Chapter 2

1. Chapter 2 The Solar System Part 2

2. Planets • Planets are bodies of considerable mass that orbit a star like the sun. • The solar system is officially comprised of nine planets. • A comparison of the planets reveals the following: • they seem to divide into rocky, dense, inner planets (called the terrestrial planets) and large outer gas giants.

3. The Planets • The orbits of the planets are ellipses though all except Mercury and Pluto are very nearly circular. • The further away from the sun, the slower a planet travels around the sun. • Venus rotates very slowly and in the opposite direction of most of the other planets. Uranus and Pluto seem to have been knocked over their axis. • All the planets except for Pluto rotate nearly on the same plane (called the ecliptic) around the sun.

4. Inner Terrestrial Planets • The inner terrestrial planets (Mercury, Venus, Earth, and Mars) are relatively smaller and denser. • They contain a much higher percentage of metal elements. Terrestrial planets all have roughly the same structure: a central metallic core made up of mostly iron with a surrounding silicate mantle. • Terrestrial planets have canyons, craters, mountains, and volcanoes. • Terrestrial planets have few moons if any.

5. Outer Gas Giants (Jovian Planets) • Jovian planets (Jupiter, Saturn, Uranus, Neptune) are composed primarily of hydrogen and helium just like the Sun but lack the size to be able to start a fusion reaction. • All Jovian planets generally have low densities, rapid rotation, and deep atmospheres. • All Jovian planets have rings (ie. Jupiter) and numerous moons.

6. Pluto … is it a Planet? • It has a moon, Charon, which is half its size. • Very similar to Neptune’s largest moon Triton. • Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). • Pluto’s orbit is quite inclined compared to the other planets. • The size and composition of Pluto resemble the sizes and composition of Kuiper Belt objects. • Officially Pluto is a planet and for historical reasons that may not change.

7. Moons (Natural Satellites) • A moon is a natural object that orbits another object. • Up to date 140 planetary moons (orbiting planets) have been discovered:

8. Planetary Moons • Mars’ moons Phobos and Deimos are irregular shaped and believed to be captured asteroids. Phobos is the larger inner moon that’s in a doomed course. In the next 50 million years it is expected to either collide with Mars or break up and form a planetary ring.

9. Galilean Moons • They are by far the 4 largest moons of Jupiter. They were discovered by Galileo in 1610 when he pointed a telescope at Jupiter thus providing solid evidence that all objects did not orbit around the Earth. • Io is the innermost of the Galilean moon best known for being the most volcanically active body in the Solar System. The high activity is believed to be caused by Jupiter and the other moons gravitational effect on Io. They squeeze and stretch the moon heating up its interior to very high temperatures. Eruptions resulting in plumes hundreds of kilometers high are then inevitable to relieve the pressure. Because of Io’s low gravitation, a lot of this material is ejected into space.

10. Galilean Moons • Europa, the next moon, has an outer layer of water 60 miles thick. The upper 12 miles are believed to be in the form of ice. This ocean is salty and holds a tantalizing possibility of life. • Europa and Io are somewhat similar in bulk composition to the terrestrial planets: primarily composed of silicate rock below the outer layer of water.

11. Galilean Moons • Ganymede is the largest moon in the Solar System. It is larger than Mercury and Pluto. • Callisto's surface is covered entirely with craters. It has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years.

12. Titan • Saturn’s largest moon is nearly the size of Ganymede. • Titan has a considerable atmosphere composed of mostly nitrogen but with traces of organic compounds. • Recent Huygens data shows that there is geological evidence for precipitation, erosion, and other fluvial activity. There processes are much the same as those shaping Earth.

13. Titan • These events are carried out in Titan a little differently. Instead of liquid water, Titan has liquid methane. Instead of silicate rocks, Titan has frozen water ice. Instead of dirt, Titan has hydrocarbon particles settling out of the atmosphere, and instead of lava, Titanian volcanoes spew very cold ice. • Titan's organic chemicals are thought by many to be the constituents that marked early Earth and that combined into complex forms to launch the long journey toward life itself.

14. Triton • It is the largest of Neptune’s moons. • It is volcanically active like Io and Titan. • Strangely it orbits the moon in the opposite direction of any other large moon.

15. Main Asteroid Belt • Located between Mars and Jupiter • It contains most of the asteroids (rocky and metallic objects that orbit the Sun but are too small to be considered planets). • The Belt is continuously disrupted by Jupiter causing asteroids within to change orbits. Many are believed to have been thrown out of the belt. Some have wondered inward passing near Earth.

16. The Kuiper Belt • The Kuiper Belt is a disk-shaped region past the orbit of Neptune extending roughly from 30 to 50 AU from the Sun containing many small icy bodies. (An AU or Astronomical Unit is the mean distance from the Earth to the Sun). • The Kuiper Belt has a large population (over 70,000) of small bodies. It is likely that the Kuiper Belt Objects are extremely primitive debris from the formation of the solar system. • It is considered to be the source of the short-period comets.

17. The Oort Cloud • The Oort Cloud is a hypothetical spherical cloud of comets approximately 50,000 AU to 100,000 AU from the Sun. • The Oort cloud is a remnant of the original nebula that collapsed to form the Sun. • No Oort Cloud objects have been detected, the existence of this classification is only inferred from indirect evidence.