Over time, Io’s modern-day eruptions will be relatively enriched in heavier isotopes of various chemical elements compared to lighter ones. This is because lighter isotopes in the upper atmosphere are more likely to escape into space. By measuring the present-day ratios of heavy isotopes to lighter isotopes in Io’s atmosphere, scientists can calculate how long it took for Io to reach this state from its original reservoir of eruptible compounds.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a team of researchers observed gases in Io’s atmosphere, particularly sulfur. They estimated the moon’s original reservoir of heavier and lighter isotopes using ancient meteorites that preserve the chemistry of the early solar system. The team found that the high proportion of heavier sulfur isotopes in Io’s atmosphere suggests that it has lost 94 to 99 percent of its original sulfur reservoir, indicating that Io has been erupting for up to 4.5 billion years.
Planetary scientist James Tuttle Keane from NASA’s Jet Propulsion Laboratory explains that the orbital dynamics of planetary satellites can be chaotic, with moons drifting in and out of stable orbits. However, Io, Ganymede, and Europa have been following a similar dance for billions of years. This consistency suggests that the Io we see today is a representation of Io throughout its long history.