Research Highlights

Nature Reviews Chemistry | https://doi.org/10.1038/s41570-019-0113-3 | Published online 18 June 2019

ASTROCHEMISTRY energy source comparable to the heating developed by the impacts of micro-meteorites on the lunar Wet side of the moon surface. The researchers found that, at temperatures mirroring those on Although the presence of water “This led to the hypothesis that the lunar surface (10–300 K), heavy ice on the lunar surface has been the water molecules formed under water was derived from the silicates. Only a acknowledged for several decades, ‘dirty’ high-vacuum conditions were “Only a combined exposure of combined its origin remains unclear. Several actually contaminants condensing lunar silicates to the solar wind and hypotheses have been put forward, from the residual gas onto the cold micro-meteorite impact can lead exposure of including degassing from the silicate surfaces in the same way ice to the formation of lunar water, but lunar silicates primordial lunar interior, asteroid builds up in a refrigerator. Therefore, not solar wind interaction alone,” to the solar delivery and, as Ralf Kaiser, Jeffrey novel experiments under ultraclean states Kaiser. The irradiated samples wind and Gills-Davis and colleagues report conditions were necessary,” were imaged by secondary electron in a new study, in situ alteration of explains Kaiser. imaging to identify the physical micro- surface minerals. Writing in the The group designed a new traces of water formation. meteorite Proceedings of the National Academy experiment in which anhydrous “Our work represents a particular impact can of Sciences of the USA, the team from olivine [(Mg,Fe)2SiO4] — a mineral analytical achievement, as the lead to the the University of Hawai’i presents commonly used to simulate the collaboration with physical chemists experimental findings that support lunar soil — is doped with hydrogen and planetary scientists helped the formation of a mechanism for low-temperature under ultraclean and ultrahigh set-up of realistic experiments under lunar water, in situ water release from protonated vacuum conditions, over ultraclean conditions,” remarks but not lunar silicates due to heating from a temperature range of 10 K to Kaiser. “This methodology and micro-meteorite impacts. 300 K. In this case, deuterium proposed mechanism can lead to solar wind Previous experiments designed ions were used to simulate the the formation of water not only on interaction to reveal the role of solar-wind bombardment of lunar silicates the moon, but on all airless bodies alone protons in the formation of water by hydrogen ions from the in our solar system and in extrasolar on the moon have led to conflicting solar wind. Deuterium, rather systems if silicates are present and results. Water and hydroxyl groups than hydrogen, was chosen to the implanted solar wind particles were found after silicate samples were discriminate experimental signals can react upon micro-meteorite bombarded with protons under arising from possible water-derived impact.” These findings provide a high-vacuum conditions, whereas no contamination. Upon warming of fundamental explanation for how water was observed under ultraclean the protonated olivine samples, water is synthesized on the moon.

and ultrahigh vacuum conditions. only D2 and no heavy water (D2O) Further experiments are needed was evolved, indicating that the to complete the story and reveal silicates are capable of storing what other factors, including possible precursors to water at low lithology and initial temperature temperatures, but that additional of the lunar soil, as well as the energy input is needed for the impact of heavier ions, might play actual formation and release of a role in the formation of water on water molecules. the moon. In search of a higher-energy Tom Henshall

source capable of deriving water from Original article Zhu, C. et al. Untangling the deuterated silicates, Cheng Zhu the formation and liberation of water in the lunar irradiated the protonated samples regolith. Proc. Natl Acad. Sci. USA https:// doi.org/10.1073/pnas.1819600116 (2019)

Credit: Carl Conway/Springer Nature Limited Nature Conway/Springer Carl Credit: using a pulsed infrared laser, an

Nature Reviews | Chemistry volume 3 | July 2019 | 401