A world of vivid scars and numerous active volcanoes, the Jovian moon Io is like no other in the solar system. Constantly churned by powerful tides, exposed lava oozes outward in snaking lines across the sulfurous yellow surface while the ground is continually peppered with the dust of new explosions. As if that were not enough, Io is also bathed in lethal radiation by the overwhelming magnetic field of its primary planet, making it one of the most hazardous places in the solar system humans could ever hope to visit - and by far one of the most interesting.
The progress of our adventure so far (current in bold):
1. The SunIo in true-color, as our eyes would see it if we were there:
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
11. Mars (Vol. 1)
12. Mars (Vol. 2)
13. Mars (Vol. 3)
14. Phobos & Deimos
15. Asteroids (Vol. 1)
16. Asteroids (Vol. 2)
17. Asteroids (Vol. 3)
19. Jupiter (Vol. 1)
20. Jupiter (Vol. 2)
26. Rings of Saturn
29. Tethys, Dione, and Rhea
32. Minor Moons of Saturn
34. Moons of Uranus
37. The Kuiper Belt & Scattered Disk
39. The Interstellar Neighborhood
Io is the fifth moon of Jupiter, and the innermost of the four Galilean moons - the four that are by far the largest, most massive, and most interesting of the Jovian satellites. It orbits the planet at a distance a few percent larger than the maximum Earth-Moon distance, although far deeper in Jupiter's gravity well than the Moon is in Earth's. Diagrams illustrating these relationships:
The second illustration above shows that Io is 16-17 times deeper in Jupiter's gravity well than the Moon is in Earth's well, which means that you would have to add 16-17 times more change in velocity (Δv) to escape the Jovian system from Io orbit than to escape Earth's gravity well from a comparable lunar orbit. Io is also dwarfed by its primary, passing like a metallic marble or bead across its face and casting a shadow on the Jovian cloud tops:
In filtered UV spectra:
According to NASA's nifty solar system simulator - which can show you both the angular size and lighting phase of any major body in the solar system from any other on any date and time - Jupiter seen from the surface of Io covers a whopping 19.5° of sky: Almost ten times the size of Earth from the Moon, and about 40 times the size of the full Moon from Earth. This makes sense because the distances are comparable and Jupiter is indeed an order of magnitude bigger than Earth. Let's imagine such a view by superimposing Jupiter as it would be seen from Io into the sky of Earth's Moon:
Of course, the surface of Io undoubtedly looks a lot different than Luna, but we have a pretty good chance of getting views like this from robotic landers at some point in the next several decades - especially as launch prices decline and space-hardened electronics achieve new economies of scale. However, it will always be relatively expensive to explore Io because of the unusual level of radiation shielding needed, so I wouldn't count on getting such images any time soon.
Io, along with its three sibling moons, all formed around Jupiter late in the planet's formative period. Some computer models indicate that the present four may merely be the final generation of a rapid era of satellite accretion early in Jovian history, the earlier products of which were devoured by the planet. Under this model, new satellites were continually forming out of material newly captured by the planet as it passed through the gas and dust of the young solar system, and would then slowly migrate inward as they too swept up material and lost orbital momentum in the process.
Once this material was largely cleared, the migration ceased and would have reversed due to tidal acceleration - the phenomenon where a rotating planet dumps momentum into a moon, causing it to spiral outward (this is happening to our Moon). However, the mutual gravitational influence of Io, Europa, and Ganymede has balanced out this effect and kept their orbits stable over time. But Io wasn't necessarily destined to be a volcanic hotspot: Only when it entered this arrangement with Europa and Ganymede did it begin to experience the competing influences that have kept its interior hot, constantly flexing under tidal forces from both Jupiter and its brethren. Without this dynamic relationship, its surface would have cooled and developed a thick ice crust like the other Galileans.
The moon's dynamism has made it unique among rocky bodies in the solar system (other than Earth) in that humans have witnessed its surface changing: Io gives us the opportunity to actually watch another world remaking itself, with both lava flows and explosive eruptions captured on camera. Some examples of Ionian history caught by humanity's robotic eyes - I'm not sure what kind of spectra each was taken in, but most seem relatively close to known coloration:
Just to give a sense of how rapidly things can change on Io, the following two images were taken only five months apart in 1997 over a volcano called Pillan Patera - and the new feature has already faded in subsequent years due to the settled dust of other eruptions:
1. Orbital and Rotational Features
The Ionian month - the time it takes a moon to orbit its primary - is 42.5 hours despite its orbit being slightly larger than the Moon's. It manages this because it travels 17 times faster than Luna, which is a result of its much greater depth in the planetary gravity well. In addition, due to the enormous size of Jupiter, Io is actually closer to the Jovian cloud deck than the Moon is to the terrestrial surface despite the latter's orbit being smaller. Nonetheless, the gravity well is so steep climbing from Jupiter to Io that going from Earth to the Moon is far easier. Time-lapse video of Io in orbit:
Io is locked in a 1:2:4 orbital resonance with Europa and Ganymede, meaning that it completes four orbits around Jupiter every time Ganymede completes one and two for every Europan orbit. This resonance is crucial both to the stability of Io's position in the Jovian system and the tidal forces that produce its volcanism. Here's why: Tides occur because one side of a body is closer to another body than the other side, meaning it experiences slightly more intense gravitational pull. As a result, the body is slightly elongated along a radial line pointed toward the gravitational center of the system, causing a tidal "bulge" in this direction and another in the opposite due to hydrostatic equilibrium (balance of pressures). In a perfectly circular orbit - which Io does not have - the force and extent of the tidal bulges would be more or less constant:
In reality, Io has a very slight eccentricity that places it 3,400 km closer to Jupiter at periapsis (closest approach to the primary) than at apoapsis (furthest point). This is only 0.8% of its average orbital radius, but the intensity of the Jovian gravity field means there are significant consequences to even the slightest deviation. When Io is closer to Jupiter, the tidal bulge is larger, meaning that over the course of its mere 42.5-hour month, the moon undergoes a cycle of flexing and deformation that heats its interior. An illustration of tidal effects in an elliptical orbit:
But over time, Jovian tides would not be enough to keep Io volcanic: On the contrary, with every orbit the constant flexing would be bleeding away the extra energy put into the moon by its eccentricity, causing its orbit to gradually circularize and reflect the first diagram above - a phenomenon known as tidal dissipation, because strong tides normally tend to dissipate eccentricity as heat. If this had happened, the Ionian surface would by now be as frigid and unchanging as those of its fellow Galileans, because the tidal bulges would not change. But because it's in resonance with Europa and Ganymede, these moons continually align to nudge Io's orbit, maintaining its eccentricity in balance against dissipation. As a result, the tidal heating never runs down, and this is why Io is the most geologically active rocky body in the solar system - far more so than even Earth.
Another consequence of Jupiter's powerful tides is that Io is tidally locked to its primary, meaning that it always presents the same face to Jupiter and only rotates once per Ionian month. This occurs because tidal bulges facing the center of the system don't "want" to rotate away from it - they have to climb a gravitational gradient to do so, and thus rotational momentum is continually bled away with every rotation until the same face always points toward the center. Thus moons that formed around their primaries are almost always tidally locked, and Io is no exception.
In more relatable terms, what this means is that there is no "Jupiter-rise" on Io: It always hangs more or less in the same place in the sky on one side of the moon, and is always invisible from the other. To see Jupiter on the horizon you would have to be at the poles or along the border between the near and far sides, and to see it rise or set you would have to be in orbit, substituting your own motion in place of the moon's lack of rotation relative to the planet. It also means that days and nights last about 21 hours and 15 minutes long, although the Sun is far enough away that the environmental effect of day/night cycles is muted.
Due to Jupiter's minimal axial tilt and large angular size from Io, a total solar eclipse occurs with every orbit when Jupiter passes in front of the Sun. The shadow of Io on the Jovian cloud tops is also a regular feature, as seen in many of the images above, and if you were floating above the Jovian clouds the moon would be about the size of Luna in the sky - assuming you could see it through atmospheric haze, which I don't know and would not be confident trying to calculate. If it can be seen through the atmosphere at all, it might be quite a sight in the night sky of Jupiter when not in eclipse. The fact that it's 4.6 times more reflective than Luna would help with its visibility, despite having less sunlight to work with.
2. Size and Mass Characteristics
Io is about 28.6% the size of Earth, and has about 1.5% the mass - slightly larger and more massive than the Moon. It is also the third largest and third most massive Galilean moon, substantially larger than Europa with almost twice the mass but considerably smaller and less massive than Ganymede and Callisto. Ionian surface gravity is about 18% of g, about 2 percentage points higher than on the Moon - so if you weigh 150 lbs on Earth and about 25 lbs on the Moon, you weigh 27 lbs on Io. Rough size comparisons of comparable objects - mouse over the image if you don't recognize an object:
Despite its volcanism, the Ionian surface is generally frigid with only small day/night swings due to the Jovian system's distance from the Sun. Excluding hotspots, the observed temperature range is 90 K (-183 °C / -298 °F) to 130 K (-143 °C / -226 °F), with the average being close to the latter. Volcanic hotspots, however, range from about twice the average temperature into low quadruple-digit figures. A small part of Io's surface temperature may actually come from Jupiter, which puts out more heat in the form of infrared radiation than it receives from the Sun. Infrared images of Io showing hotspots:
4. Internal Structure
Io's interior is still an active area of investigation, but ongoing analysis of data from the Galileo probe has given scientists a somewhat clearer understanding. For instance, no magnetic field was detected originating from Io - its only significant activity is induced second-hand from passage through the Jovian magnetosphere. This means that although the core is suspected of being liquid iron and/or iron-sulfide, and is very large for a moon, it does not appear to convect. However, models differ on the properties of the silicate mantle and at what depth the tidal heating occurs most strongly.
In one model, the mantle is fluid and is the main origin of tidal heating, transmitting heat to the crust through convection throughout the layer. Under a different model, the bulk of the mantle does not convect, but is tidally heated in the layer just beneath the crust into an asthenosphere - a transitional region between magma and solid rock, where the rock is "gooey" - that transmits heat both upward and downward through conduction rather than convection. The first model proposes that tidal heating occurs most strongly at depth, and the second that it occurs in a shallow shell beneath the crust. As far as I've been able to find, the consensus seems to lean toward the second case. A cutaway diagram showing both models - the first is on the left side of the globe, the second on the right:
Unlike Earth, the moon's internal heat overwhelmingly results from tidal effects, with the radioactive decay of heavy elements in the core being a relatively small minority contributor. Without this tug-of-war being constantly waged in its interior, it would by now have largely frozen solid and its surface would be more like those of its fellow Galilean moons, but as it stands the surface has more in common with Venus than with anything in the Jovian system.
Lava flows on Io can be either silicate or sulfurous, depending on how deep they originate, since the surface is dominated by sulfur compounds (hence the greenish-yellow coloration) while the underlying crust is silicate. Volcanoes can sometimes explode, but most often volcanic plumes are simply the result of a lava flow encountering SO2 (sulfur dioxide) ices, causing energetic outgassing that scatters solid grains of sulfur compounds over a wide area. Eventually the SO2 itself settles back to the surface and refreezes as frost deposits. However, the height and steepness of Ionian mountains require that the bulk of underlying structure be composed of silicate rock - sulfur compounds just aren't strong enough - so the sulfurous part of the surface is a relatively thin layer. Illustration:
5. Surface Features
The surface of Io is remade by lava so often that not one impact crater has yet been identified - a remarkable fact given that it's larger than the Moon and deep within the solar system's second most powerful gravity well. Not only must impacts on Io be far more frequent than on the Moon due to Jupiter's gravity, but would occur at much higher speeds, so major impacts must be truly spectacular - and yet their imprint is nowhere to be found. Instead, the surface is entirely composed of overlapping, plate-like lava plains pockmarked by huge volcanic fissures with some of the highest mountains in the solar system towering above.
Io's mountains are generally the result of tectonic uplift, not volcanism, but they achieve dizzying heights due to the intense geological forces and low gravity - the highest, Boösaule Montes, is over 17 km (10 miles) tall. Only Olympus mons on Mars, the Rheasilvia central peak on Vesta, and the equatorial ridge on Iapetus are taller. Some of the more interesting images of mountains - mouse over to see their names:
Although the taller mountains are not directly built by volcanism, they are often associated with volcanic features: In some of the images above, the dark depressions near the bases of mountains are actually volcanic craters known as paterae, and they can often be filled with molten lava. This is different from a volcanic caldera in that the material emerging from a patera does not directly build the tall mountains associated with it, but just pushes them outward as in normal tectonic mountain-building. Mountains that are built from volcanoes tend to be low (less than 2 km) due to the generally low viscosity of Ionian lava. Paterae form precisely because of this property - the lava is often not under enough pressure to actually burst out of the crust, but it can rise to just beneath and then weaken the crust through outgassing until the material above collapses. A model of patera formation:
Various paterae - mouse over to see their names:
The International Astronomical Union prescribes the following nomenclature for Ionian features: Sun gods, thunder gods, fire gods, volcano gods, mythical blacksmiths, and epic heroes associated with these themes. Regions (regio/regiones) are named after areas around the Black Sea and Anatolia. Normal terms for celestial features used elsewhere - e.g., planum/plana for high plains, mons/montes for mountains - also apply here, but the most significant features are the paterae and lava flows (fluctus). A high-level map of Io's regions, followed by more detailed maps:
Another notable feature of Io is the brown coloration of the polar regions, which is actually a result of Jupiter's magnetosphere being so intense that where the field intersects Io, sulfur compounds are chemically altered by the radiation and turn brown. The lethality of the Ionian environment is on plain display in this fact. Zoomed in from the full globe true-color image at the very top, you can see how badly irradiated the polar regions are - also note that some bright linear features that appear hazy or dusty are active lava flows. North Pole:
And since the maps above aren't very visually detailed, it's worthwhile to zoom in to see the mid-latitudes and equatorial regions of the globe shot:
It should be noted that the colors of Io somewhat fluctuate over time as volcanic activity produces new orange material, scatters greenish-yellow material to cover places that were previously other colors, and the contours of the brown polar regions shift when new surface is created. However, color differences in imagery to date are largely the result of filtered light or photo processing manipulation rather than geological changes.
Io has a negligible atmosphere of SO2, SO, sodium, potassium, chlorine, and both monatomic sulfur and oxygen. These gases are released through volcanic activity and don't stick around very long - most condense back to the surface, while some proportion escapes into surrounding space - but they're replenished frequently enough that particle bombardment due to Jupiter's magnetic field produces visible aurorae. Since Io has no magnetic field of its own, the brightest aurorae occur at the equator where the Jovian field lines pass at a tangent to the surface, going through the greatest amount of atmosphere. Green light shows sodium, red oxygen, blue from sulfur dioxide:
The cloud of sodium that surrounds the moon, seen through a green-yellow filter:
IV. Plasma Torus & Flux Tube
When the gases surrounding Io become ionized by radiation from the Jovian magnetic field, they get swept up by the field and are pushed ahead of Io by the rapid rotation of the magnetosphere (it rotates with Jupiter, every 10 hours or so). This creates a donut-shaped region of ionized gases intersecting the moon's orbit called the Io Plasma Torus, although because it follows the magnetic field equator, the torus does not exactly follow Io's orbit. Because ions in the torus are dragged along at much faster than orbital speed, the moment one becomes neutralized again and is no longer bound to the field, it slingshots out of the torus. But the region never runs out of material, since Io keeps belching it out. A diagram:
You can actually see the plasma torus surrounding Jupiter at certain wavelengths, and the neutralized atoms shooting away from it:
The movement of the plasma torus against Io generates an electrical current along the field lines intersecting it, which creates a "pipeline" of confined ions moving from Io to the Jovian magnetic poles - an even more intense region called the Io Flux Tube that generates upward of 2,000 gigawatts of power. The flux tube is what scars the polar regions of Io, ionizing and stripping the top surface layers of material, sending the particles to the polar regions of Jupiter, and triggering intense aurorae and lightning discharges in the Jovian atmosphere.
On average, the radiation environment on and around Io gives a dose of about 36 sieverts per Earth day: Equivalent to taking about 720,000 dental X-rays a day1, or about 8.3 per second. It's also roughly equivalent to 1,200 full-body CT scans a day, or one full-body scan every 1.2 minutes. The worst doses received due to the Chernobyl nuclear disaster were 20 sieverts, and in a little less than half an hour an unshielded human on Io would receive more than the highest dose received in the Fukushima disaster. The yearly background radiation on Earth would be received every 5.8 seconds on Io. A 100% certain lethal dose would occur within 7 hours, and death would probably occur within 48-72 hours due to the dosage continuing to increase. Below is a chart comparing unshielded radiation exposures, which I will add to with each new Galilean moon in this series:
V. Past Relevance to Humanity
The discovery of Io, Europa, Ganymede, and Callisto in 1610 by Galileo Galilei was the first time human astronomers had seen one celestial body orbiting another, let alone several comprising an orbital system. Since it proved conclusively that not everything directly orbits the Earth as then assumed, the door was opened to arguing persuasively that not everything orbits the Earth at all. It was a far simpler and more mathematically justifiable conclusion that the planets, including Earth, orbit the Sun rather than have moons orbiting Jupiter and Jupiter orbiting Earth. This poured fueled on the Copernican revolution, and proved the death knell of Earth-centered models of the solar system despite futile attempts by religious authorities to maintain them on the basis of Bible verses. Below are Galileo's notes on the discovery appended to a draft of a letter to the Doge of Venice extolling the virtues of telescopes, with an English translation to the right:
However, Galileo was not responsible for naming the moons he had discovered: He referred to them in systemic terms, as Jupiter I, Jupiter II, Jupiter III, and Jupiter IV. Simon Marius - who also claimed to have discovered the moons days before Galileo - came up with the theme of naming them after the lovers of Zeus/Jupiter, although these names didn't catch on until centuries later when improved telescope technology allowed them to be resolved as actual globes rather than just points of light.
The Galilean moons played significant roles in evolving a number of astronomical theories and technical practices in the following centuries, but its most profound contribution following the Copernican Revolution was inspiring Pierre-Simon Laplace to seek a theoretical explanation for the resonant orbits of Io, Europa, and Ganymede in the late 18th century. As a result of his work, integer resonances like those experienced by the moons - and which also played a major role in the evolution of the Main Belt asteroids (see Asteroids: Vol. 1) and the solar system in general - are often referred to as Laplace resonances.
VI. Modern Relevance to Humanity
By the beginning of space exploration in the 20th century, astronomers had a pretty good understanding of Io's broad properties - mass, size, density, albedo, some large-scale features like the polar regions being darker than the lower latitudes, etc. - and had vaguely deduced that its surface was rich in sulfur and virtually devoid of water. So there was some anticipation that the moon could be volcanic, especially given its orbital properties, but the data from early robotic exploration blew the experts away: Scientists were shocked and delighted to find a world totally different from the other Jovian moons, covered in lava plains, volcanic craters, and gargantuan mountains.
Io proved to be a bonanza for planetary scientists, geologists, and volcanologists, presenting humanity with an extreme case of a "fire world" where large-scale geological activity that is infrequent on Earth and virtually nonexistent elsewhere in the solar system can be routinely observed. Physicists studying planetary magnetic fields were also stunned by the strength and complexity of the electrical and particle environment around Io due to the Jovian magnetosphere, leading to much better radiation shielding on future spacecraft sent to the system, as well as flight plans that carefully avoid spending too much time in the vicinity of Io.
Due to its association with both the awesome cloudscapes of Jupiter and the potential existence of life on Europa, Io is often mentioned in passing in science fiction across various mediums, but its incredible hostility means that it's rarely the immediate scene of action. The most prominent role it has ever played in popular media was as the setting of a mining colony in the 1981 sci-fi cop thriller Outland starring Sean Connery. Although passably entertaining as a cop movie, it doesn't offer much insight into Io beyond the fact that its surface is in vacuum. Theatrical trailer:
VII. Future Relevance to Humanity
In Jupiter: Vol. 2, I laid out a plausible multi-century timeline for human colonization of the Jovian system focused on Callisto due to its being the least hostile and most accessible of the Galilean moons. So if we can build on that hypothetical foundation, I would say that even when large numbers of humans live in the system centuries from now, Io would probably remain a dangerous and expensive frontier with few attractions except for scientists and wealthy adventure tourists. Inhabitants of Callisto and/or Ganymede might see it as little more than a distant toxic wasteland to be used as a (simulated) setting in horror and action entertainment.
Io's relative inaccessibility is not only because of the radiation environment and the added expense of protecting against it, but also the moon's profound depth in Jupiter's gravity well - something that would be costly in terms of fuel or time, depending on which is more abundant. In turn, there would be steep consequences for the amount of mass that could be taken to and from Io at a given price - which is not helped by the level of heavy shielding that would be necessary. Granted, it will be vastly cheaper and faster than anything we do in space today due to rockets being reusable and utilizing advanced propulsion technologies, but it's the relative economics of things that determine the shape of human migrations.
Once freed to roam the shallow ends of the solar system's gravity wells, it would be economically disadvantageous to sink resources into deeper parts under most circumstances. In other words, it would make more economic sense for a Callisto colony to explore and settle Jupiter's Trojan asteroid swarms or even Main Belt objects than expand inward to Io, because the ongoing costs of going back and forth would be far cheaper. And there is also the small matter of Io being profoundly unstable, although there are likely places where the crust is thick enough to be safe for a sustained presence.
But despite all these reasons why Io would not be settled, gravity has a funny way of accumulating things (:D), so the relative economics merely speaks to rates of growth: I.e., I would expect gravitationally shallower regions of the Jovian system and its solar-orbit environs to be developed more quickly than those deeper in the well. Think of it this way: For centuries of Jovian system civilization, there would only be niche attractions to Io - as mentioned, scientific research and adventure tourism. And unless humans in that part of the solar system or elsewhere develop an insatiable taste for sulfur, mining isn't likely to ever be worthwhile. But eventually someone with the resources to try would choose to live there, for any of the countless reasons people migrate to wild and dangerous places: Religious or political separatism, adventurousness, stupidity, escape from criminal justice, work as a tourism employee, etc.
My bet is that it would be people from outside the Jovian system that set up shop on Io, since it's usually the case that people are much more skeptical of wild places the closer they live to them: To cite one example, it was mainly East Coast city-dwellers who migrated West in the 19th century - those who actually lived on the borders of the West were less likely to go any further, because their awareness of how bleak it was in the present clouded their ability to see the Easterners' (often unreasonable) fantasies of what it could be in the future. Of course, borderlands often benefit economically from the passage of people and materials headed to more distant points, so I could imagine a plausible scenario where the denizens of Callisto, Ganymede, or the Trojans sell a fantasy of Io to out-system rubes, along with whatever equipment and supplies would be involved.
Once anyone at all lives there, it would be just as costly to leave as to arrive, and people are very reluctant to admit they're capable of utter stupidity on the level of major life decisions - both pride and lack of resources have played major roles in trapping people in frontiers they would just as soon abandon. Ironically, that fact leads people to invest themselves even more in making a place livable, softening its edges and making it more attractive to subsequent immigrants. Beneath some fundamental limit of crowding, the more people are in a given place, the more reasons there are to go there and add to their number, so colonization can occur through sheer random accumulation of inertia even if there is no overriding economic justification.
Io's depth in the Jovian gravity field means that whatever economic resources end up being invested there, would most likely stay there and contribute to a feedback process rather than exporting a lot of things: Usually a recipe for a strong (though inward-looking) culture that bears little attraction to predatory military powers. Ultimately - and we're talking millennia now - gravity wells could come to reflect socioeconomic status, with deeper regions being poorer but having stronger cultural identities; middle regions having the optimum benefits of social cohesion and mobility; and the outermost regions (e.g., distant, irregular moons and asteroids) being individually wealthy and highly diverse but achieving little civilizational gestalt.
At over 2 terawatts, the Io flux tube has a lot of promise as a source of energy for people living on its surface, although there's no way to know how it would compare economically to the fusion technologies that become available. On a purely intuitive level it seems likely that passively harvesting a constant, inexhaustible energy source that - unlike solar on Earth - is not subject to weather or day/night variations would be cheaper at scale than an almost-closed system like a fusion generator, where complex human technology has to do everything. Passive harvesting technologies merely set up metaphorical dominoes and let nature knock them down, while all known and theorized nuclear technologies involve maintaining delicate dynamic balances. Even before anyone lives on Io, people on the other moons might find a way to efficiently harness the energy of its flux tube for their purposes.
Returning closer to the present, a key factor in when it becomes possible for humans to visit Io is the amount of shielding that would be needed to make it safe. Most likely this shielding would consist of slabs of water ice harvested from an asteroid and, of course, wrapped in something to keep it from sublimating into space. I'm not sure if the shielding properties of ice are substantially different than those of liquid water, but since you can turn ice into water if necessary - although keeping it liquid could get expensive in energy terms - let's just assume it's the same. The halving thickness of water - the increment of thickness that halves the amount of radiation penetrating it - is 18 cm. So to reduce a 36 Sv per Earth day radiation environment to a safe level for long-term exposure - let's say 0.03 Sv (30 mSv) per year / 0.083 mSv per day - would require a water shielding thickness of about 3.4 meters (~ 11 feet).
Since we would be talking about a multi-year exploratory mission, the habitat and electronics systems would have to be extensive, and thus the total mass of shielding would be tremendous. Of course, thinner shielding could be used on the way to Jupiter, and then thickened with material from a nearby asteroid, so this much mass doesn't have to be taken all the way from the inner solar system. Still, even moving that much material around the Jovian system would be quite costly in fuel terms. Now, I had fudged together a prediction for the first human mission to the Jovian system between 2070 and 2130, with my baseline being 2100 (or 2101 to be symmetrical with Arthur C. Clarke and Stanley's Kubrick's prediction of 2001), but that is definitely not my prediction for humans landing on Io. I would put it sometime in the 23rd century at the earliest, and I doubt much would come of it at the time.
There is one limitation that is more or less absolute, barring some Star Trek shielding technology that doesn't involve any kind of mass: Humans will never walk on the Ionian surface in spacesuits. As is obvious from the shielding calculation above, no spacesuit can possibly be shielded enough to make EVAs survivable - excursions would have to be inside enormous roving vehicles weighed down with blocks of ice and metal slabs. Even if a suit were made of solid, jointed lead, it would have to be several inches thick just to be survivable, and then it would be pointless because a human couldn't move under their own power.
In the exotic, very distant future - thousands of years from now - Io could be the nexus of a systemwide power infrastructure that taps into the Jovian magnetic field, extracting far more power overall than the flux tube by itself could provide. However, as the timeline extends even further, it would likely - along with all other solid bodies in the solar system - be progressively stripped of material for construction of unimaginable megastructures whose purpose and function are as beyond our comprehension as modern telecommunications infrastructure would be to a Neolithic villager. Unlike Earth and maybe Mars, I doubt that Io would ever have the depth of history and scientific interest that could cause advanced civilizations to preserve it for research or sentiment.
VIII. Future of Io
Provided humans don't go extinct or strip-mine it, Io's natural course is likely very similar to its present one for the next few billion years - being flexed and heated by tides, spewing gas into nearby space, and probably not changing very much in terms of what it does (though, of course, its surface will change constantly). However, when the Sun expands and heats Jupiter, the Jovian atmosphere will bloat to a size significantly larger than its present diameter, and one possible outcome is that there would be enough gases entering the orbit of Io to gradually bleed away its orbital momentum. If that occurred, it would spiral inward and its material would merge with the Jovian inner core and share the fate of the planet.
If that doesn't happen, then as Jupiter loses mass from the dying Sun blowing away its atmosphere, the planet would be unable to hold on to the Galilean moons and they would migrate outward until escaping entirely. Then they would orbit the solar white dwarf at a great distance, as frigid objects like Pluto covered in organic ices, and perhaps end up wandering interstellar space as rogue planets. My own semi-laymen's intuition of physics tells me the first scenario is the most likely outcome for Io, but I would defer to any planetary scientist or astronomer who says otherwise.
IX. Catalog of Exploration
1. Past & current probes:
Pioneer 10 (USA - 1973 flyby)
Pioneer 11 (USA - 1974 flyby)
Voyager 1 (USA - 1979 flyby)
Voyager 2 (USA - 1979 flyby)
Galileo (USA - 1995 to 2003, Jupiter orbiter / flybys of Io)
Cassini-Huygens (USA and Europe - 2000 flyby)
New Horizons (USA - 2007 flyby)
2. Future probes:
Juno (USA - en route, scheduled Jupiter orbiter to reach system in 2016, will flyby Galilean moons)