The new map also highlights the extraordinary effects of tidal heating on the Moon, thanks to its orbital configuration.
Io, one of Jupiter's Galilean satellites, is the most volcanically active body in the solar system. Images from space probes have shown molten lava erupting along walls forming giant lava lakes and plumes of gas and dust rising from vast calderas.
Now, a team led by planetary scientist Ashley Davies at NASA's Jet Propulsion Laboratory has prepared the first global heat flow map of Io's volcanic activity. The new map shows where magma has erupted from the moon's interior. The results, published in Journal of planetary scienceThey point to a possible magma ocean inside the satellite and highlight how common resonances in the orbits of celestial bodies could give rise to other oceans in the solar system and beyond.
Related: Io has likely been active throughout the history of our solar system.
Put on your thermal glasses
The team found 87 previously unknown hot spots using data from NASA's Juno spacecraft and 14 other similar sites from data collected with ground-based telescopes. They combined the new sites with existing ones to map a total of 343 sources of volcanic activity on the Moon.
The results show, for the first time, that Io’s volcanoes are evenly distributed on a global scale, but the heat they emit is not. Volcanoes in polar regions emit less heat than those at lower latitudes. In addition, volcanoes at Io’s north pole release twice as much energy as those in the south polar region. “That was the biggest surprise,” says Davies, “a real puzzle that we haven’t solved yet.”
The uneven distribution of heat, with more in the mid-latitudes than at the poles, is consistent with models suggesting warming on Io is occurring closer to its surface and melting underground rock. However, Davies says it's not clear how extensive the melting would be: “We don't know if it's a global or (partial) magma ocean.”
Tidal heating
One of the reasons Io is so volcanically active is that the moon is trapped in a repeating pattern of orbital configurations, called orbital resonanceswith two of Jupiter's other satellitesEuropa and Ganymede. In particular, Europa takes twice as long to orbit Jupiter as Io, and Ganymede takes four times as long as Io. Because of these resonances, the moons experience regularly repeating gravitational tugs on each other during alignments, which has made their orbits less circular. Thus, because Io moves closer to and further away from Jupiter during each revolution, tidal forces stretch and compress the moon, producing heat.
“Tidal forces create friction,” says Giacomo Lari, a mathematician at the University of Pisa in Italy and first author of another recent paper on Jupiter’s moons, published in Celestial mechanics and dynamical astronomyAnd because Jupiter is so massive and Io orbits so close to the planet, the resulting tidal heating is extreme. The effect is smaller but still present on Europa, whose orbit has become less circular and is also undergoing tidal heating, potentially allowing an ocean of water to exist beneath its icy crust.
Jupiter is not the only place where these effects matter: Saturn's moon Enceladus is also thought to have a water ocean, which may be maintained by tidal heating caused by resonances with the moon Dione.
“Understanding how resonances work and how the resulting activity has affected these incredible moons is vitally important to understanding the evolution of the solar system,” Davies says. This also includes the history of volcanism on Earth, the Moon and Mars, which likely experienced activity similar to that on Io in their early days, Davies adds.
Three are not a crowd
Lari's paper showed that the orbits of Io, Europa and Ganymede are dominated by the combined interaction of the three bodies, rather than separate resonances between pairs of moons — a finding that wasn't necessarily obvious, Lari says.
He adds that the three moons are currently in a “safe zone” where resonances are stable and should remain so for at least a billion years. Considering that this three-body resonance is the only one known between moons in the solar system, the opportunity to observe tidal effects is extraordinary.
And the effects reach far beyond our solar system. Astronomers have discovered exoplanetary systems where several major planets orbit in resonant chainsIn the future, researchers might be able to examine such systems — or a system of exomoons around a single exoplanet — for similar hot spots that could indicate tidal heating and the formation of magma or water oceans.
And in the meantime, new results from Jupiter will no doubt leave researchers hoping for more.
