Ganymede and Callisto are similar in size and are made of a similar mixture of ice and rock, but data from theGalileo and Voyager spacecraft show that they look different at the surface and on the inside. Just like Earth and Venus, Ganymede and Callisto are twins, and understanding how they were born the same and grew up to be so different is of tremendous interest to planetary scientists.
Ganymede and Callisto’s evolutionary paths diverged about 3.8 billion years ago during the Late Heavy Bombardment, the phase in lunar history dominated by large impact events. Impacts during this period melted Ganymede so thoroughly and deeply that the heat could not be quickly removed. All of Ganymede’s rock sank to its center the same way that all the chocolate chips sink to the bottom of a melted carton of ice cream. Callisto received fewer impacts at lower velocities and avoided complete melting. Ganymede is closer to Jupiter and therefore is hit by twice as many icy impactors as Callisto, and the impactors hitting Ganymede have a higher average velocity.
NASA’s Hubble Space Telescope has captured a stunning sight that happens only once or twice every 10 years. Three moons and their shadows move across Jupiter during a rare triple transit on Jan. 24, 2015. Europa is at lower left, Callisto is above and to the right of Europa and Io is approaching Jupiter’s eastern limb. Europa’s shadow is toward the left side of the image and Callisto’s shadow to the right.
(Sombrero Galaxy, the Moon (taken by me), Jupiter with its 4 Galilean Moons- Io, Europa, Callisto and Ganymede (also taken by me), the Carina Nebula, a nebula, a person standing under the Milky Way band, the night sky with a shuttle taking off, a shuttle taking off seen by a plane, a meteor shower, and Andromeda Galaxy)