Radiation Environment Inside a Lunar Lava Tube

47th International Conference on Environmental Systems ICES-2017-15 16-20 July 2017, Charleston, South Carolina

Radiation Environment inside a Lunar Lava Tube

Ronald E. Turner1 Analytic Services Inc (ANSER), Falls Church, VA, 22041, USA

Robert Kunkel2 Oklahoma University, Norman, OK, 73071, USA

Lava tubes under the lunar surface are now believed to be numerous and widespread. They could be hundreds of meters long, tens of meters wide, with 10 or more meters of overburden. This environment could be an excellent location for human habitation, providing a constant thermal environment and significant radiation protection. This project’s focus was to develop a simple model that could simulate the effects of Galactic Cosmic Radiation (GCR) inside lunar lava tubes using the NASA radiation transport model HZETRN2015. If lava tubes can be shown to adequately shield humans from GCR, they could have major implications for permanent lunar settlement missions. This study provides an updated analysis of the feasibility of lunar lava tubes as radiation shelters. The associated models in HZETRN can act as a stepping stone to enable much more complex and detailed simulation in the future.

Nomenclature A = Atomic Mass GCR = Galactic Cosmic Radiation ICRP = International Council on Radiation Protection HZETRN = High Z and Energy Transport LRO = Lunar Reconnaissance Orbiter NIAC = NASA Institute for Advanced Concepts (prior to 2011) NIAC = NASA Innovative Advanced Concepts (after 2011) Z = Atomic Charge

I. Introduction UNAR lava tubes offer the prospect of significant advantages for lunar settlements, potentially providing L significant radiation protection and a more benign thermal environment. This project’s focus was to develop a simple model that could simulate the effects of Galactic Cosmic Radiation (GCR) inside lunar lava tubes using the NASA radiation transport model HZETRN20151 2. This study served as a preliminary analysis of the feasibility of lunar lava tubes as radiation shelters and how HZETRN can act as a stepping stone to enable much more complex and detailed simulation in the future.

II. Evidence for and Characteristics of Lunar Lava Tubes Lava tubes are caves created by lava flow on a planetary/lunar surface. Typically they form as the exposed layer of lava cools and hardens, and the inner layer drains out, leaving a void behind. They exist throughout the Earth on basaltic volcanic terrains where lava flows were common. Lava tubes on Earth range in length from hundreds of meters to kilometers, with widths from a few meters up to ten to twenty meters. They have long been believed to exist also on the Moon, based on its active volcanic past and on early lunar images3. Pits seen in images from the SELENE spacecraft were suggestive of holes in the surface due to collapse of roofs over subsurface lava tubes.4 Analysis of higher resolution images from the Lunar Reconnaissance Orbiter (LRO) continued to intrigue a community of researchers enthusiastic about the prospects of read-made access to the lunar interior5. More recently, there have been extensive analysis of LRO images to find and characterize pits (skylights) that may be associated

1 Distinguished Analyst, Innovative Analysis Division, 5275 Leesburg Pike, Suite N-5000 Falls Church, VA 22041. 2 Student, University of Oklahoma Norman Campus, 1800 Beaumont Drive 1314, Norman, OK 73071.

with lava tubes. Wagner, in 2014, noted that there was evidence of over 200 such features.6 Data from the GRAIL spacecraft is now providing at least preliminary indications of the subsurface voids according to analysis published by Blair, et al. in which they determine that lava tubes with diameters in excess of one kilometer (and up to 5 kilometers) could be structurally stable. 7 Exploiting lunar lava tubes for human habitats has been explored by a number of researchers, with the most notable early article by Hörz in 1985.8 Establishing habitats in caves helps reduce the radiation exposure, which is discussed further in the remainder of this paper. It also would provide a more constant thermal environment, so system designers would not have to address the extreme and rapid temperature fluctuations from lunar day to lunar night. Data from the Diviner Instrument on the Lunar Reconnaissance Orbiter estimate daytime temperatures vary from noontime temperatures ∼387–397 K at the equator to around 95 K just before sunrise.9 The NASA Institute for Advanced Concepts (NIAC) funded Dr. Penelope Boston to study approaches for humans to explore lunar lava tubes.10 11 The successor to that NIAC, the NASA Innovative Advanced Concepts program, has funded two additional lunar lava tube studies. The Principal Investigator (NIAC Fellow) Red Whittaker looked at robotic techniques to approach, enter, and map lunar lava tubes.12 13 NIAC Fellow Jeff Nosanov is studying an innovative technique for showing that voids extend beyond skylights, using time of flight signal return from a laser on an orbiting spacecraft.14 This paper focuses on the attenuation of the radiation environment inside a lunar lava tube by the lunar regolith overburden. An example of earlier work along these lines was published by Angelis et al, 2002.15 This work applies a more recent version of HZETRN, and provides updated background information on lunar lava tubes.

III. NASA’s HZETRN2015 Model NASA’s High Z and Energy Transport code (HZETRN) started as a one-dimensional radiation transport code for high energy protons and galactic cosmic radiation that has been used by NASA since 199116 to evaluate the radiation environment behind shielding. HZETRN is a deterministic code that solves the Boltzmann equation for straight ahead propagation of the incident GCR particles and subsequent secondary particles. HZETRN has been successively expanded and improved since its inception. It was recently expanded to consider three dimensional shielding geometries in 3DHZETRN in 2014.17 The 3D model was incorporated in a further revision, HZETRN201518. HZETRN2015 retains the approach of earlier HZETRN models, but includes updated cross sections, the contribution of backscattered neutrons, and, most importantly for this analysis, the ability to create three dimensional shielding geometries from fundamental primitives (spheres, cylinders, blocks) including voids. Representations of several common shielding materials are available within HZETRN2015, most notably for this analysis, lunar regolith composition and density (Table 1)

Lunar Regolith "A17" provided within HZETRN 2015 (Density: 1.6 g/cm3 Atomic mass (A) Atomic charge (Z) Number density (atoms/g) for each atomic specie in the material 28 14 4.02E+21 16 8 1.56E+22 48 22 7.25E+20 27 13 1.29E+21 52 24 3.66E+19 56 26 1.49E+21 55 25 2.04E+19 24 12 1.44E+21 40 20 1.15E+21 23 11 7.41E+19 40 19 9.82E+18 31 15 5.96E+18 32 16 2.27E+19 Table 1. Lunar Regolith properties provided by HZETRN2015 HZETRN input includes the user definable input spectra, but also includes a library of easily selectable representations of GCR (including a variety of historical solar minima and maxima) as well as a broad selection of historical and definable solar particle events. Extensive output is available from HZETRN2015, including the particle flux verses depth of shielding (with secondary particles), the dose and dose equivalent (for a variety of quality factors) in several target materials.

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IV. Study Approach The general approach employed in this study was to use HZETRN to calculate dose and dose equivalent under various thicknesses and geometries of lunar regolith. Several simplified qualifying calculations using a slab geometry rather than a full lava tube simulation were used to get a general feel for the dose, dose equivalent, and effective dose under various slab doses of lunar regolith. This orientation used a layer of water between two slabs of regolith to determine what dimensions were necessary for effective radiation shielding. The astronaut was represented by a 20 cm radius water sphere (or 40 cm layer of water), and dose equivalent was calculated at varying depths to simulate human organ shielding. Effective dose, which is used to estimate the lifetime risk of cancer, was calculated from these values, as will be described in more detail. HZETRN’s input parameters were varied to maximize accuracy while minimizing simulation runtime. Based on those findings, a model lunar lava tube was created with variable geometry to analyze a wide range of scenarios. The input considered: • Radiation spectrum and particle composition • Number of vectors along which the radiation is applied • Direction vectors along which exposure is calculated • Location, shape, and material composition of geometries in simulation. The lava tube was initially modelled as a box of empty space surrounded on the bottom and sides with regolith thick enough to block nearly all radiation. The regolith on top varied in thickness and a cylindrical skylight cut through its center. The final geometry used in the lava tube model consisted of a 10m by 10m by 30m void box surrounded on the bottom and the sides with thick regolith. The thinner slab on top contained a skylight in its center represented by a void box. The astronaut analog was initially placed directly underneath the skylight in the center of the lava tube. To calculate effective dose, rather than dose equivalent, a sufficiently accurate human analog was needed. Matthiä et al19 discuss the differences that arise from the use of different phantoms for calculating radiation doses in low earth orbit. The phantoms in question were the Numerical RANDO, ICRP male, and ICRP female models. The report contained tables relating the organ shielding values in each phantom to equivalent shielding depths in a 20 cm radius water sphere. ICRP publication 12320 contained weight factors for the organs in the ICRP male phantom, so organ shielding values from that model were used rather than those from the numerical RANDO or ICRP female. This ensured that the calculations used values from the same model throughout the investigation. Three depths in water were chosen that encompassed all of the ICRP male organs within a ± 1.0 g/cm2 tolerance (except for the bladder, which had a tolerance of + 1.1 g/cm2). Modelling a water sphere with a 20 cm radius, was adequate to measure dose and dose equivalent at each of these three depths to produce an approximation of organ doses in the ICRP male phantom. Weight factors from ICRP publication 123 were then applied to the appropriate organs in each organ group. Summing the results from each group produced values for the effective dose in the water sphere approximation of the ICRP male phantom. This approach is summarized in Table 2. ICRP Male ICRP 123 Weighting Depth in Water (cm) Depth Approximations (cm) Total Weighting Skin 0.01 10.5 Brain 0.01 11.6 Salivary Glands 0.01 11.8 11.5 0.16 Breast 0.12 11.9 Bones 0.01 12.1 Remainder 0.12 14.3 Thyroid 0.04 14.5 Lungs 0.12 15.1 15 0.36 Testes 0.08 15.4 Red Bone Marrow 0.12 17.1 Oesophagus 0.04 17.1 Colon 0.12 17.6 18 0.48 Stomach 0.12 17.9 Liver 0.04 17.9 Bladder 0.04 19.1 Table 2. The first three columns identify an organ, the ICRP 123 organ weighting factors for the ICRP Male, and the corresponding representative organ depth in water. The fourth column is an average depth in water and grouped organs at that depth, with a nominal weighting factor for those organs.

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To put the lava tube results in perspective, one also needs to know the radiation levels on the lunar surface. NASA’s on-line OLTARIS model21 was used to get these comparisons. On the lunar surface, astronauts would experience about half the dose rate of deep space (protected from below by the moon, but increased slightly by backscattered secondary neutrons). Under minimal shielding (space suit or lightly shielded vehicle: 0.1 to 2.0 g/cm2 Aluminum equivalent), the GCR effective dose would vary from 0.3 to 0.7 mSv/day, or twenty to fifty times the nominal terrestrial daily dose rate. More detailed summary of the OLTARIS results are shown in Table 3.

Daily Effective Dose on Lunar Surface (mSv/day) Solar Minimum Solar Maximum 0.1 g/cm2 (Al) 2.0 g/cm2 (Al) 0.1 g/cm2 (Al) 2.0 g/cm2 (Al) GCR only 0.09 0.59 0.27 0.26 Lunar backscatter 0.62 0.08 0.05 0.05 Total 0.71 0.67 0.32 0.31 Table 3. Lunar surface effective dose from solar minimum to solar maximum for various aluminum shielding thickness based on the on-line OLTARIS radiation model. In comparison, US public annual radiation exposure is on the order of 5 mSv22, or on the order of 0.010 to 0.015 mSv/day.

V. Analysis The attempts to quantitatively model the lunar lava tube were not complete, largely due to the excessive run time needed to model the sides of the lava tube, but also because of issues with the way voids were treated. Voids could not extend to the boundary of a solid, rather a very thin cap had to remain. However, the slab results were useful in showing that reasonable thickness of regolith overburden reduced the radiation levels at or below terrestrial levels, as shown in Figure 1 (no shielding other than the regolith). In fact, the overburden is so effective, that there is likely not a need for a full three dimensional model to estimate the effectiveness of a lunar lava tube.

Figure 1. Summary data for the effective dose in a lunar lava tube under increasing thickness of lunar regolith overburden.

As was shown in Table 3, at solar minimum with minimal aluminum shielding, an astronaut would be exposed to radiation levels on the lunar surface that are 20 to 50 times greater than the dose experienced by the US public (5 mSv per year). By sheltering under only 500 g/cm2 of lunar regolith (a little more than 3 meters), the effective dose is reduced to roughly 20 times the public dose. Once the regolith thickness reaches 1200 g/cm2 (7.5 meters) or more, the effective dose drops below that experienced by humans on Earth, and is truly negligible for overburden of 1500 g/cm2 or greater (10 meters). The dimensional simulation did not run properly due to a number of issues that arose late in the design process, so future work would be needed to finalize the model. An important issue that is currently impeding progress is an error that occurs when parts of the geometry become too thick for HZETRN to handle. This issue is particularly tough to resolve since the lava tube model must be very large in order to accurately simulate natural conditions. A

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potential solution to this model would be to reduce the exposure vectors to a roughly hemispherical distribution that only represents radiation exposure through the surface rather than a spherical one. This would eliminate the need to use thick slabs on the bottom and sides of the tube to block undesired radiation sources. Another issue is the inability to overlap geometric elements, which requires small gaps between voids. This is particularly troublesome when attempting to include a skylight above the tube. Since small surfaces can cause trouble when implemented in a large geometry, more work will be needed to determine the tolerances that must be allowed for these two close-proximity voids. Because of these issues, it was necessary to return to the slab calculations to demonstrate that lava tubes with depth greater than 7.5 meters reduce the astronaut exposure to terrestrial levels, providing protection from penetrating GCR radiation. Although this model requires more refinement before it can become a widely used tool, the findings within this report will pave the way to its completion and expedite any future simulation endeavors.

VI. Other Factors While the assessment of regolith shielding shows that a reasonable overburden will reduce the radiation due to GCR down to “Earth-normal” background, it does not consider two additional factors. First, future lunar colonists (and especially explorers) will not likely be confined to the interior of the lava tube…they will spend some time on the surface exposed to the direct flux of ionizing radiation, protected only by their space suit or transit vehicle. Second, the interior geology of lunar lava tubes is unknown, so it is not clear what occupants would be exposed to from naturally occurring radionuclides (similar to exposure from radon and other sources sometimes found inside buildings and caves on Earth). On the lunar surface, astronauts would experience about half the dose rate of deep space (protected from below by the moon, but increased slightly by backscattered secondary neutrons). To keep the annual dose to terrestrial levels, lunar colonists would have to limit time outside the lava tube to thirty to sixty minutes per day. Because GCR are so penetrating, reasonable additional shielding, 20 to 40 g/cm2, would only extend this time by about ten to twenty minutes. While this sounds like a severe limitation, it would still provide time for minimal surface expeditions if limited to a few days per month, and would enable modest weekly time for maintenance of surface infrastructure. Note, however, a single modest solar event could lead to effective dose on the order of fifty to one hundred mSv, or ten to twenty times the annual limit. Because solar particle events generally take an hour or more to reach peak flux and then decrease over a period of tens of hours, if the astronauts are within an hour or two travel time to the lava tube base this exposure could be reduced to about the annual terrestrial limit under most cases (assuming the astronauts have access to effective means to forecast and detect solar storms). Further analysis using detailed solar event characteristics (rise time to peak, decay time, and particle energy spectrum) would be needed to quantify more precisely an operational concept keeping the risk of solar events to a minimum. Effective and reliable 12-24 hour “all clear” could further extend the range of astronaut exploration. Finally, the extent of naturally occurring radiation within lunar lava tubes is an area for further research. Terrestrial caves vary significantly in natural background, from nearly nothing to substantially higher than modern housing limits from radon.23 It may be that the natural radionuclides in lunar lava tubes would be a limiting factor in the base daily dose, but not enough is known to make that assessment. If the variability of Earth caves is any indication, then it may be that lunar lava tubes also have high variability. Analysis of concentrations of potassium and thorium (and by correlation, uranium) data from Lunar Prospector24 and more recently, Chang’E-225 support this conclusion (Figure 2). The actual distribution inside a lava tube would likely follow the surface distribution, but it could vary significantly on scales of tens of meters. The impact on radiation exposure will depending on many factors, including the concentration and distribution of the radionuclides and the details of the habitation architecture. If the habitat is essentially self contained modules emplaced or built inside the tube, then the wall shielding could be optimized for the radionuclide radiation instead of GCR. However, architectures that depend on sealing the natural access points (skylights and smaller fissures, for example) and then filling the volume with breathable air, then the inhabitants would be directly exposed to the lunar geology. The risk would then be equivalent to that posed by radon in terrestrial buildings, and similar risk management strategies would have to be considered.

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Figure 2. Thorium distribution on the lunar surface. Top panel is from Prettyman, et al. (2006) based on data from Lunar Prospector. Lower panel is from Wang, et al. based on data from Chang’E-2.

VII. Conclusion There is growing evidence that lunar lava tubes are prevalent and accessible, and could be used as shelter for future lunar inhabitants. Lunar lava tubes have promise to provide protection for humans living on the moon, both from the extreme swings of the surface thermal environment as well as the harsh lunar surface radiation environment. This study quantified the argument that lunar lava tubes could be used to provide protection from the radiation environment. A reasonable overburden of lunar regolith, as little as 7.5 meters, could reduce the effective dose from GCR to terrestrial levels. It used HZETRN2015 for the calculations, attempting to use its built in three dimensional primatives to model the cave. While that proved more challenging than anticipated, it was easy to show using the simpler slab model provided in HZETRN2015, including backscatter, that detailed modeling was not necessary to show the effectiveness of a cave for reducing the radiation levels. To represent effective dose, rather than equivalent dose, a simple approach was developed using a water sphere and dose equivalent at only three depths in the sphere. The study also found that short excursions from the cave, for up to an hour a day, or a few hours per week, could be made and still keep the annual dose down to terrestrial levels, assuming an adequate warning of solar particle events is included in an overall radiation risk mitigation strategy. To fully characterize the radiation threat, it will be necessary to understand the internal radiation environment in a lunar cave. That can likely be approximated by applying a map of the locations of lunar skylights to the known surface geology of the moon and making approximations on how the surface geology relates to the internal lunar lava tube geology. This environment could then be imposed on various habitat design architectures.

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Acknowledgments Both authors would like to thank Tony Slaba for his support as they navigated their way through HZETRN. While the code stands alone, his assistance was greatly appreciated. They would also like to thank NASA for the funding provided through a contract issued to NTT Data via a subcontract to Analytic Services Inc. Robert Kunkel would like to thank NASA for sponsoring his summer internship through the NASA Innovative Advanced Concept Program, his NASA mentor Alvin Yew, ANSER for accommodating him, and his ANSER mentor Ron Turner who oversaw this study. Finally, Ron Turner would like to express his appreciation to Nicholas Kallfa, ANSER, for the time he spent verifying many of the calculations and techniques included in this paper.

References

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