Speaker
Description
A theoretical fractional-Brownian-motion particulate-medium scattering model (fBm-PM; see Björn et al., PSJ 5, 260, 2024) is used to interpret space-based and ground-based observations of Mercury regolith in the ultraviolet--visible--near-infrared spectral range of photometric and polarimetric observations. The fBm-PM model is based on radiative transfer and coherent backscattering (RT-CB) and incorporates a densely packed regolith geometry. The overall geometry depends on three parameters. First, the packing density describes the volume fraction of material. Second, the regolith interface to free space follows fBm statistics that are described by the Hurst exponent for horizontal roughness (related to the fractal dimension) and the standard deviation for vertical roughness.
The RT-CB solution for multiple scattering is based on ensemble-averaged scattering and absorption properties of single regolith particles (Muinonen et al., JQSRT 330, 109226, 2025), giving rise to efficient numerical computations. As input for single particles, both experimentally measured and numerically computed scattering characteristics are used. Furthermore, both shadowing and coherent backscattering effects are duly incorporated into the model.
As to observational data, first, we use spectrophotometry from the Mercury Dual Imaging System instrument (MDIS; Domingue et al., Icarus 257, 477, 2015) of NASA’s MErcury Surface, Space ENvironment, GEochemistry and Ranging mission (MESSENGER). Second, we use ground-based photometric and polarimetric phase curves (Mallama et al., Icarus 155, 253, 2002; Dollfus et al., Icarus 23, 465, 1974). The MDIS data come in eight colors in wavelengths of 433.2–996.2 nm, with phase angles of 20-125 degrees, whereas the ground-based phase curves cover the phase angles of 4.7-137.2 degrees and 6.0-131.7 degrees for photometry and polarimetry, respectively. We compare our results to those from the earlier study using the MDIS data (Björn et al., PSJ 5:260, 2024).
The present fBm-PM study prepares us for the interpretation of Mercury spectrophotometry and X-ray fluorescent emission spectroscopy to be provided by the ESA/JAXA BepiColombo mission with its SIMBIO-SYS (Spectrometer and Imagers for MPO BepiColombo Integrated Observatory SYStem) and MIXS/SIXS instruments (Mercury Imaging X-ray Spectrometer/Solar Intensity X-ray and particle Spectrometer). Finally, the work aligns with the scattering modelling for asteroids (e.g., Virkki et al., Kolehmainen et al.), the Moon, and the Galilean satellite Europa (Leppälä et al.).