Exploring the Formation of Jupiter’s Galilean Moons

Scientists are diving into the mysteries surrounding the formation of Jupiter’s Galilean moons, as a new chapter from researchers at the University of Bern discusses various theories on how these large moon systems came to be. The work by Yuhito Shibaike and Yann Alibert offers insights into the unique processes that differentiate moon formation from planet formation, particularly in relation to Jupiter’s moons.

Understanding how moons form is less established than our knowledge of planet formation. While theories about the genesis of Earth’s Moon suggest a violent beginning, this scenario does not apply to larger moon systems like those orbiting Jupiter. The Galilean moons—Io, Europa, Ganymede, and Callisto—are part of what is termed the circum-Jovian disc (CJD), a structure analogous to the circum-stellar disc (CSD) that surrounds our Sun. Alongside the 93+ smaller non-Galilean moons, the CJD presents a complex environment for moon formation.

The new research identifies three primary distinctions between the formation of moons and planets. First, moon formation occurs significantly faster, estimated to be around 10 to 100 times quicker than planetary formation. Second, the CJD is continually gaining and losing material, with Jupiter at its center, influencing the dynamics of moon development. Finally, the rarity of systems with multiple large moons, especially since the advent of exoplanet discovery three decades ago, makes Jupiter and Saturn the only current examples of such systems.

To delve into the formation process of these moons, the researchers outline a three-step framework. The initial phase involves the creation of the CJD, which consists of gas, dust, and nascent moons. This idea was initially supported by a “minimum mass model” from the 1980s, which posited that the disc was static and contained a mass roughly equivalent to that of the Galilean moons.

In 2002, a new theory emerged proposing that the CJD functioned as a “gas-starved disc.” This model suggested that while the initial CJD was material-poor, it acquired additional mass through gravitational capture from the CSD. This process is thought to play a crucial role in the formation of the Galilean moons, marking the second phase of their development.

Jupiter’s massive size complicates the accumulation of smaller materials, which are necessary for moon formation. While it effectively clears its orbital path of larger debris, smaller dust particles can still navigate into the CJD without disruption. This leads to ongoing debate regarding the efficiency of this process. An alternative formation mechanism is “planetesimal capture,” where Jupiter’s gravity captures larger bodies that would otherwise become planets, resulting in their transformation into moons.

Differences in the Galilean moons themselves provide further avenues for testing these formation theories. Notably, Callisto, unlike its counterparts, does not resonate with Jupiter, which raises questions about its formation conditions. Some hypotheses suggest that Callisto might have formed under different circumstances or experienced a significant impact that altered its trajectory. Additionally, Callisto’s partially differentiated structure—having distinct core, mantle, and outer shell—contrasts with the more fully differentiated structures of Io, Europa, and Ganymede.

Models addressing pebble accretion propose that Callisto may still be in an early stage of its developmental journey, potentially evolving to resemble its more differentiated peers over time. Yet, many of these theories remain speculative without further data.

The forthcoming Jupiter Icy Moon Explorer (JUICE) mission is expected to provide valuable insights, although it will offer only one or two data sets. Until advancements in exoplanet detection technology allow for the identification of exomoons, many questions regarding the formation of large moon systems will linger. Nevertheless, as new data emerges, it will enhance our understanding of not just the Galilean moons, but also broader aspects of our solar system’s history.