"I do not know who I am, where I am going — and I am the one who has to decide the answers to these hideous questions."
A modern Callisto who still resembles the constellations that used to be her bones and she can’t remember how it is to be human. Something raw crawls beneath her skin and she is just as far away from this earth as the stars as her feet learn again how to walk upon the forests she once knew. Only her eyes are human these days, everything else is still trying to remember, though her limbs resist, knowing that on this earth, she lost all she has ever known, and all she can do is roam, only welcoming the wild into her arms. However, there are times she makes her way outside of the darkness, but only in search of ghosts, for the one that has brought her death twice, and she would still find a way to give him the world if he asked. She peers into all of the eyes of men, hoping they match her own, hoping they hold starlight. Some days, she finds them in everyone, her son is everywhere, his voice in the winds that chill her to the bone. And she prays for the stars again, for it was in the heavens that she wasn’t alone.
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.
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.
Callisto, the third-largest moon in the solar system, is tidally locked in its revolution around Jupiter so that Jupiter remains stationary in Callisto’s sky. Photo taken in 2001 as the Galileo space probe flew by.