Relative rates of optical maturation of regolith on Mercury and the Moon

Quantifying relative differences in regolith optical maturation rates is critical to interpreting multispectral reflectance measurements of airless bodies. In this study, normalized reflectance measurements of crater ejecta blankets and rays are used as indicators of the relative state of regolith maturation on Mercury and the Moon, as well as for a comparison of surface reflectance. Characterization of craters with high‐reflectance ejecta from Lunar Reconnaissance Orbiter Camera and Mercury Dual Imaging System orbital mosaics indicates that the optical maturation rate is up to 4 times faster on Mercury than on the Moon. Observations also show that there are fewer immature craters (per unit area) on Mercury than on the Moon and suggest a younger average age for mercurian rayed craters than for lunar rayed craters. A comparison of crustal reflectance of immature material yields a ratio of 1.9 ± 0.4 for the average photometrically normalized reflectance of lunar material to that of mercurian material. The difference in reflectance is attributed primarily to differences in composition between the lunar highlands and average surface material on Mercury. The new observations of immature craters per unit area and surface reflectance of immature materials are consistent with previous proposals that regolith maturation rates are faster on Mercury than the Moon, and but here we quantify the relative rate from empirical observations for the first time.