Aphelion Cloud Belt phase function investigations with Mars Color Imager (MARCI)

Abstract This paper constrains the scattering phase function of water ice clouds (WICs) found within Mars’ Aphelion Cloud Belt (ACB), determined from orbit by processing publicly available raw Mars Color Imager (MARCI) data spanning solar longitudes (LS) 42°–170° during Mars Years (MYs) 28 and 29. MARCI visible wavelength data were calibrated and then pipeline-processed to select the pixels most likely to possess clouds. Mean phase function curves for the MARCI blue filter data were derived, and for all seasons investigated, modeled aggregates, plates, solid and hollow columns, bullet rosettes, and droxtals were all found to be plausible habits. Spheres were found to be the least plausible, but still possible. Additionally, this work probed the opposition surge to examine the slope of the linear relationship between column ice water content and cloud opacity on Mars, and found a significant dependence on particle radius. The half-width-half-maxima (HWHM) of the visible 180° peak of five MARCI images were found to agree better with modeled HWHMs for WICs than with modeled HWHM for dust.