Mercury's spectrophotometric properties: Update from the Mercury Dual Imaging System observations during the third MESSENGER flyby

► A dedicated photometric observing sequence constrains Mercury's photometric characterization. ► MESSENGER-derived broadband spectral properties match ground-based observations. ► There is no evidence of phase reddening in the broadband spectral measurements. ► On large scales Mercury's regolith more closely resembles asteroidal than lunar regolith. ► Mercury regolith grain structures more closely resemble those of lunar than asteroidal grains.