Untangling the formation and liberation of water in the lunar regolith Cheng Zhua,b,1, Parker B. Crandalla,b,1, Jeffrey J. Gillis-Davisc,2, Hope A. Ishiic, John P. Bradleyc, Laura M. Corleyc, and Ralf I. Kaisera,b,2 aDepartment of Chemistry, University of Hawai‘iatManoa, Honolulu, HI 96822; bW. M. Keck Laboratory in Astrochemistry, University of Hawai‘iatManoa, Honolulu, HI 96822; and cHawai‘i Institute of Geophysics and Planetology, University of Hawai‘iatManoa, Honolulu, HI 96822 Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved April 24, 2019 (received for review November 15, 2018) −8 −6 The source of water (H2O) and hydroxyl radicals (OH), identified between 10 and 10 torr observed either an ν(O−H) stretching − − on the lunar surface, represents a fundamental, unsolved puzzle. mode in the 2.70 μm (3,700 cm 1) to 3.33 μm (3,000 cm 1) region The interaction of solar-wind protons with silicates and oxides has exploiting infrared spectroscopy (7, 25, 26) or OH/H2Osignature been proposed as a key mechanism, but laboratory experiments using secondary-ion mass spectrometry (27) and valence electron yield conflicting results that suggest that proton implantation energy loss spectroscopy (VEEL) (28). However, contradictory alone is insufficient to generate and liberate water. Here, we dem- studies yielded no evidence of H2O/OH in proton-bombarded onstrate in laboratory simulation experiments combined with minerals in experiments performed under ultrahigh vacuum − − imaging studies that water can be efficiently generated and re- (UHV) (10 10 to 10 9 torr) (29). Orlando et al. proposed that leased through rapid energetic heating like micrometeorite impacts proton bombardment of lunar regolith simulants produces chemi- into anhydrous silicates implanted with solar-wind protons. These cally bound hydroxyl (−OH) groups as potential precursors to water synergistic effects of solar-wind protons and micrometeorites liber- at elevated temperatures of at least 450 K (30)—well above the ate water at mineral temperatures from 10 to 300 K via vesicles, equilibrium noon-time temperature of the Moon of 400 K. The thus providing evidence of a key mechanism to synthesize water in authors concluded that solar proton irradiation of regolith does not silicates and advancing our understanding on the origin of water as contribute significantly to the origin of water. Therefore, despite detected on the Moon and other airless bodies in our solar system compelling evidence of the presence of water and/or hydroxyl such as Mercury and asteroids. radicals in the lunar soil, an intimate understanding of the under- EARTH, ATMOSPHERIC, lying pathways to form and release water from silicates and how its AND PLANETARY SCIENCES solar wind | water | Moon abundance might vary on diurnal timescales is still in its infancy. In this article, we use laboratory simulation experiments com- he unraveling of the formation and liberation of water in bined with imaging analysis to uncover an efficient and versatile Tlunar silicates is critical in aiding our fundamental un- pathway to generate and liberate water in lunar silicates. This is derstanding of the potential, but hitherto-elusive, water cycle on accomplished by synergistic effects of solar-wind proton implan- the Moon—in particular whether lunar water ices are primordial tation combined with thermal excursion events such as microme- water expelled from the interior of the Moon by geologic pro- teorite impacts, but not by proton implantation alone. We exploit cesses (1, 2), delivered via comets and water-rich asteroids (3–6), deuterium ions as a proxy of hydrogen ions of the solar wind or produced in situ in the regolith by bombardment of oxygen- + to simulate the interaction with nominally anhydrous olivine rich silicate minerals by solar-wind protons (H ) at energies of [(Mg,Fe)2SiO4]—a typical surrogate of the lunar material that about 1–2 keV (7). Although the origin of water on the Moon is has been widely used to simulate space weathering effects on still elusive, during the past two decades, the presence of lunar water ice was suggested based on radar (8, 9) (Clementine, Significance Arecibo) and neutron spectrometer data (Lunar Prospector) (10). The identification of lunar water culminated in the Lunar Observational evidence collected over the past two decades Crater Observation and Sensing Satellite excavation of water ice supports the existence of water on the Moon. However, the from the permanently shadowed crater Cabeus (11) along with sources and chemical and/or physical processes responsible for the first spectroscopic observation of surface exposed water ice the production of the lunar water are still unknown. Here, we (Chandrayaan-1) (12). Furthermore, three independent space- provide evidence via laboratory simulation experiments that craft instruments exploited absorption bands at 3 μm to identify 3 water can be generated and liberated through thermal shocks H2O/OH in polar regions [Moon Mineralogy Mapper (M ), induced by micrometeorite impacts on solar-wind proton- Chandrayaan-1 (13–15); visual and infrared mapping spectrom- – implanted anhydrous silicates. Our findings are of fundamental eter, Cassini (16); high resolution instrument infrared spec- importance for explaining the origin of water on the Moon as trometer, Deep Impact (17)]. Recent Earth-based telescopic well as on other airless bodies such as Ceres and for untangling observations of lunar surface water (18) revealed fascinating the present distribution of water in our solar system. latitude and time-of-day systematics consistent with measure- ments by Cassini (16) and Deep Impact (17). The neutral mass Author contributions: J.J.G.-D. and R.I.K. designed research; C.Z., P.B.C., H.A.I., J.P.B., and spectrometer on board the Lunar Atmosphere and Dust Envi- L.M.C. performed research; C.Z., P.B.C., H.A.I., and J.P.B. analyzed data; and C.Z., J.J.G.-D., ronment Explorer spacecraft has detected sporadic water signal H.A.I., and R.I.K. wrote the paper. in the exosphere of the Moon (19–22) with occurrence rate co- The authors declare no conflict of interest. inciding with periods of major annual meteoroid events (23). This article is a PNAS Direct Submission. 3 Data from the M mission reveal that H2O/OH is present on Published under the PNAS license. the Moon at all latitudes at a given local time and terrain type 1C.Z. and P.B.C. contributed equally to this work. (14, 18, 24) and strongly implicate the involvement of solar-wind 2To whom correspondence may be addressed. Email: [email protected] or ralfk@ protons in the production of water (14, 24). However, laboratory hawaii.edu. experiments, which attempted to unravel the solar-wind–induced This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. synthesis of H2O/OH in lunar silicates, have yielded conflicting 1073/pnas.1819600116/-/DCSupplemental. findings. Experiments conducted under high-vacuum conditions www.pnas.org/cgi/doi/10.1073/pnas.1819600116 PNAS Latest Articles | 1of6 Downloaded by guest on October 1, 2021 the Moon (27, 31, 32)—over a temperature range of 10 to 300 irradiation of the minerals—did not detect any ion counts at m/z = + K followed by rapid thermal processing of the irradiated sili- 4(D2 ). Consequently, the molecular deuterium detected in the cates to mimic heating effect of micrometeorite impacts under TPD phase of the irradiated olivine is clearly linked to the ion im- UHV conditions (Materials and Methods). This systematic study plantation demonstrating the capability of olivine to store deuterium. reveals the capability of lunar silicates to first efficiently store (precursors to) water from solar-wind ion implantation over a Lunar Environment Simulations Phase II. Considering the non- broad temperature range from 10 to 300 K and the key role of a detection of D2-water in the aforementioned studies, alternative subsequent rapid energetic heating of the irradiated silicates to energy sources might assist in the formation and liberation of release water into the gas phase. The results provide compelling D2-water molecules and/or D1-hydroxyl radicals potentially evidence of the significance of thermal impulse on liberating water stored in the silicates. This processing might also lead to the from solar-wind–implanted minerals, thus revolutionizing our formation of D2-water via condensation of two silicon-bound understanding of the origin and distribution of water on the Moon D1-hydroxyl groups (−Si−O−D) leading to D2-water along with and on airless bodies such as Mercury and asteroids. an oxygen bridged −Si−O−Si− moiety (39). To verify this hy- pothesis, anhydrous olivine samples were prepared and exposed Results + to a D2 ion beam under identical conditions as documented in Lunar Environment Simulations Phase I. To mimic the interaction of the aforementioned section. After the ion exposure, the sample solar-wind protons with regolith in laboratory experiments, was irradiated at 10 K by a pulsed infrared laser followed by powdered samples of anhydrous olivine [(Mg,Fe)2SiO4] were TPD. Laser pulses can create intense heating events reaching + held at 10 K and exposed to a 5-keV D2 molecular ion beam for temperatures higher than 1,400 K well above maximum diurnal − a total fluence of (1.0 ± 0.1) × 1018 deuterium nuclei cm 2, temperatures of 400 K (SI Appendix), but close to temperatures equivalent to 300 ± 30 y of irradiation at the lunar surface on produced by micrometeorite impacts (31, 40, 41). After the TPD − average, under UHV conditions at base pressures of 1 × 10 10 torr phase, the sample was exposed to the laser twice to examine (SI Appendix) (33). The use of deuterium is driven by the neces- whether water could still be generated via micrometeorite impact sity to distinguish between products formed as a result of the at surface temperatures closely approximating lunar daytime irradiation like D2-water (D2O) in contrast to possible trace temperatures.