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9. Terrestrial Planets Terrestrial Planets Mercury, Venus, Earth, Mars. Close to Sun. Closely spaced. Small radius. Small mass. Rock and metal. Solid surface. High density. Slower rotation. Weak magnetic field. Few moons. No rings. Terrestrial Planets, as seen from Earth When viewed by the unaided eye, all planets are just dots (though Mars is reddish). Mercury and Venus are always seen close to the Sun, with Venus called the “morning star” or “evening star.” With a telescope, Mercury and Venus show phases. Every two years, we get good views of Mars (when we pass it on our inside orbit). Mercury: General Characteristics Size: one third (38%) Earth’s diameter (4800 km). Distance From Sun: 0.4 AU. Density: similar to Earth’s. Surface: impact craters, maria; no surface activity. Atmosphere: none. Temperature: hot days, cold nights. No weather. Gravity: one third (38%) Earth’s gravity. Axial Tilt: none. Orbit: 3:2 spin-orbit resonance. Mercury: Surface Features Dominant impact craters show old surface, with little activity. Scarps (long cliff lines) are believed caused by shifting crust or by cooling. Maria (smooth, dark, uncratered basins) are presumably formed like on the Moon. Caloris Basin and Weird Terrain The Caloris Basin (1300 km, 800 mile) is Mercury’s largest impact site. Directly opposite the Caloris Basin the surface is jumbled and broken, called Weird Terrain. Formation of Caloris Basin and Weird Terrain The impact that formed the Caloris Basin sent seismic waves through Mercury, which intersected on the far side, creating the Weird Terrain. Kelvin Temperature Scale Astronomers use the Kelvin temperature scale, rather than Celsius or Fahrenheit. Absolute zero is the lowest temperature possible, which is zero Kelvin, 0 K. Human body (and Earth) is 300 K. Surface of Sun (and center of Earth) is 6000 K. Atmosphere and Temperature on Mercury Mercury has no atmosphere, because it’s too small (low mass), and it’s too hot. Temperature ranges from daytime 600 K (620 °F) to nighttime 100 K (-280 °F). Carbon dioxide could freeze at night, and lead could melt during the day. But, a thin layer of ice apparently exists at the poles. Mercury’s Spin-Orbit Resonance Mercury spins three times for every two orbits of the Sun. This 3 to 2 spin-orbit resonance is similar to the 1 to 1 spin-orbit resonance of the Moon to the Earth. Also called tidal locking, and synchronous rotation. Venus: General Characteristics Size: nearly (95%) the size of the Earth. Distance From Sun: 0.7 AU. Density: similar to Earth’s. Surface: volcanoes, lava plains, some impact craters. Atmosphere: very high pressure (90 times Earth’s), nearly all (96%) carbon dioxide. Temperature: hot day and night, everywhere, 750 K. Gravity: nearly (90%) the same as Earth. Spin: very slow (-243 days), backwards Venus: Surface Features Venus has been mapped by radar, showing a dominance of volcanic features: high volcanoes (Maxwell Mons), lava domes, and lava flows. Scarce cratering confirms an active geology and a young surface, less than 500 million years old. Venus: Surface Features Pancake Domes, Lava Flow, Surface Rocks, Impact Craters Venus: Atmosphere and Weather Composition: carbon dioxide (96%), nitrogen (3.5%). Pressure: 90 times Earth’s pressure, equivalent to 1300 pounds per square inch. Surface Temperature: 750K (890°F), everywhere, day and night—hot enough to melt tin, lead, and zinc—the hottest planet in the Solar System. Weather: little surface wind, but strong high-altitude winds with sulfuric acid clouds and rain Why is Venus so Hot and so Dry? Carbon dioxide traps infrared energy emitted from the surface. Heat drives carbon dioxide out of the original water, and out of the rocks, increasing carbon dioxide levels. This runaway greenhouse effect raises the temperature so high that water “boils” away and is lost to space. Venus Spins Backwards This can be thought of as a backward spin, or as an axial tip of 180°. Only reasonable explanation: A collision tipped the axis and nearly stopped the rotation. Mars: General Characteristics Size: one half (53%) Earth’s diameter (6800 km). Distance From Sun: 1.5 AU. Density: somewhat less than Earth’s. Surface: deserts, canyons, volcanic peaks, polar caps of water-ice and frozen carbon dioxide. Atmosphere: very low pressure (0.7% of Earth’s), almost all (97%) carbon dioxide; sand storms. Temperature: much colder than Earth. Gravity: one third (38%) Earth’s gravity. Axial Tilt: 24° (produces seasons). Moons: two small, captured asteroids Mars: Surface Features The obvious reddish color of the Martian soil is due to iron oxide (rust). Geologic features include: gravel plains, sand dunes, chasms, volcanic mountains, and the polar caps. Dry river beds, ancient dry lakes, and water-carved canyons are clear evidence of past water movement (plus a much thicker atmosphere and previous higher temperatures). Mars: Surface Features Sand Dunes, Dust Devil, Twin Peaks by Mars Pathfinder, Water-Ice Ground Frost, Dry River, Impact Crater, with Sand Dunes, Mars Sojourner, Polar Caps, Martian “Islands”, Olympus Mons Mars: Atmosphere Composition: carbon dioxide (95%), nitrogen (2.7%). Pressure: 0.7% of Earth’s pressure, equivalent to Earth’s pressure at an altitude of 45 miles. Surface Temperature: average is 220 K (-67°F), but varies with season, time of day, and latitude. Weather: convection-based, Coriolis-driven winds blow (similar to Earth), but with planet-wide seasonal gales and dust storms (related to polar cap thaw and freeze) Seasons on Mars Martian seasons are caused by both the axial tilt (like the Earth) and by the varying distance to the Sun (unlike the Earth). During north-hemisphere winter, the polar cap grows as carbon dioxide freezes out of the atmosphere, while the south polar cap shrinks. Air migration from one pole to the other causes planetary gales and sand storms. How did Mars lose its atmosphere? Past flowing water shows a prior thick atmosphere. Mars is a bit smaller than the Earth, making the atmosphere slightly thinner. Mars is a bit further from the Sun than Earth, making it slightly colder. Cold temperatures allowed carbon dioxide to dissolve into the water. Less carbon dioxide makes it even colder, triggering a reverse greenhouse effect. Water remains underground as permafrost. Factors Related to Martian Atmosphere Loss: size, distance from Sun, magnetic field, solar radiation, impacts, and reverse greenhouse effect Earth’s Carbon Cycle Quantities are giga (billion) tons of stored carbon. Biologic Effects on Earth’s Atmosphere Over billions of years, plant life has removed carbon dioxide from our atmosphere, replacing it with oxygen. Decomposition of plant and animal life has stored carbon in the form of sediments (fossil fuels and carbonate rocks). This results in an atmosphere depleted of carbon dioxide, with mild greenhouse warming. Life on Mars? 1890’s—Percival Lowell sketches Martian “canals.” 1965—Mariner flyby reveals a barren, cratered Mars. 1976—Viking Martian landers test soil (inconclusively) for life. 1996—Researchers announce possible life-indicators in Martian meteorite ALH84001. 1999—Analysis of Martian meteorite, Nakhla, indicates possible biological evidence Planetary Exploration: Mercury 1974-5, NASA’s Mariner 10 made first flyby. Messenger spacecraft achieved orbit March 17, 2011. Planetary Exploration: Venus Ten Soviet Venera probes landed on Venus in the 1970s and 1980s. NASA’s Magellan orbiter radar-mapped the surface in the 1990s. Planetary Exploration: Mars 1965, Mariner 4 completed first Martian flyby. 1976, Viking landers tested soil (inconclusively) for life. 2004, Mars Exploratory Rovers, Spirit (stationary) and Opportunity (mobile) began multi-year mobile geologic studies. 2011, Mars Science Laboratory rover is due to launch. Planetary Exploration: Mars 2012 Curiosity Rover lands on Mars, run by atomic power cell. Questions to be Answered: • What are the general characteristics of the terrestrial planets (as compared to the Jovian planets)? [review] • How do the terrestrial planets appear, as viewed from the Earth? • What are the surface features of each terrestrial planet, and how were they formed? • What is the composition and probable evolution of each planetary atmosphere? • What is special about Mercury’s spin and orbit, and how did this occur? • How might Venus have acquired its unusual spin? • Might Mars have supported ancient life? • What are the major differences between the terrestrial planets? • How does mass and orbital radius affect planetary evolution? • How does the development of life affect planetary evolution? • What is some of the basic history of planetary exploration?