Geology of the Solar System 10: Jupiter and its satellites


Jupiter is the fifth planet from the Sun, and is the first of the Jovian planets, or gas giants. It spins very fast, having a rotation period of just 10 Earth hours, and it takes 11.9 Earth years to revolve around the Sun. Being made almost entirely of gas, it has a low density of just 1.3 g/cm3. Its atmosphere is well-organized into light zones and dark belts with opposing wind directions, with shearing at the interfaces which creates vortices and storms like the giant red spot, and is dominated by hydrogen and helium. Jupiter’s magnetic field is 20,000 times more intense than Earth’s.

Below is a picture of its internal structure. The layer of liquid, metallic hydrogen below the atmosphere is highly charged, and the rapid spinning of the planet produces the huge magnetic field from it.

Because there is no rocky surface on Jupiter as there is on the terrestrial planets, we won’t focus on it too much, but instead look at four of its satellites.


This is the most volcanically active body in the solar system, erupting 100 times more lava than Earth from at least 80 active volcanoes. It therefore has a very young surface with little cratering. The eruptions create lava plains and massive umbrella-shaped plumes that drop volcanic ash, sulfur, and SO2 frost and snow in rings around the volcanoes.

Volatiles produced in the eruptions, such as nitrogen gas, water vapour, and carbon dioxide gas, are lost to space upon being erupted. This incredible internal heat is created by the tidal friction caused by the gravitational pull acting on Io from Jupiter and some other nearby satellites, like Europa. Jupiter’s magnetic field may also have something to do with it, as Io is so close to Jupiter (closer than our Moon is to Earth).

Io lacks the surface ice found on other satellites around Jupiter (duh, lava), and is likely dominated by silicates, based on its density of 3.57 g/cm3. It also appears to have its own magnetic field independent of Jupiter’s.


This moon is covered in a thick layer of water ice, that may have a “lithosphere” of partially liquid, slush-like material. There are very few craters, which could be either because of slush eruptions covering them over or because of the plasticity of flowing ice smoothing them out in response to gravity. I personally like the slush eruption theory the best.

There may have been, and might still be, a huge water ocean under the ice, which makes Europa a good candidate for possible life beyond Earth. The internal structure is speculative, as it’s hard to get a look at when all you’ve got is a couple of satellites, but judging by its density of 2.97 g/cm3 it is probably dominated by silicates like Io is.  There is also a small magnetic field, that might be indicative of a small iron core, or could also be caused by Jupiter’s magnetic field interacting with a salty, subsurface ocean (the better supported of the two theories).

Magnesium sulphate salts have been detected on its surface, and while the sulphur likely comes from Io’s volcanic eruptions, there is no surface source for the magnesium.  It must therefore come from the subsurface ocean, which is likely filled with magnesium chlorides (so, magnesium plus sodium and potassium), more evidence that Europa could possibly support some kind of extant, marine life. There are also a couple of large irregularities on the ice surface (which they call “chaos terrain”, cool!) which could be a means of transferring materials (like nutrients for Europan fishes?) to the ocean below the ice.


This is the largest satellite in the solar system, even bigger than the planet Mercury, and also has a freaking awesome name. Ganymede…

It seems that the farther out from Jupiter you go, the less dense the satellites get.  Ganymede’s density is only 1/94 g/cm3, suggesting a composition of lesser refractory elements and more volatiles like water.  It has an icy surface, with sections of heavily cratered terrain, suggesting a surface as old as the earliest bombardment period in our solar system, and areas of bright, grooved terrain, with fractures interrupting fractures, suggesting tectonism earlier in its history.  It has a thicker ice layer than Europa that includes a mantle, and its magnetic field suggest an iron core that has been kept hot enough to produce the magnetic field by tidal heating. Ozone has been detected in the ice, but unlike on Earth, Ganymede’s ozone is created by non-biological processes, meaning that we can no longer rely on ozone as an indicator for life on other bodies (boo, Ganymede, the fun-killer).

There is a strange phenomenon involving the previous three moons around Jupiter. In the time it takes Ganymede to orbit the planet once, Europa orbits exactly twice, and Io orbits exactly 4 times, which is how the interiors of these moons are kept hot in spite of the fact that this far from the Sun, they should have cooled and solidified for good a long time ago.


Also sporting a pretty awesome name, Callisto is the outermost satellite I will be looking at, and has the lowest density of the four at 1.86 g/cm3, further suggesting a temperature gradient moving outwards from Jupiter early in its formation. It is basically a ball of ice, with lots and lots of craters. It’s magnetic field suggests a subsurface saline ocean.


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