Effects of space weathering on the Christiansen feature position of lunar surface materials

We use observations by the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment to examine the effects of space weathering and particle size on the position of the Christiansen feature (CF) in thermal infrared (TIR) spectra of silicate lithologies on the Moon. Laboratory studies have found that both the increased surface iron content associated with formation of nanophase iron (npFe) during space weathering and decreasing grain size of regolith shift the CF position to longer wavelengths for TIR spectra measured under lunar-like conditions. This study confirms that the variation in CF position due to space weathering measured in the laboratory, is also evident at orbital scales. We have used swirls and rayed craters from lunar highlands and mare regions to examine the effect of initial composition and particle size distribution on the change of CF position. We also examine the effect of magnetic shielding at lunar swirls on the CF position. Our observations indicate that 1) for quantitatively comparable changes in albedo, the CF shift is greater for mare surfaces than for highlands surfaces; 2) particle-size is the dominant factor for the CF shift in fine-particulate regolith;3) npFe accumulation dominates CF position changes in coarser regolith and 4) the variation in CF position of lunar swirls is positively correlated to the magnetic field strength in the region. We hypothesize that the effects of particle size variations are also observed between the crater and swirl regions. However, further work is needed to constrain the effective particle size distributions of the regolith.