Io, a moon of Jupiter, has a substantial presence of volcanoes, volcanic pits and lava flows on its surface. Volcanic activity on the moon was first discovered in 1979 by Linda Morabito, an imaging scientist working on Voyager 1. Observations of Io by passing spacecraft and Earth-based astronomers have revealed more than 150 active volcanoes. , up to 400 such volcanoes are predicted to exist based on these observations. Io's volcanism makes the satellite one of only five known currently volcanically or cryovolcanically active worlds in the Solar System (the others being Earth, Venus, Saturn's moon Enceladus, and Neptune's moon Triton.)
First predicted shortly before the Voyager 1 flyby, the heat source for Io's volcanism comes from tidal heating produced by its forced orbital eccentricity. This differs from Earth's internal heating, which is derived primarily from radioactive isotope decay and primordial heat of accretion. Io's eccentric orbit leads to a slight difference in Jupiter's gravitational pull on the satellite between its closest and farthest points on its orbit, causing a varying tidal bulge. This variation in the shape of Io causes frictional heating in its interior. Without this tidal heating, Io might have been similar to the Moon, a world of similar size and mass, geologically dead and covered with numerous impact craters.
Io's volcanism has led to the formation of hundreds of volcanic centres and extensive lava formations, making it the most volcanically active body in the Solar System. Three different types of volcanic eruptions have been identified, differing in duration, intensity, lava effusion rate, and whether the eruption occurs within a volcanic pit (known as a patera). Lava flows on Io, tens or hundreds of kilometres long, have primarily basaltic composition, similar to lavas seen on Earth at shield volcanoes such as Kīlauea in Hawaii. Although most of the lava on Io is made of basalt, a few lava flows consisting of sulfur and sulfur dioxide have been seen. In addition, eruption temperatures as high as were detected, which can be explained by the eruption of high-temperature ultramafic silicate lavas.
As a result of the presence of significant quantities of sulfurous materials in Io's crust and on its surface, some eruptions propel sulfur, sulfur dioxide gas, and pyroclastic material up to into space, producing large, umbrella-shaped volcanic plumes. This material paints the surrounding terrain in red, black, and/or white, and provides material for Io's patchy atmosphere and Jupiter's extensive magnetosphere. Spacecraft that have flown by Io since 1979 have observed numerous surface changes as a result of Io's volcanic activity.
Further observations by the Juno orbiter of volcanism and volcanic plumes on Io were made during a 3 February 2024 flyby. Close flybys of Io during Juno orbits 57 and 58 (30 December 2023 and 3 February 2024), at distances of approximately 3,500–4,500 km, allowed the JIRAM infrared instrument to map Loki Patera at pixel scales as small as 400 m. These observations revealed a single counterclockwise resurfacing wave propagating at 2–3 km per day, a time-averaged thermal output of ~10 TW, and at least 20 small islands (~3 km wide) anchored to the lake floor that have remained in fixed positions since Voyager 1's flyby in 1979.
Discovery
Before the Voyager 1 encounter with Io on March 5, 1979, Io was thought to be a dead world much like the Moon. The discovery of a cloud of sodium surrounding Io led to theories that the satellite would be covered in evaporites.
Hints of discoveries to come arose from Earth-based infrared observations taken in the 1970s. An anomalously high thermal flux, compared to the other Galilean satellites, was discovered during measurements taken at an infrared wavelength of 10 μm while Io was in Jupiter's shadow. At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede. These results were considerably different from measurements taken at wavelengths of 20 μm, which suggested that Io had similar surface properties to the other Galilean satellites. Robert Nelson and Bruce Hapke attempted to explain these features in Io's spectrum by suggesting fumarolic activity as a mechanism for producing short-chain sulfur allotropes on Io's surface. It has since been determined that the greater flux at shorter wavelengths was due to the combined flux from Io's volcanoes and solar heating, whereas solar heating provides a much greater fraction of the flux at longer wavelengths. A sharp increase in Io's thermal emission at 5 μm was observed on February 20, 1978, by Witteborn, et al. The group considered volcanic activity at the time, in which case the data was fit into a region on Io in size at . However, the authors considered that hypothesis unlikely, and instead focused on emission from Io's interaction with Jupiter's magnetosphere.
Shortly before the Voyager 1 encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal Science predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io caused by its slightly eccentric orbit. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. This effect would be even greater with a differentiated Io.
Voyager 1's first images of Io revealed a lack of impact craters, suggesting a very young surface. Craters are used by geologists to estimate the age of a planetary surface; the number of impact structures increase with the age of the planetary surface. Instead, Voyager 1 observed a multi-coloured surface, pockmarked with irregular-shaped depressions, which lacked the raised rims characteristic of impact craters. Voyager 1 also observed flow features formed by low-viscosity fluid and tall, isolated mountains that did not resemble terrestrial volcanoes. The surface observed suggested that, just as Peale and colleagues had theorized, Io was heavily modified by volcanism.
On March 8, 1979, three days after passing Jupiter, Voyager 1 took images of Jupiter's moons to help mission controllers determine the spacecraft's exact location, a process called optical navigation. By processing images of Io to enhance the visibility of background stars, navigation engineer Linda Morabito found a tall cloud along its limb. At first, she suspected the cloud to be a moon behind Io, but no known satellite would have been in that location at that time, and the size of the object seen made it unlikely for it to be a new satellite, since at that size, it would have been seen from Earth before Voyager 1 arrived. The feature was determined to be a plume generated by active volcanism at a dark depression later named Pele. Following this discovery, eight other plumes were located in Voyager images of Io. These plumes were later named after mythological deities associated with fire, volcanoes, or mayhem: Loki (two separate plumes), Prometheus, Volund, Amirani, Maui, Marduk, and Masubi. Thermal emission from multiple sources, indicative of cooling lava, were also found. Surface changes were observed when images that had been acquired by Voyager 2 were compared to those taken four months previously by Voyager 1, including new plume deposits at Aten Patera and Surt.
Heat source
Io's main source of internal heat comes from the tidal forces generated by Jupiter's gravitational pull. This external heating differs from the internal heat source for volcanism on Earth, which is a result of radioactive isotope decay and residual heat from accretion. In the Earth, these internal heat sources drive mantle convection, which in turn causes volcanism through plate tectonics.
The tidal heating of Io is dependent on its distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state. Its Laplace orbital resonance with Europa and Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io from circularizing its orbit. The eccentricity leads to vertical differences in Io's tidal bulge of as much as as Jupiter's gravitational pull varies between the periapsis and apoapsis points in Io's orbit. This varying tidal pull also produces friction in Io's interior, enough to cause significant tidal heating and melting. Unlike Earth, where most of its internal heat is released by conduction through the crust, on Io internal heat is released via volcanic activity and generates the satellite's high heat flow (global total: 0.6–1.6 W). Models of its orbit suggest that the amount of tidal heating within Io changes with time, and that the current heat flow is not representative of the long-term average. The observed release of heat from Io's interior is greater than estimates for the amount presently generated from tidal heating, suggesting that Io is cooling after a period of greater flexing.
Composition
Analysis of Voyager images led scientists to believe that the lava flows on Io were composed mostly of various forms of molten elemental sulfur. The colouration of the flows was found to be similar to its various allotropes. Differences in the lava colour and brightness are a function of the temperature of polyatomic sulfur and the packing and bonding of its atoms. An analysis of the flows that radiate out from Ra Patera revealed differently colored materials, all associated with liquid sulfur, at different distances from the vent: dark albedo material close to the vent at , red material in the central part of each flow at , and orange material at the farthest ends of each flow at . This colour pattern corresponds to flows radiating out from a central vent, cooling as the lava travels away from it. In addition, temperature measurements of thermal emission at Loki Patera taken by Voyager 1's Infrared Interferometer Spectrometer and Radiometer (IRIS) instrument were consistent with sulfur volcanism. However, the IRIS instrument was not capable of detecting wavelengths that are indicative of higher temperatures. This meant that temperatures consistent with silicate volcanism were not discovered by Voyager. Despite this, Voyager scientists deduced that silicates must play a role in Io's youthful appearance, from its high density and the need for silicates to support the steep slopes along patera walls. The contradiction between the structural evidence and the spectral and temperature data following the Voyager flybys led to a debate in the planetary science community regarding the composition of Io's lava flows, whether they were composed of silicate or sulfurous materials.
Earth-based infrared studies in the 1980s and 1990s shifted the paradigm from one of primarily sulfur volcanism to one where silicate volcanism dominates, and sulfur acts in a secondary role. In 1986, measurements of a bright eruption on Io's leading hemisphere revealed temperatures of at least . This is higher than the boiling point of sulfur (), indicating a silicate composition for at least some of Io's lava flows. Similar temperatures were also observed at the Surt eruption in 1979 between the two Voyager encounters, and at the eruption observed by Witteborn and colleagues in 1978. In addition, modeling of silicate lava flows on Io suggested that they cooled rapidly, causing their thermal emission to be dominated by lower temperature components, such as solidified flows, as opposed to the small areas covered by still molten lava near the actual eruption temperature.