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

Abstract

[1] 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.

Global distribution of lunar impact melt flows

“…We made no attempt to look specifically at craters of a specific age, to avoid any biases in our survey. However, we note that more than a third of the craters with D > 10 km in our sample were identified in Braden and Robinson (2013) as Copernican, supporting the idea that impact melt is often present around the youngest craters on the Moon. The survey conducted by Braden and Robinson (2013) was limited to those craters within ±40°of the equator, where crater rays can be identified, so more Copernican craters may be present in our sample at higher latitudes.…”

“…However, we note that more than a third of the craters with D > 10 km in our sample were identified in Braden and Robinson (2013) as Copernican, supporting the idea that impact melt is often present around the youngest craters on the Moon. The survey conducted by Braden and Robinson (2013) was limited to those craters within ±40°of the equator, where crater rays can be identified, so more Copernican craters may be present in our sample at higher latitudes. If the crater was 'confirmed' to have exterior melt deposits, we documented the parent crater diameter and location, the flow direction, and the maximum distance of the melt from the crater rim (Table 1, which includes our initial observations reported in Carter et al (2012)).…”

Application of multiple photometric models to disk-resolved measurements of Mercury’s surface: Insights into Mercury’s regolith characteristics

“…It should be noted that the lunar data contained observations within the opposition region whereas the Mercury data did not, so the opposition related parameters (l 1 and m) are poorly constrained for Mercury. The difference in normal albedo is consistent with the observation that immature materials on Mercury are 30-50% lower in reflectance than corresponding materials on the Moon, suggesting the presence of a darkening component within Mercury's regolith compared to the Moon (Denevi and Robinson, 2008;Robinson et al, 2008;Braden and Robinson, 2013;Murchie et al, 2015).…”

“…Regardless, some inferences can be drawn from these comparisons. Mercury's regolith behaves as expected for particles with a high density of internal scatterers, possibly higher than lunar regolith particles, consistent with a highly space weathered surface (Hapke, 2001;Domingue et al, 2014) or one incorporating extremely fine-grained opaques (Murchie et al, 2015), which has been postulated to explain the darkening agent present in Mercury's regolith compared to the lunar surface (Denevi and Robinson, 2008;Robinson et al, 2008;Braden and Robinson, 2013).…”

“…The derived globally averaged normal albedo at 700 nm for Vesta ranged from 0.292 to 0.301, depending on the disk function used. These values are much higher than the 0.09-0.13 values derived for Mercury at 748.7 nm, commensurate with a darkening agent present in Mercury's regolith (Denevi and Robinson, 2008;Robinson et al, 2008;Braden and Robinson, 2013;Murchie et al, 2015). In a similar study, Li et al (2013) used an exponential phase function of the form:…”