Assessment of the Shielding Effect of the Earth's Magnetic Field on Lunar OH/H2O

When the Moon passes through Earth's magnetosphere, it is believed that the amount of OH/H2O that would typically be created is reduced because the incident solar wind flux on the lunar surface decreases. To confirm this, we analyzed changes in OH/H2O absorption feature in the infrared region with or without the shielding effect of the magnetosphere using Moon Mineralogy Mapper (M3) data in low‐latitude regions. We report that the strength of absorption feature near 3 μm for the region from the equator up to the 10° latitude decreased when the Moon is inside Earth's magnetosphere relative to when it was outside of it. This indicates that the amount of OH/H2O on the shielded surface is less than on the unshielded surface. We confirmed that Earth's magnetosphere affects the OH/H2O distribution at low‐latitude regions, so the OH/H2O on the surface is originated by solar wind implantation. However, this study includes certain level of uncertainties in thermal corrections, and thus, sophisticated additional attempts are needed. Plain Language Summary It is regarded that an origin of OH/H2O on the lunar surface is solar wind. Researchers suggest that the OH/H2O is formed when solar wind protons are combined with oxygen atoms in the lunar regolith. In this study, in order to determine whether the OH/H2O is originated by solar wind or not, we controlled the amount of solar wind protons by using the shielding effect of the geomagnetic field. Then, we compared the amount of OH/H2O at shielded and unshielded sites by analyzing the absorption feature of 3 μm. These results confirmed that shielded sites have less OH/H2O than unshielded sites. It implies that the OH/H2O on the lunar surface is originated by solar wind. It is believed that this research will contribute toward the studies of OH/H2O on airless bodies. Key Points Earth's magnetosphere hinders OH/H2O formation on the lunar surface There is less OH/H2O on the shielded surface with certain level of uncertainties The shielding effect on OH/H2O distribution is an evidence of solar wind origin