Over time, that would mean that Io’s modern-day eruptions will be relatively enriched in heavier versions (isotopes) of various chemical elements than lighter ones, because lighter isotopes in the upper atmosphere can more easily escape into space. If the team could measure the present-day ratios of the atmosphere’s heavy isotopes to lighter isotopes, they could calculate how long it would have taken for Io to get to that state from an original reservoir of subterranean, but eruptible, compounds within Io.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe gases in Io’s atmosphere—most notably, sulfur—de Kleer’s team did just that. They also estimated the moon’s “original” reservoir of heavier and lighter isotopes by using (among other things) ancient meteorites, which preserve a record of the average chemistry of the primeval era of the solar system.
They found that the high proportion of heavier sulfur isotopes in today’s Ionian atmosphere suggests that Io has lost 94 to 99 percent of its original sulfur reservoir. And the only way that makes sense, and fits with preexisting models of the evolution of Jupiter and its inner moons, is that Io has been erupting for perhaps as long as 4.5 billion years.
Orbital dance
“The orbital dynamics of planetary satellites can get very chaotic,” says James Tuttle Keane, a planetary scientist at NASA’s Jet Propulsion Laboratory who was not involved with the new work. Moons can drift in and out of stable orbits, sometimes colliding or potentially being ejected from the solar system entirely.
But it seems that Io, Ganymede, and Europa have been dancing about in a similar way for billions of years, and “the Io we see today is somewhat representative of Io over its long history,” says Keane.
Dr. Thomas Hughes is a UK-based scientist and science communicator who makes complex topics accessible to readers. His articles explore breakthroughs in various scientific disciplines, from space exploration to cutting-edge research.
