Mercury phase functions from MESSENGER/MDIS spectrophotometry: First results
Authors: Muinonen Karri , Domingue Deborah L. , Björn Vesa , Penttilä Antti 
Published: October 2020

Abstract. We analyze spectrophotometric observations of Mercury acquired from the Mercury Dual Imaging System (MDIS) on board NASA&#8217;s MErcury Surface, Space Environment, Geochemistry and Ranging mission&#8217;s spacecraft (MESSENGER). The spectrophotometric properties provide insight into regolith composition and structure, which in turn provide insight into surface processing. Using the Lommel-Seeliger reflection coefficient, we derive first estimates for the single-scattering phase function and the disk-integrated phase curve in eight colors. The results help us prepare for the interpretation of the Mercury observations by the ESA/JAXA BepiColombo mission.1. IntroductionPlanet Mercury continues to challenge scientific interpretation. Mercury&#8217;s principal rotational characteristics were unveiled only in 1960&#8217;s and its surface composition remains largely open. The elemental chemistry instruments on board the MESSENGER spacecraft provided constraints on many elements [1-6], however the surface mineral composition remains a mystery [7]. Mercury will soon be scrutinized by the BepiColombo mission that will examine the surface over a broader range of wavelengths, promising to identify the mineral composition.Spectrophotometry provides constraints on the physical and chemical properties of the regolith (e.g., [8]). We analyze MDIS eight-color disk-resolved photometry with the Lommel-Seeliger reflection coefficient (LS), the simplest model valid for regoliths with dark constituent materials. The LS coefficient has been recently utilized in, e.g., asteroid lightcurve inversion using disk-integrated photometry from the ESA Gaia mission [9]. The Gaia photometry unveils asteroid phase curve slopes in the phase-angle range of 15-35o. Examination of the Mercury photometry using the LS coefficient allows for the determination of the single-scattering phase function over MDIS&#8217; wavelength range, paving the way for the assessment of BepiColombo Mercury data [10,11].2. Modeling, results, and discussionWe make use of the LS reflection coefficient with the single-scattering phase function as in [9]. The disk-integrated phase function is introduced as part of the single-scattering phase function using a divisor that corresponds to the disk-integrated phase function for isotropic scattering. It follows that the data points themselves suggest the shape of the phase functions. Figure 1 shows the single-scattering phase functions as point clouds when reduced from the full MDIS data set (e.g., [8]). The filter wavelengths are as follows (in nm): &#955;(F)=433.2, &#955;(C)=479.9, &#955;(D)=558.9, &#955;(E)=628.8, &#955;(G)=748.7, &#955;(L)=828.4, &#955;(J)=898.8, &#955;(I)=996.2. In order to minimize surface roughness effects, we have required that the incidence angle is less than 40o. Figure 1: Single-scattering phase functions as indicated by the processed data points.In Figure 2, we show the disk-integrated phase curves. The phase curves look realistic for phase angles 60o, there is a&#160; bump that can be due to the simplicity of the LS coefficient.Figure 2: Disk-integrated phase curve as indicated by the processed data points.&#160;Figure 3 shows linear magnitude-scale fits to the data, with normalization to zero magnitude at 30o phase angle, when both incidence and emergence angles are required to be less than 40o. Color dependence is evident and there are indications of reddening as a function of phase angle. Although exceptions are present, the slopes tend to become shallower with increasing wavelength, in agreement with the increasing reflectance of Mercury with wavelength [12]. Figure 3: Linear fits to the single-scattering phase function data for incidence and emergence angles