Mehra reported in mid-February 2025 that the Jovian satellites may have formed in cold spots in the disk of gas and dust surrounding the infant Jupiter planet. In the early 1600s, Galileo Galilee trained a telescope of his own making on Jupiter and spotted its four largest moons.
Jupiter’s moon Ganymede—spotted here by NASA’s orbiting Hubble Space
Telescope—is the largest moon in the Solar System, although exactly how
it formed is unclear.STScI/NASA
Four centuries later, scientists are still seeking to understand exactly how those moons formed billions of years ago from a swirling disk of gas, dust, and ice that once surrounded the infant planet. Now, a new computer simulation suggests shadows cast by the inner region of that circumplanetary disk (CPD) may have created cold spots in the wispier outer region, creating the physical conditions needed for those materials to congeal into the moons Galileo spotted.
Published last month in The Planetary Science journal, the study “represents a significant step” toward linking the distribution of heat within the disk and the formation of Jupiter’s largest moons, says Zhaohuan Zhu, an astrophysicist at the University of Nevada, Las Vegas. Edward Guinan, an astrophysicist at Villanova University, adds that the effects of such shadow cooling on the disk “have not been investigated in such detail before.”
Jupiter has at least 95 moons and many more smaller moonlets. To explore the role of shadows in their formation, Antoine Schneeberger and Olivier Mousis, astrophysicists at the Origins Institute of Aix-Marseille University, used a computer to model Jupiter’s long-gone CPD in 2D, examining the disk’s cross-section and how it was structured in distance from the planet and from the midplane of the disk. They calculated several of the disk’s properties, such as its density, heat structure, and self-shadowing ability, then ran multiple simulations of how these would have influenced one another.
In the model, the evolving disk of gas, dust, and ice naturally splits into two distinct regions: a thick, opaque inner region that traps light and heat, and a thinner, transparent outer region that stretches into space. The scientists accounted for parameters such as the length of time it took Jupiter to form, its distance from the Sun, and a minimum temperature of the CPD consistent with the surrounding region at the time. They also varied the concentrations of elements heavier than hydrogen and helium, a property known as metallicity, based on previous studies, and simulated the structure’s evolution up to 3 million years after its formation.
Notably, they found that if the inner region expanded in altitude, it would cast a longer shadow on the outer portion of the disk, shielding it from heat from the newborn planet, which burned as hot as 2000 kelvins. In the model, those shadows created cold spots on the disk. Moreover, as the disk’s metallicity increased over time, the disk’s inner region became more opaque and trapped more heat. That heating caused the inner zone to expand to higher altitudes and cast even longer shadows on the disk’s outer regions.
The researchers also noticed the disk’s temperature did not grow steadily colder with increasing distance from Jupiter. Instead, the shadows led to certain spots with much cooler temperatures than neighboring regions. About 100,000 years after the formation of the CPD, a frigid region 100 kelvins colder than its surroundings appeared and endured for 80,000 years, the simulations suggest. That region reached out in the disk a distance 123.5 times Jupiter’s radius.
It was these massive cool spots, Schneeberger and Mousis conclude, that laid the groundwork for the largest moons. While tracing the evolution of volatile gases in the CPD, they found the cold regions centered 10 Jupiter radii away, or approximately 629,000 kilometers from the planet, were places where gases such as ammonia, hydrogen sulfide, and carbon dioxide could condense. The authors inferred these gases froze and turned solid, later becoming the seed for forming a moon like Europa, which today orbits in the same location as this predicted cold region.
If these cold traps did, in fact, have a significant impact on the formation of the four Galilean moons, then the inner moons, Io and Europa, should have a greater proportion of those volatile gases than the outer moons, Ganymede and Callisto. Schneeberger and Mousis expect their moon composition theory to be tested by the European Space Agency’s JUICE and NASA’s Europa Clipper missions that will analyze Jupiter and its satellites.
Such in-depth modeling “provides an excellent ground” to understand the Galilean moons and even planet formation, says Annie Dickson-Vandervelde, an astrophysicist at Vassar College. Still, the study has its limitations, she says. For example, if the moons’ formation was slower than predicted by the model, the cold shadow “would have little to no effect” on their chemical compositions, she notes. Similarly, Guinan notes variations in the activity of the young Sun could have affected the evolution of Jupiter’s CPD in ways the model could not account for. The authors acknowledge both points.
Schneeberger suggests further research should involve studying the deep composition of planets to understand their formations. He says his future research could be on the chemical composition of the disk to find any traces that could date the formation of Jupiter’s moons.
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