The progress of our adventure so far (current in bold):
1. ?The SunIn Volume 2 of our visit to Jupiter, we explore the planet's chemistry, cloud formations, and powerful magnetic field, with a brief look at its satellite system (the major moons will be explored individually in subsequent diaries). ?We will also explore what significance Jupiter has held and may yet hold for mankind.
2. ?Mercury
3. ?Venus
4. ?Earth (Vol. 1)
5. ?Earth (Vol. 2)
6. ?Earth (Vol. 3)
7. ?Earth (Vol. 4)
8. ?Earth (Vol. 5)
9. ?Earth (Vol. 6)
10. ?Luna
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)
18. ?Ceres
19. ?Jupiter (Vol. 1)
20. ?Jupiter (Vol. 2)
21. ?Io
22. ?Europa
23. ?Ganymede
24. ?Callisto
25. ?Saturn
26. ?Mimas
27. ?Enceladus
28. ?Tethys, Dione, and Rhea
29. ?Titan
30. ?Iapetus
31. ?Rings & Minor Moons of Saturn
32. ?Uranus
33. ?Moons of Uranus
34. ?Neptune
35. ?Triton
36. ?The Kuiper Belt & Scattered Disk
37. ?Comets
38. ?The Interstellar Neighborhood
4. ?Composition
Excluding the solid core, whose size and composition are as yet poorly understood, Jupiter is slightly less than 90% H2 by molecular abundance, 10% He, and very small decimal percentages of NH3 (ammonia), hydrogen deuteride (a diatomic molecule with one hydrogen and one deuterium atom), C2H6 (ethane), H2O, and NH4SH (ammonium hydrosulfide). ?These numbers differ considerably by mass, with helium contributing roughly a quarter of the mass of the planet. ?
Given its size and gravity, every element present in the primordial cloud out of which the planets formed is undoubtedly represented somewhere in Jupiter, and abundances appear to reflect their original proportions in the early solar system. ?However, the most interesting substances are the relatively complicated molecules that may be present in the cloud layer in small abundances - for instance, C6H6 (benzene), PH3 (phosphine), and various other phosphor, sulfuric, and hydrocarbon compounds, some of which are suspected in creating the diverse colors of the clouds when they well up from below and are exposed to sunlight. ?The much blander, more uniform atmospheres of the other gas giants are thought to result from having simpler cloud chemistries than Jupiter. ?
There has been speculation in fiction (e.g., Arthur C. Clarke's 2010: Odyssey Two) that biochemistry could occur in the Jovian clouds, but scientists consider the possibility remote. ?Still, the universe is a very strange place, and even on Earth we continue to find life where we had never thought to find it. ?The cloud layer of Jupiter receives heat from beneath as well as above, temperatures and pressures are amenable (see the temperature / pressure / altitude diagram in Volume 1), and water vapor is present, so there is no way to rule it out at this point. ?However, if biochemistry were occurring, it would be microscopic - the complex animal life envisioned by Clarke would be just this side of impossible due to the diffusiveness of atmospheric compounds.
5. ?Cloud Formations
As described in Volume 1, because Jupiter is a gas ball with a rapid rate of rotation, different latitudes complete a day at different rates, and this tends to shear cloud structures apart into bands that completely circle the planet. ?Because bands move at different speeds, they appear to flow in opposite directions although they are all rotating with the entire planet in the same direction. ?As a result, the boundary regions between bands tend to be violent and turbulent. ?Closer to polar latitudes, banding becomes undifferentiated, so the polar cloud regions are wide and relatively uniform.
Bands alternate in brightness between light and dark, with light bands called zones corresponding to upwelling air becoming cooler and forming ice crystals while dark bands called belts sink and become hotter. ?Rapid linear wind flows called jets occur at the inner boundaries of zones, bending zonal cloud formations into chevron shapes. The three-dimensional motions of these air currents are likely very complex, and are poorly understood at this point: Researchers do not yet know whether cloud movements reflect deeper patterns in lower regions of the planet, or if they emerge only at a given altitude on top of an otherwise stable environment beneath. ?Below are maps of Jupiter's cloud bands, the first showing a comprehensive illustration, and the second a photograph with some of the bands labeled to get a sense of the reality:
A closeup of the Equatorial Zone showing how the rapid jet winds that bound it bend the clouds inside into chevron formations:
Zonal jets also drive the swirling formations seen in adjacent belts:
The relative sizes of the bands are consistent because they arise from the planet's rotation, but their color and internal structure do fluctuate somewhat over time. ?Below is a time-lapse video of Jupiter's clouds in motion from the Voyager 1 probe in 1979, taken over 32 Jovian days (about 13 Earth days). ?Because the frame of reference is stationary, cloud bands that are moving around the planet in the same direction at different rates appear to be moving in opposite directions. ?I recommend going full screen and adjusting the resolution to 1080p (the maximum available) - it is quite amazing:
There hasn't been much photography done of the polar regions except at high angles, since none of the probes sent to Jupiter thus far have been built to withstand the powerful radiation environment near the poles. ?However, the Juno spacecraft currently en route will be inserted into a polar orbit - it has to be, because the probe is solar-powered and must remain in perpetual sunlight. ?Juno will take detailed imagery of the North and South Polar regions, as will be able to see every other latitude at every possible angle along its orbits. ?Despite the dearth of direct imagery, NASA had used existing photos from a flyby of the Cassini spacecraft on its way to Saturn to make stereographic projection images of the North and South Polar regions. ?Only the poles themselves are missing, since they are out of view from the angled perspective:
If you look closely, you see the cloud structures of the polar region are much more intricate than within the bands, which happens because the shear forces of the planet's rotation are much lower at polar latitudes. ?Clouds are thus better able to maintain coherent structures without being constantly torn apart or blasted by ultra-high winds. ?So we see a larger number of smaller vortices near the poles, as opposed to the relative handful of gigantic ones at lower latitudes with enough energy to resist the forces of the bands. ?Here are closeups of the same images to give a better sense of the difference:
Note that the "irising" effect near to each pole is due to the projection having increasingly little visual data to work with as the angle becomes close to parallel - what the actual pole would look like is probably closer in character to the region surrounding the "iris." ?And, of course, it's impossible to directly see an entire polar region in full sunlight, since the planet has very little axial tilt - about half of it is always in shadow, and the illuminated half is dim due to the incident angle of sunlight. ?For both of these reasons, as well as much lower zonal wind speed (motion along latitude) than at the equator, the air currents are more languid and able to form and maintain more delicate cloud structures as well as smaller, lower-energy vortices. ?
Theoretical models of the Jovian clouds fall under two very different frameworks: One, the Shallow Model, holds that the cloud systems are just surface manifestations with little or none of their motions determined by deep currents. ?As the cloud deck is only 50 km thick, there is no clear way to know at this time how deep their observed movements go. ?Under Shallow Model theories, the atmosphere is more or less a free-floating envelope that just does its own thing on top of the supercritical fluid mantle below (see Volume 1 for a description of supercritical fluids). ?An imperfect analogy would be how the Earth's atmospheric motions relate to ocean currents: The thermal relationship is highly relevant, but the actual motion is not - oceanic currents do not directly shape air currents. ?
The other contender is the Deep Model, which is a set of theories based on proposing deep origins for the cloud-layer manifestations we see. ?In this framework, the interior of the planet rotates at different speeds along concentric cylinders parallel to the rotational axis, and the bands correspond to the intersection of these cylindrical regions with the cloud layer. ?In other words, band boundaries would go all the way through the supercritical mantle along the planetary axis, and corresponding bands in the Northern and Southern hemispheres are just opposite sides of the same cylindrical region. ?An illustration:
Jovian storms usually occur as pale oval vortices that interrupt the normal flow of cloud bands and force zonal winds to detour around them. ?They vary greatly in size, both between storms and in the same storm over time, and there are several hundred of them active at any given time. ?Those which are low-pressure systems rotate in the direction indicated by the Coriolis effect - counterclockwise in the Northern hemisphere, clockwise in the Southern hemisphere (rotation called cyclonic, just as in terrestrial meteorology) - while the case is vice-versa for high-pressure systems (anticyclonic rotation). ?
Anticyclones always occur in zones, usually appear bright white, and are confined to a given latitude by the zonal jets that pass around them to the North and South, but they can advance or retreat along their latitude relative to other systems. ?Cyclones, however, occur in belts, are dark and dim in color, less cohesively shaped than anticyclones, and generally much smaller. ?If you look back to the closeups of the polar regions, you can see a number of examples for both types of vortex. ?Some additional cases:
When an anticyclone becomes very powerful, it may dredge up molecules from much deeper layers of the atmosphere and turn reddish, although the coloration is usually temporary. ?Scientists are unsure exactly what compounds are responsible for storms turning red, but there is definitely something more involved than the ammonia ice crystals that make most anticyclones white. ?Such storms necessarily only occur at low latitudes, since they need a lot of energy to form in the first place, let alone persist. ?The iconic Great Red Spot (GRS) is the largest and most famous example of a red storm, and is so prominent that it's visible from Earth with a decent commercial telescope.
Staring outward like the Eye of Sauron, the GRS has been a constant fixture and a symbol of the alien power of Jupiter since telescopic observations began centuries ago. ?Although it could swallow three entire Earths today, it's only a fraction of the size observed in previous eras, and continues to shrink as time goes on - though it isn't known how long it will survive. ?Its color also fluctuates from time to time, going from deep rust-color to pale pink and back again, most likely as whatever material causes the red coloration is replenished by new upward currents. ?
Scientists don't have a very good idea of how it formed or what continues to power it, with some arguing that it has more in common with a volcano than a hurricane - caused by upwellings from the deepest parts of the atmosphere or even from the supercritical mantle - while others think it is just a very energetic manifestation of otherwise shallow phenomena. ?It does appear to reflect deeper processes than other storms, but how deep is not known. ?Contrast- and brightness-adjusted images of the GRS, showing interior and environmental details:
Closeups from these images reveal a startling level of detail, in some cases so clearly that you can actually get a sense of the three-dimensional contours of the clouds in some areas:
However deep its roots go, the topmost clouds of the GRS are a full 8 km higher than the surrounding cloud deck. ?Wind speeds along its edge are also very fast and variable, ranging from 400 to over 640 km/h (about 250 to 400 mph) - as one would expect given that the region is driven by diverted zonal jets. ?However, the interior is relatively slow and stagnant - relatively, mind you - with not a lot of apparent movement across the storm boundary. ?I would be curious to know how fast air moves vertically in the GRS compared to its cyclical motion, but I suppose that will have to wait for Juno, if not even later probes. ?
Another feature is occasionally seen when openings occur in the cloud deck to lower atmospheric layers, which typically present as dark areas. ?Although they're commonplace, there are not many high-resolution visible-light images taken of the phenomenon, since its interest (as a conduit to lower, hotter layers) is mainly in infrared. ?However, there are a few, such as this one taken by Voyager:
6. ?Magnetosphere
The magnetic field of Jupiter is the second largest and second most powerful in the solar system after the Sun's, and is more powerful than Earth's by an order of magnitude. ?As with other planetary magnetic fields, its interaction with the solar wind causes it to elongate away from the Sun, but the extent of elongation is remarkable in Jupiter's case: The field is so large and powerful that its longer dimension extends most of the way to Saturn's orbit. ?Moreover, it produces radiation belts in certain regions of space near the planet vastly more intense than Earth's Van Allen Belts - a torrent of lethally energetic particles with major implications for humanity's future in the Jovian system. ?As a result, the Jovian system contains the most hazardous known places in the solar system outside the Sun.
As discussed in Volume 1, the field is generated by convection in the metallic hydrogen / helium outer core, which dominates the planet's volume - i.e., most of the gargantuan bulk of Jupiter consists of its dynamo, so the enormity of its magnetic field is a natural consequence. ?In general, magnetospheres capture particles from the solar wind as well as cosmic rays (which are actually particles, not rays) from interstellar and intergalactic space, diverting them along field lines toward the poles of a planet. ?But the more intense the field, the more of these particles are captured and concentrated into confined regions of space. ?We can thus think of Jupiter's radiation belts as the focus of a huge radiation "magnifying glass."
Despite its massive size, the most significant effects of the field are local, particularly as it concerns the Jovian moons. ?The most intense belts are relatively close to the planet and are flooded with ionizing radiation, so the innermost Galilean moon Io has the misfortune of orbiting directly within one. ?As a result, the material it continually ejects into space due to volcanism is ionized and creates a donut-shaped region of charged particles centered on its orbit called the Io plasma torus. ?And yet as hostile as the torus is, it's not even the most dangerous place in the field - that would be along the field line intersecting Io itself, where electrical interactions with the moon create an even more concentrated particle "pipeline" to the Jovian poles. ?This is called the Io flux tube, and (to my understanding) passing through it unshielded would fry electronics and kill humans instantly.
The next innermost Galilean moon, Europa, also passes through an intense (though lesser) region of the field and generates its own inferior plasma torus and flux tube. ?But the power of the Europan versions is far beneath that of Io's, and is thus much more survivable given thick shielding. ?Still, it is very hostile, suggesting that the other two Galilean moons, Ganymede and Callisto, have far better prospects for a future human presence.
The dynamics of the electrical and particle interactions between Jupiter's magnetic field and its moons are pretty complex, even with the limited data we possess today - more so than I actually care to unravel, which is fortunate since it's likely more technical than most of the people reading this would care to know. ?It involves a lot of the vagaries of electrical currents and charged particle flows in rotating reference frames, but feel free to read up on it here if you wish. ?Suffice it to say that as complicated as the study of Earth's magnetic field and reactions to solar weather, developing a practical and comprehensive understanding of Jupiter's electromagnetic environment would be the apotheosis of planetary space weather studies.
It is worth saying, however, that the field has some visually interesting consequences for the polar upper-atmosphere of Jupiter - namely, it produces huge aurorae in patterns that actually display the direct influence of the Galilean moons on the field. ?I don't know what kind of spectrum the following image was taken in, but it shows a Jovian aurora borealis with distinct spots indicating the intersection of field lines that pass through moons - in particular, the bright spot to the left with the tail trailing off it corresponds to where the Io flux tube enters the Jovian atmosphere:
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IV. ?Satellites and Rings
Jupiter has dozens of known moons, but only the four largest - Io, Europa, Ganymede, and Callisto - are larger than asteroids, with the next largest (Amalthea) being 1/12 the size of Europa. ?Only 8 of the 66 moons identified thus far orbit close to Jupiter's rotational plane - the so-called regular satellites, which occupy the inner region of the Jovian system - with the rest orbiting more distantly, at random inclinations characteristic of objects captured from elsewhere in the solar system (the irregular satellites). ?
Among the regular satellites, the inner group are four asteroidal objects - Metis, Adrastea, Amalthea, and Thebe (pronounced "theebee"). ?The inner group is responsible for contributing material to Jupiter's faint ring system. ?Amalthea is the most interesting of these objects, as it has a remarkably intense red coloration. ?A black and white size comparison followed by a few dim, blurry color photos of Amalthea:
Thebe is also somewhat interesting in that it has a rather large crater dominating one face:
Amalthea and Thebe both contribute material to eponymous "gossamer" ring systems, as well as to the inner "halo" ring and the main ring that is the most substantial. ?However, these rings are all completely trivial compared to those of Saturn, and even the main ring is visible only from certain lighting angles. ?An orbital diagram of the inner group and ring systems - note that Metis and Adrastea orbit within a thousand kilometers of each other, so the diagram makes them appear co-orbital, but they are in fact separate:
Images of Jupiter's rings:
The next four regular moons, the Galilean group, have already been mentioned - Io, Europa, Ganymede, and Callisto: They are large, spherical bodies that would be considered planets if they orbited the Sun independently. ?Io, Ganymede, and Callisto are all larger than Earth's Moon, and Ganymede is 78% the size of Mars. ?Io and Europa are remarkable for two opposite reasons: Io is the most volcanically active rocky body in the solar system, and Europa is thought to have vast liquid water oceans beneath its global ice crust. ?Ganymede and Callisto also have thick ice shells, and may have a much smaller amount of liquid water beneath, but they're less likely to have oceans because there is less internal heating from tidal forces since they are farther away from Jupiter. ?I will address each of the Galilean moons in detail in subsequent entries in the series. ?An orbital diagram, followed by a true-color size comparison:
The remainder of Jupiter's satellites are irregular and orbit in a much larger, extended sphere of space - we have no decent images of them. ?Orbital diagram showing a few of the more significant objects - as you can see, it's a bloody mess:
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V. ?Past Relevance to Humanity
As one of the brightest objects in Earth's sky, Jupiter was bound to have mythological significance to the ancients. ?The name itself is a corruption of Greek words from the same root as deus pater - "father sky god" - and the modern word "jovial" comes from the ancient belief that happiness arose from the positive mystical influence of Jupiter. ?Still, the ancients had no notion of what they were observing - they assumed the planets were stars that simply moved differently from others, and in religious terms believed they signaled the moods and intentions of the gods. ?More philosophical minds were less mystical in their interpretations, but remained fixated on abstract notions with little empirical basis.
That only started to change in the early 17th century, when Galileo Galilei first applied early telescope technology to astronomical observation. ?Upon discovering the four moons that now collectively bear his name, Galileo brought the first clear evidence that Jupiter was a system unto itself with observable physical properties - the first ever observed celestial motions not apparently centered on Earth. ?This observation, and the relative ease of confirming it, poured fuel on the Copernican revolution that theorized the Sun at the center of the solar system. ?By the end of the 17th century, large-scale motions and structures of the Jovian atmosphere were already being observed. ?It was not, however, until the modern era that we had a basic understanding of its nature as a gas giant.
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VI. ?Modern Relevance to Humanity
Prior to robotic exploration of Jupiter, scientists knew from ground-based observations that Jupiter had a banded cloud structure and a large red storm system in the Southern hemisphere, but they were completely floored by the complexity and detailed fluid movement of the clouds shown by the Pioneer and Voyager flybys. ?Below is a progression of representative photographs showing the evolution of our view of Jupiter from ground-based telescopes to high-quality probe imagery, which occurred in less than a decade from Pioneer 10in 1973 to the Voyager flybys in 1979:
The deep Jovian gravity well has been very useful for gravitational assist flybys of probes headed to more distant places in the solar system. ?Such flybys work because there is a specific trajectory where, if hit just right, the probe leaves the gravity well going much faster than it entered - essentially tapping a planet's gravity and turning it into kinetic energy. ?The more powerful the gravitational field, the greater the potential change in speed.
When mass (i.e., fuel) is constrained and mission duration is not, gravitational assist flybys are deemed preferable to the quicker but more energy-intensive Hohmann transfer orbits used to reach other bodies, and have become favored in robotic exploration. ?Voyager 1, Cassini, and the New Horizons probe currently headed toward Pluto all used gravity assists from Jupiter, saving quite a lot of mass for instruments that would otherwise have had to be fuel. ?An animation of the New Horizons flyby:
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VII. ?Future Relevance to Humanity
Humanity is not yet capable of personally visiting the Jovian system, and it would likely be an ordeal worthy of Odysseus even when transportation between the Earth, Moon, and Mars becomes routine. ?However, it can theoretically be done with evolved, scaled versions of technology currently under development - particularly continuous-thrust engines such as VASIMR that allow a rocket to accelerate over the entire course of a trip. ?Such an engine would require megawatt-scale nuclear power to achieve a reasonable transit time (less than 1 year), considerable radiation shielding, the habitat would have to spin for gravity to keep the astronauts healthy, and there would have to be many advances in environmental systems to avoid needing constant resupply, but there are no fundamental obstacles. ?Moreover, the Jovian system has plenty of ice for producing air, water, and auxiliary fuel, so that's mass the spacecraft would not have to carry with it.
Given these challenges, and the counterintuitive possibility that regular human travel around the inner solar system will make us less interested in more distant regions, I think it would be neither too optimistic nor pessimistic to imagine initial human scouting of the Jovian system around a century later than Arthur C. Clarke and Stanley Kubrick predicted - 2101. ?On the optimistic side, however, I see a reasonably plausible case for it happening a few decades earlier starting from the 2070s. ?I can also easily imagine the complexities found to be involved in long-duration human spaceflight multiplying until the closest part of the outer solar system only comes within range decades later, in the 2120s to 2140s. ?However, the technical fundamentals seem close enough to what is already being done that I don't imagine the range extending much farther into the future. ?Once human space travel is occurring in large volume in the inner solar system, extending the distance and time is just a matter of incremental scaling.
As for exploitation and settlement, the timeline is considerably longer - likely several centuries before human activity in the Jovian system becomes regular and significant, and not just due to the technical requirements: The main economic reason to set up operations in the region other than scientific research - i.e., the huge abundance of ice on the Galilean moons - applies equally well to the Main Belt asteroids, which are easier to reach, far easier to leave again due to Jupiter's overwhelming gravitational field, and don't have the radiation hazards of operating near Jupiter. ?So that fact would tend to dampen the economic motives for going there.
But what the Jovian system does have are worlds - four of them, three humanly accessible, two with relatively benign radiation environments (relative to the other two), and all with likely breathtaking surface scenery. ?Just try to imagine Jupiter rising from one of these moons, dozens of times the size of Luna from Earth, with other moons clearly visible as well. ?These are worlds with substantial natural gravity fields, vast surface areas, inexhaustible supplies of ice, and at least the theoretical possibility of harnessing both solar power and Jupiter's radiation for some fraction of energy needs. ?They also have the benefit of being near Jupiter itself, which means that everything about the planet that could become attractive in the future would also benefit the prospects of colonizing at least one of the Galilean moons.
On balance, the most likely Galilean moon to be colonized - if it becomes a zero-sum question - would have to be Callisto: ?Its radiation environment is by far the least hostile, it occupies a much shallower region of Jupiter's gravity well (i.e., it's easier to both reach and leave), and as far as we know it's geologically stable. ?So the best place for humans to be in the Jovian system is not necessarily the most scientifically interesting, but the appeal of the more fascinating moons Io and Europa would only add to the attraction of settling Ganymede and Callisto as the base of operations. ?I think the fact that Callisto is the most distant from Jupiter of the four would make it an attractive waystation for missions bound further out, as well as an embarkation/disembarkation point for transit to and from the inner solar system. ?
Economically speaking, it's very likely more efficient to have interplanetary spacecraft stay in the shallow ends of gravity wells and have people move within systems only with smaller, more specialized craft. ?That isn't to say that Ganymede and Europa wouldn't also be colonized, but whatever human establishment occurs there would be further down the road and itself somewhat benefit the growth of whatever is established on Callisto. ?Now, the picture changes drastically if life were discovered in the subsurface Europan oceans - although that would as likely put the moon off limits to settlement as to further motivate it. ?Ultimately it would become like a nature preserve, with lots of scientific installations but no cities. ?Still, the likelihood remains against Europan life, or at least against our discovering it: The ice shell is thick, the seabed where volcanic vents might power life processes are likely far below, and if there is life there is a much greater likelihood of it being microbial than animal given the environmental energy constraints.
But this only addresses the vicinity of Jupiter - the planet itself also holds some potential attractions. ?For instance, helium is exceedingly rare in the inner solar system - because it doesn't form compounds, none of it is stored in any abundance the way that hydrogen is stored in water and hydrocarbon compounds on Earth, so all we've got are the miniscule amounts trapped in bubbles within rocks since the primordial formation of the solar system. ?And once it gets released into the atmosphere, it's gone - the environments of the inner planets are too hot, and their gravity too weak to keep helium bound, so it just blows away into space. ?If there is ever some compelling economic demand for helium beyond the niche uses to which it's applied today - e.g., if fusion power comes to rely on it - then mining gas giants could become worthwhile.
We've all seen Lando Calrissian's gas mining city in the Empire Strikes Back, so the concept is at least vaguely present in popular science fiction already, but there is real scientific potential behind it. ?The concept goes like this: Because the pressure of a gas giant atmosphere - and especially Jupiter's - increases drastically with depth, it is possible to have an arbitrarily large installation floating around the clouds, so long as it's enclosed and kept at low enough pressure (Lando's wide-open place would be impossible - it would have to be air-tight). ?
But since the atmosphere is hydrogen and helium though, and human-breathable atmosphere is denser at comparable pressures, there would have be a huge hydrogen and/or helium balloon involved and the gases inside would have to be hotter than the ambient temperature - and then even so, the exact location where such a thing could exist stably is not necessarily somewhere practical. ?In Jupiter's particular case, that region of the atmosphere might be at depths and temperatures that would be impractical to access and leave on a routine basis. ?However, the picture becomes much simpler if you just stipulate that it's an unmanned operation rather than imagining a city, since then it can be designed to operate in the ambient environment.
Buoyancy isn't even the biggest obstacle to direct human settlement of the Jovian clouds though: Those would be (1)the 2.5 g's of gravity one would experience in a floating Jupiter installation, and (2)the enormous difficulty of leaving the planet again once you're so deep into its gravity well (see the gravity well diagram in Volume 1). ?Now, 2.5 g's doesn't sound like a lot given that people easily withstand it for a split second on rollercoasters, but it's very different to survive in a constant field of that strength for any length of time. ?
Being in 2.5 g is not like walking around with 2.5 times the weight on your back in Earth gravity - it's not just your muscles and skeleton experiencing the more than doubled force, but every single tissue and organ in your body: The organs hanging internally, the heart that has to pump blood against a constant force 2.5 times greater, the blood vessels in lower extremities that have to resist blood pooling in them and becoming ruptured, your brain becoming flattened inside your skull under its own weight, your intestines having to deal with stools and fluids that now weigh 2.5 times as much, and so on. ?
So we can imagine that highly-trained astronauts with a lot of centrifuge time to build up their bodies' ability to cope might be able to personally explore the Jovian clouds in a balloon station for a few days, although it's questionable whether there would be any reason for it. ?Given that it would have to be built like a tank, the astronauts probably couldn't step outside, they wouldn't likely see anything through a window at most times, and they'd have to spend the whole time on their backs in specially-designed couches, it seems like there isn't a great deal of purpose to direct human exploration of Jupiter itself.
And even if the direct effects of hypergravity were less extreme than supposed, there is still the indirect consequence that it would be absurdly difficult to leave again: Just escaping from Earth is tremendously hard - escaping from Jupiter would require an entirely different league of power, robustness, and mass from a rocket than has ever even been imagined for use on Earth. ?The Saturn V rocket would be a toy in comparison to what would be needed to get even into low Jupiter orbit from the cloud layer, let alone make it all the way to a Galilean moon. ?That doesn't mean it will never happen - just that its happening will require developments so far removed from the present that I don't see any clear economic or developmental pathway to it even from a future where the Moon, Mars, and asteroids are routine venues of human life.
Nonetheless, I think Jupiter will oversee the birth of diverse, wealthy, and powerful civilizations on some of the Galilean moons, and the awesome image of a huge Jupiter cresting an alien horizon will be the cherished birthright of future generations who come to live in the Jovian system. ?How will people who grow up with such a vast context think of Earth and the inner planets? ?Small, parochial, tribalistic places where the environment is swamped with broiling heat and chaotic life? ?I would say it's pretty easy to imagine such societies seeing Earth as an Old World with charms and history, but seriously lacking in big ideas and possibilities. ?I think the tides of Jupiter will come to be encoded in their very DNA, and when they visit other parts of the solar system, they will yearn to see its flowing clouds and swirling storms in the black sky again.
More practically, living and operating in gas giant systems will give humanity important lessons about the diversity of possible habitable environments, and expand what kind of places people will able to settle when they finally make it around to reaching other stars. ?If I were to suppose a timeline - and I admit this is pure guesswork - I would say that human exploration of the Jovian system (i.e., transient missions) would continue throughout the 22nd century and into the 23rd, with frontier settlements taking root at some point in the 23rd and growing for the next few centuries. ?Call it 2500 when they really come into their own, whatever that ultimately means. ?The sheer density of human events and history that could occur in such a huge playing field as the Jupiter system is staggering to imagine. ?How many nations would come to exist? ?What great and tragic political and natural events would unfold? ?It makes the present seem so insignificant, and yet all the more important for enabling that evolution to a larger context.
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VIII. ?Future of Jupiter
Due to far greater internal temperatures in the past, Jupiter was originally twice its current size. ?It has been steadily shrinking over the 4.5 billion years since its formation at a rate of about 2 cm per Earth year, as interior heat is radiated away and the planet contracts. ?The same radiation-and-contraction process is happening to every planet in the solar system including Earth, albeit at very different rates with very different consequences, and the endpoint is when the planet approaches temperature equilibrium with the Sun-heated ambient environment. ?Since the Sun is also evolving, the timing for each world to reach this point would depend on where the two changing conditions intersect. ?In Jupiter's case, there is still quite a lot of contraction in its future before external heating from the expanding Sun balances and then reverses it.
Of course, there is the possibility that humanity in the distant future will mine or manipulate Jupiter in unforeseeable ways that radically alter its physical future, but absent such potential the process described will unfold until it once again expands due to solar heating. ?As the Sun grows, the Jovian atmosphere will become extended and tenuous, increasingly escaping the planet entirely, and all of it will be stripped away completely when the dying Sun starts throwing off mass. ?As the Sun becomes less massive, Jupiter would also begin migrating further away, until what remains is the exposed solid core distantly orbiting the solar white dwarf in frigid exile. ?From there it may leave completely and become a wandering rogue planet.
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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)
Ulysses (Europe - 1992 flyby)
Galileo (USA - 1995 to 2003, orbiter and descender)
Cassini-Huygens (USA and Europe - 2000 flyby)
New Horizons (USA - 2007 flyby)
2. ?Future probes:
Juno (USA - en route, scheduled orbiter to reach Jupiter 2016)
Source: http://www.dailykos.com/story/2012/09/09/1124028/-Getting-to-Know-Your-Solar-System-20-Jupiter-Vol-2
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