A ”quick” tour of volcanism on Io

By Jesper Sandberg

Art: Lucasfilm, the moon Mustafar in Star Wars episode 3 Revenge Of The Sith is a good analogue of Io. Mustafar is just like Io torn by tidal forces from its nearby gas giant, on this planet Anakin Skywalker got severe burn injuries in the lava flows and had therefore to live inside the Darth Vader suit for the rest of his life. Io is Jupiters’ darth vader moon. Lava planets like Io are very common in the Sci-Fi genre.

1. The variety of moons out there and the discovery of Io’s true nature

After writing my article on the Pillan Pateras 1997 eruption, it is time to return to Io with a general overview, because there are not many articles on this. Jupiter’s Moon Io is by far the most volcanic place in the solar system. Io is so volcanically active, that it looks more like the planets did over 4 billion years ago than like any modern terrestrial celestial body. Io is a world of violent extremes, where 100’s of volcanoes are erupting at once, with huge lava fountains going up miles, huge lava lakes which can swallow small countries on Earth and with hot basaltic lava flows spreading out over 100’s of kilometers. The volcanoes on IO dwarf those on Earth in eruption scale and volume as well. Ever since I learned of Io as a small kid it has been stuff of fascination, a world so very volcanic it is more like a Star Wars world aka ”lava planet” -think planet Mustafar- rather than any normal celestial object.

The discovery of the true nature of the major Galilean moons by the Voyager and Galileo space probes, showed how moons of a planet can have an amazing diversity if they are powered by tidal heating. The probes were seeing new exciting stuff for the first time. The lava oceans of Io and the volcanic saltwater oceans of Europa are among the greatest discoveries in the history of space exploration.

IO is the only place outside Earth and Venus where hot silicate volcanism has been seen in action. In this article I will talk about in general how Ionian volcanism operates. We will take a broad overview of Ionian volcanism for the readers, looking at Io’s main eruption styles. Many VC readers may feel it is a very alien place, yet it’s volcanism is surprisingly earthlike, being basaltic, although it is far far more vigorous than Earth’s.

In the 1970’s most scientists expected Io to be cold, dead, boring, covered in impact craters like our moon. Nasa scientist Bruce Murray had to shout and lead ”a one-man war crusade” at the Voyager 1979 team to convince them to have these ”boring moons” included in the imaging time at Jupiter, but it paid off. They found Sci-Fi-like worlds. The space probes saw things that nobody had seen or imagined before. The most volcanically active body in the solar system was a small moon. This was unexpected and was a huge shift in the paradigm of what to expect in the outer solar system, where it is very cold and presumably very dead: the exotic variety and diversity of the Galilean moons surprised everyone at Nasa JPL. The galilean moons became the star of the whole Voyager – Galileo probe experience, even more interesting than Jupiter itself. Io and Europa spawned a revolution in planetary geology about what kinds of bodies are possible and where life coud exist. Other non-jovian moons powered by tidal heating processes are Titan and Enceladus.

A visit to IO would reveal 100s of active volcanoes erupting all at once!. Caused of Jupiters strong gravity and the pull from all of the other moons, IO is being pushed through a meatgrinder: the whole body of Io is being deformed by tidal forces which generate tremendous amounts of heat. This causes Io’s interior to melt and forms hot silicate magmas. In fact the whole of upper mantle of Io may be a so-called magma ocean. Io’s geothermal heat flux far far exceeds that of Earth or of any other terrestrial body. Because Io is locked in this tidal thug of war, its also locked permanently in a volcanic hellscape, an hadean era that Earth escaped from billions of years ago. For a person obsessed with volcanoes there is nothing more fascinating than Jupiter’s moon Io and it’s space volcanoes. It is also volcanism on a colossal scale: events like Laki, Thjorsahraun and bigger happen many times a year on Io, while on Earth it is once in a few 1000 years. If Earth was as volcanic as Io is, CO2 would build up and Earth would suffer a run-away greenhouse effect becoming a Venus. The irradiated volcanic wasteland of Io is the most extreme moon environment in the solar system. Perhaps the most fitting names for Io would be fire gods from roman mythology, like Vulcan.

Io is home to some of the most incredible sights in the solar system, kilometers tall incandescent hot lava fountains going up in huge tephra columns, ionized gas columns going up 300 km glowing as in green blue-ish northern lights, while Jupiter looms over 30 times larger than Earth’s Moon in the sky. It is a landscape that is unforgiving in all senses. Run-away volcanism and strong radiation for Jupiter makes Io the most dangerous moon to explore in the solar system both for men and for robots. Io’s volcanoes are deadly, but before the hot lava will get you, Jupiter’s radiation will. The radiation on the surface is enough to kill a human in minutes, so a spacecraft needs heavy-duty insulation against it.

Jupiter’s worlds: Graphic by Nasa. Tidal heating can keep a world alive. Jupiter’s moons are worlds of their own. Io closest to Jupiter is like Lucifer descended into hell, cooked by tidal heating, while Europa got the sweet spot, probably tidally heated enough for volcanism on the seafloor but not enough to boil away the oceans and ice crust. Ganymede and Callisto resemble Europa but have more ocean water and less tidal heating. The dry nature of Io probably has to do with it being cooked dry by the young, hot Jupiter. The red ring and black center is Pele Patera’s sulfur plume and dark silicate ejecta that is a product of the violent lava fountains in the lava lake.

2. The color and surface of Io

Io is definitely the most colorful world surface in the solar system. The moon’s outer crust is probably made of countless layers of basaltic lava and sulfur snow. The surface shows dark fresh silicate lava flows and yellow-to-red sulfur frost and silicate lava flows that are in different stages of being covered by sulfur snow. Violent lava fountains on Io tends tends to leave dark ring halos, that consist of basaltic pyroclasts, while more gentle sulfur gas plumes tend to leave red or orange sulfur deposits. It is very cold on Io at -130 C. That means that when sulfur rich volcanic gases escape from lava fountains and lava lakes on Io, they freeze out forming flakes of snow. The snow settles on hot fresh silicate lava flows forming the wonderful spectrum of colors. Fresh lava flows on Io start out stunningly dark. (All dark spots on Io are either active basaltic lava flows or lava lakes). The dark spots that always stay dark steel grey or black are silicate lava lakes that remain too hot for sulfur snow to settle. Cooling lava flows on IO tend to first go from black to green as the sulfur reacts with basaltic minerals forming a kind of pyrite frost. As the emplaced basaltic lava continues to cool and age, they slowly disappear under sulfur frost and snow turning orange and later yellow and ultimately yellow-beige, until a new basaltic lava flow comes along.

Io is a world of extremes, the dark basaltic spots can reach over 1300 degrees c and the light sulfur snow plains can drop below -130C  so its a world of both fire and ice. Despite very high levels of volcanic activity, most of the ionian surface consists of empty sulfur plains rather than being totally filled by lava flows. Fresh lava flows on Io would be like Holuhraun, but ”older” sulfur plains would be like a strange version of Earth’s moon, without impact craters and without a lunar dust regolith but with beige yellow sulfur that covers basaltic ground and perhaps be powdery. The sulfur snow is probably packed quite hard in older areas.

Io’s fantastic shades of sulfur frost and fresh basaltic flows make it look like an ugly rotten orange, but I think its the most beautiful moon in the solar system with all these shades. Volcanoes on Io vary from giant low shield volcanoes, lava plains to lava filled calderas and fissure systems. There are so many volcanoes and types of volcanic features in mafic variations, it is best to leave them for separate articles. Despite the large volcanic outgassing, Io has no atmosphere, probably because Io is too small and the surface too cold. On Io all volcanic gas freezes to ice on the surface forming these colors. The weather forecast on Io is – 130 degrees C or below and a light snowfall of sulfur frost or a violent rain of hot lava pyroclasts.

Graphic: http://www.elementsmagazine.org/archivearticles/e18_6/3-Davies.pdf. Cutaway block of Ionian geology, Io is very much a volcano world and may shed light of what Earth was like billions of years ago. Io erupts primarily hot silicate lavas, but sulfur volatiles play the role as dominant magmatic volatile and pure sulfur flows in some areas are not impossible to rule out.

3. Io magma

With the arrival of Voyager probes, it was thought that Ionian lavas was liquid sulfur, with the colors being different sulfur allotropes and temperatures. But Io’s very high density proved a silicate composition, with dense metal-rich rock. In the 1990’s when the Galileo Spacecraft turned its sensors on Io’s dark spots, temperatures as high as 1700 C was recorded, a sign of silicate volcanism. (Today those readings have been revised to 1330 degrees C ) but true ultramafic 1500 C readings are not Impossible to rule out. During the great eruption at Pillan Patera, Galileo Probe dectected mafic minerals in form of ortopyroxenes in that lava flow, which shows silicate volcanism, meaning Ionian magmas are basaltic. With Io’s silicate volcanism around 100 times more vigorous than Earth over the whole surface, many think that the fiery moon may have a global magma ocean under its surface.

Ionian silicate magmas are low in silica, very hot and therefore have very low viscosity, perhaps even lower than Kilauea and Nyiragongo. Having very low viscosity fits well with the Ionian flood plains of lava and huge lava seas and the almost complete lack of tall volcanic edifices other than very broad shield volcanoes. The eruption temperatures over 1280 degrees C means that Ionian lavas will erupt nearly white hot. Perhaps Ionian lavas could look like liquid iron slag when it erupts. If it is hot enough at above 1300 degrees C the magmas will have very low visocosity, lower than almost all Earthly lavas, only some hot hawaiian lavas come close to Io today. At temperatures higher than say 1330 degrees C Ionian lavas could be ultramafic, perhaps like komatiites, but the views Galileo spacecraft provided of compound active ionian lava flow fields suggest a morphology and viscosity similar to the pahoehoe flows of Kilauea, rather than flowing like water like a komatiite would do. Still Ionian magmas are hot and may have a viscosity much lower than most terrestrial examples.

Now we know thanks to Pillan that Ionian magmas are clearly silicate magmas, not unlike Earth’s basalt, but they are very sulfur rich. The sulfur gas also expands violently as the hot magma rise towards the vacuum of space, driving enormous lava fountains and gas plumes from the ionian volcanoes. Ionian eruptions are ”effusive” but with sulfur gas expanding violently in shallow conduits, are often surprisingly violent. The high sulfur content of Ionian ”basalt” is though to come from recyling: volcanoes drive out volatiles: SO2, water and CO2. Water and CO2 are lost to space but SO2 is heavier and stays behind. It freezes on the surface, either as SO2 in the coldest places or as yellow octasulfur crystals around cooling lava flows. Basaltic lava flows and sulfur snow constantly accumulate and form a layer of pancakes. It gets buried deeper and deeper until the layers re-melts at 30 to 50 kilometers depth and supplies another round of eruptions. Over time, water and CO2 are lost from the crust and only SO2 survives as a volatile magma component.

A hand specimen of Ionian lavas would be dark, heavy and resemble terrestrial basalt in any explorers hand. My best opinion on Ionian magmas composition is some kind of thoelitic basalt that is similar to the mare lavas, but ultramafic is not impossible to rule out, also the silicate composition could be a little different than Earth’s basalts. If we really want to know Io’s silicate chemistry a space probe would have to flow low over Io and fly through an eruption column there it would deploy a wafer-like collector where tiny pyroclasts would be collected and trapped, and then it flies back to Earth, reenters the atmosphere and the capsule would be captured and its mineral – glass contents could be studied.

Photo: By Galileo spacecraft, Io is the most volcanic place in the solar system, pale areas are sulfur frost/sulfur snow, while dark black areas are hot silicate volcanism (basalt) whose surfaces are too hot for snow to settle. The largest black spot is Loki Patera 220 km wide it is the largest lava ocean in the solar system, beige – white flow patterns are earlier sulfur frosted basalt flows.

4. A look at Io’s lava lakes and lava seas

Photo by Galileo Spacecraft: An excellent closeup of a lava lake at Radegast Patera, hot, dark and smooth. It is clearly a huge lake of basaltic lava, you can even see new hot dark surface as old crust sinks into the lava lake. Being 10s of km wide it dwarfs any terrestrial lava lake. And this is a ”small” lava lake on IO

On Earth a permanent active lava lake surface larger than 1 kilometer wide simply does not exist. Kilauea and Nyiragongo may be able to do it once in a while, but most open conduit lava lakes seem restricted to max 300 m wide. Most open conduit Earth lava lakes are merely pools as big as a tennis court. On Io, a large lava lake is 230 kilometers wide, that is, bigger than Rhode Island/ They are better called ”lava seas”. Small lava lakes on Io are 20 km wide (you can fit St Helens inside!). Some of these really large lava lakes I imagine as holes into IO’s inner asthenosphere magma ocean. There is something mesmerizing about searing space lava lakes as large as some of the US great lakes and some smaller states.

Io’s giant lava lakes are easy to spot on visible light Galileo imagery as huge dark black spots that do not change shape and are almost always too hot for sulfur snow frost to settle on it. Ionian lava lakes come in two behaviours in their thermal emissions as seen by Galileo Spacecraft: placid and active lava lake surfaces. They are both open conduited but vent heat and gases very differently.

Photo: Voyager: at over 220 kilometers Loki Patera is the largest lava sea in the solar system.

Graphic by myself: Loki Patera lava sea compared to Iceland, the largest lava sea on Io.

The very largest Ionian lava lakes are ”placid lakes” that are most of the large black stationary spots on Io with a quite cool but thin and calm surface crust that are only episodically resurfaced. Resurfacing happens in a front that spreads over the lava lake as heavy old crust founders and gets replaced by hot fresh gassy basalt. It can take months for the largest lava lake surfaces on Io to repave itself. For Galileo spacecraft and Earth based instruments it is seen as a very hot front that is creeping over the constantly warm surface. Loki Patera, the solar systems largest lava lake, is one such monster, a 230 km wide lava lake pit whose behaviour involves periodic overturns of its surface. If you were standing on the cliff walls of Loki Patera it is so big the lava lake would curve with the Ionian horizon. At 21 000 square km Loki Patera is a million times the surface area of the average Earth lava lake. With most of Io’s heat flow coming from these dark (100 – 200 km wide spots) it is possible that a lot of Ionian heat gets released through placid lava lake overturns rather than by eruptions. The high geothermal heat flux keeps these huge lakes molten and active and prevents them from getting a solid thick stable crust. Similar overturning crusts have been seen in much smaller lakes in Hawaii like at the Kupainiaha vent.

The surface of Loki Patera also seems to be jammed full by ”icebergs” of volatile rich rocks that are broken off from the patera rims and are slowly drifting around in this sea of lava, not unlike the lavabergs in some Kilauea lava lakes. In Loki the biggest lavabergs are as large as some of the smaller Azores islands, so the same scale as some of the larger icebergs in Antarctica. In some of these types of lava lakes like at Daedalus, Hephaestus, Amaterasu paterae, the dark lava lakes are surrounded by older bright flows that radiate from them, so perhaps sometimes these gigantic lava lakes rise and have gigantic lava overflows covering over 1000 s of square km, on a scale of nothing recorded on Earth in recent times.

In March 8, 2015 a rare orbital alignment occurred between Io and Europa, two of the moons of Jupiter, that allowed researchers to distinguish heat being emitted from Loki Patera. The astronomers were able to accomplish this because Europa’s surface is covered in water ice which reflects small amounts of sunlight at infrared wavelengths. Scientists were able to determine that there were two waves of resurfacing lava, which explains the change in brightness on Loki Patera every 400–600 days. Loki Patera had until recently had a relatively predictable crustal resurfacing time, but has changed its timetable, leading to longer times between resurfacing. A new theory to explain this is that the upper crust is gassier and lower in density and more un-willing to turn over.

Graphic https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2006JE002703 Crust resurfacing of the largest Ionian lava lakes. This is what a spacecrafts sensor would see thermally. This graphic fits Tupan patera features (photo below) perfectly.

Photo Galileo Spacecraft: Tupan Patera is an excellent example of a placid Ionian lava lake that has been stagnant and was caught overturning. But what is in the image here? 2 is freshly overturned basalt surface, too hot for sulfur snow to settle, the overturning front is slowly creeping towards the right consuming the old cold crust. 1 is old cold lava lake crust that is covered by sulfur snow. 3 is an interesting example where the crust is overturning in many places, some are dark and fresh and some are green and have pyrite frost on them meaning not all overturns are active in that part. The scale here is gigantic. The lava lake is 75 kilometers wide and the walls are 2 to 3 km tall.

Another class of Ionian lava lakes are ”active lava lakes”. They have a much faster circulation and faster supply by gas and their surfaces are much more active and disturbed by lava fountains and crustal disturbances. A famous example on Io is Pele Patera which is Io’s most thermally active lava lake. Much more hot lava surfaces are revealed to space than the placid lakes that only overturns episodically. The crust on these lakes are thin and overturns regularly. It is probably circulating, and is disturbed by huge fountains and lava upwellings. Pele Patera consist of an elongated lava lake pit that sits on the Northern edge of the fiery tectonic canyon called ”Danube Planum”. It was constantly very bright in Galileos Spacecrafts sensors, in fact it is Io’s consistently most powerful constant thermal source, suggesting a constant supply of gas rich magma from depth which manifests itself as persistent fountains in the lake.

The style of activity may be similar to Halema’uma’u s 2008 – 2018 open lake phase or even Marum, but the scale is off any charts here on Earth. Standing on the 2 to 3 km tall walls would reveal a lava lake that stretches beyond the horizon. And unlike the placid Loki Patera, Pele Patera would probably show a long line of lava fountains and upwellings the size of skyscrapers stretching for miles along the nightmarishly steep walls of this caldera, just waiting to consume any explorer in radiation proof suit that slips on some rock. It is perhaps similar to standing on a Grand Canyon thats lava filled. To be at the rim of Pele Patera would perhaps be like Mustafar in Revenge Of The Sith. Pele Patera would be a wonderful deadly sight up close.

The biggest lava fountains in Pele Patera are a constant source of of dark pyroclasts, that carpet an area half of the size of Spain around Danube Planum. Features like that are a good sign of massive lava fountains in the lake, evoking the sight of a constant ”fire rain” around this area, with curtains of lava chunks falling kilometers before they are swallowed up by the lava lake. Any observers on the other rim far away from the fountains would be showered by hot falling silicate pyroclasts that constantly falls in the area. The violent activity at Peles lava lake release a lot of gas, mainly sulfur, and a large sulfur plume rises 300 kilometers into space above this lava lake which deposits sulfur snow in thick layers. Closer to the lava lake pit the landscape is mantled by silicate pyroclasts. It is entirely possible that the really large lava lake pits on Io may grow from lakes like Pele, with the pit walls caving in growing the lava lake into a larger and more calm lava ocean like Loki patera.

A safer view of Pele Patera would be from the mesa cliffs at Danube Planum overlooking the lava lake and the massive plume coming up from it. Pele is home to a permanent 1200 km wide and 400 kilometers high gas plume of sulfur allotropes, like a dome, and like most gases it scatters blue light from the sun, so inside it the radiation proof suited traveller may see a dark blue sky as he stares up in the void above, a welcome sight from the otherwise harsh black vacuum of Io’s surface. Despite the vigorous fountaining at peles lava lake, most of this lava lake is covered by a black floating crust, thinner than at Loki, with the lava fountains mostly confined to the edges. One can imagine waves caused by lava fountains along the lava lakes edge, spreading under the thin crust of Pele Patera and causing the kilometers wide crustal plates to bob and heave with the waves.

Photo: Galileo thermal spacecraft closeup of Pele Pateras lava fountains, the most violent lava lake in the solar system. While similar to terrestrial lava lakes in behaviour, the scale of the activity is off any scale on Earth.

Art: by myself showing a cross section of the monsterous Pele Patera volcano. While perhaps similar for example to Halemaumau, the lava lake scale is once again off the charts. I modified a graphic from Dr Jani Radebaughs papers. The size of magma stoorage is ( very speculative )

Art by myself. The lava lake at Pele is 60 km long, its edges are marked by thermal emissions. For comparison, here it is shown placed just north of Vatnajökull glacier.

5. Lava flows and styles of lava flow eruptions on Io

Moving away from Io’s raging lava lakes its time to take a look at another type of Ionian activity: it’s lava flows. There is no shortage of lava flows here, in fact Io is entirely covered by basaltic lava flows of various ages. In some areas the flows are fresh hot and black and active, in other places old lava flows are buried under meters of sulfur snow and become sulfur snow fields rather than lava flow fields. In some ” older” areas on Io the constant rain of sulfur snow packs into thick layers and I even imagine sulfur glaciers. Sillicate flows and sulfur snow on Io haves a complex relationship: when hot silicate lava flows over frozen volatile layers it causes violent degassing and explosions. Lava flows resurface large parts of Io’s surface over time and is the major building blocks of Ionian crust. The fresh lava crusts are black and crunchy basaltic very earthlike, but the sulfur snow would be an alien environment but perhaps not unlike fallen snow on Earth. Imaginary explorers in radiation proof spacesuits would leave yellow footsteps behind them and a rover car would leave yellow tracks on the sulfur plains.

Fresh lava flows on Io are surprisingly Earthlike and can be divided into tube fed pahoehoe and fountain fed Aa flows, clearly seen so by their morphology in spaceprobe photos. But if the eruptions are identical to, for example, Hawaii and Iceland, the scale of them are huge compared to any historical earthly lava flows, just like Io’s lava lakes are huge compared to Earth’s ones. On Earth lava flows rarely much beyond 10 km, the eruptions are simply not large enough. Hawaii and Iceland, Earth’s two strongest volcanic powerhouses, are capable of sending flows 10’s of km long or even over 100 km, but on Io it is very common for both tube fed and fountain fed lava flows to spread out for many 100s of kilometers. Some older preserved lava channel features on Io suggest apocalyptic eruptions where lava flows of 1000s of km long may be possible. One such feature is the gigantic Tawhaki Vallis lava channel. The lava production on Io’s surface is probably a few 100s of km3 every Earth year, Earth doesn’t even produce 1 km3 of lava a year on its dry surface and most that is emplaced in Hawaii and Iceland.

Ionian silicate lava flows can be divided into two categories by their morphology seen in Galileo Spacecraft photos: slow and fast flows. It is time to have a look at them.

Galileo spacecraft photos showing the large sizes of some of Io’s compound tube fed lava flows, with the large Lei Klung Fluctus dwarfing any recent historical lava flow on Earth. to the left is Carrizozo Malpais Lava Flow shown for comparison with these Io flows.

6. Slow tube lava flows (prometheus style eruptions)

These are most common types of lava flows on Io s surface and are a major part of Ionian crust formation and landscape resurfacing. They are like massive versions of the Puu Oo eruption with a constant relatively slow lava output that carpets large areas of Ionian ground over timespans of years. Such lava flows are tube fed and constantly supplied from a magma source in the Ionian asthenosphere. The Galileo spacecraft observed close-up imagery of three such pahoehoe fields on Io, on Amirani, Prometheus and Zamana volcanoes. On Prometheus and Zamana pahoehoe lava fields Galileo were shown to be identical in flow morphology to Kilauea’s pahoehoe fields when seen from above. The first Ionian volcano that confirmed that tube fed pahoehoe existed outside Earth was Prometheus and the eruption style was named ”Promethean Style eruptions” or compound pahoehoe lava flow fields in some papers, On Io they form lava plateaus and plains rather than any shield volcanoes.

Photo: Galileo spacecraft showing the tube fed prometheus lava flow field, clearly showing dark fresh paheohoe breakouts. The dark ejecta around the flow front are the result from explosive interaction between hot silicate lava and cold sulfur snow ices. The morphology is clearly pahoehoe, and reminds me of Kilauea’s coastal benches when sea salt settles on lava, and dark hot fresh flows contrasts with older salty white. The lower photo shows sulfur dunes formed by freezing gas jets as hot basalt flows over sulfur ice.

Such lava flows can be active for decades or perhaps 100s of years and slowly resurface the Ionian surface. While similar to Kilaueas lava flows during shield building, the scale is larger, the Ionian Amirani – Maui tube fed lava field is 500 km long and apparently fed by a 300 kilometer long lava tube system.

An observer on the Ionian ground would see a landscape very familiar to Hawaii’s Puu Oo fields but the lava supply rate to the flow front is much faster than Puu Oo so the lava flow breakouts along the front or inside the flow field will probably be much larger. Changes in Ionian flow fields documented by Galileo spacecraft suggest a far larger resurfacing rate than Puu Oo s lava fields. Individual breakouts of these Ionian Pahoehoe flows may surge forward at 10s of cubic meters a second as the inflated crusts cannot hold the pressure. Any imaginary explorer will have to run and watch his step. The lava breaking out is very hot and highly fluid. First the breakouts exposing 1320 C lava are white hot then yellow, then orange and finally red turns to black as the hot Ionian stuff hides itself below a black blanket. But because of the low gravity even these fluid Ionian flow breakouts haves a swollen look, it is not pushed down as much by gravity as on Earth.

Despite the placid nature of these lava flows they are dangerous for imaginary metal clad Sci-Fi explorers. When the hot pahoehoe lava flows over Ionian ground, they flow over sulfur snow layers that have been accumulating for a long time. The hot Ionian lava have a far higher temperatures than the boiling point of the sulfur, so the breakouts vaporize the covered sulfur and huge gas pockets build up causing explosive phreatomagmatic activity along the flow fronts. And indeed all active Ionian lava flows have dark silicate dust fans along their edges from sulfur gas explosions sending lava spatter flying everywhere. The vaporization of sulfur snow also forms gas jets, and this piles the sulfur up into dunes around the lava flow front. Because of this violent interaction of lava and sulfur, it may not be safe to explore the active flow fronts at all, as Ionian lava flow fronts is often prone to violently explode. Behind the flow front the scenery is more peaceful and the lava flows breaking out can be watched safely because the ground is fresh dark black basalt and there may only be a slight coating of green ”pyrite frost” as sulfur snow chemically reacts with hot basalt. But close to the flow front the land is mostly pure fresh glassy basalt, ropey lava and lobate fluidal breakouts dot the scenery. Large inflation mounds and tumulus structures are found too forming a hummocky but smooth, hilly landscape. The surface maybe cracked by inflation pressure from incoming lava from the tube system and a fierce yellow hot glow can be seen in these meters deep cracks, just like it does on inflation cracks in pahoehoe fields on Earth. Sometimes lava flows violently breaks out from inflation mounds making this an uneasy unsafe imaginary place to camp the rover habitat for well needed sleep.

Photo: Galileo spacecraft. The Amirani lava flow field is the largest tube feed active lava flow in the solar system. Galileo showed massive resurfacing between different visits as it orbited Jupiter. Fresh hot basalt is black, green is pyrite frost that happens when sulfur snow settle on warm basalt. The lava expansion rate is much higher here than in any historical terrestrial tube feed eruption.

A few 10s of km further into the flow field the crust grows thicker and colder as the tube system is well developed here. The ground has a clear coverage of sulfur frost. In the silent radiation filled vacuum, sulfur snow sourced from active vents and frozen volcanic gases silently falls down and settles on the basalt. The pahoehoe features here are not black but yellow or green or beige as sulfur frost and snow is building up on the basalt surface. The alien yet familiar landscape is dotted with infernal glowing pits, lava tube skylights. Here imaginary explorers can sample white hot Ionian silicate lava without the worry of being blown up. The tube skylights maybe as large as houses, so are real ”blast furnaces” and one does not want any imaginary manned rover to fall into these. Large amounts of frozen ”sulfur snow” also vents from these glowing skylights as gases freeze in the -150 C Ionian environment. The lava flow rates flowing inside will be much faster than for example in the Puu Oo eruption, and could be as high as 100s of cubic meters a second to explain the large 800 square kilometers resurfacing seen by Galileo spacecraft at the Amirani pahoehoe lava flow fields in just a few months difference. The lava flow field is also dotted with innumerable gas vents where gas escapes and freeze spewing sulfur snow like snowblower machines, the immense Jupiter dominates the horizon brooding and ever present, it is an unearthly sight.

Compound tube fed lava flows like these resurface large parts of the Ionian surface over time. There are very few holocene continental pahoehoe fields that comes close to any of Io’s. Most Ionian fields are vastly larger than any earthly pahoehoe fields. The magma production at these kinds of Ionian eruptions are quite notable. Kilauea, Earth’s most productive volcano in the long term, covered around 5 to 8 square miles every month during the Puu Oo eruption, but the Aminarani Ionian flow field covered 250 to 470 square miles in new lava surface during the same timespan. Io’s upper layers are probably full of lava tubes, that get compressed and crushed further down as new lava layers get added above. Many geologists think these lava flows coud be analougus to many prehistoric flood basalt units on earth. Many large LIP flow fields have turned out to be paheohoes so the Io examples coud be like them.

Photos. Galileo Spacecraft showing pahoehoe flows on Io, identical to the fluid hawaiian pahoehoe at Kilauea but far larger in scale. Puu Oo flow fields are 16 km wide in comparison.

Graphic: The interaction of hot basalt and sulfur ice/snow is a major process on Io s surface, forming dark tephra plumes around active lava flows when trapped gas explode. https://www.nature.com/articles/s41467-022-29682-x

7. Fast lava flow eruptions (pillanian style eruptions)

Photo: By New Horizons spacecraft showing a spectacular lava fountain eruption at Tvashtar Catena that was caused by a major fissure eruption. These are the most violent lava eruptions in the solar system and perhaps analogous to flood basalt vents.

Now we are getting into the most fascinating side of Ionian volcanism, the violent high output eruptions, that are very different from the slow compound lava flows and lava lakes that we looked at above. These are fast violent fissure eruptions, or paroxymal eruptions from central vents or fissures caused by fast magma ascent and violent depressurization of volcanic gases, resulting in large volumes of lava being erupted over short timescales. Such eruptions are named after the spectacular 1997 eruption at Pillan Patera that I wrote on before in this article https://www.volcanocafe.org/ios-pillan-patera-eruption-in-1997-the-largest-lava-falls-ever-seen/ . These eruptions involves violent short lived lava fountains which shine extremely brightly in infrared sensors on space probes, giving them the name ”outburst eruptions”. There have been many of these from Io over the years with some famous examples being from the Surt volcano 2001 that probably produced 200 times more thermal energy than Laki did at its peak. Other famous lava fountain outbursts occured around Pillan many times in the 2010s and a particularly powerful one in late 2020s.

These are indeed immense eruptions compared to what we seen on Earth in the last 100 years of modern equipments. Leilani and Holuhraun who both made more than 1 km3 in a few months with average eruption rates of a few 100 cubic meters a second, but on Io some of these ”pillanian” style eruptions could perhaps produce 100 km3 in a week. The Surt 2001 eruption had an eruption rate that has been estimated at 10 km3 per hour, or even per minute at the opening phase with a rapidly dropping eruption rate afterwards. Many of these powerful fountain eruptions last for weeks or months at different eruption rates shining bright in a spacecrafts sensor. The high temperatures oversaturated Galileos infrared spectrometers giving readings as high as 1800 degrees C. Such high eruption temperatures have today been questioned unless tidal heating from Jupiter is stronger than expected, but the current models on the lava temperatures on Io land more on 1280 to about 1400 degrees C, still very hot. and true 1500C komatiitic temperatures are not impossible as I mentioned earlier in this article.

Pillanian style eruptions are the most violent eruption events in the solar system. Their violence and size comes from two factors. The first is that because the much lower gravity of Io generates less buoyancy forces on magma chambers rising through the Ionian crust, the magma chambers are thought to be much larger than those on Earth, so that eruptions can be far more voluminous. The second factor is of course that far more magma is available on Io, as tidal heating is a superior magma producer compared to Earth’s radioactive and primordial heating. On Earth a volcano with a base supply of just a few cubic meters a second is a real monster like Kilauea, while Io may have some volcanoes with a deep lithospheric supply of perhaps 100 s of cubic meters a second. The larger asthenospheric supplies sets up the stage for frequent large scale fissure volcanism. Io’s vacuum is a factor that makes these eruptions so violent, because the magmatic volatiles degasses violently when they rise towards the surface: an eruption of any size on Io is more violent than an eruption on same size on earth.

Eruptions that have been seen like Tvasthar and Pillan and others around Pillan and Thor volcanoes can erupt astonishing amounts of lava over short timescales. The eruption fissures appear to be many 10s of kilometers long. An imaginary eruption fissure that is 25 km long and even with only 5 cubic meters a second per meter of fissure would produce 1 km3 of lava in just a few hours, with a total eruption rate of 125 thousand cubic meter per second. The Ionian outburst eruptions that have been seen since Galileo appear to be in many 10 s of thousands of cubic meters a second, and perhaps many 100 s of thousands cubic meters a second at start up, to explain the intense thermal radiation. These eruption rates are of course much higher than the magma supply from the Ionian lithosphere so they don’t last very long at such rates.

Eruptions like these will form spectacular curtains of lava fountains, like walls of fire going through the ground for countless miles. The fountains would be large and optically dense meaning the lava clasts heat themselves by their own thermal radiation, forming clastogenic lava flows that rapidly move forward at high speeds, and unlike the slow lava flows of promethean style eruptions here it is a real flood of fire, with lava flowing over the ionian surface in turbulent and laminar flows. With very large amounts of lava rapidly moving over thick deposits of sulfur snow, these ”flood basalts” will create some very violent interaction with a cold icy volatile rich surface, with large phreatic blasts of sulfur gas blowing through the raging lava stream. Such interactions between flood lavas and the cold Ionian surface may result in the formation of rootless cones as seen on recent flood lava flows on Mars when they too quickly flow over an icy volatile rich surface.

Large fissure eruptions on Io also produce a lot of volcanic gas, that expands very quickly in vacuum as it leaves the lava fountains, and tends to form a dome like plume of gas, scattering blue sunlight. It was such a 400 kilometers high blue gas plume that gave Io away as an active world 40 years ago when JPL engineers looked at spacecraft imagery. Such plumes deposits sulfur snow over large areas as the volcanic gases freeze out. Closer to pillanian type eruptions, the lava fountains deposits dark silicate pyroclasts, leaving dark halo like areas on Io. These dark ejecta blankets can cover 10 s to 100 000 s of square kilometers. Large eruptions like these are also having an interesting relationship with Jupiters magnetosphere as charged sulfur and silicate particles that escape gets trapped by Jupiters magnetosphere which funnels them towards Jupiters polar atmosphere where they crash into the upper gas layers and dramatically increase the jovian polar light activity.

The scale of the eruptions thats been observed on Io appear to to be a middle form between Laki and true LIP flows in scale in Earth terms. They are bigger than Laki in volume and speed (50 to over 100 cubic km3) but not as large as true LIP flows on Earth that often have many 1000s of km3 per individual flow. In terms of eruption rates some of these basaltic eruptions are probably at or near LIP scale. We don’t know how LIP flows behaved on Earth, and Io’s ”outburst eruptions” could perhaps teach us. LIP flows on Earth may have had constant eruption rates lasting for decades, but many clues point at real flood basalts being much faster, at least some of them.

Photo by Galileo Spacecraft. Tawhaki Vallis is Io’s largest lava channel at a few 100s of km long, a sign that Io may be capable of eruptions that involve a few 1000s of km3 of basalt.

There are clues that Io can produce and has produced lava flows of perhaps many 1000s of km3 in a single VEI 8 lava flow. A good example of that is Tawhaki Vallis. It is Io’s largest lava channel. It could be a collapsed lava tube, but most likely it is a lava channel due to its morphology and length of a few 100s of kilometers. The eruptions that carved it are definitely larger in scale than the eruption styles that we discussed here. Tawhaki Vallis may have been formed by extremely hot ultramafic lava, that even melts the ground below it, and allows it to be scraped away by the moving lava flow. Such phenomena with violent thermally eroding lava flows are thought to have carved the lava channels on the Moon, Mars and Mercury and best fits an ultramafic temperature of over 1500 degrees C. The high volcanic rates of Io means that surfaces are quickly buried by lava flows and ash, so Tawhaki Vallis is probably no older than a few 1000 s of years. Exploration of Io is just barely over 40 years old by this writing, so we have not seen yet what Io is fully capable of.

Photo: Galileo Spacecraft, showing the eruption of Thor volcano that was probably a larger version of Laki. Like the Pillan eruption was as well, earlier yellow flows are older basalt covered with sulfur snow. How does it look today 20 years later?

8. Summary

Io truly has the most extraordinary features of any moon ever found, giving it the name ”lava planet”. Ionian eruptions are on a really large scale, perhaps showing us how Earth was like in its Hadean youth. Ionian eruptions also shed some light how large prehistoric eruptions behaved on Earth. In general the volcanism on Io is quite similar to Earth’s effusive eruptions, but Ionian volcanoes are much larger and more powerful than the ones on Earth. A surprising amounts of stuff has been learnt about Ionian volcanism, despite the relatively few closeup photos and measurements that have been taken. We need more probes for Io as its pretty much the most exciting place in the solar system. In February 2024 Juno Spacecraft will come as close as 1500 kilometers to Io and should get some very good photos of the volcanic activity, so I’m excited indeed. But we need in reality more probes that are specially engineered for Io. The technology and radiation proofing for that exist, while expensive. I do think the future for Ionian exploration is very bright and hope for more probes and even lander rovers. Having finished this one, I will later make more articles on Individual Ionian volcanoes and eruptions when I have time. Just 60 years ago it was nearly impossible to think that moons like Io and Europa could exist. Nature can create celestial objects that borders on science fiction.

Jesper Sandberg, November 2023


Recommended read: Volcanism on Io: A Comparison with Earth Ashely davies et al.

















199 thoughts on “A ”quick” tour of volcanism on Io

  1. A truly fascinating article and really enjoyed reading this. Many thanks for this.

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