Nature of the Universe

Chapter 9

Jovian Planets

Jovian planets or Jupiter-like planets are Jupiter, Saturn, Uranus and Neptune. They don't have a solid surface and they are big.

Diameter (Earth=1)Mass ratio (Earth=1) Density (Water=1)Revolution period Sidereal rotation periodNo. of satellites
Jupiter11.2317.91.3 11.9 years10 hours~16
Saturn9.495.20.7 29.4 years10.7 hours~18
Uranus4.014.51.3 83.8 years17.2 hours~17
Neptune3.917.21.8 163.7 years16.1 hours~8

Jupiter

Courtesy STScI.
Jupiter is the largest and most massive planet in our solar system. It is almost completely made up of gases. The rotational period of Jupiter is about 10 hours, and such a high velocity flattens Jupiter at the two poles. The mass of Jupiter is about 300 times that of the Earth, however its density is low. In fact, these are general features of Jovian planets.

Jupiter is mainly made up of hydrogen, helium, and a small amount of methane and ammonia. The atmospheric pressure is extremely high, over 1000 times than that of the Earth. Because of the great pressure, the core of Jupiter is made up of metallic hydrogen. The rapid rotation of such metallic core explains the strong magnetic field of Jupiter.

Courtesy STScI.

Because of the great mass, the inner parts of Jupiter undergo gravitational collapse, and the gravitational potential energy turns to thermal energy. As a result, Jupiter radiates more energy than it receives from the Sun.

Usually, we can only observe the upper atmosphere of Jupiter. There are bands and spots when we observe through telescopes. The atmosphere of Jupiter is in fact in great turbulence. The Great Red Spot, which is a typhoon in the atmosphere, is red in color due to the presence of sulphuric compounds. Unlike the typhoon on Earth, it is much larger and persistent. The size of the Great Red Spot is 3 times the size of the Earth and it has been lasting for more than 300 years. The wind speed of Jupiter is more than 1000km/hr.

Courtesy JPL/NASA. Courtesy JPL/NASA.

Like Saturn, Jupiter has a ring system. However, it is very pale and cannot be detected by ground based telescopes. In fact, having a ring system is a common feature of the 4 Jupiter-like planets.

Courtesy JPL/NASA.

Courtesy JPL/NASA.
Jupiter has a large number of satellites, including the four Galilean satellites, Io, Europa, Ganymede and Callisto. Io is famous for its active volcanic activity, which emits sulphuric compounds, and has a geologically young surface. Ganymede is the largest satellite in our solar system. And we believe there are water lakes under the icy surface of Europa and Callisto.

Saturn

Courtesy JPL/NASA.
Saturn has large size and mass, but it has the lowest density among all the planets, even lower than the density of water. In fact, Saturn is very similar to Jupiter in many ways, like the high speed rotation, the internal heating mechanism, the atmosphere composition, the presence of metallic hydrogen core and the strong magnetic field, etc.

Courtesy JPL/NASA.

Saturn is most famous for its ring system. In close up view, it consists of many thin ringlets. The thickness is about 1km. Actually, the ring is made up of tiny dust and ice, size ranging from millimeter to meter. The formation of ring is due to the great tidal force by Saturn, tearing one or more satellites into smaller parts. In closer look, there are many gaps between the ringlets, the most obvious one is the Cassini division. For some ringlets, there are two satellites lying inside and outside the ringlet, they are called shepherd satellites, that keep the stability of the ringlet. The following photo shows the shepherd satellites of the ring of Uranus.

Courtesy JPL/NASA.

Saturn has many satellites, the most famous one Titan has an atmosphere, which is mainly made up of nitrogen.

Courtesy JPL/NASA.

Uranus

Courtesy JPL/NASA.
One of the most interesting facts about Uranus is that its rotational axis is almost parallel to the orbital plane. As a result, Uranus seems to be rolling on the orbital plane.

It is blue in color, owing to the scattering of blue light by methane. It is similar to Saturn and Jupiter in many respects, like the atmosphere composition, the internal heating mechanism, the presence of magnetic field, existence of many satellites and a ring system, etc. (The streaks in the following photo are background stars not related to the ring.)

Courtesy JPL/NASA.

However, unlike Saturn and Jupiter, Uranus has a rocky core and the atmosphere seems to be less dynamic, not much surface features can be observed.

Neptune

Courtesy JPL/NASA.
Neptune is similar to Uranus in many ways, it has a rocky core, a ring system, many satellites, a magnetic field, etc. However, it seems to be more dynamic; its surface has a Great Dark Spot, which is a typhoon similar to the Great Red Spot in Jupiter. This typhoon is less persistent, lasted for only two years.

Courtesy JPL/NASA.

Previous chapter. Next chapter.

Title page.