Hubble 's view is so sharp that astronomers can see features on ganymede 's surface , most notably the white impact crater , tros , and its system of rays , bright streaks of material blasted from the crater .
Ganymede completes an orbit around jupiter every seven days .
ganymede环绕木星运行一圈的周期是7天。
Specifically , dr kipping reckons , they would need to have at least a fifth of the mass of earth . Ganymede , jupiter 's most massive satellite ( and also the biggest in the local solar system ) , has only a fortieth of the earth 's mass .
