How was Io, the hellish and volcanic moon of Jupiter, born?

“We see Io as it has always been,” says Jani Radeboe, a planetary geologist at Brigham Young University in Utah, US, who was not involved in the new work. This turns Io into a kind of time machine, whose tireless heat engine (powered by gravitational tides) can tell us about worlds near and far.

“This process occurs throughout the solar system, as well as on exoplanets,” explains Catherine de Cleere, a planetary astronomer at the California Institute of Technology and lead author of the study. “We are studying Io to better understand this universal process.”

The solar system may not seem all that changeable from a human perspective. But this, of course, is on an astronomical time scale. For example, in recent years, scientists have discovered that Saturn’s iconic rings are not permanent features, but rather recent decorations: they formed several hundred million years ago and will disappear within the same period of time.

So Io may not have always been the volcanic host it is today. But to find out, we need to understand how volcanism works and why it is so impressive.

In 1979, two major scientific events laid the groundwork: NASA’s Voyager 1 spacecraft flew through the Jupiter system and photographed huge plumes of volcanic material rising from Io’s surface, and an independent team of scientists calculated that Io may have powerful but unusual thermal emissions . source.

This mathematical prediction arose from the strange journeys of Europa and Ganymede, a pair of moons close to Io. Every time Ganymede makes one full orbit around Jupiter, Europa makes two, and Io makes four. This rhythm, known as resonance, changes Io’s own orbit, giving it an elliptical rather than a circular shape.

When Io is closer to Jupiter in its curved orbit, it experiences a stronger gravitational pull; When Jupiter is further away, Jupiter’s gravitational pull is slightly weaker. This causes tides on Io similar to those caused by the Earth’s Moon in the seas and oceans of our planet. But in this case, the tides are so strong that Io’s surface rises and falls to a height of 100 meters, comparable to the height of a small skyscraper.

All this movement creates a lot of friction, which generates a surprising amount of heat. Inside Io, this heat melts a significant amount of rock, possibly creating an ocean of magma. And it causes truly ferocious eruptions on its surface, often in the form of winding rivers of lava longer than most aquatic versions of Earth, towering confetti columns of sulfur-rich lava, and calderas of liquid rock that act as portals to Io’s underworld. .

“It’s impressive,” says de Kleer; “There are volcanoes on the Moon that give us a window into what’s going on inside the Moon that we don’t usually have.”

The extreme nature of volcanism is not limited to eruptions. In addition to sulfur compounds, it emits sodium and potassium chloride gases. On Earth, we use them to season our food. “It’s like table salt as a gas coming out of volcanoes,” says de Kleer.

Much of the erupted material may also be ejected into space through Io’s thin atmospheric bubble. There it mixes with sunlight, is electrically excited, and then falls into Jupiter’s magnetized sky and explodes into powerful auroras—the gas giant’s version of Earth’s northern and southern lights.

Io’s heat source (a mechanism known as tidal heating) is ultimately responsible for all this planetary wizardry. The scientific community wanted to know whether such tidal heating had always existed inside the Moon. But because it is so volcanically active, lava flows quickly and repeatedly covered its surface, obscuring any evidence of ancient geological processes.

“You can’t look at the surface of Io and tell anything about what happened more than a million years ago,” says de Cleere. That’s why she and her team took a different approach and looked at their skies.

Io loses up to three tons of material every second into space through volcanic degassing and atmospheric erosion. “You could say that Io is losing mass like a comet,” says Apoorva Oza, a planetary astrophysicist at NASA’s Jet Propulsion Laboratory who was not involved in the new work.

Over time, this will mean that Io’s current eruptions will be relatively richer in heavier versions (isotopes) of various chemical elements than in lighter ones, since lighter isotopes from the upper atmosphere can more easily escape into space. If the team could measure the current ratio of heavy atmospheric isotopes to lighter isotopes, they could calculate how long it would have taken Io to reach this state from the original reservoir of underground but eruptible compounds within Io.

De Cleere’s team used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe gases in Io’s atmosphere, especially sulfur. They also calculated the Moon’s “original” supply of heavier and lighter isotopes using (among other things) ancient meteorites that hold a record of the average chemical composition of the early solar system.

They found that the high proportion of heavier sulfur isotopes in Io’s current atmosphere suggests that Io has lost between 94 and 99 percent of its original sulfur reserves. And the only way for this to make sense and fit with pre-existing models of the evolution of Jupiter and its inner moons is for Io to erupt, perhaps within 4.5 billion years.

“The orbital dynamics of planetary satellites can be very chaotic,” says James Tuttle Keane, a planetary scientist at NASA’s Jet Propulsion Laboratory who was not involved in the new work. Moons can enter and leave stable orbits, sometimes collide or be ejected from the solar system entirely.

But it appears that Io, Ganymede and Europa have been dancing in the same way for billions of years, and “The Io we see today in some ways represents Io throughout its long history,” Keane says.

This is unusual in itself, but it also has implications for Io’s neighbor Europa. Beneath the icy shell of this ice ball is not only an ocean of liquid water, but it is also believed to be kept warm and liquid by tidal heating. This means that if Io was volcanically active for billions of years, Europa’s ocean could be just as primitive.

“This may have implications for the long-term habitability history of Europe,” says de Kleer. We still don’t know if there is life in this ocean. But if this is so, then it owes its existence to the same eternal force that makes it burn with volcanic fire.