  This study constrains the lower bound of the scattering phase function of
Martian water ice clouds (WICs) through the implementation of a new observation
aboard the Mars Science Laboratory (MSL). The Phase Function Sky Survey (PFSS)
was a multiple pointing all-sky observation taken with the navigation cameras
(Navcam) aboard MSL. The PFSS was executed 35 times during the Aphelion Cloud
Belt (ACB) season of Mars Year 34 over a solar longitude range of
L_s=61.4{\deg}-156.5{\deg}. Twenty observations occurred in the morning hours
between 06:00 and 09:30 LTST, and 15 runs occurred in the evening hours between
14:30 and 18:00 LTST, with an operationally required 2.5 hour gap on either
side of local noon due the sun being located near zenith. The resultant WIC
phase function was derived over an observed scattering angle range of
18.3{\deg} to 152.61{\deg}, normalized, and compared with 9 modeled phase
functions: seven ice crystal habits and two Martian WIC phase functions
currently being implemented in models. Through statistical chi-squared
probability tests, the five most probable ice crystal geometries observed in
the ACB WICs were aggregates, hexagonal solid columns, hollow columns, plates,
and bullet rosettes with p-values greater than or equal to 0.60,
0.57,0.56,0.56, and 0.55, respectively. Droxtals and spheres had p-values of
0.35, and 0.2, making them less probable components of Martian WICs, but still
statistically possible ones. Having a better understanding of the ice crystal
habit and phase function of Martian water ice clouds directly benefits Martian
climate models which currently assume spherical and cylindrical particles.
