52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 1882.pdf

PROBABLE RESERVES OF WATER ICE IN THE LUNAR POLAR REGIONS. E. A. Grishakina1, E. N. Slyuta1, 1Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, Kosygina Str. 19, Russia, [email protected].

Introduction: Investigations of the , carried estimated at 5.6 ± 2.6 wt%. In the Сabeus crater, the out most recently by spacecraft, have shown that the concentration of the water equivalent of hydrogen in Polar Regions may contain significant reserves of the sum for the weakly bound and frozen components hydrogen-containing compounds, which include water can reach 10.9 + 5.1–3.3 wt%, and in the Shoemaker ice, as well as a number of other frozen and buried crater - 9.4 + 2.7–2.0 wt% [8]. There is no clear volatile compounds. The source of water (water ice) correlation between the distribution of hydrogen- present in the lunar regolith could be comets, asteroids, containing anomalies and the degree of illumination. meteorites, interplanetary dust [1], cosmic weathering Nevertheless, hydrogen-containing anomalies with the of regolith minerals under the influence of solar wind highest hydrogen concentration are located, as a rule, ions, which results in the formation of proton water at the bottom of large permanently shaded depressions [2], and finally, degassing lunar bowels. (craters) [8] (Fig. 1). Frozen volatile in the lunar soil: The cumulative To assess the probable reserves of water ice in the effect of sublimation, sputtering, Lyman-alpha lunar regolith in the fields located in the South and ultraviolet radiation and micrometeorite bombardment North polar regions, anomalies with a hydrogen water does not allow water ice deposits to persist on the equivalent (WEH) content of 0.33 wt% or more were surface of the regolith [1]. Preservation of water ice is identified and delineated. If we assume that all this possible only if it is buried by regolith. The minimum hydrogen is contained in water ice, then this thickness of the dry regolith layer covering the buried corresponds to the content of water ice in the lunar deposits of water ice is about 40 cm and, depending on regolith of 2.97 wt% or more. The maximum content the temperature conditions on the surface, it can of WEH within the identified anomalies reaches 0.54 change [3, 4]. The deposits of water ice in the regolith, wt%, which corresponds to 4.86 wt% of water ice. apparently, should be layered or stratified, in direct Thus, within the South Polar Region, 10 deposits were proportion to the history of formation of the regolith identified with the maximum water ice content in the layers. Deposits of frozen volatiles in the regolith layer lunar soil, which are of the greatest practical interest, at the Poles can extend to a depth of 2 m or more. The and 7 deposits in the North Polar Region (Fig. 2). age of this regolith layer can reach 2 billion years or To estimate the probable reserves, the minimum more. According to various data, the content of water value of the water ice content within the identified ice is estimated from 150 g t-1 (0.015 wt%) to 5 wt% fields was used, equal to 2.97 wt%, the depth of [1]. The frozen volatiles appear to be present in the occurrence in the lunar regolith was down to 3 meters, open pore space in the form of micron-sized particles and the average density of the regolith was 1.9 g cm – 3 () on the surface of the regolith particles. The [9]. In the South Polar Region, the area of the presence of massive deposits of water ice in the form identified deposits ranges from 8.8 to 1939.9 km2 with of lenses in the regolith is assessed as unlikely. If we probable reserves from 1,490,000 to 328,405,000 tons take the water ice content in cold traps (constantly of water ice (Table 1). shaded craters) equal to 1.5% [5], then, taking into Table 1. Probable reserves of water ice in the fields of the account the obtained minimum area of traps in the South Polar Region. northern and southern Polar Regions, according to [6], № Field Coordinates Square, Water ice 8 the minimum probable ice reserves will be 4.5×10 and (lat., lon.) km2 reserves, t 3.9×108 tons, respectively. According to the radar 1 1S -84.5°, -47.3° 964.10 163 212 000 survey of the Chandrayan-1 spacecraft, the probable 2 2S Shoemaker -88.0°, 53.8° 1939.90 328 405 000 reserves of water ice in the northern Polar Region at 3 3S Haworth -87.3°, 1.8° 836.20 141 560 000 the bottom of 40 trap craters with a diameter of 3-12 4 4S Scott M -84.8°, 32.3° 351.50 59 505 000 km are estimated at about 6×108 tons [7]. 5 5S -88.8°, -107.3° 111. 00 18 791 000 Estimation of probable reserves of water ice: As 6 6S Hale Q -77.8°, 80.8° 56.80 9 616 000 7 7S Amundsen -83.6°, 99.8° 30.70 5 197 000 a result of the LCROSS experiment, an extensive 8 8S Idel’son L -82.9°, 127.3° 23.70 4 012 000 composition of hydrogen-containing volatiles (H2O, 9 9S Idel’son L -82.9°, 127.4° 18.20 3 081 000 − H2S, NH3, C2H2, CH3OH, CH4, OH ) was found in the 10 10S Newton E -78.9°, -36.1° 8.80 1 490 000 ejection cloud, according to observations in the NIR 11 Total 4340.90 734 869 000 and UV spectral ranges, in addition to water and OH− In the North Polar Region, the identified fields, in [8]. According to the data of the same experiment, the contrast to the fields in the South Polar Region, are water content in the regolith of the Cabeus crater is characterized by closer sizes ranging from 136.80 to 52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 1882.pdf

445.20 km2 and, accordingly, close probable reserves total probable reserves in the North Polar Region are ranging from 23,158,000 to 75,368,000 tons (Table 2). estimated at 341,356,000 tons, or 3.4×108 tons. Thus, Table 2. Probable reserves of water ice in the fields of the in general, the estimated forecast reserves in the South North Polar Region. and North Polar Regions are in good agreement with № Field Coordinates Square, Water ice the minimum preliminary estimate calculated taking (lat., lon.) km2 reserves, t into account the minimum area of constantly shaded 1 1N Erlanger 87.3°, 64.3° 351.20 59 455 000 cold traps (craters) according to [6], as well as those 2 2N 86.2°, 51.3° 284.80 48 214 000 calculated from the radar survey of the Chandrayan-1 3 3N Plaskett 80.3°, 176.8° 445.20 75 368 000 at the bottom of 40 craters in the north polar region [7]. 4 4N Rozhdestv. U 85.5°, 139.3° 136.80 23 158 000 References: [1] Arnold J.R. (1979) J.G.R. 84, 5659– 5 5N Whipple 88.8°, 116.3° 153.20 25 935 000 5668. [2] Slyuta E.N. et al. (2017) Geochem. Int. 55(1), 27– 6 6N Rozhdestv. U 84.5°, 153.8° 242.70 41 087 000 37. [3] Feldman W.C. et al. (1998) Science 281, 1496–1500. 7 7N Milankovic E 78.0°, -170.8° 402.50 68 139 000 [4] Lawrence D.J. et al. (2006) J.G.R. 111(E08001), 1-19. [5] 8 Total 2016.40 341 356 000 Feldman W.C. et al. (2000) J.G.R. 105(E2), 4175-4195. [6] Conclusion: The total reserves in the fields of the Bussey D.B.J. et al. (2003) G. Res. Let. 30(6), 1278-1281. [7] Spudis P.D. et al. (2010) LPSC XXXXI, ID: 1224. [8] South Polar Region of greatest practical interest are 8 Sanin A. B, et al. (2017) Icarus 283, 20-30. [9] Slyuta E.N. estimated at 734,869,000 tons, or 7.3×10 tons. The (2014) Sol. Sys. Res. 48(5), 330–353.

Fig. 1. Map of the distribution of the water equivalent of hydrogen (WEH) in the North Polar Region (left) and in the South Polar Region (right) according to [8].

Fig. 2. Deposits of water ice in the North Polar Region (left) and South Polar Region (right) on the Moon with a water ice content in the lunar regolith of more than 2.97 wt%.