How do two similar objects form in similar conditions yet end up in vastly different final states? Whether studying human twins, planets, or moons, differences provide powerful tools in understanding the subject’s development. One such example, Ganymede and Callisto, Jupiter’s two largest moons, has puzzled astronomers for decades.
The gross features of Ganymede and Callisto (shown below) indicate the two moons should look pretty much the same. They both have a radius around 2500 km and densities around 1.9 g/cm3 (Callisto’s radius and density are about 10 percent smaller than Ganymede’s). Given this density, the compositions of both moons consist of half rock and half ice. Furthermore, the moons formed in a similar environment. Yet they differ greatly. Ganymede’s grooved surface suggests extensive interior evolution, and measurements indicate all its rock is consolidated in its core. In contrast, Callisto’s surface appears ancient with no significant signs of resurfacing, and measurements indicate that a substantial amount of its rocky material resides outside its core.
Ganymede and Callisto
Recent research traced the marked differences between Ganymede and Callisto back to a cataclysmic period in the early solar system. About four billion years ago, the planets and large moons in the solar system experienced numerous impacts from comets and asteriods. Known as the Late Heavy Bombardment (LHB), this period, which lasted a few hundred million years, wreaked havoc throughout the solar system. On Earth for example, these collisions sterilized the planet surface, turning it into an ocean of liquid rock. Yet these impactors also delivered the seeds for the eventual formation of human technology and civilization.
Although Ganymede and Callisto are similar in many important aspects, Callisto orbits Jupiter at almost twice the distance of Ganymede. A team of scientists at the Southwest Research Institute modeled how this orbital difference affected the two moons during the LHB.1 Because it resides deeper in Jupiter’s gravitational well, Ganymede experienced twice as many impacts, with greater characteristic impact velocities, during the LHB as Callisto. The greater amount of energy deposited on Ganymede by the impacts meant the moon’s icy outer region melted completely. Thus, the rocky material sank to the core and the tumultuous cooling period resulted in the large surface features. Because the impacts on Callisto deposited less heat, less quickly, the moon’s ice did not melt as deeply and the rocky material remained distributed throughout the planet. Also, the cooling process occurred more placidly resulting in an ancient and much smoother surface.
The resolution of the differences between these two moons highlights two points. First, the scientists’ model places limits on how much material was in the solar system disk during the LHB. They determined that somewhere between five and thirty times the mass of the Earth were required to explain the differences between Callisto and Ganymede. This estimate agrees with other calculations derived from trying to reproduce the architecture of the outer solar system. Second, this research continues to demonstrate how catastrophic processes in the early period of planet formation lead to drastic difference in planet development. Though disruptive, these processes serve important functions in preparing Earth to support life.
1. Amy C. Barr and Robin M. Canup, “Origin of the Ganymede–Callisto Dichotomy by Impacts during the Late Heavy Bombardment,” Nature Geoscience, (January 24, 2010), http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo746.html .