Io was always extremely volcanic, evidence indicates

The most volcanically active object ever observed in the solar system is Io. Its massive and sustained eruptions, extensive lava flows, and huge lava lakes are more substantial than any on Earth and have led researchers to wonder: Has Io always been that way? The question is hard to answer because its surface is constantly changing. In just a million years—which is short by geological standards—Io’s entire surface has been repaved by volcanic deposits, destroying craters and other evidence of geologic history over its 4.5-billion-year existence.

Now Katherine de Kleer of Caltech and colleagues have developed a way to study Io’s volcanic history by tracking its sulfur. With new isotope evidence and geochemical modeling, they found that Io’s volcanism was likely even more pronounced in the past and may have been present over the moon’s entire history.

The researchers observed sulfur gases in Io’s atmosphere with the Atacama Large Millimeter/Submillimeter Array in Chile. When a gas molecule loses angular momentum, it transitions from a higher rotational energy state to a lower one and, in the process, emits microwave radiation. Each molecule has many unique spectral emission lines that can be observed. Even molecules made with different isotopes—say, with sulfur-32 and sulfur-34—are distinguishable. With the molecular emission data, the researchers determined the sulfur isotope composition of Io’s atmosphere and found that it contains much more ³⁴S than ³²S.

In fact, no other measured body in the solar system has such a high ratio of ³⁴S/³²S. To interpret Io’s unusual sulfur enrichment, one of the paper’s coauthors, Ery Hughes, led the development of a geochemical model of Io’s interior and its atmosphere. The sulfur ratio likely reflects a geochemical process that begins when sulfur dissolved in magma rises through Io’s mantle and erupts from volcanoes. The lighter ³²S diffuses into the upper atmosphere more readily than ³⁴S, and then the atmospheric sulfur collides with stray plasma from Jupiter’s magnetosphere before being lost to space.

The high ³⁴S/³²S ratio may indicate an especially volcanic past. The geochemical modeling shows that Io could have been losing sulfur in the past at a rate as high as five times what it is today. The extreme volcanism necessary to cause that much sulfur loss comes from tidal heating. Jupiter’s intense gravity flexes Io, and the resulting tidal energy dissipates in Io’s interior as frictional heat. Io’s tidal heating is possible only because of its elliptical orbit around Jupiter, caused by interactions with other Jovian moons. So by learning more about Io’s volcanic history, researchers may unravel more about the formation of Jupiter and its moons—and potentially other systems subject to strong tidal forces. (K. de Kleer et al., Science, 2024, doi:10.1126/science.adj0625 .)