Jupiter’s satellite Io “Io” is the most volcanically active planet in the solar system. It orbits Jupiter once in about 1.8 Earth days. Due to the tidal force caused by Jupiter, the scale of volcanic eruptions is larger than any volcano on Earth.
The orbits of the first three of Galileo’s four known satellites: Io, Europa, and Ganymede are called Laplace resonances. For each orbit of Ganymede around Jupiter, Europa completes exactly two orbits, and Io completes four orbits. Under this rule, the satellites’ gravitational pull pulls each other into an elliptical orbit.
This orbit causes Jupiter’s tidal forces to heat the moon’s interior, causing volcanic activity on Io and adding heat to Ganymede’s icy underground liquid ocean. But how long has Io been experiencing volcanic upheaval? Or how long do Jupiter’s moons stay in this orbit? New research measuring the Io atmospheric sulfur isotope determined that the satellite has been trapped in this resonance for billions of years. Europa’s liquid ocean is considered a potential candidate for the evolution of life beyond Earth, and understanding how long the satellite has been in orbit is crucial to determining long-term habitability. The research results were published in the journals Science and JGR-Planets.
▲ Photos of Io taken by Galileo. (Source: JPL)
Earth can analyze fossils and craters to find traces of past events, but Io’s surface is constantly changing at an extremely fast rate. The surface is only about a million years old, but it is about 4.5 billion years old. To understand how long Io experienced volcanic activity, researchers measured atmospheric chemicals.
The main component of the Io volcanic eruption gas is sulfur, and there is no water on the surface, resulting in an atmospheric sulfur dioxide content as high as 90%. Io dynamic volcanic cycle, gas close to the surface will be sucked back into the interior and then erupted into the atmosphere. The study shows that the lighter sulfur-32 isotope of Io’s atmosphere may be at the top and the heavier isotope sulfur-34 may be at the bottom closer to the surface. In addition to the constant renewal of Io’s surface due to geological activity, the atmosphere is also dispersed into space at a rate of one ton per second due to collisions with charged particles in Jupiter’s magnetic field. The top of the atmosphere at the collision site is where sulfur-32, the lightest form, is more abundant and is disproportionately depleted compared to the heavier isotope sulfur-34. Understanding the extent of sulfur-32 loss could provide clues as to how long the volcanic activity lasted.
▲ Schematic diagram describing how Io is subject to Jupiter’s tidal forces, causing geological activities such as volcanic eruptions and how volcanic eruption gases escape. (Source: Chuck Carter and James Tuttle Keane / Keck Institute for Space Studies.)
Researchers used the ALMA telescope in Chile to measure the sulfur isotopes of Io. From the remnants of the early solar system, that is, meteorites, we know that the ratio of sulfur-32 to sulfur-34 was about 23:1 when the solar system was formed. If Io has not changed since its formation, it should still be the same ratio now.
However, new research shows that Io has lost 94% to 99% of its original sulfur-32, which means that there have been volcanic activities for billions of years, constantly dissipating sulfur-32 into space. The timing of Io’s volcanic activity shows that Io entered orbital resonance with Europa and Ganymede shortly after the formation of Jupiter’s moons, which supports 20-year model predictions that the three moons should enter a resonance state soon after their formation.
The Jupiter system is just one of many examples of moons and even extrasolar planets that share this type of resonance, and the tidal heating caused by the resonance is the main source of heat for satellites and drives geological activity. Io is an extreme example, and it serves as a perfect laboratory for understanding tidal heating in general.
(This article is reprinted with permission from the Taipei Planetarium; the first picture shows Jupiter and Io-4, source: NASA)
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