28.3 Satellites of Other Planets

1. 28.3 Satellites of Other Planets

2. A. Other Natural Satellites • All of the planets in our solar systems except Mercury and Venus have moons. In addition, Saturn, Jupiter, Uranus, and Neptune all have rings. B. Moons of Mars • Mars has two tiny moons named Phobos and Deimos (pronounced fo-bahs and dee-mahs, respectively), which revolve around Mars relatively quickly. • Phobos and Deimos are irregularly shaped chunks of rock and are thought to be captured asteroids. • The surfaces of Phobos and Deimos are dark like the maria on earth’s moon, and both have many craters.

3. C. Moons of Jupiter • Galilean moon - any one of the four largest satellites of Jupiter—Io, Europa, Ganymede, and Callisto - that were discovered by Galileo in 1610 • In addition to the four large moons discovered by Galileo, scientists have observed dozens of smaller moons around Jupiter. • Of Jupiter’s four Galilean moons, three are larger than Earth’s moon.

4. C. Moons of Jupiter Io (Pronounced E-oh) • Io is the innermost of the Galilean moons, and is the first extraterrestrial body on which active volcanoes have been seen. • Volcanoes on Io eject thousands of metric tons of material each second. • Io moves inward and outward in its orbit around Jupiter because of the tidal forces, or gravitational pull, of the other moons of Jupiter. • Calculations show the tidal forces cause Io’s surface to move in and out by 100 m (328 ft). The friction from this movement results in the melting of the interior of Io and leads to volcanism.

5. C. Moons of Jupiter Io • Data gathered from the Galileo spacecraft show that Io has a giant iron core and may possess a magnetic field. • Much of what scientists know about Jupiter’s moon comes from the information gathered by the Galileo spacecraft, which orbited Jupiter from 1995 to 2003.

6. C. Moons of Jupiter Europa (Pronounced Yoo-row-pa) • Europa is the second closest Galilean moon to Jupiter. Europa is about the size of Earth’s moon, but is much less dense. • Astronomers think that Europa has a rock core that is covered with a crust of ice that is about 100 km (62 mi) thick. • Scientists have concluded that an ocean of liquid water may exists under this blanket of ice. • If liquid water exists, simple life forms could also exist there.

7. C. Moons of Jupiter Ganymede (Pronounced Gan-eh-meed) • Ganymede is the third Galilean moon from Jupiter and is also the largest moon in the solar system. • Ganymede has a relatively small mass because it is probably composed mostly of ice mixed with rock. • The surface of Ganymede has dark, crater-filled areas, and light areas that may be ridges and valleys. • The Galileo spacecraft provided evidence to support the existence of a magnetic field around Ganymede. • Ganymede and Io are the only Galilean moons that have strong magnetic fields. • Both moons’ magnetic fields are completely surrounded by Jupiter’s much more powerful magnetic field.

8. C. Moons of Jupiter Callisto (Pronounced Keh-lis-to) • Callisto is the farthest Galilean moon from Jupiter. • Callisto is similar to Ganymede in size, density, and composition, except Callisto’s surface is much rougher. • Callisto may be one of the most densely cratered moons in our solar system.

9. D. Moons of Saturn Titan • Saturn has over 30 moons, but only five moons are fairly large. • Saturn’s largest moon is Titan, which has a diameter of over 5,000 km (3107 mi). • Titan’s has a thick atmosphere composed mainly of nitrogen. Saturn’s Other Moons • Most of Saturn’s moons are small, icy bodies that have many craters. • Saturn’s icy moons resemble Jupiter’s icy Galilean moons. • Saturn’s other smaller moons have irregular shapes. Scientists think that many of the smallest moons were captured by Saturn’s gravitational pull.

10. E. Moons of Uranus and Neptune • Uranus’s four largest moons, Oberon, Titania, Umbriel, and Ariel, were known by the mid-1800’s. • A fifth moon, Miranda, was discovered in 1948. Scientists now know that Uranus has over two dozen moons. • Neptune has at least eight moons. One of these moons, Triton, revolves around Neptune in a retrograde orbit.

11. F. Pluto’s Moon • The relationship between Pluto and its moon Charon (pronounced kar-en) is unusual because Charon is almost half the size of Pluto itself. • Because Pluto and Charon are similar in size, some scientists consider them to be a double-planet system. • Charon completes one orbit around Pluto in 6.4 days, the same length of time as a day on Pluto. Because of these equal lengths, Charon stays in the same place in Pluto’s sky.

12. G. Rings of the Gas Giants • Saturn’s set of rings was discovered over 300 years ago. Each of the rings circling Saturn is divided into hundreds of small ringlets, which are then composed of billions of pieces of rock and ice. • Scientists think that the rings are the remains of a large comet-like body that entered Saturn’s system and was ripped apart by tidal forces. • Jupiter has a single, thin ring made of microscopic particles that may have been given off by Io. • Uranus also has a dozen thin rings. • Neptune’s relatively small number of rings are clumpy rather than thin and uniform.