During the last few weeks we discussed the terrestrial planets--that is those planets that are most like the Earth in size, composition, and structure. While the geologies of the inner four planets have some things in common, we have found that each one is quite different from the others. The next four planets we will talk about--Jupiter, Saturn, Uranus, and Neptune--are very different to the terrestrial planets in that they have no real solid surfaces, being dominated by thick, deep atmospheres. Thus, there is no "geology" to speak of, and we talk more about the meteorology (weather) of these gas giants. Note their low densities:
Jupiter is the fifth planet from the Sun, and is the largest planet in the solar system. Its average distance from the Sun is 5.2 AU (778,330,000 km), and its equatorial radius is 68,700 km (almost 11 times that of Earth):
Though Jupiter is very massive (having 318 times the mass of the Earth), it is not very dense: it has a mean density of 1.4 gm/cm3. Just slightly more dense than water! Jupiter is 75% hydrogen, 24% helium, almost identical to the Sun. Suprisingly, even though Jupiter is enormous, it spins very rapidly, completing one rotation in slightly under 10 hours. It takes 12 years for Jupiter to circle the Sun. Jupiter is a swirling ball of clouds arranged in rotating storms and in complex bands:
These bands are caused by rising and descending air, as shown in the textbook:
Atmospheric temperature profile:
Jupiter is mostly a large ball of hydrogen and helium gas with a few trace elements:
This picture illustrates the internal structure of Jupiter. The outer layer is primarily composed of molecular hydrogen. At greater depths the hydrogen starts resembling a liquid. At 10,000 kilometers below Jupiter's cloud top liquid hydrogen reaches a pressure of 1,000,000 bar with a temperature of 6,000 K. At this state hydrogen changes into a phase of liquid metallic hydrogen. In this state, the hydrogen atoms break down yielding ionized protons and electrons similar to the Sun's interior. Below this is a layer dominated by ice where "ice" denotes a soupy liquid mixture of water, methane, and ammonia under high temperatures and pressures. Finally at the center is a rocky or rocky-ice core of up to 10 Earth masses.1 The central pressure of Jupiter is 80 Million Bars, and the temperature is 25,000 K. In fact, Jupiter radiates about twice as much energy as it receives from the Sun. This is mostly believed to be heat left over from formation, but "shrinking" can also supply some of this heat.
The rapid rotation of Jupiter creates an incredible amount of shear in the atmosphere, and all kinds of rotating structures form. The biggest of these, the "great red spot" was discovered by Cassini in the 17th century:
The red spot is simply a high pressure area like those seen on the Earth:
While it was first observed about 300 years ago, it has begun to shrink and may disappear in the next decade or two. Jupiter has no real solid surface, with the clouds gradually compressing to a liquid state, and this further compresses into a "metallic" state that surrounds the rocky core. The temperature of Jupiter at the cloud tops is very cold, 152 K, but increases dramatically as you go inwards, reaching to 25,000 K in the core.
Because Jupiter spins so rapidly, and it is a big ball of gas, it is actually a lot larger in diameter at the equator than at the poles--this is called "oblateness", and all of the gas giants are oblate. Here's the physics:
Jupiter has an equatorial radius of 71,492 km, but a polar radius of 66,854 km! The polar radius is only 94% of the equatorial radius (this can easily be noticed in a small telescope). The Earth is also oblate, but not as dramatically: REq = 6378 km; RPol = 6357 km (oblateness = 6357/6378 = 0.997).
Jupiter has an enormously strong magnetic field--it is about 14 times the strength of the Earth's field ("14 Gauss"). It is believed the rapid rotation leads to the generation of this field, enabling electric currents in its central regions (liquid/metallic hydrogen conducts electricity like a normal metal). The magnetosphere of Jupiter is one of the largest structures in the solar system, extending all of the way to Saturn!
Where this magnetic field enters the atmosphere we get aurorae (like those on Earth):
The Solar System's Vacuum Cleaner
Because Jupiter is so massive, its gravity influences many of the smaller bodies in the solar system---it also eats the ones that get too close. In 1994 a comet, Shoemaker-Levy, got too close, first being "captured" by Jupiter, and then breaking-up into a bunch of pieces:
And then these pieces crashed into Jupiter one at a time:
Eventually they all did, leaving dark plumes of material in Jupiter's atmosphere:
This has certainly happened numerous times in Jupiter's past, as this "crater chain" on one of Jupiter's moons (Ganymede) reveals:
The Moons of Jupiter
Jupiter has a large number of known moons (about 67 of them), but most of these are tiny little rocks (< 5 km in radii) and most of the outer ones are probably captured asteroids. There are four major moons of Jupiter: the "Galilean satellites" Io, Europa, Ganymede, and Callisto (shown in order, below--that tiny oblong rock to the left of Io is one of the smaller moons Almathea, whose orbit is inside that of Io). The densities of these four moons are Io: 3.5 g/cm3, Europa: 3.0 g/cm3, Ganymede: 1.9 g/cm3, and Callisto: 1.8 g/cm3. Just like the terrestrial planets, the densities decrease with distance. This is surely due to the fact that when Jupiter formed it was much hotter, thus driving away the water and other low density atoms/molecules to more distant reaches.
The innermost of these four moons is Io, and its surface is covered with lava flows from the numerous volcanoes on its surface (go to pages 323 to 325 of the textbook for more images).
In fact, Io has the youngest surface in the solar system, being constantly reshaped by volcanic eruptions. The yellowish-orange color of Io is due to the large amount of sulfur that is discharged in these eruptions. Here is a possible lake of sulfur:
All of these eruptions have left a trail of sulphur in orbit around Jupiter:
The reason Io has so many volcanoes is that the intense gravity of Jupiter, and that of the other three moons, constantly tugs on Io, stretching and compressing it (see page 325 of the textbook). The the three inner moons are also in orbital resonances:
For every four orbits of Io, Europa makes two, and Ganymede one. This means the constantly line up in the same configurations every once in a while, enhancing the tidal stresses. That generates an enormous amount of heat that liquifies the insides of Io. This process is also at work on the next moon out from Jupiter, Europa. Europa has one of the smoothest and brightest surfaces in the solar system. It is believed to be frozen water ice. Because ice is plastic (it flows), features with large elevations (mountains, craters, etc.) are not sustainable, and thus the surface is relatively smooth, but is covered with a large number of cracks:
Here is an image of ice on the Earth's surface in the polar regions, it is remarkably similar to Europa in appearance:
It is now believed that Europa has a large ocean of water beneath its frozen crust. This is also probably the case for the largest moon of Jupiter, Ganymede. Here are cutaway drawings of the internal structures of Jupiter's four moons (blue signifies water):
Here is a close-up of two possible models for Europa:
Recently, geysers of water were discovered on Europa, proving that it has liquid water below the ice layer.
Plans are being drawn-up to explore Europa and/or Ganymede as they may be ideal sites for life to have evolved and to currently exist. For more on Europa, go here. A recent mission (arrived in orbit July 5th, 2016), Juno, will probe the gravity of Jupiter and its moons.
The largest planetary satellite in the solar system is Ganymede (radius of 2,600 km).
Ganymede is slightly larger than Mercury, and twice as massive as the Earth's moon. Because of its low density, however, Mercury is much more massive than Ganymede. Thus it is both the largest and most massive moon in the solar system. Ganymede is the only satellite known to have its own magnetic field, suggesting a liquid, iron rich core. Presumably the tidal forces that act on Io are at work on Ganymede helping the keep the internal regions hot. The surface seems to be nearly all water ice. Like Europa, there is much evidence for grooved terrain caused by tidal forces on the icy crust (go here for a big map of Ganymede).
The final Gallilean satellite is Callisto, with a radius of 2400 km, about the same size as Mercury (but only 1/3 its mass). It has one of the most battered surfaces of any object in the solar system:
It shows no evidence of structures that would have resulted from internal
heating like the other Gallilean satellites. Callisto may have a completely
unaltered surface from the time it was created--only impacts appear to
exist, nothing else. Its low density means mostly ice with a rocky core.
All of the Gallean moons have small "exospheres", where molecules of the
rocks and ices that make up their surfaces are blown into space.