Download Chapter 8: The Terrestrial Planets
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CHAPTER 8 The Terrestrial Planets CHAPTER OUTLINE 8-1 Mercury Mercury as Seen from Earth 1. The least seen planet by people on Earth. Because of its proximity to the Sun it can be seen low on the horizon just before sunrise in the east or just after sunset in the west. 2. Mercury exhibits phases like Venus. 3. Features on Mercury are hard to discern from Earth because Mercury is small and, since it is seen near the horizon, its light passes through more of the Earth’s atmosphere than if it were overhead. Mercury: Comparison with the Moon 1. Mariner 10 flew by Mercury in 1974 (and subsequently twice more), returning a total of 4,000 photographs for the three fly-bys. 2. MESSENGER made three flybys of Mercury in 2008 and 2009 and then began orbi ting the planet in 2011. 3. Mercury appears similar to our Moon; both are covered with many impact craters. Mercury’s craters are less prominent and its surface has less extensive ray patterns; the planet’s surface gravity is twice that of the Moon so loose material does not stack as steeply and doesn’t travel as far. 4. Mercury lacks the larger maria seen on the Moon. Because Mercury cooled more slowly, meteorites were able to penetrate its crust over a longer period, which allowed lava to flow out and obliterate older craters. This resulted in the smooth plains seen between the evenly spread craters. 5. More scarps—long cliffs in a line—are found on Mercury than on the Moon. Most are believed to have formed from the shrinking of Mercury’s crust after cooling. 6. A large “bulls-eye” impact crater called Caloris Basin (the size of Texas) is visible in Mariner’s images. 7. Mariner confirmed that Mercury has negligible atmosphere. Structural Characteristics 1. Mercury’s density is slightly smaller than Earth’s while its surface gravity is only 38% that of Earth’s. Since surface rocks on Mercury look similar to Earth rocks, Mercury must have a very large iron core, perhaps 65%–70% of its total mass.
2. Astronomers speculate that early on a collision with a large asteroid blasted away most of Mercury’s rocky mantle. 3. Mariner 10 did detect a magnetic field on Mercury, but it is 1% as strong as Earth’s. Mercury’s magnetic field suggests that part of its metallic core must be molten in order for the dynamo effect to operate. Recent measurements of heat loss indicate that its core is not molten, but radar measurements of Mercury’s rotation support the idea that the core is partially molten. 4. The dynamo effect explains the generation of magnetic fields as a consequence of circulating electric charges, such as in an electric generator or in molten magnetic material within a planet’s core. Mercury’s Motions 1. Compared to the other planets, Mercury circles the Sun in less time (88 days), moves faster in its orbit (48 km/s) and of the eight planets has the most eccentric orbit (varying between 46 and 70 million km from the Sun). 2. Radar observations show that Mercury rotates once very 58.65 Earth days (precisely 2/3 of its orbital period of 87.97 Earth days). This sidereal day is quite different from Mercury’s solar day, which is 176 Earth days long. Thus a Mercurian solar day lasts two Mercurian years. 3. The coupling between Mercury’s rotation revolution periods is probably due to the unbalanced nature of its mass. The leading hypothesis is that the impact object that created the Caloris Basin was very dense, and its presence under Mercury’s surface has caused the planet to be lopsided. The Surface of Mercury 1. High temperatures on Mercury can reach 450°C (840°F), well above the melting point of lead (330°C or 626°F). On the night-side of Mercury, temperatures can fall to -150°C (-238°F). 2. Radar observations show a high albedo—fraction of incident sunlight that an object reflects—for Mercury’s polar regions, suggesting the presence of ice. Yet ice should evaporate over the ages. The presence of ice for so long remains a mystery. 3. Volcanism played a very important role in shaping the planet through much of its history. The large expanses of the smooth plains on Mercury’s surface are volcanic deposits, several kilometers in thickness. 4. Mercury’s surface composition remains a mystery, but data suggest the rocks are ironpoor and consist mainly of calcium-magnesium silicates, sulfur materials, and a varying mix of metal oxides.
8-2 Venus 1. Like Mercury, Venus is visible only in the evening sky after sunset or in the morning sky before sunrise. Structural Characteristics 1. Venus is Earth’s sister planet with a bit smaller diameter (95% of Earth’s), smaller mass (82%) and smaller density (95%). 2. The surface gravity of Venus (91% of Earth’s) and the similarities between the two planets’ surface rocks suggest than Venus has a dense interior with probably a metallic core. 3. Venus does not seem to have an intrinsic magnetic field. It is possible that its field is now in the process of reversing. Venus’ Motions 1. Its orbit is almost circular with a period of 225 days; orbital speed is a nearly constant 35 km/s (78,300 mi/hr). 2. Venus’ surface is shrouded by heavy clouds. Since 1961 we have been bouncing radar signals off its surface to learn about its rotation rate and surface features. 3. Venus’ sidereal rotation period is 243 days, its revolution period is 225 days, and these combine to produce a solar day that is 117 Earth days long. 4. Venus’ axis is tilted 177°; because the angle is greater than 90°, Venus’ direction of rotation is backward compared to most other directions of rotation and revolution in the solar system. 5. The definition of a planet’s North Pole is based on the right-hand rule: grab the planet with your right hand so that your fingers point in the direction of the planet’s rotation; your thumb then points to its north pole. The Surface of Venus 1. Since 1962, many spacecraft from the U.S. and Russia have visited Venus. The former Soviet Union landed 11 spacecraft, some of which produced close-up photos of the surface. 2. Photos of sharp-edged rocks confirm that winds at the surface are fairly calm. 3. Orbiting probes Pioneer Venus 1 (1978) and Magellan (1990) have produced detailed radar maps of Venus’s surface. About two-thirds of Venus’s surface is covered with rolling hills. Highlands occupy
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