Volcanism on Io

Jupiter's moon Io is one of the most volcanically active worlds known in our solar system. The tidal heating produced by Io's forced orbital eccentricity has led the moon to become one of the most volcanically active worlds in the solar system, with hundreds of volcanic centers and extensive lava flows. During a major eruption, lava flows tens or even hundreds of kilometers long can be produced, consisting mostly of basaltic or ultramafic silicate lavas. As a by-product of this activity, sulfur and sulfur dioxide gas and pyroclastic material are blown up to 500 km (310 mi) into space producing large, umbrella-shaped plumes, painting the surrounding terrain in red, black, and white, and providing material for Io's patchy atmosphere and Jupiter's extensive magnetosphere.

Discovery

Discovery image of active volcanism on Io

Prior to the Voyager 1 encounter with Io on March 5, 1979, Io was thought to be a dead world much like the Earth's Moon. The latest theories suggested that Io would a world covered in evaporites due to the discovery of a cloud of sodium surrounding Io. Hints of the discoveries to come did come from observations in the Infrared in the 1970s, as Io was found to have an anomalously high thermal flux during an eclipse compared to the other Galilean satellites. At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede. But these results, taken at a wavelength of 10-μm, were considerably different from similar measurements at longer wavelengths (at 20-μm), which suggested that Io had similar surface properties to the other Galilean satellites. In retrospect, the greater flux at shorter wavelengths was due to the combined flux from Io's volcanoes and solar heating, while solar heating provides a 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 a region on Io 8000 km² in size at 600 K. 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. 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 Io's 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.

When Voyager 1's first images of Io came back in early March 1979, an obviously lack of impact craters was noted. Impact craters are used by geologists to estimate the age of a planetary surface. While Callisto, another Galilean satellite, was found to be saturated with impact craters and thus has an ancient surface, no obvious impact craters could be found on Io in Voyager's images, suggesting a very young surface. Instead of impact structures, Voyager 1 observed a multi-colored surface, pockmarked with irregularly-shaped depressions, which lacked the raised rims characteristic of impact craters, flow features formed by some low-viscosity fluid, and tall, isolated massifs that did not resemble terrestrial volcanoes. The surface observed suggested that, just as Peale et al. had suggested, the surface was heavily modified by volcanism on the surface.

On March 8, 1979, shortly after the encounter, Voyager 1 took several images of Jupiter's satellites for optical navigation, to determine the position of the spacecraft by comparing the position of the satellites to background stars. Navigation engineer Linda Morabito, while processing Io images to enhance the visibility of the background stars, found a 300-km tall cloud along the limb of Io. Once the possibility of background solid body was ruled out, the feature was determined to be a plume generated by active volcanism at a dark depression later named Pele. Following this discovery, other plumes were discovered in earlier Voyager images of Io, as well as thermal emission, indicative of cooling lava, from several others.

Paterae

Io's surface is dotted with volcanic depressions known as paterae. Paterae generally have flat floors bounded by steep walls. These features resemble terrestrial calderas, but it is unknown if they are produced through collapse over an emptied lava chamber as with their terrestrial cousins. One hypothesis suggests that these features are produced through the exhumation of volcanic sills, with the overlying material either being blasted out or integrated into the sill. Unlike similar features on Earth and Mars, these depressions generally do not lie at the peak of shield volcanoes and are normally larger, with an average diameter of 41 km (25½ mi), the largest being Loki Patera at 202 km (125½ mi). Whatever the formation mechanism, the morphology and distribution of many paterae suggest that these features are structurally controlled, with at least half bounded by faults or mountains. These features are often the site of volcanic eruptions, either from lava flows spreading across the floor of the paterae, as in Gish Bar Patera in 2001, or in the form of a lava lake.^, Lava lakes on Io either have a continuously overturning lava crust, such as at Pele, or a episodically overturning crust, such as at Loki.^,

Lava Flows

Lava flows represent another major volcanic terrain on Io. Magma erupts onto the surface from vents on the floor of paterae or on the plains from fissures, producing inflated, compound lava flows similar to those seen at Kilauea in Hawaii. Images from the Galileo spacecraft revealed that many of Io's major lava flows, like those at at Prometheus and at Amirani are produced by the build-up of small breakouts of lava flows on top of older flows. Larger outbreaks of lava have also been observed on Io. For example, the leading edge of the Prometheus flow moved 75 to 95 km (46½ to 59 mi) between Voyager in 1979 and the first Galileo observations in 1996. A major eruption in 1997 produced more than 3,500 km² (1,350 sq mi) of fresh lava as well as flooding the floor of the adjacent Pillan Patera.

Analysis of the Voyager images led scientists to believe that these flows were composed mostly of various compounds of molten sulfur. However, subsequent Earth-based infrared studies and measurements from the Galileo spacecraft indicate that these flows are composed of basaltic lava with mafic to ultramafic (magnesium-rich) compositions. This hypothesis is based on temperature measurements of Io's "hotspots," or thermal emission locations, which suggest temperatures of at least 1200 K and some as high as 1800 K.

Plumes

Sequence of New Horizons images showing Io's volcano Tvashtar spewing material 330 km above its surface.

The discovery of plumes at the volcanoes Pele and Loki were the first sign that Io is geologically active. Generally, these plumes are formed when volatiles like sulfur and sulfur dioxide are ejected skyward from Io's volcanoes at speeds reaching 1 km/s (0.62 mps). Additional material that might be found in these volcanic plumes include sodium, potassium, and chlorine.^, These plumes appear to be formed in one of two ways. Io's largest plumes are created when sulfur and sulfur dioxide gas dissolve from erupting magma at volcanic vents or lava lakes, often dragging silicate pyroclastic material with them. These plumes form red (from the short-chain sulfur) and black (from the silicate pyroclastics) deposits on the surface. Plumes formed in this manner are among the largest observed at Io, forming red rings more than 1000 km (620 mi& in diameter. Examples of this plume type include Pele, Tvashtar, and Dazhbog. Another type of plume is produced when encroaching lava flows vaporize underlying sulfur dioxide, sending the sulfur skyward. This type of plume often forms bright circular deposits consisting of sulfur dioxide. These plumes are often less than 100 km (62 mi) tall, and are among the most long-lived plumes on Io. Examples include Prometheus, Amirani, and Masubi.

Heat Source

Unlike the Earth and the Moon, Io's main source of internal heat comes from tidal dissipation rather than radioactive isotope decay. Such heating is dependent on Io's distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state. Its Laplace-resonant orbit 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 100 m (330 ft). The friction produced in Io's interior due to the varying tidal pull from Jupiter between the periapsis and apoapsis points in Io's orbit is enough to cause significant tidal heating within Io's interior and creating a significant amount of melt. This heat is then released from the interior in the form of volcanic activity and generates its high heat flow (global total: 0.6-1.6×10¹⁴ 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.