Regal and brutal, awesome and menacing, Jupiter reigns supreme among its fellow planets while ruling its own mini-solar system of subordinate worlds with an iron fist. It is a titanic ball of perpetually raging storms on an unimaginable scale, and by far the most exotic object in the solar system: Halfway between the rocky planets we can personally experience and the stars that illuminate our universe, a world made almost entirely of crushing atmosphere in violent motion. Looming in inconceivable might and strangeness, the King of the Gods dispels all hubris.
The progress of our adventure so far (current in bold):
1. The Sun
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
I. Context
Jupiter is the fifth planet from the Sun, and orbits about five to five-and-a-half times the Earth-Sun distance (4.95 - 5.46 AU). It is both the largest and most massive planet in the solar system, and anchors its own retinue of world-sized moons and a far larger number of asteroidal objects. The adjectival form of the planet is Jovian, after the alternate Latin form of Jupiter, Jove. Diagrams of Jupiter's solar system location and path of motion:
Due to the inverse-square law of electromagnetic radiation, the intensity of sunlight arriving in the location of Jupiter varies between 1/25th and 1/30th that received by Earth. However, this is still bright enough for solar-powered unmanned space probes like the Juno mission currently en route to be practical, although it will have to remain in perpetual light by entering a Sun-facing polar orbit of the planet. A comparison of the apparent size of the Sun from Earth and Jupiter:
The angular size of the Sun from Jupiter at its largest is somewhat larger than Venus in Earth's sky at inferior conjunction, although far brighter - and thus its glare would make it appear much larger. Because of the decline in solar intensity, objects at this distance would begin to seem visibly less bright to the naked eye in full daylight, although the effect is modest due to human perception of light being roughly logarithmic (i.e., changes with factors of ten in objective intensity) rather than linear. In other words, things are not 25-30 times dimmer at Jupiter than Earth because of the light being 25-30 times less intense, but only a few shades dimmer.
Jupiter is also the second most intense gravitational influence in the solar system after the Sun, with a field more powerful than all other planets combined. Below is a representation of the solar system's nested gravity wells, modified from the well-known xkcd illustration for the sake of clarity, although I highly recommend looking at the full-sized original for more detailed (and very humorous) explanations.
II. History
The majority of stellar systems involve two or more stars, but there wasn't enough material in the pre-solar nebula that formed the Sun to produce a companion for it. Instead, the bulk of leftover matter went into forming Jupiter and, to a much lesser extent, the other gas giants - i.e., it is an embryonic kernel of a star that failed to reach the necessary mass needed to be born. For gravitational pressure to produce the temperatures needed to ignite fusion, the mass of Jupiter would have to be dozens of times greater than it is. So, in a sense, Jupiter is to a star what Ceres is to a planet - the remnant of an incomplete process.
Under the current model of planetary formation, the protoplanetary disk of gas and dust heated by constant collisions began to cool as the material accreted into fewer and fewer bodies. The density of material was greatest in the area of Jupiter's formation and likely resulted in a number of very large, rocky protoplanets, all of which shortly ended up either colliding into a single object or being ejected. This object, with many times the mass of Earth, would become the core of Jupiter. A simulation showing how a process like this can occur - although this particular model involves somewhat different parameters than apply to Jupiter:
As it swept through the dense, rapidly cooling cloud of hydrogen and helium, the protoplanet accumulated a thick envelope of gas close to its present mass in only a few million years, quickly evolving from a super-Earth into the gas giant it is today. Even with the relatively intense gravity of the rocky core, this would not have been possible had it formed closer to the Sun: The temperature of the hydrogen and helium would have been too high to become gravitationally bound, which is why the atmospheres of terrestrial planets like Venus, Earth, and Mars come exclusively from release of gases trapped in rocks rather than having accumulated directly from the disk as with gas giants.
That, in turn, is why terrestrial planet atmospheres are overwhelmingly composed of heavier molecular gases like nitrogen and CO2 rather than hydrogen and helium. Although some H2 and He is trapped in rocks and later released through volcanism, it ultimately ends up escaping into space or, in the case of hydrogen, recombines with elements like oxygen, carbon, or sulfur that make it heavy enough to remain bound under the prevailing temperatures of the inner planets. Thus, the boundary in a solar system where gas giant formation becomes possible is referred to as the Frost Line - the region where temperatures become low enough in full sunlight for volatile compounds like water to remain solid in vacuum without sublimating (i.e., transitioning directly to gas and escaping). Comets are spectacular demonstrators of this boundary, erupting once they pass within it and going dormant again once they pass in the other direction.
So, the two primordial conditions needed to form a gas giant in a given region are density and cold - although gas giants can subsequently migrate inward to high-temperature regions, as seen with some planets around other stars. Jupiter itself migrated somewhat inward from its region of formation in its early history, which occurred because constant collisions with gas molecules, dust, and occasionally rocky bodies were bleeding away its orbital velocity. This is typically how planetary migration occurs among gas giants, although smaller planets may also migrate as a secondary consequence of larger ones doing so.
For instance, Neptune was thought to have formed inward of Uranus (which could explain the former's heavier mass), but at some point the migration of Jupiter put it into a 2:1 orbital resonance with Saturn that dumped momentum into Neptune and sent it spiraling outward to its present orbit. A number of inner planet impacts may also have occurred due to this resonance, and it is one possible explanation for the Late Heavy Bombardment period of the solar system that saw a spike in collisions.
Another major consequence of the Jupiter-Saturn resonance was the disruption of the early Kuiper Belt - the distant region of numerous icy bodies beyond the planets. It was originally much denser than it is today, but apart from sending Neptune careening through it, the resonance also sent large numbers of Kuiper Belt constituents into eccentric orbits that placed them on collision courses with other planets or the Sun, and others were ejected from the solar system entirely. The result is that the Belt is much more widely-dispersed and more distant than in the past, and the overall size of the solar system's planetary region was expanded. A diagram of this resonance-driven evolution - the innermost green orbit is Jupiter, and the disruption of Neptune (dark blue) is shown:
The principal moons of Jupiter (Io, Europa, Ganymede, and Callisto) - also known as the Galilean moons after their discoverer Galileo Galilei - are thought to be survivors of a larger number of massive moons formed from the same material as the planet. As the Jovian system moved through the gas and dust of the protoplanetary disk, older moons would spiral inward due to having their velocity bled off and be swallowed by the planet while newer ones formed from new material captured into the system. Since the disk no longer exists, the orbits of the current four are relatively stable.
Closer to home, the formation of Jupiter was responsible for the existence of the Asteroid Belt, having perturbed its constituents into eccentric orbits that prevented planetary formation (see Asteroids (Vol. 1) for a more detailed description). Some of these objects or fragments thereof have come to impact inner planets, such as the asteroid that extinguished the dinosaurs - a fragment of a larger one thought to have been shattered in an earlier collision due to interference from Jupiter. However, it has captured, absorbed, or ejected a much larger number of objects than it has sent into the inner solar system, so its relationship to Earth has been mostly protective.
Humanity witnessed the planet performing this role in 1994, when it tore apart Comet Shoemaker-Levy 9 and then devoured the fragments. These fragments entered Jupiter's atmosphere at over 216,000 km/h (134,000 miles per hour) with thousands of times more energy than all the nuclear weapons on Earth combined, punching holes through the upper cloud deck into dark, lower regions that would never otherwise be seen. The resulting transient scars in the atmosphere were the size of the entire planet Earth, and easily visible to the Hubble Space Telescope and a few ground-based observatories. Unfortunately, the Galileo spacecraft was not yet on hand to gather detailed imagery, being still en route to Jupiter. Still, the imagery is profound:
III. Properties
1. Orbital and Rotational Features
Although Jupiter's orbit is relatively circular and well-behaved, the size of the orbit is large enough that its modest eccentricity translates to a 76,000,000 km difference between perihelion and aphelion (closest and furthest points from the Sun, respectively) - a nontrivial consideration for solar-powered technology. The much larger circumference of this orbit - and the lower speed along it due to conservation of angular momentum - is also why it takes 11.86 Earth years to complete one Jovian year. It also has a low inclination, which is unsurprising given how much of the solar system's planetary mass it absorbed - and thereby the net orbital characteristics of the original disk.
One of the complications of a gaseous body is that rotational periods differ with latitude because the motion is fluid rather than that of a solid object - i.e., the rotation of the equator has only a weak effect on the rotation of the poles, so two points placed along a longitude but spaced significantly apart by latitude would move progressively farther from each other over time. The shearing caused by this phenomenon is the origin of Jupiter's characteristic cloud bands, which are also found to varying degrees in all the other gas giants of the solar system. An illustration of the principle:
I should note that the actual rotational difference is not very large, so it would take a significant amount of time in reality for the divergence shown above to manifest - it would not occur in one or a few rotations as in the illustration above, but hopefully it gets the point across about how cloud bands form. Think of the bands as hurricanes whose Northern half has been stretched all the way around the planet in one direction, and whose Southern half has been stretched all the way around in the other direction, connecting together into a storm ring that completely circles the globe. Rather than forming a vortex, bands flow linearly in opposite relative directions (although still rotating in the same direction with the whole planet). Only powerful storms that have enough energy to interrupt the bands, like the Great Red Spot, behave as coherent vortices in this environment.
At the equator, the Jovian day is a little less than ten hours - an amazing thing given the titanic size of the planet. In fact, the linear speed at the equator (45,300 km/h) is so high that if Earth rotated that quickly, it would fly apart into pieces. Only Jupiter's gargantuan gravity keeps it together, and even then the planet is noticeably warped by its rotation, bulging nearly 5,000 km at the equator beyond its polar radius. As a result, it is by far the most oblate planet in the solar system. It is worth noting, however, that not all images of Jupiter show its full oblateness: The higher the angle of perspective relative to its equator, the less evident the warping is - and, of course, it appears perfectly spherical from the poles. This shot from the Cassini spacecraft on its way to Saturn gives a decent impression (the black spot is Europa):
The time-lapse video below gives a sense of how Jupiter looks in motion, although it's somewhat artificial since it was pieced together from images taken over five Earth days (i.e., nearly a dozen Jovian rotations), and of course it moves much more quickly than a real Jovian day. Despite the amount of time involved, you won't see much cloud motion - the planet as a whole moves on order of a hundred times faster than its highest wind speeds, and many of these cloud features are the size of entire planets or larger, so even five Earth days wouldn't show much evolution in the weather:
2. Size and Mass Characteristics
Jupiter's equatorial diameter is about 11.2 Earth diameters wide, and 10.5 Earth diameters from pole to pole. If you can imagine flying a 747 around the planet at the equator, you would have to be on the plane for three weeks straight to make a complete circuit. It's also 2.9 times the size of Neptune, 2.8 times the size of Uranus, 18% larger than Saturn, and a tenth the size of the Sun - so we can say that in terms of scale, Earth is to Jupiter as Jupiter is to the Sun. Some visual comparisons:
The mass of Jupiter is 317.8 times that of Earth, 21.9 times the mass of Uranus, 18.5 times the mass of Neptune, 3.3 times the mass of Saturn, and slightly less than 1/1000th the mass of the Sun. Every other object in the solar system combined - planets, dwarf planets, asteroids, comets, and Kuiper Belt Objects - would not even be half of Jupiter's mass. The reason an object can be so much more massive while only being fractionally larger - or in the case of Uranus and Neptune, the more massive can even be smaller - is that higher mass means more intense gravity, and gas giants are capable of tremendous compression. In fact, a planet more massive than Jupiter would actually shrink to a certain extent before increasing in size again with added material. Higher temperatures and/or lower-density composition are the main reasons a gas giant would be larger than Jupiter - higher mass only weakly contributes to size growth.
Contrarily, the density of a gas giant is far below that of a rocky planet, so the "surface" gravity experienced by an object floating in Jupiter's upper atmosphere would be nowhere near reflecting the scale of its mass. For instance, a payload carried by a balloon would only experience about 2.5 g. Although that doesn't sound like much, it would be medically prohibitive for humans to be there due to factors I explore later, so this is mainly a fact useful for designers of robotic probes. But even if gravity were not a direct problem for humans, the absurd level of power needed to get back into orbit definitely would be.
3. Internal Structure
At the center of Jupiter is a solid core with 10-20 times the mass of Earth and a few times its size - the remnant of the original protoplanet, along with all the rock and metal added to the planet over the eons. Although the temperature of the core is 20,000 - 40,000 K, it also contains ices because the pressure is so high that otherwise volatile compounds like water, methane (CH4), and ammonia (NH3) remain solid even in the intense heat. [Correction: These compounds, when found in gas giant interiors, are referred to as "ices" only in shorthand by planetary scientists to distinguish them from hydrogen, helium, metals, and silicates - they are actually liquid, and the term "ice" in this context does not necessarily denote a solid state]. Moreover, the core itself is probably differentiated to some extent - i.e., the core has a core. The innermost region would be the heavy, rare, and radioactive metals, then above that a region of crystalline rock and ice.
The next layer up consists of liquid metallic hydrogen, and strongly dominates the mass and volume of the planet. This region is under such intense pressure that hydrogen becomes degenerate - basically a free-flowing, electrically conductive soup of protons and electrons. It's called metallic because of its conductive properties, and its flow is responsible for Jupiter's awesomely powerful magnetosphere.
There is unlikely to be any clear "surface" at the boundary between this layer and the solid core, despite the stark transition in properties: Liquid metallic hydrogen is so corrosive that it has been continually eating away at the core since Jupiter formed, dissolving ice and rock on an ongoing basis and possibly making the transition region into a frothing, dirty "muck." The implication of this process is that the core was larger in the past, and that in the far future it may dissolve completely if the rate of corrosion is sufficiently rapid.
Helium is also present in the metallic layer, although its exact role is an area of active investigation. Under the consensus view, He from the gaseous region loses heat and precipitates downward into the metallic hydrogen where it is subsequently heated again, helping to drive convection. However, there is some evidence that under the conditions of the metallic layer, hydrogen and helium can mix and form a liquid alloy. If that were the case, there would not be enough convection occurring to generate the observed heat coming from the planet, so proponents of the idea will have to find an alternate heat source to strengthen their case. The Juno mission will help provide answers when it arrives in 2016.
Beyond the metallic region, the next layer up consists of molecular hydrogen and helium in an exotic fluid state called supercritical (not to be mistaken for a very different state called superfluidity, which generally occurs at very low temperature, and is not associated with Jupiter). Because it moves fluidly and is not a gas, the supercritical phase is often inaccurately identified as liquid in popular explanations of the Jovian atmosphere, but there are key differences between the two phases: A supercritical fluid has no surface tension and diffuses completely into a given volume like a gas, but can also behave as a solvent like a liquid. Below is a generalized phase diagram showing the relationship of this phase to others - specific substances have different shapes for each area of the diagram:
All gas giants have thick supercritical envelopes, although even terrestrial planets can experience this phase under some conditions: The lower atmosphere of Venus, for instance, consists of supercritical CO2 because it's under such intense pressure and temperature. A number of super-Earth exoplanets are also thought to have supercritical conditions, some of which may consist of water - albeit in a form completely useless to life as we know it. If you're curious to know what this looks like, here is a video of liquid CO2 transitioning to supercritical - you'll notice that the liquid-gas boundary present at the beginning disappears:
4. Atmosphere
As we ascend, the supercritical layer transitions smoothly to the atmosphere proper, where hydrogen and helium exist in normal gaseous form - albeit still at enormous pressures and high temperatures through the vast majority of it. Not much is known about the atmosphere below the visible cloud deck: We have only the vaguest sense of the upper reaches of that region, mostly via the behavior of storms at higher altitudes and a few minutes of direct data from a kamikaze probe. The Galileo spacecraft had dropped a descender package into the Jovian atmosphere in 1995, but despite being built like a tank, it only got 140 kilometers beneath the clouds before failing - a small fraction of the way through the 5,000 kilometer-thick gaseous region.
The descender wasn't equipped with a camera, but it wouldn't have missed much even if it had: Scientists operating from the gathered data describe the area below the clouds as a vast empty space so large the horizons would be undiscernable - a blank, featureless, dark-yellowish sky in every direction devoid of structure but with surprisingly hot and violent air currents. A diagram relating pressure, temperature, and altitude in the upper regions of Jupiter's atmosphere:
As you can see, the cloud layer is a relatively nice place in terms of temperature and pressure, varying roughly from less than 1 Earth atmosphere to about 10, and from 150 to 250 kelvin (-123.15 to -23.15 °C / -189.67 to -9.67 °F). However, it's not very extensive, covering a region only about 50 km thick. What we think of as the face of Jupiter occurs entirely within this layer: Below is a fathomless cauldron, and above just thin air, but among the clouds otherwise invisible currents take form and show us the magic and terror of a truly alien world.
Jupiter's wind speeds have been observed at over 500 km/h, although speeds vary greatly across latitude and within storm systems. By far the highest winds occur in the tropics, which is unsurprising for all the same reasons it occurs on Earth: Namely, receiving the biggest share of sunlight, and also having storm motion partly driven by planetary rotation. But regardless of latitude, Jupiter at the cloud level is a Wagnerian world full of rending, shrieking, lightning-streaked drama and towering cloud-mountains the size of nations - a roiling, howling nightmare more profound than anything written by H.P. Lovecraft. And yet its beauty from a distance is beyond description.
(Continued in Volume 2)
