A RADIATION SAFETY ANALYSIS for LUNAR LAVA TUBES G. De

Lunar and Planetary Science XXXIII (2002) 1417.pdf

A RADIATION SAFETY ANALYSIS FOR LUNAR LAVA TUBES

G. De Angelis1,2,3, J.W.Wilson2, M.S.Clowdsley4, J.E.Nealy3, D.Humes2 and J.M.Clem5 1Istituto Superiore di Sanita', Rome, Italy; 2NASA Langley Research Center, Hampton, VA; 3Old Dominion University, Norfolk, VA; 4National Research Council, Washington, DC; 5Bartol Research Institute, Newark, DE

Introduction. An analysis of radiation for lunar lava tube size parameters have been ex- safety issues on lunar lava tubes as potential habitats tracted and incorporated into the transport calcula- has been performed. Lava tubes are basically formed tion.. The “minimum” values are such with respect to when an active low viscosity lava flow develops a the currently available imagery, with a tube a roof continuous and hard crust, which thickens and forms thickness of e.g. 3 m being unobservable. a roof above the still flowing lava stream, and at the Scenarios. The analysis has been performed end of the extrusion period an empty flow channel considering radiation from galactic cosmic rays now free from molten magma is left [1]. Lava tubes (GCR) interacting with the lunar surface. The surface are commonly observed on the Earth [2], on basaltic has been modeled as a 5 m regolith layer, followed volcanic terrains, with typical sizes of the order of 1- by rock. The regolith density profile has been ob- 2 km of length, and few meters for cross-sectional tained by combining data from groundbased radio- parameters (i.e. height and width). Under lunar con- physical measurements and from in-situ analysis data ditions (lower gravity field, absence of atmosphere), from the Luna, Surveyor and Apollo missions, as lava channels and tubes are at least an order of mag- modeled in [6], whereas for the rock layer a constant nitude larger in each size dimension [3], i.e. hundreds value of 3.3 g/cm3 has been used as typical of mare of meters wide by hundred of meters or more deep basalt rock. The composition has been adopted as the and tenths of kilometers long. For many years [1-3] it same for both surface and rock layers, and has been has been suggested that these natural cavities on the chosen as an average of the Apollo 12 surface sam- Moon could provide an ideal location for a manned ples [7, 8, 9] taken at the Oceanus Procellarum land- lunar base, by providing shelter from various natural ing site, the region with the largest number of lava hazards, such as cosmic rays radiation, meteorites, tube candidates in the catalog. Two different scenar- micrometeorites impacts, and impact crater ejecta for ios have been considered, namely a Lunar Night example, and also providing a natural environmental (Tsurface = 100 K) and a Lunar Day (Tsurface = 400 K) control, with a nearly constant temperature of –20o C scenario, with temperature profiles for regolith and unlike that of the lunar surface showing extreme rock as from data from the Apollo 15 and Apollo 17 variation in its diurnal cycle. The purpose of this landing sites measurements [10, 11, 12]. work is an assessment of the lunar lava tube radiation Analysis. For the initial conditions a pri- characteristics and an evaluation of the their actual mary spectrum of GCR (p, α, HZE) for Solar Mini- safety features. mum conditions modulated at 510 MV has been Lava Tubes Geology. The formation of adopted as background radiation, and a spectrum lava tubes is generally associated to the formation of with particle fluxes equivalent to four times the in- “sinuous rilles” [4], frequently observed on the lunar tensity of the 29 September 1989 event has been basalt surface, especially in the maria floors, which adopted for Solar Particle Events (p). All primary formed from high extrusion and very low viscosity particles heavier than protons have been approxi- magma which filled the existing basins. In contrast to mated as individual nucleons, e.g. α He4 nuclei have the so numerous flow channels in the form of sinuous been transported as 4 individual protons. Radiation rilles, real lava tubes cannot be easily observed on profiles given by natural and induced radioactivity the Moon, for the reason of being subsurface objects, (α, β, γ) have been taken into account. Particles (p, n, therefore unobservable in surface imagery, and only K+, K-, π+, π-, µ+, µ-, e-, e+, γ) have been trans- those with at least a partially collapsed roof are ob- ported with the three-dimensional Monte Carlo servable. Moreover, lunar surface imagery is at most transport code FLUKA [13]. The evaluation of the at medium resolution, so rilles or tubes smaller than radiation safety-related quantities, namely the Effec- few meters wide are not observable with present lu- tive Dose (E) and the Ambient Dose Equivalent nar imagery. A catalog of lava tube candidates has (H*10), from particle fluence has been performed been obtained by analyzing Lunar Orbiter and Apollo with the conversion coefficients given in ref. [14], imagery along lunar rilles on the lunar nearside [5], whereas the physical quantity Absorbed Dose (D) has and more than 90 candidates were identified in been obtained by rescaling with the use of the maria, namely Oceanus Procellarum, Mare Imbrium, ICRP60 [15] radiation weighting factors wr. Both Mare Serenitatis and Mare Tranquillitatis, as discon- downward and upward (backscattering) particles tinuous rilles alternating between open lava channel have been considered in the dose evaluation, so the segments and roofed-over segments. This catalog results obtained are to be taken as radiation exposure provided a large lunar lava tube data set, from which upper limits. typical values for minimum, average and maximum

Lunar and Planetary Science XXXIII (2002) 1417.pdf

LUNAR LAVA TUBES RADIATION SAFETY ANALYSIS: G. De Angelis et al.

Results. No significant differences in the results have been observed between the Lunar Night and the Lunar Day scenarios. After 6 m of depth no effects of radiation due to or induced by GCR in both quiet and disturbed scenarios are observable in the simulation, and after far less than 1 m no effects of radiation due to or induced by SPE particles are observable. Natural and induced radioactivity seems not to play a significant role in the lava tube exposures. Mesons can interact in a quite dense material like lunar rocks before decay, and this is the reason why this usual component, mainly µ+µ-, is not present at larger depths. As a by-product of the transport results, the particle fluence from arriving GCR particles and from upward backscattering just at Moon surface and the relative dose equivalents have been obtained. Also in the very shallow and presently unobservable lunar lava tubes, with roof thickness of the order of 1-2 m, the doses are well below the monthly, annual and career limits given by NCRP 132 [16]. The radiation safety of lunar lava tubes environments has been demonstrated.

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