PASADENA, CA.- Jupiter's moon Io is the most volcanically active place in the solar system. During its 1.8-day orbit, this moon is gravitationally squeezed by Jupiter, leading to volcanic eruptions larger than any on Earth today.

Io, Europa, and Ganymede are in an orbital configuration known as a Laplace resonance: For every orbit of Ganymede (the farthest of the three from Jupiter), Europa completes exactly two orbits, and Io completes exactly four. In this configuration, the moons pull on each other gravitationally in such a way that they are forced into elliptical, rather than round, orbits. Such orbits allow Jupiter's gravity to heat the moons' interiors, causing Io's volcanism and adding heat to the subsurface liquid ocean on icy Europa.

How long has Io been experiencing volcanic upheaval? In other words, how long have Jupiter's moons been in this configuration?

Two new studies from Caltech researchers measure sulfur isotopes within Io's atmosphere and determine that the moons have been locked in this resonant dance for billions of years. Europa's liquid ocean has long been considered a potential location for life to evolve, and understanding exactly how long these moons' orbits have been this way is crucial for characterizing its long-term habitability. The papers appear in the journals Scienceand JGR-Planets on April 18.

On Earth, we can find signatures of past events through fossils and craters. Io, however, is perpetually transforming, so its surface is only about a million years old, while the moon itself is around 4.5 billion years old. To understand how long this Jovian moon has been experiencing volcanism, the researchers examined the chemicals in its atmosphere.

Io has no water, so the main component of the gases spewing from its volcanoes is sulfur, leading to an atmosphere that is 90 percent sulfur dioxide. During Io's dynamic volcanic cycles, the gases near the surface become subsumed back into the interior and are regurgitated again into the atmosphere.

The sulfur atoms on Io have a few different forms, or isotopes. Isotopes are variants of a given element with different numbers of neutrons. For example, both sulfur-32 and sulfur-34 have the same number of protons (16), but the former has 16 neutrons, and the latter has 18. Extra neutrons make an element physically heavier, so in Io's atmosphere, the lighter isotopes are more likely to be located at the top while heavier isotopes are more likely to be at the bottom, near the moon's surface.

The surface is not the only ever-changing feature on Io—its atmosphere is also being siphoned into space at a rate of 1 ton per second due to collisions with charged particles in Jupiter's magnetic field. As the lighter sulfur isotope, sulfur-32, is more abundant near the top of the atmosphere where these collisions occur, that isotope gets depleted disproportionately in comparison to its heavier counterpart. Understanding how much of the light sulfur is missing can give clues to how long the moon has been volcanic.

To do this, the researchers utilized the ALMA (Atacama Large Millimeter/submillimeter Array) telescope in Chile—a telescope that is itself surrounded by volcanoes—to measure sulfur isotopes on Io.

From meteorites, which are remnants from the early solar system, researchers have determined that the solar system formed with a ratio of roughly 23 atoms of sulfur-32 for every one atom of sulfur-34. If Io had been unchanged since its formation, it would have this same ratio today. However, the new study showed that Io has lost 94 to 99 percent of its original sulfur—and that means the moon has been volcanically active for billions of years while losing sulfur to space the entire time.

The duration of Io's volcanism indicates that it became locked into an orbital resonance with Europa and Ganymede very soon after the moons' formation. This supports predictions from models over the past 20 years that show these Galilean moons—Io, Europa, Ganymede—should enter this resonance very early on after their formation.

"The Jovian system is just one of many examples of moons, and even exoplanets, that occur in these types of resonances," says Katherine de Kleer, assistant professor of planetary science and astronomy, Hufstedler Family Scholar, and the Science paper's first author. "The tidal heating that is caused by such resonances is a major heat source for moons and can power their geological activity. Io is the most extreme example of this, so we use it as a laboratory for understanding tidal heating in general."

In the JGR-Planets paper, led by former Caltech postdoctoral scholar Ery Hughes, the team conducted sophisticated modeling of Io's sulfur system to explore potential scenarios for the moon's history, including some in which Io was even more volcanically active in the past than it is today.

"Because lots of the light sulfur is missing, the atmosphere we measure today is relatively 'heavy' in terms of sulfur. Key to achieving such heavy sulfur in Io's atmosphere is the process of burying the heavy sulfur back into Io's interior, so that it can be released by volcanoes over and over again," says Hughes, now a volcanic fluid geochemist with GNS Science in New Zealand. "Our modeling shows that sulfur gets trapped in the crust of Io by reactions between the sulfur-rich frosts, which are deposited from the atmosphere and the magma itself, allowing it to be eventually buried into Io's interior."

The researchers next aim to learn what other gases Io may have lost over the course of its long dynamic history. For example, while Io appears to contain no water, the other Galilean moons have plenty. Did Io once have water in its interior and subsequently lose it through volcanism?

The Science paper is titled "Isotopic evidence of long-lived volcanism on Io." Additional Caltech co-authors are Hughes and John Eiler, the Robert P. Sharp Professor of Geology and Geochemistry. Other co-authors are Francis Nimmo of UC Santa Cruz; Amy Hofmann (PhD '10) of JPL, which Caltech manages for NASA; Statia Luszcz-Cook of New York University; and Kathy Mandt of NASA Goddard Space Flight Center. Funding was provided by the National Science Foundation, the Caltech Center for Comparative Planetary Evolution, the NASA ROSES Rosetta Data Analysis Program, JPL, and the NAASC.

The JGR-Planets paper is titled "Using Io's Sulfur Isotope Cycle to Understand the History of Tidal Heating." Ery Hughes is the study's first author. Additional Caltech co-authors are de Kleer and Eiler. Other co-authors are Nimmo, Mandt, and Hofmann. Funding was provided by the 3CPE, the National Science Foundation, the NASA ROSES Rosetta Data Analysis Program, and JPL.