  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 degree
peak of five MARCI images were found to agree better with modeled HWHM for WICs
than with modeled HWHM for dust.
