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Learning Objectives

1. Solar systems are groups of planets orbiting a central star. The Sun is the star at the center of our Solar System, which is composed of nine planets, innumerable asteroids orbiting primarily between Mars and Jupiter, and comets that reside mostly in the outer reaches of the Solar System in the Oort cloud and Kuiper Belt.

2. The inner planets of the Solar System are Mercury, Venus, Earth, and Mars. They are similar in that they are composed of silicate minerals forming rocks around iron and nickel cores. The outer planets of the Solar System are Jupiter, Saturn, Uranus, Neptune, and Pluto. These planets are composed primarily of hydrogen and water, perhaps surrounding small rock and iron cores (See Fig. 22.5 of your textbook).

3. Astronomical distances in the Solar System are measured in astronomical units (A.U.). One A.U. is equal to the mean distance of the Earth from the Sun, approximately 150,000,000 (150 million) kilometers. Pluto, the most distant planet, is approximately 40 A.U. from the Sun.

4. Our Solar System is part of the Milky Way galaxy which in turn is part of the Universe. Astronomers believe the Universe formed about 14 billion years ago.

6. Our Sun, the planets, and other celestial bodies in the Solar System are believed to have coalesced from a very large, somewhat flattened rotating mass of gas and dust about 4.6 billion years ago. The gas and dust from which the Solar System formed is called the solar nebula. Similar nebulae (plural of nebula) are very commonly observed throughout the Universe with telescopes, and each star or planet is thought to have formed from the gravitational collapse of a nebula or portion of a nebula.

7. The Moon is a satellite that orbits Earth, and is the only other body in the Solar System on which humans have walked. The surface of the Moon is thoroughly covered by craters, surface depressions formed by impacts with meteors of various sizes. The fact that the Moon's surface shows so many craters is evidence that many geologic processes found on Earth are not active on the Moon. Brightly reflective areas of the Moon are known as lunar highlands whereas dark areas are called maria. The difference in color of these two lunar provinces is due to differences in the composition of rocks that form them. Highland areas are formed of an intrusive igneous rock called anorthosite while maria are composed of a volcanic rock called basalt.

8. Mercury is the planet closest to the Sun, and its surface is completely covered with impact craters. It is too small and close to the sun to retain an atmosphere. As a consequence, it is subject to broad temperature extremes. The side of the planet facing the Sun can be as hot as 430 degrees Celsius, while the side facing away from the Sun can be -173 degree Celsius.

9. Venus is the 2nd planet from the Sun. The surface of Venus is not visible because it is cloaked by thick clouds in the atmosphere. Venus' atmosphere is composed almost entirely of carbon dioxide, creating an extreme greenhouse effect that raises the surface temperature of the planet to 480 degrees Celsius. Radar can penetrate the thick clouds, and has provided scientists with very detailed maps of the planetary surface revealing impact craters, volcanoes, lava flows, and other tectonic features the look similar to some geologic features on Earth.

10. Mars is the 4th planet from the Sun. Mars has a very thin atmosphere composed mostly of carbon dioxide. The surface of Mars has many Earth-like features including sand dunes, river channels, lake basins, canyons and gullies, and volcanoes. Mars also has ice caps (composed of both water and carbon dioxide ice) in both of its polar regions. Networks of valleys and channels across the Martian surface suggest the presence of liquid water flowing across the planet's surface during its early history.

11. Jupiter is the 5th planet from the Sun, and the largest planet in the Solar System. Jupiter is composed primarily of hydrogen gas, but may have a small solid core. One of Jupiter's moons (Io) is known to have active volcanoes. Another moon, Europa, is believed to have a liquid water ocean beneath a surface cover of ice.

12. Saturn is the 6th planet from the Sun, and the 2nd largest planet in the Solar System. It is surrounded by a number of prominently visible rings. The rings are composed of innumerable rock fragments orbiting the planet, believed to represent fragments generated by collisions among many very small moons or between a small moon and a comet whose orbit came too close to the planet.

13. Uranus is the 7th planet from the Sun. Relatively little is known about this planet because it is so far away. A few space vehicles have passed close enough to Uranus to provide pictures of the planet. Its atmosphere is composed mostly of hydrogen and methane. An unusual feature of Uranus is that its rotational axis is inclined such that it rotates "on its side" relative to the other planets.

14. Neptune is the 8th planet from the Sun, and like Uranus, has an atmosphere composed mostly of methane and hydrogen. Pictures of Neptune show that there are "storms" in the atmosphere similar to those observed in the atmosphere of Jupiter.

15. Pluto is the most distant planet in our Solar System. Little is known of this planet because it is so far away. It was discovered by astronomers in 1930.

16. Asteroids are small rock bodies orbiting in a broad region between Mars and Jupiter known as the asteroid belt. Asteroids are thought to be fragments of small planets (planetesimals) that failed to coalesce into a larger planet from the solar nebula due to the gravitational field of Jupiter.

17. Comets are small bodies in the farthest reaches of the Solar System that are believed to be composed of ice of various compositions mixed with dust and rocky debris. Some comets have highly eccentric orbits that bring them through the inner Solar System where they can be illuminated by the Sun and visible from Earth without the aid of telescopes. Some comet orbits are very well known, making it possible to predict when they will be seen from Earth.

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