Io is a volcanically active body of the Solar System and one of the largest moons. Long-term volcanic activity is extremely unusual for bodies this size because they do not have deep enough depths and internal heat does not last long on its own. But Io flies in orbital resonance with Jupiter’s other two moons, Europa and Ganymede. Due to the influence of tidal gravity, this “dance” warms the inside of Io. Of course, Jupiter also plays a role, but neighboring moons increase the effect. As the simulation showed, the triplet echoed shortly after formation. The moons then experience tidal heating throughout the entire history of the Solar System. So Io was volcanically active for 4.57 billion years: either all the time or in cycles.

It is impossible to test this assumption by examining the surface of Io. Volcanoes update the topography of the satellite at a rate of 0.1-1 centimeter per year. Therefore, behind craters from asteroids, scientists can only “unravel” the last few million years of their history. But the entire “story” may be preserved by the atmosphere.

“If Io has been volcanically active throughout its history, the volume of material emitted on its surface must be tens or even hundreds of times greater than the volume of its mantle,” explains planetary scientist Kathrine de Kleer from the California Institute of Technology. lead author of the new study

It turns out that to maintain this rhythm, Io must “recycle” expelled matter back into the mantle. At the same time, during the explosion, volatile compounds enter the atmosphere. The lightest of these (those with lighter isotopes) rise high enough to escape into space, while the heavier ones sooner or later fall. Thus, then the “classical” ratio of isotopes of elements is violated – an abnormally large number of compounds with heavy isotopes turns out. This is exactly what scientists checked in the new study.

The authors of the new study examined gases in Io’s atmosphere using the ALMA radio telescope complex. Data were collected in May 2022. They monitored changes in sulfur monoxide, sulfur dioxide, sodium chloride and potassium chloride. Their goal was to determine the ratio of sulfur-34 to sulfur-32 and chlorine-37 to chlorine-35. The results of the study were published in the journal Science. Also in the magazine Journal of Geophysical Research: Planets published a companion paper on the history of tidal heating and the sulfur isotope cycle on Io.

Scientists know from analysis of meteorites that during the formation of the Solar System, there were 23 light atoms of sulfur-32 for one atom of sulfur-34. They differ from each other by two neutrons. If volcanoes haven’t been erupting on Io for that long, the current rate shouldn’t be radically different. Analysis of ALMA data showed that this ratio ranges from approximately one to 16.8 on Io.

After performing calculations, scientists concluded that Io had lost 94-99 percent of its sulfur. To lose this amount of sulfur, Io would have had to lose mass 0.5-5 times faster throughout its history than today (1-3 thousand kilograms per second). This suggests that there were periods in the past when Io lost matter much more rapidly. It is important that we are talking about sulfur that is not in the core (sulfur in the core accounts for 80-97 percent of all sulfur in the satellite). Chlorine isotope ratio analysis confirmed these findings.

“Tidal heating from resonance is an important source of heat in satellites and can fuel geological activity. Io is the most extreme example of such an effect, so we use it as a laboratory to understand the mechanism of tidal heating.” de Clear explained.

Meanwhile, the multiple “recycling” of Io’s mantle means that planetary scientists have a unique opportunity to accurately learn the chemical composition of the satellite’s deep layers. For this, samples must be taken from the expelled substance.

The duration of volcanism on Io means that tidal influence continues throughout this period. Therefore, an orbital resonance has long been established between the satellites, which is important for all participating organizations, including Europa. It turned out that this satellite had been heated from the inside all this time. Europa is one of the most promising objects in the Solar System. Proving the existence of a constant heat source is of great importance for hypotheses that this satellite can support life.