Are the Moons of Jupiter Unique?

Abstract

The practice of astronomy is in many ways an intrinsically introspective endeavour. A significant fraction of astronomical motivation is derived from the desire to understand whether a habitable planet such as the Earth is a unique object, and, by extension, whether the inhabitants of the Earth themselves collectively represent a unique phenomenon. By definition, a world is considered potentially habitable if it is theoretically capable of supporting liquid water at its surface. But by focusing solely on strictly Earth-like planets, we risk overlooking other potentially habitable options. In fact, the majority of the worlds known to host liquid water oceans in the solar system are not Earth-like at all. These other worlds do however share a singular defining characteristic: they are the icy moons that orbit the gas giant planets. Their oceans are concealed below kilometers of frozen crust. In the solar system at least three moons are known to host an ocean with a high degree of confidence (Europa, Enceladus, and Titan), and another four are suspected (Ganymede, Callisto, Mimas, and Dione). Hence, any hope of answering the question as to how unique the phenomena of life on Earth really is may hinge predominantly on answering a seemingly unrelated question: how unique are the icy moons?

The formation of gas giants appears to be accompanied by the formation of moons, as, at least in the solar system, the two appear inseparable. The gaseous planets Jupiter, Saturn, and Uranus, are each attended by a retinue of moons both regular and irregular. The regular satellites tend to orbit in a single plane, in the same direction, and on nearly circular paths. These peculiar properties are also exhibited by the planets, and hence it is considered likely that some similar process has been at play to form them both. That process is the formation within a swirling disk of gas and dust. Planets form within disks that surrounded a star (a circumstellar disk) while the moons would have formed within a disk surrounding their planet (a circumplanetary disk, or CPD).

The last decade of strides made in the observation of circumstellar disks has revolutionized our understanding of the planet formation process. To what extent might the moon and planet formation process be similar? To what extent might we be able to extrapolate our understanding of circumstellar disks down to the scales characteristic of circumplanetary disks? Is there a smooth continuum in physical processes, connecting the formation of large moons, with the formation of the smallest planets? In this work we have extended the theoretical tools used to explore planet formation down into this new regime. On the observational side, as the scale of the astrophysical object shrinks, the capabilities of the instrument must rise commensurately to observe it. We are now at the earliest possible stage of directly observing CPDs to gain insights beyond the theoretical into how giant planet moon formation actually proceeds…