Spatial Distribution of Ray Craters on Callisto: Implications for Ray Retention and Impactor Sources on Jovian Satellites

Ray craters are good indicators for deciphering the most recent cratering rate and surface alteration rate on a solar system solid body. We utilize Voyager and Galileo images to examine the distribution of ray craters (D > 10 km) on Callisto. We identify a total of 57 ray craters and 279 rim deposit craters and find that ray craters (D > 25 km) on Callisto show a pronounced decreasing apex-antapex asymmetry (i.e., crater density decreases from the center of the leading hemisphere to the center of the trailing hemisphere), which would be consistent with a heliocentric origin, but the degree of the asymmetry is much lower than the theoretical expectation for ecliptic comets. A possible explanation for this is the preferential surface alteration rate on the leading hemisphere. The retention times of ray craters on Callisto are slightly longer than those on the dark terrain of Ganymede; this is possibly because of the substantially weaker micrometeorite gardening or/and sputtering on Callisto than those on Ganymede. The slope of the size-frequency distribution of ray craters on Callisto is shallower than that of all impact craters, which is due to the higher erasure rate for the smaller craters' rays.