Jupiter’s Moon Io is the most volcanic place in the solar system. The most powerful active volcanoes known to exist cannot be found here on Earth, but on Io, a small moon of Jupiter, and it is therefore an enormous fascination for me. Io is a volcanic powerhouse, so volcanic it is very much a real-world” Mustafar”. Io is so volcanic because of tidal forces from Jupiter and the rest of its moons so it is more like the planets were like in the Hadean era than any modern-day moon or planet. Io’s intense volcanism surprised everyone in 1979 during the Voyager era. It is the most volcanic place in the solar system and it is just a small moon, that was so unexpected, indeed a complete shock of what to expect from a moon in the outer solar system, where it is very cold and presumably very dead. Instead, they discovered a celestial object that is in many ways more interesting than the other planets themselves. This is my first article on a specific Ionian eruption, one of many in the last 20 years.
The Ionian eruption at Pillan Patera 1997 involved one of the most spectacular basaltic volcanic events ever seen by instruments, an event so spectacular it deserves its own article for Volcanocafe. The event involved the largest lava falls ever seen as an effect of filling a Patera ( Ionian caldera ) with lava from a fissure eruption. In this article, I will talk about this event that was the first time a major lava flow fissure eruption event was seen on another celestial body in some detail.
This eruption resulted in a dark, pyroclastic deposit more than 400 kilometres (250 miles) across, and a fresh dark lava flow emplaced with an area of 5000 square kilometres. There have been other more intense Ionian eruptions in thermal output since then, like 1990, 1991, 2000, 2010 and 2020 events that may have done ten’s of km3 in an hour, but in terms of scenery, Pillan Patera perhaps wins the contest. Still, the February 2001 Surt eruption produced 100 times more energy than Pillan Patera did in thermal emissions.
Io is truly an extraordinary moon, and the eruption at Pillan definitively proved that not only Earth has hot active silicate volcanoes in this solar system. Almost 30 years ago while the poor atmospheric probe was disappearing into the jovian abyss, the Galileo mothership itself put on the brakes and slipped into orbit around Jupiter. The staff at JPL knew when they entered orbit in 1995 that Io would be the most fun among the Jovian satellites, and what they discovered was extraordinary. Io bubbles with volcanoes, very large powerful volcanoes too and because of that I will write a separate article later about Ionian volcanism in general. Jupiter’s moon Io is so unique and strange compared to other moons in the solar system, that it almost borders on Sci-Fi.
The opening of the 1997 event far exceeded historical lava fissures in strength, One reason for this is that more magma was available, but it is also because magmatic gases expand more violently in a vacuum. So the lava fountains at Pillan and other fast ”outburst type” eruptions on Io are extraordinarily violent. The eruption continued for months at high intensity and was seen numerous times by the Galileo mothership from its many orbit flybys of Io. The intense thermal emission and creeping warm dark flows were a sure sign of a large-scale emplacement of fast lava flows. At peak output, the Pillan Patera eruption outshone all other Ionian volcanoes combined. Even before the Galileo mission violent lava fountains have been proposed to be the cause of such powerful thermal emissions. These outbursts on Io provided one of the best opportunities to measure the temperatures of Ionian lavas because of the sheer amount of hot materials exposed. Despite the sheer size of the event, it had many similarities to large-scale fissure eruptions that have been seen on Earth. Pillan’s eruption was the first high eruption rate Ionian eruption seen at close view, so fast high output Ionian fissure lava eruptions have been given the name ”Pillanian eruptions”. The event was a wake-up call of how volcanic Io is compared to Earth and a sure sign of high-temperature silicate volcanism on a body outside the Earth. It surprised the experts at JPL even if eruptions had been seen there in 1979. The eruption at Pillan formed the largest lava falls ever seen from remote instrument sensors and is the main reason I wanted to write this article about this Ionian eruption and the events that made it so spectacular. Despite the few photos and data taken from this alien eruption, a surprising amount was learnt from what was recorded by Galileo. While the lava falls are the main star of this extraterrestrial eruption I of course need to explain the eruption itself as well.
The name Pillan itself comes from Mapuche (Araucanian) peoples of Chile. Pillan is a god of thunder, volcanoes and fire and is said to reside in many Chilean volcanoes, like Villaricca (Ruka Pillañ or Quetrupillán as examples), but on Io, Pillan was given a far grander home for sure as Ionian volcanoes dwarf these. On Io, all volcanoes and volcanic features have to be named after gods, demons, mythological beings of fire, earthquakes or the sun. Below follows an explanation of the year 1997 Ionian eruption 25 years ago and its high points of fun and fascination.
The 1997 eruption site and lava flows
During the Voyager era and the very first Galileo probe years, Pillan was a quiet region, a 70 km wide and 3 km deep ”caldera”, called a ”patera” in planetary geology. It was dusted with sulfur snow from the nearby volcano Pele. To the west of the caldera was a rifted mesa and the mountain ranges of Pillan Mons. Only a very small thermal hotspot and an older flow field in this area was suggesting any active silicate basaltic volcanism. Galileo’s probe arrived at Jupiter’s system on December 7 1995. That it arrived at all was a triumph of technology and solutions to stubborn problems. Still, the Galileo mothership was not without issues. The biggest plague was that the HGA antenna failed to open leaving the probe with reduced data capabilities with the Low Gain Antenna. But the probe was still very capable of photography and collecting sensor data.
Galileo returned to the inner Jupiter system orbits in late June 1997 during orbit C9, allowing an eruption to be seen in daylight and an eclipse on June 28, 1997. In the case of images returned back, a new plume fallout ~110 kilometres tall was observed over Pillan on the limb of Io. Before that observation, no earlier volcanic plume had ever been observed at Pillan. On June 28 1997 (during orbit C9) a massive eruption just east of the mountain complex was in progress. The thermal emission was so intense that even at 1.4 million kilometres distance it overloaded Galileo’s SSI instrument. A 140 km high silicate tephra plume shot up from it as well with an intense thermal radiation emission at its base. What happened was a huge intrusion of gas-rich basaltic magma had reached the surface, resulting in a long line of violent lava fountains showering 1000s of square kilometres of the surface with molten lava, Galileo’s instruments also showed a creeping expanding dark area in visible light, but that also was very hot in the infrared and was large scale turbulent lava flows fed by the lava fountains from the fissures.
Large powerful eruptions like these at Pillan and other similar Ionian outbursts erupt from vast fissure systems, with fissures many 10’s of kilometres long. This is certain for Pillan’s eruption fissures, looking at the few photos that were taken: the fissure system appears to perhaps be 50 kilometres long, or because the lava flows emerged from a line, that may give the fissure system a length of 75 – 80 kilometres when compared to the 75 km wide Pillan “caldera”, that exceeds even large historical Icelandic rifting eruption fissures in length. But because spacecraft imagery resolution of the eruption site itself is low, we will never know for sure how long the eruption fissures were at Pillan, but many 10s of kilometres of lava fountain curtains are certain, like fiery walls going up into the void. From low orbit Pillan’s start would be a spectacular sight at the night terminator of Io, its gas plume would be seen as a faint blue dome as it scatters the sun’s light with an intensely glowing spot at its centre, which is the violent start of the eruption. On the ground, effects almost similar to a Plinian eruption on Earth probably happened with the massive lava fountains throwing out an ejecta plume with falling materials around them as seen with the dark pyroclastic sheet left by the eruption around the lava flows. The start-up of the 1997 eruption must have been a formidable sight with miles-long walls of calbuco-like glowing pyroclastic fountain columns when eruption rates were at their most intense. The violent opening of the eruption could have been many 100,000s of cubic meters a second, while the lava flows flowing out from the enraged moon was fed at 10,000s to 100,000s of cubic meters a second at the first phase of the eruption, declining to many 1000s of cubic meters a second as the eruption vented on for months. Despite the sheer violence of it, it all happened in the silent vacuum of airless Io, it was a geological rock concert without sound, spraying lava into the silent void. The long-term eruption rates for the Pillan eruption have been estimated at 6000 – 8000 cubic meters a second with a much faster start for quite a while. The Holuhraun eruption had around 250 to 300 cubic meters a second, to make a comparison to Ionian eruptions. Even at average lava flow output Pillan was somewhat or even much higher than Laki lava flow emplacement, with its start far exceeding terrestrial fissure examples. Estimating eruption rates from only a few handfuls of hot pixels and grainy closeups is a complicated mathematical process.
At these eruption fissures, massive lava fountains kept going for months or even years with decreasing intensity. Most of the lava volume emplaced by Pillan 1997 was likely emplaced during the first weeks. Io’s low gravity and lack of atmosphere means lava fountains are much larger than on Earth. The fountains would probably be many kilometres tall and throw out large amounts of lava as sulfur gas nucleation in the magma conduit is much more violent than on Earth in Io’s vacuum, spraying lava pyroclasts on ballistic trajectories in all directions. The inner part of these huge fountains are very thermally and optically dense, so the lava clasts heat each other by their own radiation and the lava doesn’t cool much as it was thrown everywhere feeding the clastogenic lava flows from the fissure system. These huge fountains can be thought of as liquid-gas geysers when a magmatic sulfur volatile violently decompresses driving the magma curtains miles into space. It probably goes up with the speed of a high-powered rifle, and during the long fall down the pyroclasts also hit the Ionian ground at high speed, leaving impact craters in earlier pyroclastic sheets. Because of the lava fountains spraying tephra everywhere, an imaginary visitor on the ground near Pillan may experience a constant stone rain of crumbly gassy mafic pyroclasts and visibility would likely be poor. The Galileo Spacecraft later pointed its instruments spectrometers at the eruption deposits and discovered Inosilicate minerals in the group of orthopyroxenes. The mineral enstatite was identified and it may suggest pure ultramafic lava was involved in the event rather than normal basaltic lava, or perhaps even a mafic silicate basalt composition unique to Io. The discovery of mafic minerals in this eruption confirmed the presence of high-temperature silicate magmas on Io. Lava fountains on airless Io should behave exactly the same as lava fountains would do on Earth’s airless moon and may result in similar volcanic deposits. Apollo 17 astronauts discovered deposits of small glass spheres near old fissures on the Moon that were the result of volatile rich lunar lava fountains, the same lava droplets surely was produced in the Pillan event and was deposited far and wide. The pillian eruptions dark pyroclastic deposits helped researchers to identify what all the other Ionian colors are from. Pale deposits are sulfur snow that froze out, while dark diffuse deposits are the tephra fallout from silicate lava fountains.
The eruption’s energy and superheated gases thrown out formed a vast plume of dark basaltic tephra and sulfur gas that fans out in a dome like plume shape and rains down over a 400 kilometers wide spot. Pillan’s lava fountains rained these dark silicate rich tephra over a vast region. The lighter sulfur gases plume rose to 140-km-high similar to the constant gas plume that exist over Pele’s Lava lake just to the west of Pillan, the sulfur gas ejected by this eruption spread out as a dome and would scatter blue light, so near the eruption site any space traveller walking the surface would perhaps see a dark blue sky and a fiery plume going up into it. It is a spectacular light show, any imaginary expedition team in metal clad suits, likely dazzled in awe as the area east of Pillan Mons becomes a huge wall of ”fire” as the gas rich dyke reached the surface sending countless lava pyroclasts that fills the skies like meteors. Lava fountains like fiery geyser pillars also erupt from Pillan Mons itself. This is a rifted mesa plateau and probably was used as an easier pathway for the ascending magma as many Ionian lava lakes and lava flows do emerge from leaky faults along crustal blocks. Similarly fault guided, caldera fault eruptions have also been seen at other violent outburst eruptions like the Tvasthar Catena eruptions that had some similar dynamics as the Pillan Patera eruption.
Violent Ionian eruptions can be thought of as ”sulfuroaticmagmatic” eruptions as the rising hot magma interacts violently with thick layers of sulfur ice in the low density volatile Ionian crust. Such interactions may cause even more volatile release in the already decompressing volatiles. Here on Earth same phenomena happens when magma erupts in a glacier or in waterlogged ground. On Io it happens when magma gets crammed in together with sulfur ”ice”. But most of the eruption pressure at Pillan came from decompressing pre-existing magmatic sulfur volatiles.
On October 11, 1999, Galileo performed its first close Io passover since the spacecraft entered orbit around Jupiter in December 1995 just after I was born. The close range of the encounter over Io’s trailing hemisphere provided a chance for Galileo’s cameras to observe the new still warm Pillan lava flows that had flowed out in the years before. They were photographed at high resolution (9-18 meters or 30-60 feet per pixel). Unfortunately most of the images were mostly scrambled, requiring extensive processing to bring out useful information. Despite the attempts of processing these closeup photos, most of the images were heavily degraded by radiation, and included a dark stripe down the center of each frame. The cropped full-frame mode image, shown at the lower left of the above mosaic, did not suffer from this anomaly. A rough lava flow emplaced a broad sheet (rather than a channelized flow) was revealed in the close-up photo of the lava flows, with pits and lava channels across parts of the flow. By looking at length of the shadow along the edges of the new lava flows on the left and right sides of the mosaic, the Pillan lava flow had an average thickness height of 8-11 meters and had a slabby Aa structure at the front and sheet pahoehoe at the middle, In fact most Pillan flows had a ”flood pahoehoe” appearance. The features that were seen in Pillan’s flows are quite similar to the flood lavas on Mars that been seen in the Athabasca Valles region that consist of thin lava flows with large chunks of rafted crust. This is a sure sign they too was formed from lava flows rather than water or mud. These lava flow surfaces are quite rare on Earth and only some really fast historical fissure eruptions in Iceland, Hawaii and Africa have produced similar surfaces. The raging lava flows that engulfed ionian ground at Pillan was probably partly turbulent at least near the fissures, could also explain the chaotic mix of slabby and sheeted pahoehoe that was seen the close-ups from the Galileo Spacecraft during its close pass in 1999 when most of the eruption had died down, most of the lava was quick laminar flows and formed a lava flow surface, that is neither Aa or pahoehoe but something that is reserved only for very fast eruptions.
Lava flows emplaced, fed at an estimated many 10s of thousands of cubic meters a second, flowed south towards the Pillan Patera. At these high eruption rates it would be a massive, fast moving sheet Aa or pahoehoe mix flows fed by fluid open channels. Near the vent fountains it would be flowing at car speeds over kilometers wide fronts, erupting cubic kilometers in just hours. Further away from the fissures the energy of the flow would be spread out and the moving lava front would flow slower. Still, many lava flow fronts each 30 km wide and 11 m high would be quite a sight indeed, moving at a few kilometers a day towards the caldera pit of Pillan Patera. The massive lava flows would engulf thick layers of sulfur snow quickly, and especially so during the first days and weeks of eruption when flows where fastest close to the vents. That would result in spectacular ”phreatomagmatic” explosions when hot lava engulfed these volatile frosts. On IO this is possible as well if such fast moving lava flows would flow over a ”sulfur glacier” or thick layers of sulfur snow/ frost.
Most of the flow emplacement style at Pillan was probably large single ”sheet type pahoehoes” not unlike those close to the vents of Mauna Loa lava flows but on a much larger scale, with a very violent opening, and miles long curtains of fire with vast sheets flowing out. Later the vent erodes the conduits forming more gentle activity, the later stages of the eruption likely had an impressive lava channel system when the eruption focused on a few locations rather than miles long fountain fissures. With such intense eruption rates all the lava simply flows away from the rift system without forming any spatter ramparts. Eruptions like these maybe analogous to ancient lunar mare eruptions with basalt flooding large areas, with Io’s Pillan eruption being a slightly smaller version of lunar ”mare eruptions”
The rough crumbly texture of the Pillan lava surfaces was likely the result from a number of factors, including the interaction between the hot lava and the cold, volatile-rich surface it flowed over, high eruption rates and low viscosity encouraging turbulence, and the disconnect between the high effusion rate (the volume of lava flow for each meter of the vent fissure) and the speed of the flow front. In the first case, gas created by the heating of sulfur dioxide when fast moving lava flows buries it would explode through the cooled crust of the lava flow. This action would rip up the cooled lava crust and likely formed ”phreatomagmatic” vents that provide even additional gas for the plume seen from Pillan in 1997. On Earth and Mars when large amounts of fast moving lava quickly engulf a wet or volatile ground that may also result in formation of pseudocones. Such interactions have been confirmed in martian pseudocones. One such channel can be seen in the left side of the mosaic. A very fast lava effusion flow within the lava flow ( many 10 000s of cubic meters a second) would cause the outer cooled crust to break up into blocks or rafts. These rafts flowing with in the lava river stream can also disturb the cooled crust as it is moved downstream by the still molten lava beneath. It would be a spectacular sight for sure, a little like an ice flood, with huge slabs and sheets of crust and large lava bergs moving in a fast moving lava stream, but being fiery hot instead of the cold colors of water. In terms of appearance the fast moving lava flows at Pillan or at least the flood sheet channels maybe very similar to the lava flows seen at Mustafar in Star Wars Ep 3 Revenge Of the Sith. Think a fiery spring glacier flood with lots of debris in it, spreading out. The edges of the advancing lava flows at Pillan would perhaps resemble large masses of slabby pahoehoe or even Aa digging into thick layers of sulfur snow, behind that is a faster moving jumbly mass, with more speed and fluidity the further behind the flow fronts you go. Most of the lava surface had a jumbly flexible crust. Fast moving lava flows spreading out over 1000 s of square kilometers is an impressive thought, such an eruption on Earth would form massive pyrocumulus and dramatic skies because of rising heat, but in Io’s vacuum that does not happen, but it would still be an incredible sight. The lava flows would move forward with spectacular volatile-rich explosions raging through them.
Gas buildup from ices (all volcanic gases on Io, which is as cold as hell, tend to snow out and settle as snow), can cause these plates and raft textures and swollen forms to form in the flow field, like the one seen below and to the left of the dark pit on the close-up photo of the lava flows. The lava flow field also have large slabs of crust indeed suggestive of fast emplacement with the high lava effusion rate would lead to a crumpling of the lava crust as the flow front likely Aa lava advanced at a speed of 1 kilometer per day, the result of the leading edge giving up heat to vaporize surface snow. This could perhaps create the rubbly surface texture found in the October 1999 images together with the fast eruption rates. Some suspected a komatiitic composition because of Galileo’s remote temperature measurements and the lower viscosity than that of the most mafic lavas that form inflated pahoehoe flows. Some think that it is just this combination of different eruptive and rheologic characteristics produced the unfamiliar surface lava morphology that was seen at Pillan. The original assumptions was made, in the year 2000 by Nasa geologists, that the presence of what appear to be rafted plates of rock on the upper flow surface suggests rapid lava surges or turbulence variations in a turbulence flow emplacement, they assumed Pillan lavas were low-viscosity, perhaps ultrabasic silicates because of that as the viewed the Galileo data. Another interesting point is that the apparent flows are wide, thin sheet flows rather than channelized flows which strongly suggests fast eruption rates. But sheet-like flows with a strange surface morphology fed by lava channels have been found in ancient terrestrial komatiite flow outcrops in Western Australia although they lack rough upper surfaces. The channelization of lava would be expected over rough topography, although there is not currently not a good close-up resolution of the pre-eruption ground substrate. As told earlier another explanation for the strange flow surface morphology is that it might be due to fragmentation of the flow by degassing or disruption of the lava flow by explosive interaction with the Ionian sulfur snow or icy substrate rather than a komatiitic composition. But it is also true that a very hot basalt will also be incredibly fluid and there are similar lava features on Mars.
When the Galileo Spacecraft looked at the eruption from a far distance with its SSI/NIMS temperature instrument, data collected for the 1997 Pillan eruption showed temperatures of 1590 degrees C. This value is close to the estimated eruption temperatures of a Commondale komatiite/Io analog composition, which was calculated by geologists to erupted at 1611 C using the igneous petrology program MELTS. The identification of orthopyroxene minerals in the Pillan lava flows and the fact that that most magmas erupt below full liquid temperatures (in which the two- temperature-area model fit provides only a lower limit on magmas liquidus temperature), some astronomers and geologists thought that the Commondale composition is a useful analog for Pillan. We can evaluate flow emplacement with this composition for the range of eruption temperatures consistent with Galileo data. The average thickness of the 1997 Pillan lava flows is 9 meters. Assuming that these values are typical of the Pillan lava flows, they may seem too thick for komatiitic flows and flows of temperatures over 1350 C. But long flakes of olivine crystals may cause some komatiitic flows on Io to appear thicker than they are. In recent data analysis from the Galileo Mothership the suspected komatiitic temperatures have been questioned due to inaccurate ways to understand and process the data from the Galileo Spacecraft, and the instruments on Galileo were not ultra-specialized for study of ionian eruptions. Today temperatures of around 1320 degrees C are generally more accepted for Ionian outburst eruptions as they over-saturated the instruments that Galileo had. Some readings where so high (1900 C) that they came into problems with how much tidal heating is possible at current situation. But still 1320 C is at the upper end of normal basaltic temperatures and borders on ultramafic eruption temperatures, Ionian magmas may be analogous to lunar mare basalt that are also thought to be hotter than most typical basalts on earth, but cooler than komatiites. But in all because of low resolution of thermal data and different ways to study them, ultramafic temperatures are not completely ruled out for the Pillan eruption, but the thickness of the lava flow fronts may suggest basalt rather than Komatiite. Still the viscosity was very low for these lavas, as low as Halema’uma’u and Nyiragongo, perhaps much lower still lava flows also thickens up as it cools when it leaves the vent. On Io s frozen surface the hot lava will bleed of heat into the cold icy surface of Io as it flows over causing mineral crystals to form in the lava, these mineral crystals increase viscosity. Thoelitic Basalt fits well in the data sets, as possible higher temperatures is not necessary to explain the instrument data.
Covering around 5600 square kilometers of Ionian ground outside Pillan Patera, with a 10 to 11 meter thick lava flow requires around 56 km3 of lava erupted, a volume beyond any recent historical eruption on Earth, and that is just what was emplaced outside Pillan ”caldera”. Most of the lava volume flowed into the caldera, over the years when the event where going, so the volume that was erupted as a whole could be twice as large, giving 112 km3 erupted by Pillan 1997. We don’t know how much erupted as we lack high resolution imagery of the lava levels in the caldera, before and after. Since Pillan Patera is 70 kilometers wide and 3 km deep, that allows eruptions of many 1000s of km3 to happen without covering much ionian lands surface, as much of the lava would flow into Pillan’s caldera and become trapped there. The amounts of lava erupted by the 1997 eruption is unknown because of this, but up to 75 – 140 km3 is maybe possible, it depends on eruption rates and how long those lasted. It is worth to remember that 1 km3 is around the volume of the entire Leilani eruption, so the event is much larger than any modern human historical lava flow here on Earth. Laki was 15 km3 so much smaller too. But Earth’s largest flood basalts can have some individual lava flows that reach over 10,000 km3, far larger than Pillan. It seems that Pillan’s flows where something between Mauna Loa, Laki and Siberian Traps in scale in terms of eruptions on Earth, far larger than for example Holuhraun and Laki, but far smaller than LIP flows on Earth. It is important to note that Io has gigantic lava channels like the old Twhaki Vallis, that may suggest that Io too is capable of LIP scale (VEI 8 by volume) flows. In addition to the lava flows, the 140-km-high plume eruption that deposited dark, orthopyroxene-rich pyroclastic material over 125,000 square kilometers also yields many of additional cubic kilometers of basalt erupted from this extraterrestrial eruption. On Earth the 1997 event would be a severe eruption and cause severe local gas pollution and some global weather effects and frequent eruptions of Ionian scale and frequency would degrade the global environment. In short because Pillan’s lava flowed down into Pillan Patera and collected there the real eruption volume is probably much larger than old conservative estimates. The eruption lasted for years at slower pace meaning even more unknown lava volume. The eruption at Pillan gained quite a lot of attention from researchers in planetary sciences, volcanology, remote sensing and geology and perhaps can be seen as the firework celebration for the future golden age of galilean satellite geology studies and exploration.
The lava falls at Pillan Patera
With many 10s of km3 of basalt being pumped out just north of Pillan Patera, there was no doubt it would flow into it and that is exactly what Galileo Spacecraft saw as it was flooded by dark lava when photos of the caldera was compared before and after 1997. The filling of Pillan Pateras floor formed the largest and tallest lava falls that have ever been observed. That is the reason I wanted to talk about this eruption because it would be an incredible sight to be there when it happened. Lava flows filling pit craters, forming lava falls, are common on Earth. The 1969 Mauna Ulu lava falls are the most famous example. When fast moving fluid lava from Mauna Ulu arrived at ‘Alae pit crater at 11 p.m. on August 5, 1969, the lava flows crashed into it. The Mauna Ulu lava falls were taller than Niagara falls at well over 150 meters and they were over 300 m wide in some areas. Another much more extraordinary example of lava falls and the largest case seen by cameras on Earth so far was Sierra Negra in 2005. The 2005 eruption erupted along its caldera wall and formed a 3 to 4 kilometers wide lava fall clearly seen in this video, and is the largest lava fall caught on Earth so far, that event can only be found in three youtube videos.
But the filling of Pillan Patera’s floor which is 70 km wide and surrounded by 3 kilometers high pit walls means a much grander sight than Mauna Ulu or Sierra Negra for sure. It was during the first weeks of the eruption at Pillan when lava output was at its most intense and the lava falls most spectacular. The lava flows plunged into Pillan at three places, with each lava fall at least at the start 23 kilometers wide and a mind-boggling 3000 meters tall. The scene would be most spectacular at the start when large sheet flows from the fissure eruption plunged into the abyss, a little like when massive sheet aa, or flood pahoehoe emerges from fast fissures on Earth, creating lava falls almost as wide as the caldera itself. Later on the diminishing eruption would feed smaller but still intense lava falls into the pit. At their max the three Ionian 1997 lava falls would have been 3 to 4 times wider than the entire Sierra Negras caldera itself and the falls perhaps as tall as Etna. To put this in other scales, lava falls filling of pit crater floors at Kilauea involve craters a few hundreds meters to over a kilometer wide, while the Pillan Patera event was a pit crater the size of Reunion Island, dwarfing the terrestrial examples.
The lava falls at Pillan when most intense would far exceed any known lava fall on Earth in size and speed with many 10s of Leilani volumes going over the edge in perhaps a matter of days falling kilometers down. The lava covered the 2500 square kilometer floor of Pillan rather quickly, so the lava must have been very hot and fluid to avoid from cooling much and the enormous eruption rates help to kept it liquid as it fell into the ”Pillan’s caldera”. Io’s vacuum also helped to keep the lava liquid as it can only cool by radiation. In other words the falling curtains of liquid stone heated itself and allowed the lava to fall 3 km without cooling much, so it could rapidly flood the 2500 square kilometers floor of Pillan Patera without clogging up and gaining viscosity.
Just by looking at surface are Pillan’s floor has, we can understand the scale of it. The 2014 holuhraun lava flow field is about 84 square kilometers, here Pillan’s floor is 2500 square kilometers, far far larger than even Eldgja’s 780 square kilometers. Pillan Patera being basically a ”caldera pit” can accommodate a lot of lava without taking up more Ionian surface so a lot of lava from the eruption ended up here as I mentioned above. The lava falls would be an extraordinary sight, as the lava vent over the edge it would first fall slower than on Earth but picking up speed, huge clouds of dark pyroclasts would surround these immense glowing lava falls, a little like waterfall spray, forming huge curtains of black droplets that fall down,.Large amounts of heat and light are also released, so the falls would perhaps resemble a huge mix of burning oil and liquid iron slag, that is the best analogue of something of this scale. Large lava fountains on Earth form such falling ejecta curtains, and these may be formed in a massive lava fall as well. They were literal dam bursts, walls of lava falls going on for miles and kilometers deep. It is kind of difficult to imagine the scale of the lava falls at Pillan and exactly what it may have looked like. A visitor in radiation proof futuristic suit, would still have to see the spectacle from far ahead to avoid being overheated by thermal radiation. Think it as if large areas of the Grand Canyon began to fill with liquid iron, that is the scale of Pillan Patera’s lava falls. There have been lava falls in the Grand Canyon as recent as the Pleistocene, but the Ionian events at Pillan dwarfs them.
The lava falls could have either be single 3000 m high or be terraced into many huge falls over each other. That depends how Pillan’s walls are structured, but whatever is the correct picture, they where huge as Pillan’s walls are very steep so you gets an enormous flow rate from that, at these huge scales as well the lava flowed more like water than liquid rock, and the real viscosity was probably lower than earthly basalts. The lava falls also exposed large amounts of hot surface giving large amounts of thermal radiation, in other words Pillan lit up strongly and oversaturated Galileo’s sensors. Neither the lava falls or active lava flows were observed in close details by Galileo Spacecraft who did close-up photography, just after the eruption was in max phase. After the eruption and filling of Pillan’s floor the lava flows turned green as the hot basalt degassed and reacted with falling sulfur from other volcanic vents.
More strong thermal Ionian outbursts eruptions around Pillan Patera and Pillan Mons occured in the year 2007, in 2008 , March 8 in 2015, March 31 in 2015, May 5 in 2015 and February 18 in 2015. These events like the summer 1997 also produced spectacular lava flows as there is no doubt they too flowed into Pillan Patera forming incredible lava falls as well. It’s very likely that in the far future that Pillan Patera maybe completely filled by lava flows. Filling of Ionian volcanic depressions (pateras) are likely a very common occurrence on Io, either from fissure eruptions flowing in as lava falls or eruptions inside it filling the floors. The patera floor of Pillan have layers of basaltic flows from numerous earlier eruptions. The March 2015 eruption had a fissure that pierced the patera wall as thermal radiation came from there, unlike the 1997 event that erupted nearby at Pillan Mons. None of the later thermal outbursts were observed by space probes in orbit around Jupiter. The immense Ionian lava falls at Pillan Patera have been overlooked, so I felt I needed to write about them, to further increase their ”famousity” and awareness among geologists.
I asked Justv23 to paint them for me as well, so I could put it in an article for VC. As spectacular as the lava falls at Pillan patera were, they were like the Kraffts used to say about most eruptions, an amazing theater show playing in front of an empty public, with only the brooding Jupiter looking on.
I have known since 10 years old that Io has volcanoes, but actually that a volcanic event on this grand scale was captured is just astonishing and really shows how Ionian eruptions can dwarf any recent historical events on Earth. In other words, the geologically activity on Io is off the scales when it comes to Earth’s currently active volcanoes. Even more cool and astonishing is that it was seen by a little machine that man sent there, a billion kilometers from Earth. The 1997 Pillan Patera are by far the largest lava falls that have ever been seen. With multiple 10’s of cubic kilometers of erupted lava it is the largest basaltic eruption caught that has been recorded by instruments so far, with perhaps more than half of that ending up in the caldera. (But very possible there have been much larger Ionian eruptions from other long lived recorded thermal outbursts.) I plan other articles on Ionian volcanoes later as well. The lava falls at Pillan is the most spectacular volcanic event that has ever been recorded in action by remote sensors, far exceeding in my option 2022 the Tonga event. In other worlds on Io we can observe volcanic activity that would be catastrophic on Earth and large eruption processes that only happens perhaps once every 100 000s of years on Earth. Some of Io’s largest volcanoes may have never existed on Earth since the Hadean era, like the huge lava sea of Loki Patera. Large scale volcanic activity on Io gives an insight to old ancient eruptions on Earth and therefore is of tremendous interest to planetary geologists to understand our own planet. Some geologists think that the Columbia River Basalt Flows CRB sequences are quite analogous to Pillan Pateras lava flows, as they share many of their suggested morphologies as inflated ”flood sheet pahoehoes” and indeed perhaps Pillan was a LIP scale flow on Earth in terms of eruption rates, But perhaps not like true LIP flows in size.
Also huge thanks to JustV23, Justinas Vitkus for painting the 1997’s lava falls event for me, as he is a very skilled self taught space artist for sure and is likely going to do more Io artwork for me later. The lava falls at Pillan Patera haves to be the volcanic event of the century, because how spectacular it was. A surprising amount can be learned from the few photos and measurements that was taken from the Pillan eruption. And really shows the value of cameras and remote sensing when humans cannot go in person. Because Io is the most volcanically active body in the solar system, I constantly daydream about it, the thought of huge lava fountains blazing in front of the looming Jupiter is an unreal sight. In 2003 the Galileo probe was getting sick from jovian radiation its instruments began to fail, and the high command at JPL de-orbited the probe, burning up in Jupiter’s atmosphere at insane speeds.
In February 2024 Juno Spacecraft will come as close to Io as 1500 kilometers and we should get some good close-up imagery, seeing over 20 years of change on the solar systems most geologically active object.
Jesper Sandberg, May 2023
This article written in my own words and explanations is based very much on Ashley Davies and Rosaly Lopes works on Io and their books ”Volcanism on Io a comparison with Earth” and ” Io after Galileo, a New view of Jupiter’s Moon”. Dr Davies is a volcanologist at Nasa JPL who did a lot of complicated work on Galileo’s thermal data from Ionian eruptions and worked out mathematical models for these data streams. Dr Rosaly Lopes is a planetary geologist, volcanologist who has named many of Io’s volcanoes and found many earlier unknown Ionian volcanoes as well. Everything is based on my own thoughts of the eruption and what I finds interesting. Neither Davies or Lopes et al talks much at all about the gigantic lava falls, that was the highlight of the Pillan eruption, so that is why I wrote about them.