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Transcript
Chapter 6 The Terrestrial Planets Mercury Mercury: Basic Characteristics Mass = 3.302×1023 kg (0.055 Earth) Radius = 2,440 km (0.383 Earth) Density = 5,427 kg/m³ Albedo (reflectivity) = 0.11 (Earth = 0.39) Average distance from Sun = 0.387 A.U. Travel time from Earth = 110 days (each way) Mercury: Key Concepts (1) Mercury has a 3-to-2 spin-orbit coupling (not synchronous rotation). (2) Mercury has no permanent atmosphere because it is too hot. (3) Like the Moon, Mercury has cratered highlands and smooth plains. (4) Mercury has an extremely large iron-rich core. (1) Mercury has a 3-to-2 spin-orbit coupling (not synchronous rotation). Every 2 years Mercury will rotate 3 times. Mercury’s Unusual Orbit Orbital period (Revolution) = 87.969 Earth days Rotation period =58.646 Earth days = (2/3) x 87.969 days Mecury is NOT in synchronous rotation (1 rotation per orbit). Instead, it has 3-to-2 spin-orbit coupling (3 rotations for 2 orbits). Time between one noon and the next is 176 Earth days. Sun is above the horizon for 88 Earth days at the time. Daytime temperatures reach as high as: 700 Kelvin (800 degrees F). Nighttime temperatures drops as low as: 100 Kelvin (-270 degrees F). Another Strange Mercurial Feature: Mercury has scarps, or cliffs, as much as 3 km high, 500 km long: They are probably the result of cracking as the crust cooled. Venus Venus: Basic characteristics Mass = 4.868×1024 kg (0.815 Earth) Radius = 6,052 km (0.949 Earth) Density = 5,204 kg/m³ Albedo (Reflectivity) = 0.65 (Earth = 0.39) Average distance from Sun = 0.723 A.U. Travel time from Earth = 110-150 days (each way) Venus: Key Concepts (1) The surface of Venus is hidden from us by clouds of sulfuric acid. (2) The atmosphere of Venus is hot because of a runaway greenhouse effect. (3) The surface of Venus shows volcanic activity but no plate tectonics. (4) The interior of Venus is similar to that of the Earth. Venus: Revolution and Rotation Orbital period (Revolution) = 225 Earth days Rotation period = 243 Earth days Rotation of Venus is retrograde (backwards) On Venus, the time from noon to noon = 117 days Venus is sometimes called Earth’s twin: Radius = 95% of Earth’s Mass = 82% of Earth’s Surface gravity = 90% of Earth’s (2) The atmosphere of Venus is hot because of a runaway greenhouse effect. Dense and high-pressure atmosphere: Air pressure at the surface of Venus = 90 times air pressure on Earth (2/3 of a ton per square inch). • • • • 96.5% carbon dioxide (CO2) 3.5% nitrogen (N2) 0.015% sulfur dioxide (SO2) almost no water vapor CO2 is a greenhouse gas: so Venus has a Enormous greenhouse effect. Surface temperature = 733 Kelvin (860 degrees Fahrenheit) Even hotter than Mercury! Temperatures drop very little during the long night, thanks to the thick atmospheric blanket. Escape velocity for Venus = 10.4 km/sec Venus: many volcanoes, few impact craters. More than 1600 major volcanoes. Estimated average age of crust = 500 million years (older than Earth, much younger than Mercury or Moon). Perspective view of Venusian volcanoes (Warning: vertical scale is exaggerated by a factor of 10): Mars Mars: Basic Characteristics Mass = 6.418×1023 kg (0.107 Earth) Radius = 3,377 km (0.531 Earth) Density = 3,934 kg/m³ Albedo = 0.15 (Earth = 0.39) Average distance from Sun = 1.523 A.U. Travel time from Earth = 150-300 days (each way) Mars: Key Concepts (1) Mars has a tenuous (thin) atmosphere, with little water vapour and few clouds. (2) Mars has large volcanoes and a huge rift valley, but no plate tectonics. (3) Robotic ―rovers‖ have given us a close-up look at Mars. (4) Mars has two small irregular moons, Phobos and Deimos. (5) Life on Mars?? Mars: Revolution and Rotation Orbital period (Revolution) = 1.9 years Rotation period = 24 hours, 37 minutes Rotation axis of Mars is tilted by 25 degrees. Martian seasons similar to Earth seasons (only longer). Low density atmosphere implies: (1) small greenhouse effect (2) large temperature swings. Summer on Mars: High = 270 Kelvin (0oC) Low = 200 Kelvin (-70oC) Summer in Earth’s deserts: High = 310 Kelvin (40oC) Low = 290 Kelvin (20oC) (2) Mars has large volcanoes and a huge rift valley, but no plate tectonics. Two hemispheres of Mars are very different from each other. North: recent volcanic activity (like Earth and Venus). South: heavily cratered highlands (like Moon and Mercury). BIGGEST VOLCANO in the Solar System: Olympus Mons About the size of Arizona or Poland (~600 km across). Last eruption about 300 million years ago. (3) Robotic ―rovers‖ have given us a close-up look at Mars. Spirit and Opportunity (still going since January 2004!) are taking images and spectra of Mars rocks. Success!! (4) Mars has two small irregular moons, Phobos and Deimos. Phobos (―fear‖): 28 kilometers long. Deimos (―panic‖): 16 kilometers long. Irregular in shape, undifferentiated. Probably captured asteroids. The Moons of Mars Phobos 1410 km Deimos 86 km Compositions of main-belt Asteroids. Comets & Asteroids Comet ● Definition: – ● Wild 2 Comet Stardust NASA mission Dirty snowball (long haired star) Origin: – Theory: During spherical nebular phase, ice chunks formed in outer part of cloud. ● – These became comets in Oort cloud (outside solar system) Theory: Some formed later from plantesimals in outer solar system, now in Kuiper Belt Evidence for Comet ● ● ● ● Comets can have a short or long period – Short: < 200 yrs – Long: 1 – 30 million years Comets can have orbit in the plane of the planets or not. Composition: Water ice, CO, ammonia, methane, rock Which type of comet are in the Oort cloud? Long or short period? Parts of a Comet ● ● ● ● Comets glow only as they near the sun Ice chunks in the Kuiper belt aren’t luminescent—too cold Nucleus: solid part, a few km across Coma: luminous ball around nucleus Tails of Comet ● ● Comets have 2 tails – Dust Tail – Ion Tail – Range in size up to 1 AU! Tails always point away from the Sun, due to solar wind Do comets melt? ● ● Estimates are that comets lose ~0.1% of initial mass each trip around the Sun. Particles from comet can continue to orbit even after comet disintegrates – ―Meteor‖ Showers Asteroids ● Asteroid belt was discovered in 1800, when Ceres was first thought to be a planet. – ● After other similar objects in same orbit discovered, it was reclassified. Asteroids have similar composition to rocky planets – Origin: ● ● Planet that never formed Planet that broke apart What’s the difference between a meteoroid and asteroid? ● Size! – ● Usually > 300 m -> asteroid Asteroid ―Ida‖ and her moon Meteor’s name depends on where it is: – Meteoroid is in space – Meteor is falling through Earth’s atmosphere – Meteorite is on the ground.