This table of Io's active hot spots has been compiled from identifications of hot spots in data from Voyager IRIS (Pearl and Sinton, 1982; McEwen et al., 1992, 1996); SSI (McEwen et al., 1997, 1998a; Keszthelyi et al., 2001a; Geissler, 2003; Turtle et al., 2004); NIMS (Lopes-Gautier et al., 1997, 1999, 2000; Lopes et al., 2001, 2004); PPR (Spencer et al., 2000b; Rathbun et al., 2004); and terrestrial telescopes (Goguen et al., 1998; Marchis et al., 2001; de Pater et al., 2004; Marchis et al., 2005). Hot spots in bold type have temperatures in excess of 1000 K as determined from SSI data (summarized in Geissler [2003]), NIMS data (Davies et al., 1997, 2000b; Lopes-Gautier et al., 2000; Davies et al., 2001, 2003b), and ground-based instruments (Marchis et al., 2002). This list will no doubt expand as more data are obtained and analyzed. Provisional names for features are in italics. Detections are indexed as follows: I = Voyager IRIS; S = Galileo SSI; N = Galileo NIMS; P = Galileo PPR; E = Earth-based or Earth-orbiting telescopes; C = Cassini ISS. All of the hot spots are plotted in Plates 13b and 16a.

The positions of hot spots are subject to uncertainties caused by different observation resolutions and movement of thermal sources, as well as by reference to nearest feature rather than the hot spot itself.

Chapters 6 and 7 describe models that are used to quantify thermal emission from volcanic processes and the subsequent use of those results to quantify the movement of magma from the interior of a planet to the surface. This chapter demonstrates how different types of eruptions generate different thermal emission spectra, characteristic “thermal signatures” of eruption style, and how those spectra change with time. Determination of eruption style is important because it allows application of the appropriate model to determine effusion rate. Observed temporal behavior allows eruption mechanism to be constrained.

Because most ionian volcanic thermal-emission data are at resolutions rendering the entire emitting area sub-pixel, it is necessary to understand how eruption style affects the shape of the integrated thermal emission spectrum. Such analyses are dependent on, first, adequate spectral coverage and, second, temporal coverage. Spectral resolution need not be particularly high so long as data at appropriate wavelengths are available. Two wavelengths (2 μm and 5 μm are particularly effective) can be used to constrain the silicate volcanism mode because the resulting 2-μm to 5-μm ratio (2:5-μm ratio) is very sensitive to the changing surface temperatures of lava surfaces of ages ranging from seconds to months, sometimes to years (Davies and Keszthelyi, 2005). Table 8.1 shows the timescales of different volcanic processes and the instruments best suited to observe them.

Volcanism has shaped the surface of the terrestrial planets. Earth, the Moon, Venus, and Mars have all been heavily modified by volcanism during at least some part of their history. Active high-temperature volcanism, with magma at temperatures in excess of 1000 K issuing onto the surface, however, has been observed on only two planetary bodies – Earth and Io (compared in Table 4.1). This high-temperature volcanism is clear evidence of the triumph of interior heating processes over planetary heat loss mechanisms. Interior heating has melted at least part of the planetary mantle to form silicate magmas. The forces primarily responsible for heating within these two bodies, however, are very different.

Why, then, are Earth and Io currently volcanically active? What are the origins of the heat that is generating and being lost through volcanism, and why is Io unique in the jovian system in having high-temperature volcanism? How have these bodies evolved over time?

Global heat flow

First and foremost, it is necessary to quantify surface heat flow in order to model heating and heat transport mechanisms. Determination of heat flow from remote-sensing data is complex, being dependent on the thermal properties of surface and subsurface materials. In Io's case, these properties and materials are not known with any certainty. In comparison, measurement of heat flow on Earth is easily accomplished with in situ measurements.

Using Galileo data, volcanic activity at individual volcanic centers was monitored and quantified on timescales of weeks to months, for a period of seven years. This monitoring was a huge step beyond the two snapshots of activity, four months apart, obtained by the Voyager spacecraft. Galileo's most exciting discovery was the large number of active volcanoes on Io, of which at least 50 had temperatures >1000 K (identified in Appendix 1). Having identified the locations of these high-temperature lavas, similar locations (e.g., dark-floored paterae; McEwen et al., 1985) were inferred also to be the result of silicate volcanism (see Chapter 9). By the end of the Galileo mission, more than 160 active hot spots had been identified from Voyager, Galileo, and Earth-based telescope data (see lists in Lopes et al. [2001, 2004]). In NIMS regional observations around Prometheus, where spatial resolution improved by a factor of 10 (Plate 7), the number of detected hot spots increased by a factor of ≈ 3 (Lopes et al., 2004), confirming earlier predictions that these small hot spots were present (Blaney et al., 1998, 2000). PPR detected many more low-temperature thermal sources not detectable by SSI or NIMS (Rathbun et al., 2004). Extrapolating to a global scale suggested as many as 300 large and small active hot spots on Io (Lopes et al., 2001), although the region covered by the aforementioned NIMS observation has a particularly high density of volcanic features (Schenk et al., 2001).

Because of its persistent red deposits, Pele is the most distinctive of Io's volcanoes (Plates 9a, b). Appropriately named after the Hawaiian goddess of volcanoes, Pele is the source of a giant plume more than 300 km high, as seen by Voyager (Strom et al., 1981). The reddish deposits laid down by the plume are ≈ 1200 km across and are rich in sulphur and sulphur dioxide (Geissler et al., 1999; Spencer et al., 2000a). Closer to the vent, dark pyroclastic material, most likely of silicate composition, streaks the surface (Strom et al., 1981; Geissler et al., 1999). SSI and the Hubble Space Telescope (Plate 9f) have shown that the Pele plume can exceed 400 km in height (Spencer et al., 1997b; McEwen et al., 1998a).

Voyager observed dramatic changes in the shape of the plume deposits between encounters when the deposits changed from a “cloven hoof” shape (almost certainly caused by a vent obstruction of some kind) to a circular appearance (Figures 1.3d, e). The change in the shape of the plume deposits took place in the four months between Voyager encounters, so the resurfacing for some period of time was relatively rapid. Although the Pele plume was not seen by Voyager 2, it may have changed to a more tenuous, gas-rich form – a “stealth” plume (Johnson et al., 1995).

Galileo's arrival at Jupiter opened a new era of exploration of Io's volcanism. Plates 1 and 6 show Io as seen by Galileo. A global mosaic is shown in Plate 4, created using the best Galileo and Voyager image data (Becker and Geissler, 2005). Appendix 2 contains maps of Io with feature names.

After the disappointment of the loss of imaging during Jupiter orbit insertion, the first observations of Io by the Solid State Imaging experiment (SSI) and Near-Infrared Mapping Spectrometer (NIMS) were finally obtained in late June, 1996. The closest approach to Io during this encounter (Orbit G1) was 696 000 km, not as near as Voyager 1 got to Io (20 570 km) but closer than Voyager 2 (1 129 900 km). Eclipse observations by SSI showed high-temperature hot spots glowing in the darkness (McEwen et al., 1997). NIMS obtained spectra indicating the presence of silicate-temperature volcanism (Davies et al., 1997).

The first sunlit images of Io showed a surface that was, if anything, even more colorful than that seen by Voyager. Io's surface was dominated by black, red, yellow, white, and green hues, representing different mixtures of silicate and sulphur compounds. Over the next 3 years, Io was periodically observed by Galileo, mostly at long range (hundreds of thousands of kilometers) but culminating with a sequence of fly-bys as close as 182 km (Table 3.2).

The surface of Io is covered in sulphur dioxide (SO2), sulphur, and silicates. Based on images of lava flows, estimates of lava eruption temperatures, and observed topography, silicate flows are common. The absence of steep-sloped (>10°) volcanic edifices (e.g., Clow and Carr, 1980; Schenk et al., 2004) suggests that lavas are fluid and of low viscosity, which indicates they are low in silica and therefore more akin to basalt or ultramafic composition lavas than silica-rich, high-viscosity lavas such as terrestrial rhyolite and dacite.

Volcanogenic elemental sulphur forms deposits on the surface of Earth and Io. Secondary sulphur flows, comprised of re-mobilized deposits of volcanogenic sulphur, are found on Earth and are probably quite common on Io as well.

SO2 dissolves in silicate and sulphur magma under sufficient pressure. It is a common volatile in terrestrial magma and is ubiquitous on the surface of Io (Carlson et al., 1997). SO2 plays an important part in Io's volcanism, driving most plume activity. SO2 in the lithosphere and on the surface of Io is readily mobilized by contact with hot silicates and sulphur.

This chapter reviews the physical and thermodynamic properties of these materials and their role in Io's volcanism. The first part of the chapter considers silicate magmas. The second considers sulphur and sulphur dioxide.

The Galileo mission was designed to be a detailed, in-depth investigation of Jupiter's atmosphere, the nature and evolution of its satellites and rings, and its magnetospheric environment. The Galileo spacecraft (Figure 3.1) and instrumentation were already under development by the end of the 1970s. Galileo consisted of an orbiter and an atmospheric probe that was destined to plunge into Jupiter's atmosphere. Galileo was a large spacecraft, weighing 2717 kg at launch. Of this mass, 925 kg was usable propellant, 339 kg was the atmospheric probe, and 118 kg was dedicated to orbiter science instruments. To maintain stability, the spacecraft rotated about its central axis at rates of up to 10 rpm. A “de-spun” section rotated at the same rate in the opposite direction in order to provide a stable platform for the imaging instruments. Four instruments (Table 3.1) – three imagers and an ultraviolet sensor – covering the electromagnetic spectrum from the extreme ultraviolet to the far infrared were mounted on a movable scan platform that allowed pointing of the instruments and the slews necessary to compensate for blur during fast satellite fly-bys. These instruments were bore-sighted to allow complementary imaging of a target by all of the imagers.

Galileo arrived at Jupiter on December 7, 1995. That it arrived at all was a triumph of human ingenuity over adversity, a decade of stubborn tenacity, and brilliant engineering solutions to some of the most intractable problems ever encountered with a spacecraft.

To unlock the secrets of Io's volcanism and, therefore, the story of the formation and evolution of the jovian system, observations must continue to extend the time-series data that have proven to be so valuable up to now. Taking a broader view, no other body in the Solar System is subject to as much tidal heating as Io, but tidal heating does play an important dynamic role in heating other planetary satellites, such as Europa and Enceladus. To better understand the process, it is therefore logical to study Io, where tidal heating is at its most extreme. Observations of Io can be made from spacecraft and from telescopes, both on the ground and in space. It will be interesting to see how Io changes over the next 20, 50, and 100 years in observations at increasing temporal and spatial resolutions.

Spacecraft observations

At the time of this writing, the only high-spatial-resolution spacecraft observations of Io that are likely in the next decade will be in February, 2007 from the NASA New Frontiers Program New Horizons spacecraft, as it passes at high velocity through the jovian system on its way to a rendezvous with Pluto in 2016. The following is a description of planned Io observations as presented by John Spencer, a New Horizons Science Team Member, at a meeting of the ad hoc Io Working Group in June, 2006.

Volcanic plumes are the most impressive manifestations of volcanism. It was fitting, therefore, that the first detection of active volcanism on Io was of a volcanic plume (Figure 1.2, Morabito et al., 1979). In the wake of discoveries made by Voyager, the Galileo mission greatly advanced understanding of Io's plumes and revealed the importance of their role in the resurfacing of Io.

Large volcanic plumes, which can reach heights of hundreds of kilometers, and their resulting surface deposits are the most visible indicators of ongoing or recent volcanic activity on Io. A variety of mechanisms form these plumes. The largest plumes are the result of explosive volcanic activity with the greatest excess pressures produced by an abundance of volatiles in the magma. Smaller plumes are formed by the interaction of recently erupted lava and surface deposits of sulphur and SO2. Even smaller plumes are formed by the relatively quiescent escape of volatiles from erupted lava and fumaroles.

Explosive activity on Io and Earth

Explosive volcanic activity is driven by the release of volcanic gases dissolved in magma at high pressure, the interaction of magma with external volatiles (e.g., with groundwater on Earth or a deposit of sulphur or sulphur dioxide on Io), or a mixture of both processes.

Effusive volcanism takes place where magmas are fluid and volatile-poor. The density difference between magma and crust and the pressures driving the ascent of magma are insufficient to create lava fountains. On Io, the most closely studied volcanoes where effusive volcanism is taking place, leading to the emplacement of extensive lava flow fields, are Prometheus and Amirani. At Prometheus in particular, study of the thermal emission and calculation of the effusion rate reveals the interior mechanisms and structure beneath the volcano by which magma is supplied to the surface. The closest terrestrial analogue to this style of activity is the current Pu'u 'O'o-Kupaianaha eruption of Kilauea, Hawai'i, where the typical mode of emplacement is that of inflating pahoehoe flows after magma transport through lava tubes over several kilometers.

Volcanic activity at Prometheus

Prometheus (Plates 6 and 7) is one of the most persistent of Io's volcanoes and the site of a volcanic plume that has been observed in every appropriate observation by Voyager and Galileo (e.g., Lopes-Gautier et al., 1999; Davies et al., 2006c). Study of the persistent eruption at Prometheus provides an insight into the mechanism of magma supply and heat transfer from the interior of Io to the surface and invites comparison with terrestrial analogues.

Located at ≈ 310°W and 12°N, Loki Patera (Plate 11a) is Io's most powerful and most intriguing volcano and one of the most prominent features on Io. Loki Patera appears as a low-albedo (and, by inference, relatively hot), sub-circular feature more than 200 km in diameter. A feature that looks like an “island” or “raft” takes up ≈25% of the area of the patera. This “island” is fractured, and pieces appear to have broken off in a manner akin to the calving of a terrestrial ice shelf. The “island” feature did not change appearance in the years between Voyager and Galileo so it is almost certainly immobile. It is likely to be either a resurgent dome (similar to that found in the terrestrial Long Valley Caldera) or possibly even a foundered mountain block (T. V. Johnson, pers. comm., 2005). The appearance of the Loki Patera region changed between Voyagers 1by the time of Galileo, looked very similar to its appearance as see and 2 (Smith et al., 1979c) but, n by Voyager 1, another indication of Io's tendency to exhibit long-term surface color and albedo stability (McEwen et al., 1998a). Additionally, the eruption mechanism at Loki Patera – the manner in which considerable volumes of lava yield their heat – has to maintain this appearance.

Loki Patera is therefore something of a paradox, showing little visible change while undergoing a high level of volcanic activity.