As a newborn planet, Jupiter shone brightly in the sky and eclipsed today’s sun from the perspective of the gas giant’s largest moons. This early radiation – and upcoming visits from multiple spacecraft – could help solve a 40-year-old mystery about the composition of these satellites.
For decades, scientists have struggled to understand the strange density differences of Jupiter’s four Galilean moons, which, in order from closest to the planet to furthest away, are Io, Europa, Ganymede and Callisto. Although these natural satellites must have formed from the same raw material and therefore have similar compositions, density measurements suggest that Callisto and Ganymede are much more icy than Europa, while Io has no ice. at all. New research revealed at a conference last month by Carver Bierson, a planetary scientist at Arizona State University, may shed some light on the matter.
Giant planets form by clumping together and compressing huge volumes of gas and dust. This process releases tons of excess energy and gives newborn giants a literal youthful glow that can last millions of years. It’s more than a theory: astronomers regularly use this glow to image young giant exoplanets that would otherwise be lost in the glow of their nearby stars. But the less conspicuous question of how such glimmers might shape the moons that accompany them has remained scarcely explored. In the case of Jupiter, computer modeling by Bierson and his colleagues suggests that the planet’s early brightness would have illuminated its nascent moons and boiled off most of their water within a few million years.
“It gave people a completely new process to think about,” says Francis Nimmo, who studies icy moons at the University of California, Santa Cruz, and was not part of the research.
Four satellites, one origin
The differential compositions of the four Galilean moons have puzzled researchers for decades, ever since the first high-quality density measurements from the satellites were obtained. Trapped inside Jupiter’s radiation belt and heated from within by the planet’s powerful tidal forces, which knead the moon’s innards like dough, Io is a completely ice-free world of volcanoes. hyperactive. Europa, slightly further away, is also in the grip of Jupiter’s radiation and tides. But more modest levels of internal heating gave the moon a subterranean ocean and an icy crust rather than lava-spewing calderas. Both Ganymede and Callisto are relatively inert, ice-rich, and much farther from Jupiter than Io and Europa.
Although variations in Jupiter’s gravitational grip clearly explain some of the differences between the moons, planetary scientists still struggle to understand how these objects could share a common origin while being so radically divergent from each other. Similar to how planets emerge from swirling protoplanetary disks of gas and dust around fledgling protostars, large moons can form from smaller mini disks that arise around the assembly of gas giant worlds. Current thinking dictates that Jupiter gained most of its mass very quickly, during the first 10 million years of the solar system’s life, before light, stellar winds from the ever-brighter sun blew it all away. gas from the protoplanetary disk.
This relatively tight schedule means that Jupiter had to rapidly and voraciously swallow gas to grow to its current size, which would have caused it to heat up and glow as it reached estimated temperatures of 1,160 degrees Fahrenheit (627 degrees Celsius). For the Galilean moons, which presumably formed around the same time as Jupiter itself, the planet would have shone like a star in the sky and overpowered the light emanating from the farthest sun. By carefully simulating the effects of Jupiter’s increased brightness on the Galilean moons, Bierson and his colleagues discovered that this flood of light could perfectly solve the puzzle of the current varied composition of the satellites.
Freshly Baked Sweet Moons
Torn apart by Jupiter’s gravity, Io today is a hellish landscape of volcanic eruptions and is the most active body in the solar system. But the team found that Jupiter’s youthful glow could have initially given Io Earth-like temperatures, and possibly even an ocean. “I think it’s likely that at the time of Io’s formation or just after it ended, there was water on the surface,” Bierson says.
That would have changed quickly because Io received about 30 times more energy from Jupiter than it receives from the sun today, according to Bierson. If Io had started out with as much water as its sibling Ganymede currently has, all that moisture would have been quickly washed away and any remnant of an ocean would have boiled over within the first million years of the moon’s existence.
Europa, farther out than Io, would have had slightly cooler surface conditions, but possibly still warm enough for this moon to lose a significant portion of its water. Even farther out, at Ganymede, Jupiter would have appeared barely brighter than today’s sun – a level of insolation with no significant impact on the moon’s ice. For distant Callisto, relegated to the periphery of the Jovian system, Jupiter’s brilliant youth would have had no effect. (All of this assumes the moons were in their current positions. However, they likely formed closer before migrating to their current locations, meaning the study results are likely just a lower bound on the amount of cooking of each moon by Jupiter.)
“The good thing about this hypothesis is that there are tests you can apply,” says Nimmo.
A JUICE-y proposal
If Europa had lost most of its ice in its lifetime rather than forming with less ice than its siblings, the hydrogen and oxygen left behind would have a different isotopic fingerprint than the ice on Ganymede and Callisto. So an isotopic comparison of Europa with one or both of the outermost moons could finally reveal the truth about how these satellites diverged from their common origins. “The more comparisons you can make [among the chemistries of the moons]the more you understand how things evolve in this early era,” says Berson.
That’s a pretty juicy proposition, considering the recent launch of the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) mission. Between 2031 and 2034, JUICE will make 35 flybys of Europa, Callisto and Ganymede before going into orbit around Ganymede. The extended visit can go a long way in determining whether all Galilean moons were born with the same amount of ice. JUICE is carrying a mass spectrometer that Nimmo says could make important measurements of the hydrogen and water vapor that can emanate into space from moons, especially Ganymede.
“The question is whether Ganymede provides enough material at the altitudes that JUICE can sample,” says Nimmo. He remains convinced that this will be the case.
Even if JUICE’s studies can’t solve the problem, it won’t be the only spacecraft to scan the moon around the Jupiter system. NASA’s Juno mission is already in orbit around the gas giant, and the space agency’s Europa Clipper mission is set to launch next year for a trip to the mission’s eponymous moon. The Clipper data should provide a solid comparison for JUICE’s view of Europa’s ice that would be sufficient to extrapolate and distinguish anything the European spacecraft sees at Ganymede and, potentially, Callisto.
“Comparisons between moons are going to be extremely important,” Bierson says. “It’s so exciting that we’ll have both JUICE and Europa Clipper there almost at the same time and maybe overlapping a bit.”