Pele's plume can be seen when it rises above Io's limb.
THE SURFACE OF IO
From a distance Io appears very red. Close up, the most striking array of colours becomes visible. Io's extraordinary colour palette has led to some very intense debate. No one is absolutely sure of the process or combination of processes which yield the exotic colours.
There is some consensus though that the various sulphur compounds erupted onto the surface have different colour properties. However, to complicate matters, the molecular arrangement of pure sulphur changes as it cools. The different types, or allotropes, of sulphur range from black when hot, through dark red and orange, to yellow when chilled.
Sulphur at different temperatures explains the colours within Creidne Patera.	Different wavelengths identify surface chemistry - red shows hotspots, green identifies sulphur, and blue represents sulphur dioxide frosts.
The gravity of Jupiter focuses asteroids close to it, and Io should therefore have plenty of impact scars - but no impact craters can be seen. The craters have probably been buried by rapidly accumulating volcanic material from lava flows, and fall-out from eruption plumes.
There are three types of terrain on Io. There are vent regions (volcanoes), plains regions, and mountains.
Creidne Patera
Mountains
Mountains are a rarer feature on Io. All together they make up just 2% of Io's surface. They are typically up to 200 kilometres in diameter and up to 10 kilometres high. They are rather isolated mounds, unlike the chains of jagged mountains we are used to seeing elsewhere in the Solar System. They are paler in colour than the surrounding areas. Io's mountains are thought to be formed from blocks of silicate rock, more common in the deeper interior than the crust. How they came to be on the surface is not known.
One of the largest mountains on Io is Haemus Montes near the south pole. It rises 10 kilometres above the surrounding plains. The edges of Haemus Montes appear to be overlain by materials which form the plains. It therefore pre-dates them, possibly forming quite early in Io's history.
Haemus Montes rises 10 kilometres above the surrounding plains.	Sunset on Io - Haemus Montes catching the sunlight.
Plains
Plains are simply the level regions between mountains and volcanoes. They are yellowish in colour and have a fairly uniform texture. Some of the plains surfaces are layered.
Some surfaces on or near volcanoes have a sheet-like or layered structure. The origin is still not clear. They may have been built up in layers by eruption fall-out or pyroclastic flow deposits. In a pyroclastic flow, a cloud of hot ash moves like a fluid from the vent leaving behind a dense crusty layer. The layered plains though appear very thick and very uniform, so these are not very satisfactory explanations.
Nemea Planum, in Io's southern polar region.	Ridges within Nemea Planum catch the evening sunlight.
The layers have eroded margins. It has been suggested that the layer forms an aquifer (storage) for liquid sulphur dioxide, which undermines the layer and causes collapse around the margins. Clearly there is still a lot to learn about the surface of Io.
Plains regions near Pele (Pele is at the top right).
Nemea Planum
The changing surface
The first volcanic eruption recorded on another planet was found during routine star checks for accurate location and navigation of the Voyager spacecraft. Contrast enhancement, to make stars easier to see, revealed a fuzzy blob on the limb of Io. Further processing revealed an umbrella-shaped plume.
The discovery was arguably the most important made by the Voyager mission. It demonstrated that rocky worlds other than our own have dynamic interiors and experience intense and ongoing geological activity. The plume, which was later named Pele after the Hawaiian volcano goddess, was 300 kilometres high. The area it covered was 1000 kilometres across. In fact the plume had been seen on earlier pictures but was overlooked as it was thought to be an artifact, a product of Voyager's imaging system.
The large plume from Pele rising above Io's limb.
The images returned by Voyager previously were then carefully searched for more plumes. In total eight plumes were identified during Voyager fly-bys. The plumes were seen to start and stop frequently. In the time between the two Voyager encounters, some plumes had ceased to be active, while others had started. Two additional eruptions were inferred from changes in the surface deposits.
The Galileo spacecraft found changes in the surface since Voyager. Even during the Galileo mission, on successive orbits, substantial changes in the surface were detected. Galileo found a new dark spot 400 kilometres across and possibly new lava flows surrounding Pillan Patera, a large volcano. A few months before Galileo found the new surface deposits, a 120 kilometres high plume was observed over Pillan Patera by both Galileo and the Hubble Space Telescope.
The plume of Pillan Patera volcano.
Volcano morphology
As Voyager neared Io, the dark spots which were first thought to be large impact craters were identified as volcanic calderas. The calderas are large, some are several hundred kilometres across, but with very gentle slopes. Ra Patera, one of the largest volcanoes on Io, is only 2000 metres high.
Volcanoes on Io have a large central crater several tens of kilometres across. Erupted from these are dark petal-like flows. These flows resemble the anemone volcanoes on Venus. The caldera craters have more extreme topography than the main edifice. They are relatively deep and have steep walls. They have scalloped or curved inside edges where material appears to have collapsed inwards from the rim. Some have several layers, indicating that there have been episodic eruptions.
The distinctive shape of Loki Patera and its lave flows.	Changes in the Loki region as observed by three spacecraft.
Loki Patera
Calderas are the dominant morphology, but there are also volcanoes which comprise a circular mound. They have a summit crater and at the base a steep scarp. Such a configuration is reminiscent of Olympus Mons on Mars.
Eruption types
Analysis of Voyager images which show plumes, in conjunction with studies of surface detail, have led to the conclusion that there are two types of volcanism on Io. The most frequently observed eruptions are called the "Prometheus" type (after Prometheus Mons) - eight were recorded by Voyager. The other type was observed once only, in the eruption of Pele.
The Prometheus type have plumes 50 to 120 kilometres high and leave pale deposits which cover an area 200 to 300 kilometres in diameter. The temperature of the erupting material is about 400 K. They are relatively long lived. Pele was very different. It was far more intense than the other eruptions, but it was not sustained. Pele's plume reached at least 230 kilometres above the surface. It formed a huge dark deposit 1500 kilometres across.
The flanks of Pele, one of Io's larger volcanoes.	Changes at Pele as witnessed by the two Voyagers, and Galileo.	Pele, at the time of Voyager, and Galileo.
Pele
Pele, because it had a much bigger plume, is inferred to have ejected gas at a far higher velocity (1000 metres per second). The plume also had a different shape. It was more umbrella-like, whereas the typical eruption plumes are more dome-shaped and compact. Dark filaments radiating from the vent around Pele show that amongst the plume, streams or jets of material have erupted.
The difference in eruption style is ascribed to the different volatiles (gases and liquids) involved in the eruption with different origins. They may have originated at different levels, but it is equally likely that the crust is not uniform. Long-lived Prometheus type volcanoes are driven by the rapid escape of sulphur dioxide. It is heated to boiling point (393 K) by molten sulphur. Liquid sulphur dioxide has a very low viscosity so it can move around freely. This means it can travel easily through fissures in the rocks underneath the volcano and can supply the eruption for long periods.
Pele is thought to have been driven by vaporised sulphur. It vaporises at high temperatures (600 K) and erupts violently. The sulphur magma has a high viscosity - it is stickier and cannot move around freely. This means that the magma reservoir is not replenished and the eruption is short lived.
The colour of materials erupted by the two types of volcano supports the distinction. Prometheus type volcanoes have pale plume deposits and Voyager's infrared spectrometer identified sulphur dioxide in the plumes it examined, firmly establishing sulphur dioxide as the propellant. The deposits about Pele are very dark, indicating the eruptions were driven by vaporised sulphur
17 years after Voyager 1's flyby, Galileo detects a new fissure at Marduk.		A new fissure, and frost deposits, observed by Galileo but not present at the time of Voyager.
Plumes
Eruption plumes reach heights far greater than those of terrestrial volcanoes because of Io's low surface gravity and tenuous atmosphere. There is no friction to slow the ejection of gas, so it escapes very quickly (600 metres per second) and travels very far (100 kilometres). Plume shape and height is used to calculate the speed with which the gas exits the volcano vent. The Voyager images indicate that some eruptions may originate from multiple vents located near one another.
The plume from the volcano Loki.
Loki
Lava flows
The composition of the lava flows on Io is not known. The flows may be sulphur, or have a silicate composition like terrestrial lavas. Even though Io is largely silicate, sulphur and its compounds play a significant role in Io's volcanism.
Lava flows within Lerna regio (top right)
Lerna Regio
At eruption temperatures sulphur is quite viscous, but there is a point as it cools when instead of becoming more solid, it actually becomes runnier! It changes in character from a thick syrupy consistency to one of oil. When it cools down a bit more it does eventually solidify. These properties are consistent with the flows seen on Ra Patera. The flows begin short and dark and fan out, just as if the lava had suddenly become very runny.
Lava flows from RA Patera.		Changes at RA Patera, as seen by Voyager 1, Voyager 2 , and Galileo.
Some of the fresh deposits found by the Galileo spacecraft however are lighter, more greyish in colour and possibly richer in silicates. High temperatures recorded by NASA's Infrared Telescope Facility on Mauna Kea (which also reveals new eruptions periodically) also suggest that there is silicate volcanism.
Comparing two Hubble Space Telescope images taken 16 months apart, a new hotspot can be seen on Io.