Io (moon)

Io is the innermost of four large satellites orbiting Jupiter, the largest planet in the solar system. Io is the most volcanically active body in the solar system. Tidal friction occurs constantly on Io, heating its core. Internal thermal energy is vented through immense volcanoes that spew sulfur and sulfur components into space, which fall back, resurfacing the satellite. Io has one of the youngest surfaces in the solar system.

Overview

Until 1979, Io was known as little more than a pinpoint of light, even when seen through large telescopes. Io is one of the four satellites of Jupiter that were first observed telescopically by Galileo in 1609. With roughly the same size, density, and surface gravity as Earth’s Moon, Io was expected to have similar features. Earth-based spectroscopic observations during the 1970s, however, raised speculation that Io was quite different. Suspicions were dramatically confirmed by the flybys of Voyagers1 and 2 in 1979 and by multiple encounters with the Galileo spacecraft between December 1995 and September 2003. The closest encounter with Io, early in the Galileo mission, was at a distance of 22,000 kilometers; the probe approached no closer because of concerns about the intense radiation environment the spacecraft would encounter. During the late stages of the extended Galileo mission, however, the spacecraft passed within a mere 180 kilometers of Io’s surface; at this late point in the mission, the science return was worth the risk to the spacecraft’s health.

Voyager 1 made the remarkable and completely unsuspected discovery that active volcanoes dot the surface of this planet-like Jovian moon. From Voyager and Galileo evidence, augmented by Earth-based observations, scientists identified Io as a world dominated by volcanic eruptions. Most of the moon’s surface features are transformed daily by heated liquid with gaseous emissions onto the surface and into the otherwise nonexistent atmosphere. On Io, impact craters are not formed as they are on the other, extremely cold satellites of the solar system. Io’s impact features are apparently absorbed by molten lava flows that extend across Io’s surface. Volcanic activity is not sporadic but virtually continuous. Of the eight active volcanoes observed by Voyager 1, seven were still spewing gaseous plumes when viewed by Voyager 2 four months later. Pictures taken by the two spacecraft portrayed these huge eruptions against the backdrop of the black sky over Io’s limb (its visible horizon) and from above the red-orange surface with its active calderas. However, Io’s volcanic activity changed in the time between the Voyager flybys and the arrival in orbit of the Galileo spacecraft on December 7, 1995.

With its image enhanced by spacecraft instruments, Io looked like a giant pizza with wide plains of different hues punctuated by darker and lighter active regions. The latter are calderas, at least two hundred of them, which dot the surface each with a diameter of more than twenty kilometers. The largest include eleven observed plume-emitting volcanoes named by the International Astronomical Union (IAU) for mythological gods of early and primitive religions. Two of these, separated along the Loki fissure, are associated with a lava lake 200 kilometers wide, known as Loki Patera, which apparently is the major outlet for the planet’s internal heat. Its temperature, like those of other Io hot spots, averages about 300 kelvins. The hot spots contrast with the remaining 98 percent of the surface, which at 130 kelvins is considerably colder, as would be expected for an atmosphere-poor body so far from the Sun.

Mountains tend to cluster near the polar regions on Io. Some have peaks as high as ten kilometers, but they do not appear to have been formed by plate tectonics (the shifting of continental geologic structures). They lack cone-like tops and could not have been formed by volcanism. It is speculated that some of Io’s upper crust may detach and float about the molten plains in a manner analogous to icebergs. Also, erosion scarps form near emission calderas. The fluid surface suggests that Io might receive a new surface ten micrometers thick every year, making it unique in a solar system of much older, inactive, and heavily cratered planets and satellites.

The chemistry of Io helps explain this satellite’s dynamic volcanism. Previously thought to have a solid interior, like other satellites of the solar system, Io appears to have a molten silicate core. Planetary geologists hypothesize that about four billion years ago, heated sulfur dioxide lying just below the surface became the driving force for the volcanoes, ejecting Io’s internal heat in gaseous eruptions similar to geysers. Long-lived eruptions, like Loki, eject materials ballistically at 0.5 to 0.6 kilometers per second. Short-duration powerhouses, typified by Pele (and possibly Aten and Surt, seen by Voyager 2), do so at twice that velocity. These eruption rates are significantly higher than that of Earth’s volcanoes (0.1 kilometers per second). Sulfur compounds are ejected as majestic mushroom-shaped plumes to a maximum height of 300 kilometers, enabling the lighter compounds, such as water and carbon dioxide, to escape into space. The heavier compounds, such as sulfur and sulfur dioxide, fall back to the surface as frozen, whitish, snowlike matter. Flows of molten matter, therefore, are low-viscosity sulfur and sulfur compounds rather than silicate rock lavas typical of Earth’s volcanoes. Io’s surface color results from the various sulfur compounds.

Io’s geologically active behavior is caused by its proximity to massive Jupiter and to sister-Galilean satellites Europa and Ganymede. Jupiter’s gravitational pull causes Io to “flex” along its axis ten kilometers toward Jupiter, while the combined attractions of Europa and Ganymede cause torques that give Io a slightly eccentric, noncircular orbit. The result is two opposing tidal forces stretching Io from within as it orbits Jupiter every 1.77 days. The ensuing friction raises Io’s internal heated power to sixty to eighty trillion watts, partially melting the silicate compounds of the crust and generating volcanic eruptions. Furthermore, because Io’s orbit lies entirely within Jupiter’s radiation belts, the satellite is bombarded by charged particles and affected by the powerful electrical currents produced by Jupiter’s magnetic field. These phenomena also influence Io’s internal heating, as does the spontaneous radioactive decay of isotopes, which is typical of all planetlike bodies.

Io’s volcanic emissions account for its irregular atmospheric pressure, first detected by Pioneer 10 in 1973. Atmospheric pressure variations result from the heat differential associated with the anomalous hot spots and the typically cooler surface. Earth-based observations in 1974 and 1975 detected a cloud of neutral sodium and potassium extending along Io’s orbit for more than 100,000 kilometers. This observation was explained as a “sputtering” process whereby potassium atoms are ejected into space from Io because of the impact of charged particles from Jupiter’s magnetosphere striking Io’s surface. Their rate of ejection is greater than ten kilometers per second, well above the necessary escape velocity of 2.5 kilometers per second. By comparison, Io’s most active volcano, Pele, has an ejection speed of only one kilometer per second. Volcanoes, then, are an indirect rather than a direct cause of this sodium-potassium cloud; they bring the elements to the surface in molten and gaseous states for emission into space.

In 1976, additional Earth-based observations revealed a plasma torus, or faint ring of excited glowing gas, belonging to Io’s orbit. This torus occupies space within Jupiter’s magnetosphere and results from the sputtering process. Electronic cameras and filters carried aboard the National Aeronautics and Space Administration’s (NASA) Kuiper Airborne Observatory recognized sulfur in 1981 and oxygen in 1982, as well as sodium and potassium, escaping from Io. The entire cloud assemblage supplies the torus with raw materials for further breakdown into discrete atoms and ionization. Thus energized, these ionized elements join the Jovian radiation belt that helped create them. The discovery of the torus was as unexpected as the volcanoes. Pioneer 10 and Voyager 1 flew directly through the torus in 1973 and 1979, respectively, but provided only knowledge supplementary to the major data obtained through continuous Earth-based monitoring. On its way to orbit insertion in December 1995, the Galileo spacecraft flew a relatively safe distance from Io, one that had originally been considered to be the closest that Galileo would ever get to this innermost Jovian satellite, and then passed quickly through the plasma torus. No significant radiation damage was incurred by the spacecraft.

The volcanoes of Io offer the greatest promise for resolving the details of the complex relationship between Jupiter and Io. Of the eleven volcanoes discovered by Voyagers 1 and 2, the two-part vent of Loki is the most important. With a height of about 225 kilometers and a width of more than 430 kilometers, Loki and its lava lake, Loki Patera, appear to be the major outlet for Io’s internal heat, as suggested by thermal emission polarization measurements in 1984. The thermal output from Io’s greatest volcanoes, Pele (305 kilometers high and 1,200 kilometers wide), Surt, and Aten, is also significant. One of the three apparently ceased eruptions in 1986, according to Earth-based observations. The rest are 100 kilometers or less in height. Known calderas make up about 5 percent of Io’s surface. Some, like Loki and Pele, have asymmetrical plumes and surface flows, which probably are consequences of irregular vent shapes. Others, like Prometheus, have symmetrical, fountainlike plumes and circular flows.

The Galileo spacecraft confirmed the massive scale of Io’s volcanism, detecting a fresh volcanic deposit the size of the US state of Arizona and establishing that the satellite is rich in silicates. Increasingly sophisticated Earth-based instruments and orbiting telescopes have provided additional analysis. During eclipses in 1985, ice-covered Europa passed through Io’s shadow, reflecting sunlight that revealed the uniform distribution of sodium about Io. The Hubble Space Telescope spotted an active plume in June 1997, which Galileo also detected.

Knowledge Gained

Voyager 1 and Earth-based observations revealed that Io was Jupiter's only volcanically active satellite. Volcanic gases and molten lava flows were seen being emitted from eleven major fissures. The most notable is the large dual vent of Loki Patera, which appears to be the focal point of the satellite’s heat emissions from its interior. Because these eruptions are continuous, the ejected heavier compounds of material steadily move across the surface, eroding low-lying scarps and erasing the craters formed by objects striking Io. At least one lava lake and mountains near the poles were discovered. Emissions appear to be generated by Io’s molten interior, from which sulfur compounds are emitted onto the surface and into the atmosphere. The heating mechanism for this activity is apparently internal friction caused by the gravitational pull of massive Jupiter on one side and the satellites Europa and Ganymede on the other.

Because Io lies within Jupiter’s magnetosphere, charged particles strike the satellite’s volcanic surface, causing jets of potassium, sodium, sulfur, and oxygen to be ejected into its atmosphere—a “sputtering” process that feeds a cloud of those neutral elements. This cloud, in turn, supplies raw materials for a torus of excited gas along Io’s orbital path.

Determination of Io’s bulk density and moment of inertia has revealed the satellite to be a differential body composed of a silicate mantle with a metallic core that makes up as much as half of Io’s radius. Comparative studies of the four Galilean satellites indicate that they share several similarities in their cores and differences due to their environment. Io, being active, has lost any significant icy shell it might have had over its core. Ganymede and Callisto, being colder and far less active, have thick, icy shells over their cores, with craters on the icy surfaces of each. Europa has activity that gives it an icy crust that breaks up and flows over what is believed to be a liquid ocean beneath that crust.

As a result of Galileo’s repeated observations of Io, hundreds of volcanic sites were identified, as were about two hundred dark surfaces believed to be fresh silicate lava. The orbiter’s near-infrared mapping spectrometer and solid-state imager identified a hundred active hot spots through thermal emission.

Context

Io emerged from the Voyager 1 and 2 missions as unique, not only among the satellites of Jupiter but also among all planets and satellites within the solar system. Io is recognized as the most volcanically active planetary body in a solar system where only a few other worlds display volcanism, and many of those display a type of cryovolcanic activity quite different in nature from Io’s volcanoes. Io’s volcanism is of particular interest because, like Earth, it is a dry body with a molten interior, and its sulfur-enriched chemistry may mimic volcanic conditions that existed during Earth’s early history. Earth’s active volcanoes convert water to steam for geothermal output. Io’s sulfur-based volcanism provides an active laboratory for the study of planetary evolution because volcanic eruptions constantly resurface its crust.

Io exists well within the Jovian magnetosphere. Electromagnetic fields affect Io’s surface, allowing lighter elements emitted through its volcanic vents to escape into the atmosphere and feed the torus that encircles Jupiter as part of the radiation belt. The Jupiter-Io connection serves as a laboratory for the study of large-scale magnetic forces in the solar system. One of Io’s effects on the Jovian system is a very gradual slowing of the rotation of Jupiter and erosion of the orbits of Europa and Ganymede.

Because active satellites like Io had not been anticipated before the 1979 Voyager flybys, their study has enhanced the evolving field of comparative planetology. Io was joined by Enceladus and Triton as satellites in the outer solar system that display unexpected volcanic activity.

Naturally, Io remains a high-priority target in planetary science for further study by robotic spacecraft. However, the radiation environment makes it difficult to dispatch probes into close proximity to the satellite. Human exploration is highly unlikely.

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