Galactic sandbox: From quartz clouds to lunar water generation

The simplest chemical pathway leading to formation of both circumstellar dust grains and ultimately the lunar crust begins with reactions of silicon monoxide (SiO) with water at temperatures of 1643 K (1916 °C, 2498 °F) and above in an article published in the October 11, 2023 issue of the journal ACS Earth and Space Chemistry by Flint and Fortenberry. Continued addition of SiO and H₂O molecules is predicted to result in growth of the silica (SiO₂) mineral precursor. Quantum chemical generation of this reaction pathway produces precursors as large as the silicon dioxide trimer, which can be extrapolated to the silicon dioxide hexamer, a stand-in for larger silica grains.

The reverse pathway, which corresponds to water generation through mineral destruction, proceeds through a number of energetically uphill processes. However, the energy input required for the most energetically costly of these steps falls within the ultraviolet and is accessible through simple exposure to sunlight.

Since the confirmation of the presence of water on the moon in 2020 by a Hawaii-based team, the lunar surface has been eyed as a potential wellspring for water generation. Experiments done by a group at The Open University on simulated lunar crust show that processing of this material, composed of silicon-, magnesium-, aluminum-, calcium-, and iron-containing minerals, through hydrogen gas exposure produces water that can be condensed and collected. This hydrogen reduction, while successful, is done at high temperatures. If this processing could occur readily at conditions that more closely resemble those achieved naturally throughout the lunar day, it would significantly ease the engineering challenges associated with producing large quantities of water on the lunar surface.

Fig. 1. Computed (SiO₂)₆ structures. The top structure of Ci symmetry is a result of Si₃O₆ stacking, while the bottom structures are a likely result of bond breakage and orbital reorganization of the Ci structure.

Computational approaches to such questions are useful in order to assess the utility of undergoing the relevant laboratory experiment. However, computational solid-state simulations must often greatly sacrifice chemical accuracy of the results, as most quantum chemical treatments are not feasible on large systems. Rather than simulating the degradation of the solid oxide mineral through lower-accuracy methods, a higher-accuracy treatment can be used to generate a chemical model of the construction of the mineral grain. This reaction pathway, when carried out until the smallest unit that resembles the larger solid, can be viewed in reverse to approximate grain degradation as done by Flint and Fortenberry. This view also provides new insights into the microscopic first stages of grain formation near stars from the very smallest constituents.

Fig. 2. Amounts of energy required to traverse uphill steps in the Si₆O₁₂ decomposition process (vertical lines) set against the air mass zero emission spectrum of the Sun.

The recent observation of larger silica nanoparticles in the atmosphere of the exoplanet WASP-17b through JWST’s Mid-Infrared Instrument from the October 2023 issue of The Astrophysical Journal supports a silica formation scheme like the one computed by Flint and Fortenberry. In addition to the presence of silica, H₂O depletion from the atmosphere is also observed, consistent with a silica formation mechanism that consumes H₂O. The barrier to silica formation is surpassable at both the WASP-17b equilibrium temperature (1771 K) and the predicted necessary temperature for silica formation in the WASP-17b atmosphere (approx. 1300 K). With additional JWST infrared observations, these modeled pathways can continue to inform silicon chemistry within warm sources.

This article made use of the following publications:

Honniball et al. 2020, NatAs, 5, 121-127

Sargeant et al. 2021, P&SS, 205, 105287

Flint and Fortenberry 2023, ESC, 7, 2119-2128

Grant et al. 2023, ApJL, 956, L32