Spectral Trends of the Surface Regolith in Lunar Craters

Using topography‐corrected reflectance maps of the Moon, we present a new methodology for quantifying the spectral distribution of craters in the 750 nm reflectance and 950 nm/750 nm reflectance ratio space (R‐C space) and investigate how the spectral trend changes as the crater matures and degrades. Considering more than 3,495 craters across the Moon, we calculate three parameters to describe spectral trends of the pixels in each crater: 95th‐percentile length, clockwise rotation angle from the horizontal line, and statistical skewness along the principal axis of the distribution, as well as median values of the reflectance and the reflectance ratio. We find that the shape of the spectral trend of a crater in R‐C space becomes shorter, steeper, and more skewed toward the upper left as it optically matures and topographically degrades. Skewness shows a similar relationship to topographical degradation in both mare and highland craters. The skewness is useful in age estimation because it can be calculated easily from reflectance data and is applicable to craters of large size or in low‐iron‐content regions where a topographic model is currently not available. Using these three parameters, one can define the representative trends of a group of craters. We show representative trends of craters with various iron contents at different latitudes. The trends are parallel to one another, with offset controlled by latitude and radial rotation controlled by local iron content; these observations integrate the results of previous studies that reported the two behaviors individually.