7-2 Seven large satellites are almost as big as the terrestrial planets

All the planets except Mercury and Venus have moons (also called satellites). At least 170 satellites are known: Earth has 1 (the Moon), Mars has 2, Jupiter has at least 66, Saturn at least 62, Uranus at least 27, and Neptune at least 13. Dozens of other small satellites probably remain to be discovered as our telescope technology continues to improve. Like the terrestrial planets, all of the satellites of the planets have solid surfaces.

You can see that there is a striking difference between the terrestrial planets, with few or no satellites, and the Jovian planets, each of which has so many moons that it resembles a miniature solar system. In Chapter 8 we will explore this evidence (as well as other evidence) that the Jovian planets formed in a manner similar to the solar system as a whole, but on a smaller scale.

Seven of the Jovian satellites are roughly as big as the planet Mercury! Table 7-2 lists these satellites and shows them to the same scale. Note that Earth’s Moon and Jupiter’s satellites Io and Europa have relatively high average densities, indicating that these moons are made primarily of rocky materials. By contrast, the average densities of Ganymede, Callisto, Titan, and Triton are all relatively low. Planetary scientists conclude that the interiors of these four moons also contain substantial amounts of water ice, which is less dense than rock. (In Section 7-4 we will learn about types of frozen “ice” made of substances other than water.)

The satellites listed in Table 7-2 are actually unusually large. Most of the known satellites have diameters less than 2000 km, and many are irregularly shaped and just a few kilometers across.

Interplanetary spacecraft have made many surprising and fascinating discoveries about the satellites of the solar system. We now know that Jupiter’s satellite Io is the most geologically active world in the solar system, with numerous volcanoes that continually belch forth sulfur-rich compounds. The fractured surface of Europa, another of Jupiter’s large satellites, suggests that a worldwide ocean of liquid water may lie beneath its icy surface. On Saturn’s moon Titan, lakes of hydrocarbons, the primary source of energy on Earth, have been found near the poles—some the size of Lake Superior! In 2008, the space probe Cassini flew right through the plumes of geysers on Saturn’s moon Enceladus as they ejected water-ice, dust, and gas. While Jupiter and Saturn lack a solid surface, their many moons contain a rich variety of features.