By the end of Galileo, over 160 active hot spots were identified. Eruptions were classified into lava flows, lava lakes, lava fountains, sheet flows, and activity within paterae, which could have been lava lakes, ponded lava flows or active lava flows. There is no distribution of volcanic activity that appears to be driven by global tectonics, leading to the conclusion that the crust is for the most part immobile. Paterae nearer the poles are fewer in number and larger in size than those at lower latitudes. This may mean the style of volcanism varies on a global scale.
Active volcanism from mid-sized hot spots deposit an estimate 43 km3/year on the surface. Most of the resurfacing comes via plume deposits.
Pele is one of the most distinctive of Io’s volcanoes. It has both a massive lava lake and a persistent plume over 300 – 400 km high. The reddish deposits are 1,200 km across and were laid down by the plume. They are rich in sulfur and SO2. Dark pyroclastic material is closer to the vent. The shape of the plume has changed over time. It was not seen by Voyager 2 and may have shifted into a more gas-rich stealth plume. No other giant plume is as persistent as the one from Pele.
Thermal imagery of Pele shows it to be relatively hotter and long-lived hot spot on a decades long scale. Closer observations by Galileo led to the conclusion that Pele has a vigorous eruption under way. There are no lava flows emplaced, meaning that Pele has an active lava lake with at least part of its surface being regularly overturned. IR emissions are measured from 17 km2, equivalent to a lava lake 4.5 km in diameter, though the actual lake is likely larger. There are no extensive, cooling lava flows at Pele observed by IR.
A lava lake like this is thought to be the top of an active magma column. There is one other spot on Io that looks like Pele in IR observations, Janus Patera. It does not have a plume. The presence of volatiles in the plume make Pele unique on Io. These are thought to be added to the magma on the way to the surface, perhaps encountering a liquid SO2 “aquifer” a short distance below the surface.
While active lava lakes are present on earth at Erta Ale and Erebus, the best analog for Pele is Kilauea.
Eruptions at the next two locations are much different from Pele. Pillan began explosively and transitioned to an effusive eruption. Pillan erupted ultramafic magmas, high in magnesium. These dominated eruptions on earth billions of years ago, giving volcanologists perhaps another analog to study.
While there is an atmosphere on Io, it is very thin, so expansion of volatiles into the vacuum drives explosive activity. Expansion of a small amount of gas in a magma column can create exit velocities over 1 km/sec with plume tops 350 km high. On the moon, explosive eruptions were driven by CO2, on the earth, water and CO2, while on Io, the volatile is SO2. This all means that a huge volume of lava can be erupted from a fissure in a short time, perhaps 1 km3 in a couple hours.
The 1997 Pillan eruption started explosively and was so vigorous that it saturated the Galileo IR sensor. It put a dusty plume to at least 140 km. The combination of dusty plume and IR led to the conclusion that the source was a massive lava fountain outbreak. The plume deposited material 200 km from the vent. Lava flows broke out from heavily fractured mountains, entered the Patera eventually covering 5,600 km2 with an estimated 56 km3 volume. Pyroclastics covered 125,000 km2. All of this was done in an eruption lasting 99 – 167 days.
Also began with a spectacular lava-fountain episode in 1999. This one was also sufficiently vigorous to overload the IR sensor and cause image saturation. Tvastar is a chain of nested calderas. The eruption deposited red material over 900 km in diameter. The plume was over 400 km high. The eruption took two years to complete. Flow rate estimates from the 25 km long fissure are very close to the maximum rate that can ever be erupted from magma in any fissure eruption on Io or earth.
Earth analogs to these two eruptions would be Loki Skaftar Fires in 1783 – 1784 and The Columbia River Flood Basalts, with the Columbia River basalts being closest in total volume erupted from these two systems. Io has lava flow fields considerably larger than Pillan or Tvashtar, one of which is still warm. These may be the Ionian equivalent of terrestrial flood basalts.
Prometheus is one of the most persistent active volcanoes with an active plume observed as long as we have looked at it. The plume is thought to be caused by boiling off SO2 deposits by advancing lava flow fronts. Lava flows appear to be laminar, thin, and mobile, covering previous flows. New flows covered 60 km2 in 4 months during 2000. Individual flows are thought to be in the vicinity of 1 m thick. New flows are almost entirely limited to the existing flow field. This activity is most similar to Kilauea.
There are two plumes at Prometheus. One is a small sulfur-rich plume from the magma source, the other SO2-rich plume centered on the westernmost flows. Flows from here covered over 6,700 km2 from 1983 – 2004.
Amirani is another persistent hot spot with a flow field. It is the largest active flow field in the solar system, over 300 km long. 620 km2 of new lava was emplaced in 134 days in 2000. This system had intermittent Prometheus-style plume activity centered on the flow front. It is difficult to determine how many individual active centers feed this system.
Loki is one of the most powerful IR emitters on the entire moon, totaling 10 – 25% of all thermal emissions from Io. Yet this emission comes from less than 0.1% of the total surface area. This means that it must be a very active region. This high output also makes it easy to observe from earth.
Unlike almost all other large volcanic sites, this activity has not changed Loki’s appearance over the years, which is yet another paradox. The Loki Patera is around 200 km in diameter. There is a feature that looks like a raft covering perhaps 25% of the area of the patera. The island may be a foundered mountain range or a resurgent dome. If it is the latter, it will be the only dome discovered on Io. While there was some visible difference in the patera in the months between Voyager 1 and 2, its appearance returned to that of Voyager 1 during Galileo’s stay in the system.
Two plumes were observed around 200 km north of Loki by the Voyagers. They were not observed by Galileo.
Changes in the floor of the patera itself have been observed mainly by movement of IR peaks. These are interpreted as either resurfacing the floor with new lavas or overturning of the crust on a massive lava lake. The Davies book leans toward the lava lake explanation, which would make it the largest active lava lake in the solar system. Close IR sensor passes with Galileo gathered data that could only be explained by the foundering lava lake crust model. Foundering takes place in roughly a 540-day long cycle. Given its size, Loki has been described as a magma sea rather than a lava lake. There is no evidence of convection on the lake. The crust passively sinks without lateral movement.
The previously described volcanic centers are primarily silicic eruptions, though sulfur and SO2 are most certainly involved. There are numerous other sites with varying levels of sulfur or SO2 eruptions. If we use SO2 as a water analog on Io, perhaps the primary sulfur eruption sites are more properly compared to hydrothermal systems at and near earth volcanic centers.
Tupan Patera is one of the most colorful volcanic centers. It is 75 x 50 km. It looks like it has an active lava lake that overturns quietly like Loki.
Culann Patera has both silicate and sulfur volcanism. It is also highly colored and appears to have lava tubes. It is a persistent thermal source that may indicate yet another lava lake that quietly overturns.
Zamama was one of the first hot spots detected by Galileo. It has low shield volcanoes and primary lava flow field over 140 km long. Newer flows appear to be emplaced on top of the existing flow field.
Emakong Patera is 75 – 65 km and appears to be the location of extensive sulfur volcanism. It also has a lava lake with a ring around it, much like benches on earthbound lava lakes.
Balder and Ababinili Paterae have very bright floors, with an abundance of SO2 coverage. This is thought to be due to SO2 eruptions.
Final interesting feature are the plumes at Surt and Thor. The 2002 eruption at Surt was the largest thermal emission detected on Io. It was first discovered by the Keck telescope and almost doubled Io’s thermal emission for a while, classifying it as an outburst eruption. Thor erupted the tallest plume observed on Io at over 500 km in 2001. It erupted lava flows and laid down a white SO2 ring around the patera.
Eruptions and Plumes
Eruption style in patera are either lava lakes of flows across the surface. These eruptions can be as large as lava eruptions spreading across the plains. Lava lakes appear to be directly connected to a magma source below and are typically covered with a thin crust. These crusts overturn from time to time, with the colder, heavier crust foundering and sinking. This typically increases infrared (IR) signature of the lake until a new crust forms.
Flow dominated eruptions are called Promethian volcanism. These are extensive and are a major terrain type on Io. Magma is erupted from either floors or edges of patera or from surface fissures. The flows are similar to what we see in Hawaiian volcanism, with flow rates holding relatively steady for an extended period of time (years to decades). Individual flow thicknesses are thought to be around a meter, though layering of multiple flows has built substantial structures whose total thickness are unknown. Active flow fields more than 300 km long have been observed. IR signature of these flow fields indicates that there are active lava tubes from the source vent to the breakout of new magma at the edge of a field. There is a known inactive field measuring over 125,000 km2.
The final type of eruptions on Io are explosive or Pillanian volcanism. There are two types of these. The first is due to interaction of hot magma with SO2 snow / ices on the surface. The second is an old fashioned Plinian / super-Plinian plume complete with pyroclastic flows. These are also called outburst eruptions, are typically short duration (weeks to months), have large volume ejection rates, and are visible from earth due to significant infrared (IR) emissions. A typical eruption starts with a fissure and extensive lava fountains. Outburst fountains at Tvashtar in 1999 and 2007 produced a 25 km long, 1 km tall curtain above the surface.
Explosion-dominated eruptions can also produce large volumes of lavas. They generally last longer than the lava fountains, at weeks to months as compared with the fountains at a few days to a week. A 1997 eruption at Pillan Patera produced over 31 km3 over a few months. Its lava flows were ten times the thickness of the typical Io flow at an estimated 10 m thick. Flow rates are similar to those at Iceland’s Loki in 1783.
Explosive eruptions will also produce large pyroclastic and plume deposits. The 1997 Pillan eruption covered 400 km2 of dark silicate materials and bright sulphur dioxide snow. Two eruptions from Tvastar in 2000 and 2007 generated 330 km tall plumes.
Volcanic plumes are relatively uncommon, though appear to be the major player in resurfacing Io and erasing its cratering record. At least 20 active plumes were observed by Voyager and Galileo.
The most common of these are Prometheus-type dust plumes that are created when lava flows vaporize underlying SO2 frost and snow. These are typically less than 100 km high with eruption velocities less than 0.5 km/sec. Dust plumes like this generally have an umbrella-like appearance. These are frequently seen at flow-dominated eruptions. Many of these have an outer halo of gas rich material reaching heights of the larger plumes.
The most vigorous plumes are Pele-type eruptions, with significant sulfur and SO2 involvement from the vent itself. They carry silicate pyroclastic material with them and are usually associated with explosion dominated eruptions. The exception to this is Pele, which the plume sources from a long-lived lava lake eruption. They have higher vent temperatures and pressures, generate plume velocities up to 1 km/sec, and reach altitudes of 300 – 500 km above the surface. There is little dust at altitude, making them somewhat difficult to observe.
Just to make sure things are interesting, some plumes do not fit neatly into the Prometheus or Pele types. One example would be a pair of Loki plumes observed by the Voyagers, classified as a hybrid of the two types.
The tidal interaction of the four moons with each other and Jupiter is sufficient to maintain liquid oceans. However, the tidal forces drop with distance from Jupiter, so none of the other moons have active volcanoes. The next moon out, Europa has a kilometers thick ice cover on an ocean that may be 100 km deep. The third and largest moon, Ganymede is thought to have several oceans interlayered with ice and rock. Calisto, the fourth Galilean moon may not have any oceans at all. As it is not in a strong resonance with the other moons, it is the least dense of the four moons. It is made up of a mix of rock and ice that have not differentiated much during its lifetime.
There is no consensus on the thickness of Io’s crust, though at least 30 km is needed to support 18 km tall mountains. Io’s magma is even more difficult to figure out. If the current level of activity has been constant through the lifetime of the solar system, the entire silicate crust would have undergone hundreds of episodes of partial melting. This would eventually yield high silica content, viscous magmas, of relatively low melting point and density.
Yet we see low viscosity magmas on a global scale. This may mean a highly efficient recycling process or delamination of buried lithosphere by a vigorous mantle convection. Another possibility is that Io’s tidal heating is cyclic, and it goes through extended periods of quiet followed by a period of intense heat buildup, formation of a global magma ocean, and widespread intense volcanic activity until the heat is dissipated.
Like most bodies in the solar system, the more we look at Io, the more we figure out that we don’t know. For instance, the actual location of volcanoes does not agree with predictions where they should be. We don’t have a good idea yet where all the magma comes from. Subsurface magma ocean? Crystal mush in the asthenosphere? Why does SO2 behave much like water does on earth with snow and what appears to be an aquifer layer just under the surface? Although there are a few shields and cones visible, most activity is related to patera. What causes the patera? Better yet, what locates them? Happily, we have figured out how to observe Io from afar and can at least see large outbursts. The Davies book has two pages of great questions pp 292 – 293.
There is an Io orbiter in the planning stages for a return to the Jovian system. When and if it flies is anyone’s guess. I find it fascinating that silicic volcanism halfway across the solar system can be used to model what goes on here on earth.