Orbital Evolution of Impact Ejecta from Ganymede

We have numerically computed the orbital evolution of ∼103 particles representing high-speed ejecta from Gilgamesh, the largest impact basin on Ganymede. The integration includes the four Galilean satellites, Jupiter (including J2 and J4), Saturn, and the Sun. The integrations last 100,000 years. The particles are ejected at a variety of speeds and directions, with the fastest particles ejected at 1.4 times the escape speed vesc≡2GMG/RG of Ganymede. Ejecta with speeds v<0.96vesc follow suborbital trajectories. At v∼0.96vesc there is a transition characterized by complex behavior suggestive of chaos. For v>0.96vesc, most particles escape Ganymede and achieve orbits about Jupiter. Eventually most (∼71%) of the jovicentric particles hit Ganymede, with 92% of these hitting within 1000 years. The accretion rate scales as 1/t. Their impact sites are randomly distributed, as expected for planetocentric debris. We estimate that most of the resulting impact craters are a few kilometers across and smaller. The rest of the escaping ejecta are partitioned as follows: ∼3% hit Io; ∼10% hit Europa; ∼13% hit Callisto; 2% reach heliocentric space; and less than ∼1% hit Jupiter. Only two particles survived the entire 105-year integration. Ejecta from large impact events do not appear to be a plausible source of large craters on the Galilean satellites; however, such ejecta may account for the majority of small craters.