By Sabine Stanley Ph.D., Johns Hopkins University
Saturn and Jupiter are both significantly bigger than the Earth, with similar gas compositions. Yet, the storms, the color, the density, the rings, the planet behavior, and other distinctive features differ. How can the same composition lead to so many differences in two planets of almost similar size?
The density of Saturn and Jupiter
Saturn and Jupiter have attracted as much similar gas as they could, to form two biggest gas giants in our solar system. But they have done things in different ways and caused differences. The first difference is the planets’ densities.
Saturn is significantly less dense than Jupiter. Saturn’s average density is less than 700 kilograms per cubic meter, 30% lower than water. Despite common belief, Saturn will not float on the water since it is not a homogeneous body and does not have constant density. What would happen if Saturn was put in a bathtub big enough to hold it?
The outer atmospheric layer would dissolve, and the denser interior would sink to the bottom of the tub. This is the best possible way, not because Saturn’s temperature would boil the water away or compress it significantly with its gravity until it is no longer liquid.
The hydrogen, helium, and a bit of heavier elements in Saturn take up almost the same space as Jupiter, but Saturn’s mass is half of Jupiter’s. The reason is Jupiter’s strong gravity and its compression. Even if Jupiter attracted more amount of gas, it would remain a planet of the same size since the gas would get more compressed.
Material Organization in the Two Planets
Saturn and Jupiter have different material organization. Saturn has a big rocky core at the center. In both planets, hydrogen changes from a gas to liquid metal, but in Saturn, it happens much deeper. Saturn is less dense, so the pressure required for liquid metal is at lower depths. The temperature at Saturn’s center is about 18 million bars, with temperatures reaching over 10,000°.
In Jupiter, the metallic liquid hydrogen region is the main reason for its giant magnetic field. As in Saturn, this region is much below the surface, the field strength for Saturn is around ten times weaker than for Jupiter. Saturn’s magnetic field is almost as strong as Earth’s, despite their significant difference of size.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Axisymmetry in Saturn
Axisymmetry refers to how Saturn’s magnetic axis completely aligns with the rotation axis. Magnetic fields of Jupiter and Earth are mainly dipolar, and the dipole is tilted by about 10° from the rotation axis.
The axisymmetric magnetic field in Saturn made measuring its rotation speed much more difficult than it could be. Gas giants have no solid surface, and their atmosphere is full of winds that can have different speeds. So to figure out the rotation rate of a giant planet, we would normally look at its magnetic field, since that’s generated in the deeper interior. With Jupiter, for example, we can track periodic radio signals associated with the tilt of the planet’s magnetic field. But Saturn’s magnetic field has no tilt, so there is no periodic signal associated with the rotation of the planet!
Learn more about Jupiter’s Planetlike System of Moons.
Saturn’s Rotation Speed
In the early 1980s, the Voyager missions detected a periodic radio signal for Saturn, based on which the rate was calculated as 10 hours and 39 minutes. In 2004, the Cassini mission detected the same radio signal, which now showed the period to be six minutes longer!
The period is, in fact, a changeable feature in Saturn’s magnetosphere, not a measure of Saturn’s rotation period. Scientists finally estimated the speed by using Saturn’s gravity field. Saturn is a bit flattened as a direct result of its speed. Saturn’s rotation rate has now been determined to be about 10 hours and 33 minutes. That number is about 7 minutes shorter than the rotation period inferred from the Voyager measurements. So Saturn is rotating a little faster—and the days are a little shorter—than we originally thought.
Learn more about Saturn’s Moons: Titan to Enceladus.
Mass Distribution in Saturn
The deepest part of Saturn is stratified, i.e., not churning and producing convection. However, waves can still travel through this region. The waves in the interior move mass a bit, which then creates slight changes in Saturn’s gravity field, felt by the rings. Consequently, the ring particles oscillate and create waves that travel through the rings. The frequency and amplitude of these ring waves reveal facts about Saturn’s interior, density, and stratification.
Even though Jupiter and Saturn are made of the same thing, small differences make the resulting planet so different that even gathering information about the two gas giants needs significantly different methods.
Common Questions about Saturn and Jupiter
Saturn and Jupiter have different densities, behaviors, and organization of material. There are also appearance differences. For example, Saturn’s rings are much bigger than Jupiter’s.
Even though both Saturn and Jupiter are composed of similar gasses, Jupiter is bigger and denser than Saturn.
Saturn and Jupiter are gas giants, i.e., they are composed of different gases at different phases. A gas occupies much more space than solid material, and both planets have managed to attract as much gas as possible.
Neither Jupiter nor Saturn has solid surfaces. They are gas giants, and their surfaces are merely points where gas pressure equals the surface pressure on Earth: one bar.
