface exploration, Planetary and Space Science 74(1): 3-14, https://doi.org/10.1016/j.pss.2012.06.002.
3 H.C. Urey, 1952, On the early chemical history of the Earth and the origin of life, Proceedings of the National Academy of Sciences U.S.A. 38: 351-363, 010.1073/pnas.38.4.351.
4 K. Watson, B.C. Murray, and H. Brown, 1961, The behavior of volatiles on the lunar surface, Journal of Geophysical Research 66(9): 3033-3045, doi:10.1029/JZ066i009p03033.
5 P.O. Hayne, O. Aharonson, and N. Schörghofer, 2020, Micro cold traps on the Moon, Nature Astronomy, doi:10.1038/s41550-020-1198-9.
6 J.A. Zhang and D.A. Paige, 2010, Correction to “Cold-trapped organic compounds at the poles of the Moon and Mercury: Implications for origins,” Geophysical Research Letters 37: L03203, doi:10.1029/2009GL041806.
and clathrates are more volatile than water. However, because the temperatures in the PSRs can be so low (<50 K), with seasonal temperatures between 40 and 110 K,7 various volatiles—for example, H2O, NH3, CO2, SO2, CO, and H2S—are stable against sublimation and can exist for billions of years if undisturbed.
There is remote sensing evidence that some of these expected volatiles are present within lunar PSRs, including the presence of water surface deposits8,9 and possible surface CO2 deposits.10 Anomalous 1064 nm reflectance values from polar surfaces at temperatures of around 200 K, and possibly 300 K, were found, and the results suggested that these reflectance values may arise from the presence of volatiles stable at higher temperatures, such as sulfur or organics.11
Observations from the Lunar Crater Observation and Sensing Satellite (LCROSS) mission, which produced an impact in Cabeus crater, indicated a number of volatiles relevant to questions of prebiotic chemistry and the origin of life, including H2O, CO2, H2S, SO2, NH3, CO, Hg, and H2. There is also an indication of some simple hydrocarbons, including CH4, C2H4, and CH3OH (see Table 2.1).12 CH4 is found in the exosphere and will likely contribute to concentrations of CH4 in polar cold traps.
Measurements by the mass spectrometers on both Apollo 17 and LADEE spacecraft detected exospheric CH4, some of which could make it to stable locations in the PSRs. The abundances of the various molecules relative to water have some similarities to comet inventories. However, the presence of sulfur compounds may be indicative of endogenic (volcanic) sources.13 There also may be similarities to abundances in the molecular environment of high-mass, star-forming regions.14
Analyses of Apollo lunar samples, although not from the PSRs, identified amino acids at low concentrations, ranging from 0.2 ppb to 42.7 ppb. Compound-specific carbon isotopic ratios of glycine indicate that the primary source of these amino acids is terrestrial contamination.15 However, nonproteinogenic amino acids are also observed, which suggests the possibility of some contribution at the PSRs from exogenous sources such as carbonaceous chondritic micrometeoroids, comets, and interplanetary dust particles (IDPs).
7 P.O. Hayne, A.M. Siegler, D. Paige, P. Lucey, and E. Fisher, 2019, “Carbon Dioxide Frost at the Poles of the Moon: Thermal Stability and Observational Evidence from the Lunar Reconnaissance Orbiter,” 50th Lunar and Planetary Science Conference 2019, LPI Contrib. No. 2132, https://www.hou.usra.edu/meetings/lpsc2019/pdf/2628.pdf.
8 S. Li, P.G. Lucey, R.E. Milliken, P.O. Hayne, E. Fisher, J.-P. Williams, D.M. Hurley, and R.C. Elphic, 2018, Direct evidence of surface exposed water ice in the lunar polar regions, Proceedings of the National Academy of Sciences U.S.A. 115: 8907-8912, doi:10.1073/pnas.1802345115.
9 E.A. Fisher, P.G. Lucey, M. Lemelin, B.T. Greenhagen, M.A. Siegler, E. Mazarico, O. Aharonson, J. Williams, P.O. Hayne, G.A. Neumann, D.A. Paige, D.E. Smith, and M.T. Zuber, 2018, “Evidence for Surface Water Ice in the Lunar Polar Regions Using Reflectance Measurements from the Lunar Orbiter Laser Altimeter and Temperature Measurements from the Diviner Lunar Radiometer Experiment,” Lunar Polar Volatiles 2018, LPI Contrib. No. 2087, https://www.hou.usra.edu/meetings/lunarvolatiles2018/pdf/5011.pdf.
10 Hayne et al., 2019.
11 Fisher et al., 2018.
12 G.R. Gladstone, D.M. Hurley, K.D. Retherford, P.D. Feldman, W.R. Pryor, J.-Y. Chaufray, M. Versteeg, et al., 2010, LRO-LAMP observations of the LCROSS impact plume, Science 330: 472, doi:10.1126/science.1186474.
13 P.G. Lucey, E. Costello, D.M. Hurley, P. Prem, W.M. Farrell, N. Petro, and M. Cable, 2020, “Relative Magnitudes of Water Sources to the Lunar Poles,” 51st Lunar and Planetary Science Conference 2020, https://www.hou.usra.edu/meetings/lpsc2020/pdf/2319.pdf.
14 A. Colaprete, P. Schultz, J. Heldmann, D. Wooden, M. Shirley, K. Ennico, B. Hermalyn, et al., 2010, Detection of water in the LCROSS ejecta plume, Science 330: 463, doi:10.1126/science.1186986.
15 J.E. Elsila, M.P. Callahan, J.P. Dworkin, D.P. Glavin, H.L. McLain, S.K. Noble, and E.K. Gibson, 2016, The origin of amino acids in lunar regolith samples, Geochimica et Cosmochimica Acta 172: 357-369, doi:10.1016/j.gca.2015.10.008.
TABLE 2.1 Abundances of Volatiles, Relative to Water, Detected from the LCROSS Mission
|Compound||Percent Relative to Water Vapor|
SOURCE: E.A. Fisher, P.G. Lucey, M. Lemelin, B.T. Greenhagen, M.A. Siegler, E. Mazarico, O. Aharonson, J. Williams, P.O. Hayne, G.A. Neumann, D.A. Paige, D.E. Smith, and M.T. Zuber, 2018, “Evidence for Surface Water Ice in the Lunar Polar Regions Using Reflectance Measurements from the Lunar Orbiter Laser Altimeter and Temperature Measurements from the Diviner Lunar Radiometer Experiment,” Lunar Polar Volatiles 2018, LPI Contrib. No. 2087, https://www.hou.usra.edu/meetings/lunarvolatiles2018/pdf/5011.pdf.
Distribution of Volatiles at the Lunar Poles
There are many unknowns about the distribution of lunar polar volatiles, given differences in data sets, sensing depths, and spatial coverage. The distribution of lunar volatiles is thought to be heterogeneous, both horizontally and vertically, within the PSRs, with water molecules adsorbed onto small surface grains, trapped in minerals, condensed as ice, and buried in patchy and discrete depth horizons in the subsurface. Estimates on the abundance and thickness of ice deposits varies from about 1 to 2 percent by weight to 1 m depth, to ~30 to 50 percent ice by mass at more substantial depths (on the order of 10 m). Although the prevalence of regions of thick ice has not been confirmed, up to 100 billion metric tons of water ice may be contained within a few meters of the lunar surface.16
Orbital observations do not indicate that there are extensive contiguous ice sheets in the top meter of regolith, in striking contrast to Mercury. Interstitial ice at the few percent level is more likely. There is the possibility of larger blocky exposed boulders at the decimeter scale.17,18 The vertical extent and heterogeneity at scales less than tens of kilometers is not known. Impact gardening19 probably generates significant heterogeneity in the lateral distribution at scales between 10 and 100 m20 on timescales of tens of thousands of years.
17 LRO Miniature Radio Frequency radar observations of circular polarization may indicate this in the Cabeus crater region.
18 C.D. Neish, D.B.J. Bussey, P. Spudis, W. Marshall, B.J. Thomson, G.W. Patterson, and L.M. Carter, 2011, The nature of lunar volatiles as revealed by Mini-RF observations of the LCROSS impact site, Journal of Geophysical Research (Planets) 116: E01005, doi:10.1029/2010JE003647.
19 Gardening is the process by which the top layers of lunar surface are mixed and disturbed when impacts form new craters. See D.M. Hurley, D.J. Lawrence, D. Benjamin, J. Bussey, R.R. Vondrak, R.C. Elphic, G. Randall Gladstone, 2012, Two-dimensional distribution of volatiles in the lunar regolith from space weathering simulations, Geophysical Research Letters 39: doi:10.1029/2012GL051105, and E.S. Costello, R.R. Ghent, M. Hirabayashi, and P.G. Lucey, 2020, Impact gardening as a constraint on the age, source, and evolution of ice on Mercury and the Moon, Journal of Geophysical Research (Planets) 125(3): e06172. doi:10.1029/2019JE006172.
20 K.M. Cannon and D.T. Britt, 2020, A geologic model for lunar ice deposits at mining scales, Icarus 347: 113778, doi:10.1016/j.icarus.2020.113778.
Orbital neutron observations can imply that subsurface water is present,21,22 with concentrations ranging from 150 ppm to several percent. Some craters exhibit a morphology suggesting the possibility of significant water at depth.23 Recent analysis of the LCROSS debris plume is consistent with as much as 10 percent water ice buried several meters deep.24 Such deeper water deposits may be a relic of early deposition of water.25 Whether such deeper massive ice deposits exist, how they are related to the surface deposits, whether the two putative reservoirs have similar or different sources and sinks, and whether there is exchange between the two, are not clear.
Different sources of volatiles are expected to result in different distributions of frozen material on the lunar surface. Solar wind-derived volatiles, continually delivered, may result in frozen volatiles at the surface. Impact-delivered volatiles might be present in the subsurface in a patchy distribution. Internally derived volatiles may form discrete but broader horizons of volatiles in the subsurface26 (see Figure 2.1).
21 W.C. Feldman, S. Maurice, A.B. Binder, B.L. Barraclough, R.C. Elphic, and D.J. Lawrence, 1998, Fluxes of fast and epithermal neutrons from lunar prospector: Evidence for water ice at the lunar poles, Science 281: 1496.
22 D.J. Lawrence, 2017, “Permanently Shaded Regions: Future Exploration of a Unique Solar System Environment,” Planetary Science Vision 2050 Workshop 2017, LPI Contrib. No. 1989, https://www.hou.usra.edu/meetings/V2050/pdf/8053.pdf.
23 Rubanenko et al., 2019.
24 K.M. Luchsinger, N.J. Chanover, and P.D. Strycker, 2021, Water within a permanently shadowed lunar crater: Further LCROSS modeling and analysis, Icarus 354: 114089, doi:10.1016/j.icarus.2020.114089.
25 Siegler et al., 2016.
26 Rubanenko et al., 2019.
Comparison with Other Solar System Bodies
Because there have been several observations of organics, of varying complexity, throughout the solar system at low temperatures, it can be useful to consider the organic compounds likely present in the lunar polar regions, in the context of organics elsewhere in the solar system. Observations of the north pole of Mercury, including by radar, neutron spectroscopy, and imaging from the MESSENGER spacecraft, show significant amounts of water ice accumulated in PSRs. Dark mantles on Mercury are associated with the ice deposits that might be organic material that were either generated in situ or delivered by external sources, including impacts of comets, asteroids, and IDPs.27 Similar evidence exists for water ice within lunar PSRs. However, these lunar deposits do not appear to be as ubiquitous as the Mercury deposits, with large variability between PSRs. Furthermore, there does not appear to be any dark material associated with lunar water deposits. The explanation for these differences is an area of active research, with possible reasons including lunar deposits being much older and/or differences in chemical processing mechanisms associated with Mercury’s magnetic field.
Additional examples of solar system organic detections include the dwarf planet Ceres, which exhibits spectral signatures of aliphatic organics in specific locations on its surface.28 Organic grains have been discovered at comets Halley29 and 67P.30 Saturn’s moon Enceladus produces a plume of water and organic material from the subsurface ocean and disperses these grains, via the E ring,31 throughout the inner Saturnian system.32 Titan, Saturn’s largest moon, hosts a wide inventory of organic molecules in its atmosphere, on the surface, and in its lakes. The leading hemisphere of Saturn’s moon Iapetus appears to be continually bombarded by organic-rich dust from its outer companion Phoebe.33 Some outer main belt asteroids (e.g., Themis) have been shown to exhibit evidence for organic signatures on their surfaces.34,35 Organics are suspected to be responsible for optical reddening, for instance on regions of Europa and Charon.
To conclude, the lunar polar regions, along with Mercury, are important inner solar system end members for understanding transport of organics and other volatiles throughout the solar system, and they may provide a unique record of ancient volatiles brought to the Earth-Moon system. Little understanding exists, however, regarding the full suite of volatiles present, their quantities, ages, sources and time of introduction, and the processes affecting delivery, retention, and distribution.
27 M.L. Delitsky, D.A. Paige, M.A. Siegler, E.R. Harju, D. Schriver, R.E. Johnson, and P. Travnicek, 2017, Ices on Mercury: Chemistry of volatiles in permanently cold areas of Mercury’s north polar region, Icarus 281: 19-31, doi:10.1016/j.icarus.2016.08.006.
28 M.C. De Sanctis, E. Ammannito, F.G. Carrozzo, M. Ciarniello, M. Giardino, A. Frigeri, and S. Fonte, et al., 2018, Ceres’s global and localized mineralogical composition determined by Dawn’s Visible and Infrared Spectrometer (VIR), Meteoritics and Planetary Science 59: 9, https://doi.org/10.1111/maps.13104.
29 J. Kissel and F.R. Krueger, 1987, Organic dust in comet Halley, Nature 328: 117, doi:10.1038/328117b0.
30 F. Capaccioni, G. Filacchione, S. Erard, G. Arnold, D. Bockelee-Morvan, M.C. De Sanctis, C. Leyrat, et al., 2015, “The Nucleus and Coma of 67P/Churyumov-Gerasimenko: Highlights of the Rosetta-VIRTIS Results,” EGU General Assembly Conference Abstracts 17: 12375.
31 F. Postberg, N. Khawaja, B. Abel, G. Choblet, C.R. Glein, M.S. Gudipati, B.L. Henderson, et al., 2018, Macromolecular organic compound s from the depths of Enceladus, Nature 558: 564-568, doi:10.1038/s41586-0180246-4.
32 A.R. Hendrix, G. Filacchione, C. Paranicas, P. Schenk, and F. Scipioni, 2018, Icy Saturnian satellites: Disk-integrated UV-IR characteristics and links to exogenic processes, Icarus 300: 103-114, doi:10.1016/j.icarus.2017.08.037.
33 D.P. Cruikshank, C.M. Dalle Ore, R.N. Clark, and Y.J. Pendleton, 2014, Aromatic and aliphatic organic materials on Iapetus: Analysis of Cassini VIMS data, Icarus 233: 306-315, doi:10.1016/j.icarus.2014.02.011.
34 H. Campins, K. Hargrove, N. Pinilla-Alonso, E.S. Howell, M.S. Kelley, J. Licandro, T. Mothé-Diniz, Y. Fernández, and J. Ziffer, 2010, Water ice and organics on the surface of the asteroid 24 Themis, Nature 464: 1320-1321, doi:10.1038/nature09029.
35 A.S. Rivkin and J.P. Emery, 2010, Detection of ice and organics on an asteroidal surface, Nature 464: 1322-1323, doi:10.1038/nature09028.
SCIENTIFIC VALUE OF PSR VOLATILES
The LEAG Response to the COSPAR Planetary Protection Inquiry described volatiles trapped in lunar PSRs as “of significant interest in the study of chemical evolution (i.e., primordial chemistry as precursor to the origin of life on Earth and potentially other planets, moons and planetary bodies).”36 There was a general consensus in the U.S. and non-U.S. responses to the COSPAR survey that lunar PSRs “represent maybe the most accessible reservoir of solar system volatiles and could potentially answer fundamental questions about volatile transport to the Earth-Moon system.”37
These interests point to the Moon as a natural laboratory and the lunar polar regions being of particular importance, for example, for studying the effects of radiation on silicate grains at very low temperatures and the effect of low temperatures in inhibiting the formation of glass that is ubiquitous elsewhere on the Moon. 38 In its section on high-priority science concepts, The Scientific Context for Exploration of the Moon noted that
(4) The lunar poles are special environments that may bear witness to the volatile flux over the latter part of solar system history; …
(6) The Moon is an accessible laboratory for studying the impact process on planetary scales;
(7) The Moon is a natural laboratory for regolith processes and weathering on anhydrous airless bodies.
Under this general framework, the scientific value of the Moon, and particularly the PSRs, for prebiotic chemistry and the origins of life falls into the following three categories: (1) PSRs as inventories of volatiles, (2) PSRs as prebiotic reaction records, and (3) PSRs as a record of information to advance the understanding of Earth’s habitability.
PSRs as Inventories of Volatiles
The early supply of organics and other volatiles to the Earth-Moon region may be related to the beginnings of life on Earth. Thus, study of the ancient record of lunar organics may provide insights into the organic inventory that may have been available during the initiation of life on Earth. Although there is strong evidence for the presence of volatiles that are of interest in studies of prebiotic chemistry, the sources of lunar volatiles are not well-understood. Seven possible sources of lunar volatiles have been proposed: proton deposition by the solar wind, the Earth during formation of the Moon, internal degassing from the Moon, comets, asteroids, interplanetary dust, and giant interstellar molecular clouds.39 Solar-wind deposition and impacts with interplanetary dust and small meteoroids are thought to be important continuous sources of volatiles,40 but volatiles deposited by larger impacts could yield important
36 Lunar Exploration Analysis Group, 2020, “Rapid Response Specific Action Team in Response to COSPAR Planetary Protection Inquiry,” May 8, https://www.lpi.usra.edu/leag/reports/COSPARRRSAT_v2.pdf.
39 S.T. Crites, P.G. Lucey, and D.J. Lawrence, 2013, Proton flux and radiation dose from galactic cosmic rays in the lunar regolith and implications for organic synthesis at the poles of the Moon and Mercury, Icarus 226(2): 1192-1200.
40 P.G. Lucey, 2009, A lunar waterworld, Science 326(5952): 531-532, doi:10.1126/science.1181471.
The early supply of organics to the Earth-Moon region can also inform studies of the transport of prebiotic or organic material throughout the solar system, such as through giant planet migration.43 Early evidence of organics on Earth has been erased because of geologic processing, thus sampling the organic record at the Moon is critical. When studying lunar polar organics, it is important to keep in mind that organics deposited in any era may have undergone some level of processing (thermal, radiolytic) that would alter the original state of the organic material.
PSRs as Prebiotic Reaction Records
The exact conditions, chemical species, and reactions associated with the origin of life remain unknown. However, in the presence of an energy source, sufficient mobility of ions, and suitable precursors, organic synthesis reactions may occur. In lunar PSRs, proton bombardment could provide an energy source for organic synthesis up to 1 m below the regolith surface.44 The Moon is not the only option for obtaining this information, because similar information may also be stored in ices associated with comets or outer solar system satellites. However, the Moon is easier to access and provides a record of information for the Earth-Moon system.
PSRs Inform Understanding of Earth’s Habitability
The presence of liquid water on Earth is key to the planet’s habitability and understanding life as we know it. Theories on the origins of Earth’s water include delivery by comets or asteroids, but neither source has been clearly substantiated. The isotopic ratio of deuterium to hydrogen (D/H) is a useful metric for determining water sources. However, the few data points that exist for comets and asteroids are highly variable and differ from that of Earth’s oceans. Determination of D/H ratios of lunar water ice may provide a more robust estimate of comet or asteroid derived water, informing our understanding of the sources of water on Earth.9
Finding 1: The scientific potential of the Moon’s poles and PSRs is significant, including for studies of prebiotic chemical evolution that have long been within the scope of national and international planetary protection policy.
Finding 2: Understanding of the lunar poles and PSRs has advanced but remains incomplete concerning many scientific questions, including how “cold traps” on the lunar surface function with respect to volatile and organic chemicals, the nature and composition of water and other volatile deposits in PSRs, and how the water and other ice deposits inform the scientific understanding of prebiotic chemical evolution in the solar system.
41 I.A. Crawford, M. Anand, CS. Cockell, H. Falcke, D.A. Green, R. Jaumann, and M.A. Wieczorek, 2012, Back to the Moon: The scientific rationale for resuming lunar surface exploration, Planetary and Space Science 74(1): 3-14, https://doi.org/10.1016/j.pss.2012.06.002.
42 M. Lingam and A. Loeb, 2019, “Searching the Moon for Extrasolar Material and the Building Blocks of Extraterrestrial Life,” arXiv: Earth and Planetary Astrophysics, https://arxiv.org/abs/1907.05427.
43 A. Morbidelli, H.F. Levison, K. Tsiganis, and R. Gomes, 2005, Chaotic capture of Jupiter’s Trojan asteroids in the early solar system, Nature 435: 462-465, doi:10.1038/nature03540.
44 P.G. Lucey, 2000, “Potential for Prebiotic Chemistry at the Poles of the Moon,” pp. 84-88 in Instruments, Methods, and Missions for Astrobiology III (R.B. Hoover, ed.), Proceedings of SPIE, Vol. 4137.
The large number of landed missions to the lunar polar regions being planned for the next 5 to 10 years suggests that answering these key questions about the poles and PSRs is needed sooner rather than later. Such information to guide realistic planetary protection approaches needs to be available in advance of large-scale missions for human exploration and lunar resource extraction.
Finding 3: Tapping the scientific potential of the lunar poles and PSRs requires accelerating lunar science across orbital and in situ missions and building “ground truth” about these regions to inform planning for planetary protection approaches for future scientific, exploration, and commercial activities on the Moon.
POTENTIAL THREAT OF ORGANIC AND BIOLOGICAL CONTAMINATION
The interest in the Moon as a scientific target for investigations of prebiotic chemistry brings up questions about the potential for, and consequences of, organic and/or biological contamination from human activities. Given the incomplete knowledge of volatile transport across the lunar surface, it would be wise to exercise caution when interacting with these special deposits so that their value can be assessed and tapped. For instance, the level of ongoing transport of volatiles to and inside the PSRs is unknown; distinguishing these naturally transported volatiles from volatiles brought in from spacecraft would best be accomplished before many more missions land on the lunar surface.
Below, the committee considers both direct contamination from missions directly landed onto PSRs, as well as indirect, transported contamination from rocket exhaust. In either case, surface contamination is possible, but the risk of contaminating deep subsurface deposits from downward transport, through natural processes, is low. Contamination to subsurface regions by drilling or excavations for in situ resource utilization exploration and operations will be an exception to that conclusion. The discussion also explains why biological contamination is not a threat to astrobiological studies of the lunar surface and PSRs.
Direct Contamination from PSR Missions
How will landings of spacecraft directly into PSRs contaminate those regions? Without better knowledge of the regolith conditions within PSRs, only estimates can be made. Apollo missions provide a useful example, because Apollo mission data can provide some indication of the damage or contamination to the lunar surface by a crewed mission. However, one must remember that the Apollo missions landed at low- to mid-latitudes where thermal conditions and the space environment were significantly different. Thermal conditions, in particular, are critical to acknowledge when considering the behavior of volatiles.
The Apollo missions affected the regolith in a number of ways, including the following:
- Scouring by lander exhaust during descent/ascent;
- Residual Lunar Module (LM) descent propellant venting;
- LM cabin atmosphere venting, crew member direct interaction with the regolith; and
- Excess mass left on the surface.
Optical effects (e.g., brightening of the surface) around the landing blast zone (BZ) were likely caused by smoothing of the regolith and removing the fine-scale roughness of the regolith. No chemical differences between Apollo samples collected in the BZ and undisturbed regions have been reported (this includes
core samples in and out of the BZ).45 The lack of any correlations found of amino acid content in lunar samples with proximity to the Apollo 17 LM led to the conclusion that LM exhaust was not a primary source of amino acid in the samples.46 It is not clear how exhaust in the direct vicinity of PSRs might affect the nature of the surface and chemistry, given the unknown nature of the regolith in the PSRs. PSR terrains may be very different from low-latitude regolith, so a PSR BZ (or even a high-latitude BZ) may respond differently than the Apollo BZs.
Rocket Exhaust from Landers Distant from Polar Regions
Combustion byproducts of spacecraft propellants were a commonly cited concern in the LEAG response to the COSPAR planetary protection inquiry. Chapter 3 of this report addresses this topic in detail, where the committee concludes that this contamination is likely to be minimal and poses little threat to many studies of the PSRs. Elevated abundances of organics are expected to be present at depth relative to the surface, as a result of deposition over billions of years. Conversely, exhaust plumes are expected to deposit smaller quantities of organic contaminants directly at the surface. The resulting differences in abundances and potentially, types of volatiles, may impart different signatures to volatiles of different provenance. Organic contaminants from exhaust plumes from landers distant from polar regions are expected to accumulate and remain in the topmost surficial layer of the regolith on timescales of tens of thousands of years.47
Possibility of Subsurface Contamination by Diffusion or Impact Gardening
Lander exhaust and other outside contamination sources48 will have the most effect on surface deposits of volatiles and organics in PSRs. Below the surface, relic ice and organics are likely to be protected, except when actual subsurface sampling occurs. Molecular diffusion of water (and presumably other molecules) is expected to be very slow at temperatures found in the PSRs.49 Thus, surface water and organic deposits will exchange with the subsurface (~10 cm) on timescales of ~104 to 105 years. The regolith overturn/gardening rate of the top several cm is ~80,000 years.50 Both mechanisms of exchange act over longer timescales than are important for the lunar missions being discussed here.
Contamination associated with the sampling hardware may constitute a greater contamination risk than that from previous landed missions. This problem is not dissimilar to contamination risks in terrestrial settings (e.g., subsurface lakes in Antarctica) where contamination control and mitigation
45 R.N. Clegg, B.L. Jolliff, M.S. Robinson, B.W. Hapke, and J.B. Plescia, 2014, Effects of rocket exhaust on lunar soil reflectance properties, Icarus 227: 176-194, doi:10.1016/j.icarus.2013.09.013.
46 J.E. Elsila, M.P. Callahan, J.P. Dworkin, D.P. Glavin, H.L. McLain, S.K. Noble, and E.K. Gibson, 2016, The origin of amino acids in lunar regolith samples, Geochimica et Cosmochimica Acta 172: 357-369, doi:10.1016/j.gca.2015.10.008.
47 P. Prem, D.M. Hurley, D.B. Goldstein, and P.L. Varghese, 2020, The evolution of a spacecraft-generated lunar exosphere, Journal of Geophysical Research (Planets) 125: e06464, doi:10.1029/2020JE006464.
48 Given the limited time available to the committee to complete its analyses, the report focuses on potential for contamination from lander exhaust; however, the committee recognizes that other potential sources of contamination may exist, such as leakage of volatiles from spacesuits. See, for example, P. Lee, M. Rosenthal, G. Quinn, T. Chase, and J. Rohrig, 2020, “EVA H2O Release: Need for Measurements and Monitoring During Human Exploration of the Lunar Polar Regions,” white paper submitted to NASA Artemis Science Definition White Paper Call, https://www.lpi.usra.edu/announcements/artemis/whitepapers/2116.pdf.
49 N. Schorghofer and J.-P. Williams, 2020, Mapping of ice storage processes on the Moon with time-dependent temperatures. Planetary Science Journal, in press.
50 E.J. Speyerer, R.Z. Povilaitis, M.S. Robinson, P.C. Thomas, and R.V. Wagner, 2016, Quantifying crater production and regolith overturn on the Moon with temporal imaging, Nature 538: 215-218, doi:10.1038/nature19829.
protocols are developed and applied. Similar mitigation steps will need to be applied to any lunar sampling activity. Requirements imposed to preserve the scientific integrity of individual sampling investigations of lunar prebiotic chemistry will be far more stringent than planetary protection requirements as a consequence of rigorous cleanliness provisions for the sampling instruments.
Low Threat of Biological Contamination
One cannot rule out the possibility that deeply frozen, ancient ice deposits in large PSRs contain remnants of microorganisms transported to the Moon by ejecta from terrestrial meteorite impacts over geologic time. Evidence for such biotransfer events might be preserved within stratified layers of future PSR ice cores. Embedded microbes and their biomolecules would have great scientific value, but they would not represent a planetary protection threat because of their terrestrial origin. Thus, sterile techniques applied during lunar sample collection and return missions would be sufficient to protect the biological integrity of frozen samples. Whether or not bacteria, fungi and their viruses could remain viable for tens of millions of years or longer within PSRs is an open question, but theoretically feasible.51,52
The NASA Interim Directive on Planetary Protection Categorization for Robotic and Crewed Missions to the Earth’s Moon added the requirement in Section 1.1.4 to “provide an inventory of the biological materials (living and dead) included in spacecraft hardware and payloads.” In an effort to characterize the potential for biological contamination of the Moon or of samples collected from the Moon, a “Lunar Microbial Survey” model was developed53 to estimate the remaining bioburden on all Moon-landed spacecraft to date, after accounting for the combined effects of solar ultraviolet, vacuum, high-temperature, and ionizing radiation on bioburden bacterial cells or spores. The results showed that the external surfaces of all landers to date should have been free of all bioburden as of 2019, as a result of log10 reductions in bioburden of −2479 per lunation at the equator and −1163 per lunation at latitudes above 60º.54 Reduction rates would be lower on internal surfaces; however, the range of total bioburden predicted to exist in deep internal surface areas of landers by 2030 ranged from 0 to 10 cells for all spacecraft except for the Chang’e/Yutu lander (1.29 × 105 cells or spores), based on log reductions of −0.02 to −188 per lunation (deep and shallow internal surfaces, respectively). The rapid log reductions in surface bioburden suggest that cells or spores transported by landers are unlikely to result in lunar surface contamination and therefore do not require special consideration.
Planetary protection levels needed to protect and preserve the volatile small-molecule content of pristine PSR environments (Level II-L) will equally protect and preserve macromolecular biological molecules. Thus, the risk of contamination of PSRs with microbes transported on robotic missions to the lunar South Pole is judged to be negligible and need not be factored in planetary protection decisions for uncrewed missions to the Moon. Therefore, the requirement for a biological inventory specified under NASA Category II-L is not scientifically justified. On the other hand, intentional biological payloads should be reported, because they are likely to introduce a bioburden in excess of that accounted for in the modeling study cited above and their behavior cannot be easily predicted.
51 R.C. Richmond, R. Sridhar, Y. Zhou, and M.J. Daly, 1999, Physico-chemical survival pattern for the radiophile D. radiodurans: A polyextremophile model for life on Mars, pp. 210-222 in Instruments, Methods, and Missions for Astrobiology II, Proceedings of SPIE 3755, doi:10.1117/12.375078.
52 L.R. Dartnell, S.J. Hunter, K.V. Lovell, A.J. Coates, and J.M. Ward, 2010, Low-temperature ionizing radiation resistance of Deinococcus radiodurans and Antarctic dry valley bacteria, Astrobiology 10: 717-732, doi:10.1089/ast.2009.0439.
53 A.C. Schuerger, J.E. Moores, D.J. Smith, and G. Reitz, 2019, A lunar microbial survival model for predicting the forward contamination of the Moon, Astrobiology 19: 730-756, doi:10.1089/ast.2018.1952.
Finding 4: Inventories of biological materials for spacecraft and other lunar equipment are unimportant for planetary protection purposes because (1) the Moon’s surface does not support indigenous forms of life or the proliferation of terrestrial organisms brought to the Moon; (2) biological contamination of the lunar surface will not contaminate the lunar subsurface through the operation of natural processes on the Moon; and (3) any biological material identified in samples from the lunar surface or subsurface can be tested against terrestrial organisms to determine its source.