Hydration Observations of Reiner Gamma Lunar Swirl in Partial Eclipse

Abstract:

Spacecraft observations of the Moon in the near infrared revealed a 3 micron absorption feature associated with O-H bonds. A key hypothesis for the origin of this hydration on the lunar surface is the chemical reaction of solar wind hydrogen with oxygens in lunar surfaces. Magnetic fields can be used to test this hypothesis because they may shield the surface from solar wind and could allow different degrees of solar wind flux to be observed. Lunar Swirls are bright albedo features that are linked to the presence of local crustal magnetic fields and could serve as natural laboratories for exploring the role of solar wind hydrogen implantation on the surface while keeping other parameters such as viewing geometry and surface composition constant. This study uses ground based observations from NASA’s InfraRed Telescope Facility’s SPeX spectrograph to observe a lunar swirl named Reiner Gamma in the 1.7-4.2 micron spectral range. The measured radiance from the surface of the Moon in this spectral range contains both reflected sunlight and thermal emission, however the feature of interest is only in the reflected component of the light and experts disagree about how to separate the two. One method for decreasing the unwanted thermal component is to observe during partial lunar eclipse when temperatures are reduced on the surface. Our observations were taken as the Moon emerged from the Jan 21 2019 UTC lunar eclipse. We found the high albedo areas of Reiner Gamma were observed to have less hydration than the surrounding maria. However, lower albedo regions in the swirl’s interior, known as ‘dark lanes’, seem to be an area of greater hydration. The reduced hydration on the swirl indicates that the magnetic field is protecting the high albedo surfaces below from the solar wind hydrogen. Greater hydration in the dark lanes may be a result of solar wind hydrogen being deflected along magnetic field lines to places where they intersect the surface.