Mercuryʹs integral phase curve: Phase reddening and wavelength dependence of photometric quantities

We present results of a five-filter photometric study of Mercuryʹs integral phase curve in the Johnson–Cousins UBVRI system, performed with the 0.90-m Westerlund Telescope in Uppsala, Sweden. CCD observations were made of the integrated disk for the phase angle range 22–152°, and the study is the first to cover the extended visible spectrum of Mercury. The observations are analyzed with Hapkeʹs semi-empirical radiative transfer-based light-scattering model and photometric quantities are derived. A statistically significant phase reddening effect of - 9 ± 6 mmag / deg / μ m is determined for Mercury based on color index observations, which is similar to that of the Moon. Phase coefficients fit to integral absolute magnitude data and Hapke models in combination with color index data provide a phase reddening effect of - 11 ± 13 mmag / deg / μ m which does however not provide statistically significant evidence for its presence. Phase coefficients indicate that phase reddening may be decreasing in magnitude with wavelength. As for the case with the Moon, the value of the phase integral increases with wavelength, but at an eight times higher rate. This value is consistent with the difference in the rate of change in the spectral slope–emission angle relation for the two bodies. We attribute these differences with Mercuryʹs redder spectral slope and an increase with wavelength of the backscattering lobe amplitude in the double Henyey–Greenstein particle phase function formulation. The normal albedo of integral Mercury at 1064 nm, pertinent to the return pulse energy of the BepiColombo laser altimeter (BELA), is estimated to 0.23 ± 0.06 with a range of 0.13–0.33 for 99% of the surface.