With the possible exception of Venus, Jupiter’s innermost Galilean moon Io is the most volcanically active body in the solar system. Volcanic activity was first discovered in 1979 by Voyager 1’s imaging team. Flybys by other probes since 1979 and telescopic observations have identified over 150 active volcanoes and perhaps 400 volcanoes in all.
The moon is in a slightly elliptical orbit. It is relatively close to three other major moons also in slightly elliptical orbits. The tidal interaction between the elliptical orbits and Jupiter and the resonance between the orbits knead and heat up Io’s interior to the extent that it has sufficient eruptible magma to power these active volcanoes. We do not know at this time if the magma source is a magma ocean under the crust or a mantle with an appreciable percentage of melt interlaced through it.
Initial observations of massive amounts of sulfur on the surface of the moon led to speculation that sulfur was the primary erupted material. This changed upon further analysis which demonstrates that most of the erupted material is silicic in nature. SO2 is produced in large quantities and ends up as a substantial snow layer on the surface. Many of the less vigorous explosions observed are thought to be lava interaction with this snow cover.
Io orbits Jupiter at 422,000 km, about the distance between the earth and moon. It is slightly larger than our moon. It interacts with Jupiter’s magnetic field, creating significant electrical charge across the surface. This interaction also creates auroras. Tidal interaction with Jupiter has locked its rotation so that the same side always faces Jupiter. The other Galilean moons are similarly tidally locked, just like our moon. It takes Io about 1.77 days to make a complete orbit of Jupiter.
Io ejects a significant amount of sulfur dioxide (SO2) from its surface. Due to its low gravity, gasses easily escape and have created a torus of gas that the moon travels through. Jupiter’s magnetic field sweeps through that torus every ten hour long rotation and quickly ionizes the gas into an ion plasma. This is called the Io plasma torus. The interaction between Jupiter’s magnetic field and the torus creates what is called the Io Flux Tube. All of this working together makes Io a very difficult place to visit due to incredibly high radiation and electrical conditions locally.
There was a great explosion of publications on Io following the Galileo mission. Among the best is Davies’ 2007 355-page Volcanism on Io: A Comparison with Earth. It is available on Amazon in all three formats including Kindle. And for some reason, the Wiki Volcanology of Io is surprisingly complete and a decent place to start.
I pulled the majority of these two posts from those two sources.
The Galilean moons were discovered by Galileo Galilei in January 1610 using his newly invented refracting telescope on Jupiter. He was able to resolve the other moons and published his observations in March. There was a competing claim dated perhaps a week earlier that was not published until the news of Galileo’s discovery spread. For the next several centuries, improved telescopes allowed astronomers to resolve features on the moons a little better each year. Spectroscopic observations of Io did not show the water the other three moons have in abundance. On the other hand, sulfur and sodium salts are present in large quantities on the surface.
The first spacecraft to pass near Jupiter were Pioneer 10 and 11 in 1973 and 1974 respectively. These were high speed fly-bys on their way out of the solar system. The flybys discovered a thin atmosphere on Io and intense radiation belts in the vicinity of Io’s orbit. Trajectory analysis showed that Io was the densest of all four satellites, with a density similar to the terrestrial planets. The only good photo of Io was taken by Pioneer 11. Close-up images by Pioneer 10 were lost due to the intense radiation environment.
Voyager 1 and 2 flew by Jupiter on their way outbound to the Grand Tour of the gas giants of the outer solar system. Both flybys were in 1979. Both spacecraft took the first photos close enough to Io to show actual volcanoes in eruption with active plumes. Voyager 1 images showed nine active plumes. This activity was predicted by a paper published by Peale et al shortly before the encounter. Comparisons between images taken on both flybys showed that seven of the nine plumes were still active, with a single volcano shutting down in the four months between flybys. Both Voyagers are still active, recently departing the heliosphere.
The next visitor to Jupiter was the Galileo orbiter in 1995. This orbiter had a significant early problem when its main antenna refused to deploy. The mission figured a way around the problem that allowed it to gather and return data and images, though not nearly as many images as originally planned. Still, it did outstanding science regarding Io, discovering large numbers of active volcanoes, lava flows on the surface, mountains, and changes on the surface from orbit to orbit. Its mission ended in 2003.
Cassini flew by in 2000 on its way to Saturn and scientists were able to observe the system with two probes (Cassini and Galileo) simultaneously. New Horizons flew by in 2007 on its way to Pluto, and the Juno Jupiter orbiter arrived in the system in 2017, though its target is Jupiter itself rather than the moon system. All three of these probes have taken distant photos and gathered data on Io.
Io is the Galilean moons closest to Jupiter. It does not appear to have any residual water or ice left and is mostly silicate rock.
Europa is a bit smaller than Io at 3,126 km in diameter. It has an ice crust on a global ocean that may be up to 100 km deep. There is a mixture of rock and ice until a solid rock core. It is tidally locked to Jupiter and takes 3.55 days to complete an orbit. It is in a 2:1 resonance with Io.
Ganymede is the largest natural satellite in the solar system at 5,248 km in diameter. It may have several ice and water ocean layers under a frozen surface. It is not as tectonically active as the inner two moons. Like the other moons, it is tidally locked to Jupiter and takes 7.15 days to make an orbit, a 4:1 resonance with Io.
The final Galilean moon is Callisto. Like the others, it is tidally locked and takes 16.69 days to complete an orbit. It is 4,821 km in diameter and has a rock-ice crust overlying a mixed rock-ice interior. It is not in a strong resonance with the other three moons, though it is close to a 10:1 resonance with Io. It is the farthest from the intense radiation belts of Jupiter.
These are not the only moons around Jupiter, with another 75 and a small ring system present as well. 8 of the 79 are in regular orbits around Jupiter’s equator. The rest of them are in irregular orbits, a substantial portion of them in retrograde orbits, and were likely captured by Jupiter’s gravity.
One of the early mysteries of Io was its yellow color. The volcanoes pock marking its surface made it look not unlike a pizza. Early analysis of Voyager images concluded that it was primarily erupting sulfur, as the lava flows varied in color not unlike the various forms of sulfur, with different colors at different distances from the vent. Voyager infrared (IR) and spectroscopy sensing (IRIS) were consistent with sulfur volcanism.
The problem with this is that the instrument was not able to handle wavelengths generated by higher temperatures. Voyager scientists deduced active silica volcanism based on the overall density of Io and the steepness of slopes on patera (caldera) walls. Sulphur deposits being relatively soft, are mechanically unable to hold large steep slopes even in 1/6th earth gravity.
IR studies over the next 20 years shifted the conclusion to one that silica volcanism was the primary form of volcanism with sulfur being a secondary player. An eruption in 1986 had temperatures at least 600 C, well above the boiling point of sulfur. Modeling lava flow temperature change indicated that they cool quickly, so early temperature measurements were seeing the cooled crust or erupted magmas.
Galileo confirmed silicate volcanism, with very hot basalts with temperatures during eruptions ranging 900 – 1,300C. Actual chemistry of the basalts may be close to terrestrial komatites and ultramafic magmas may or may not exist. Measured temperatures are higher than expected and may be due to some sort of a mechanism that superheats the magma during ascent. It may also be due to limitations of remote sensing instruments and the interpretation of what they are seeing.
Among the landforms on Io are caldera analogues called patera. We do not know if these structures are formed by either terrestrial analogue – explosive eruptions like we see in subduction volcanoes or simple subsidence like we see with Hawaiian and other oceanic volcanoes. Either way, they tend to be larger at an average of 42 km in diameter with steep walls. Patera are not exclusively located at the tops of shield volcanoes, though there are some. Depths have only been measured for a few of these and are typically over a kilometer deep. The largest of these is Loki Patera at 200 km across. These may be structurally controlled with half bounded by faults or mountains. Patera may be formed by thermal interaction between silicic magmas and overlying layers of thick, frozen volatiles (SO2) which erupt in plumes and collapse. After volcanic activity stops, resurfacing quickly removes the newly formed patera unless it has a continuous magma supply.
Only 45 steep shield volcanoes have been identified so far. The steep portion is typically 40 – 60 km in diameter, with long, radial lava flows. There are many low shield volcanoes. These are much larger at 450 km in diameter. Most are topped with a patera. Some of them are nested like we see on Hawaiian volcanoes, though much larger.
There are lava channels carved into ice-rich surface. The longest of these is over 190 km near the Tawhaki Patera hot spot.
Mountains on Io are up to 18 km above surrounding plains. There are nearly 150 mountain features identified. These include mesas, plateaus, peaks, ridges, massifs, mixed types, and unclassified types (usually due to poor imaging). The largest mountains appear to be tilted blocks of crust that have been uplifted. Mechanism for formation is not clear but is thought to require some sort of crust tens of kilometers thick and faulting. Although plate techtonics do not exist on Io, eruptive products are continually being deposited on the existing surface, layering the upper crust. There seems to be some connection between mountains and adjacent patera as the distribution of both is somewhat higher than expected.
Next post: We will take a look at some individual volcanoes, plumes, active processes and tectonics on Io in Part 2.