Depth profiling of implanted D⁺ in silicates: Contribution of solar wind protons to water in the Moon and terrestrial planets

¹ Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang (Guizhou), PR China
² State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macao, PR China
³ Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, Hefei (Anhui), PR China
⁴ Key Laboratory of Space Manufacturing Technology, Chinese Academy of Sciences, Beijing, PR China
⁵ Key Laboratory of the Earth and Planetary Physics, Chinese Academy of Sciences, Beijing, PR China
⁶ Analysis and Test Center, Guangdong University of Technology, Guangzhou (Guangdong), PR China
⁷ College of Resources and Environmental Engineering, Guizhou University, Guiyang (Guizhou), PR China

^★ Corresponding author; tanghong@mail.gyig.ac.cn

Received: 26 May 2024
Accepted: 9 October 2024

Abstract

Context. The solar wind protons implanted in silicate material and combined with oxygen are considered crucial for forming OH/H₂O on the Moon and other airless bodies. This process may also have contributed to hydrogen delivery to planetary interiors through the accretion of micrometre-sized dust and planetesimals during early stages of the Solar System.

Aims. This paper experimentally investigates the depth distribution of solar wind protons in silicate materials and explores the mechanisms that influence this profile.

Methods. We simulated solar wind irradiation by implanting 3 keV D₂⁺ ions in three typical silicates (olivine, pyroxene, and plagio-clase) at a fluence of ~1.4 × 10¹⁷ ions/cm². Fourier transform infrared spectroscopy was used to analyse chemical bond changes, while transmission electron microscopy (TEM) characterised microstructural modifications. Nanoscale secondary ion mass spectrometry (NanoSIMS) was employed to measure the D/¹⁶O ratio and determine the depth distribution of implanted deuterium.

Results. The newly produced OD band (at 2400-2800 cm⁻¹ ) in the infrared spectrum reveals the formation of O–D bonds in the irradiated silicates. The TEM and NanoSIMS results suggest that over 73% of the implanted D accumulated in fully amorphous rims with a depth of 70 nm, while 25% extended inwards to ~190 nanometres, resulting in partial amorphisation. The distribution of these deuterium particles is governed by the collision processes of the implanted particles, which involve factors such as initial energy loss, cascade collisions, and channelling effects. Furthermore, up to 2% of the total implanted D penetrated the intact lattice via diffusion, reaching depths ranging from hundreds of nanometres to several micrometres.

Conclusions. Our results suggest that implanted solar wind protons can be retained in silicate interiors, which may significantly affect the hydrogen isotopic composition in extraterrestrial samples and imply an important source of hydrogen during the formation of terrestrial planets.