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Cratered Worlds:

Cratered Worlds:. The Moon & Mercury Chapter 7. The Moon. Mass 1/80 of Earth’s mass Gravity 1/6 of Earth’s Atmosphere no real atmosphere few volatiles (elements that evaporate at relatively low temperatures; e.g. water). Apollo 17 Dec 1972. Lunar Surface.

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Cratered Worlds:

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  1. Cratered Worlds: The Moon & MercuryChapter 7

  2. The Moon • Mass 1/80 of Earth’s mass • Gravity 1/6 of Earth’s • Atmosphere • no real atmosphere • few volatiles (elements that evaporate at relatively low temperatures; e.g. water)

  3. Apollo 17 Dec 1972

  4. Lunar Surface • Dominant Features • Craters • Maria = “seas” • Dark features • Radioactive dating (moon rocks) • age 3.3 - 4.4 billion yrs • older than rocks on Earth • Earth-Moon system about 4.5 billion years old

  5. Geological Features • Surface dominated by impacts • Highlands • heavily cratered, light colored • oldest parts of surface • silicate rocks • Maria • 17% of surface, mostly on near side • dark material, fewer craters • youngest parts of surface • volcanic plains of basalt • lava-filled impact basins

  6. Composition & Structure • Average density • 3.3 g/cm3 • lower than Earth’s mantle (5.5 g/ cm3) • Similar to Earth’s crust • Composition • mostly lighter silicates • depleted in iron • similar to Earth’s crust • Mantle • solid • little seismic activity • Core • Small, possibly iron rich • solid and cold Not geologically active (now) DifferentiationMoon’s interior molten in past

  7. Impact Craters • Not erased by erosion (no atmosphere) • preserved record of impacts • indicator of solar system history • Crater Origin • not volcanic • meteor impacts • surface “explosions” • creates circular craters • typical characteristics • bowl-shaped • turned up rims • central peaks

  8. Crater Counts • Number of craters indicates age • many craters = old • few craters = young • Ongoing impacts • during last 3.8 billion yrs • Compare: highlands vs. maria • maria formed 3.8 billion years ago • Based on number of craters • highlands have many more craters • highlands older • Conclude: • Period ofheavy bombardment • prior to 3.8 billion years ago

  9. A moon covered with numerous and very old craters created by meteorite impacts likely • has no ocean to cover the craters. • orbits a large Jupiter sized planet. • has a cold, solid interior. • has no protective magnetic field.

  10. Formation of Moon Giant Impact Theory

  11. Mercury • Closest planet to sun • Surface similar to Moon • heavily cratered • no atmosphere • Only one spacecraft flyby • Mariner 10 flew by 3 timesin 1974-75 • New mission Messenger is in planning stages

  12. Mercury - Structure • Second smallest of planets by mass and size • Mass 1/18 of Earth’s mass • Radius less than half of Earth’s • Pluto is only planet smaller • Density 5.4 g/cm3 • denser than Earth’s mantle • quite different from Moon • Dense iron-nickel core • 60% of total mass • almost size of Moon!

  13. Mercury - Surface • No evidence of tectonic activity • Scarps (cliffs) in middle of craters • crust shrank and cracked(after craters formed)

  14. Earth-Like Planets:Venus and Mars Chapter 7

  15. View from Earth • Venus shows phases • otherwise featureless • Obscured by clouds • Mars appears red • due to iron oxides (rust) • polar ice caps • surface markings • Seasonal changes

  16. Martian Canals • Schiaparelli (1877) reported canale on Mars • Italian = “channels” • English = “canals” • Led to suggestion of intelligent Martian civilization • War of the Worlds (H.G. Wells) • Percival Lowell primary American proponent • Eventually shown to be optical illusion

  17. Highlights • Venus: • seems a “twin” of Earth (but not!) • massive atmosphere • large greenhouse effect • surface temp about 750 K • Mars: • quite cold • once much warmer • very thin atmosphere • once much thicker • had flowing water in past • possibility of life?

  18. Venus: General Properties • Mass • 82% of Earth’s mass • Radius • about same as Earth • Density (5.3 g/cm3) • about same as Earth

  19. Surface of Venus • Problem: how to see through cloud cover? • Spacecraft exploration • Venera 7 (USSR) • landed on surface (1970) • lasted 23 minutes! • Magellan Orbiter (USA) • use radar imaging (from orbit) • mapped surface at 100m resolution

  20. Radar Map of Venus

  21. Surface of Venus • Surface features • produced by volcanic and tectonic activity • butno plate motion • 75% low lava plains • produced like lunar maria • very “young” surface (few craters) • 25% mountains/mountain ranges

  22. Craters on Venus • Few small craters • small objects burn up in dense atmosphere • Use large (>30 km) craters to estimate age • lava plains 500-600 million yrs • vast geologic activity then • not much since

  23. Atmospheric Conditions • Extreme atmospheric pressure / density • 100x greater than Earth • Very high surface temps • around 750 K • little day/night variation • due to thick atmosphere • Generally hot and dry • no water • Occasional “acid rain” • sulfuric acid clouds

  24. Atmosphere of Venus • Layers of sulfuric acid clouds • 30-60 km above surface • Composition • 96% carbon dioxide • 3% nitrogen • remove CO2, then atmos. like Earth’s • Runaway greenhouse effect • Greenhouse gasses -> high temps • water evaporates • carbon dioxide CO2 released from rocks • increases greenhouse

  25. Mars: General Properties • radius • about 1/2 of Earth’s • density 3.9 g/cm3 • less than Earth; more than Moon • mostly silicates • possible metal core • Rotation • period 24 hours, 37 min (like Earth) • tilt of axis about 25º; orbital period 1.88 years • seasons similar to Earth’s • duration ~ 6 months (instead of 3)

  26. Surface Conditions • Temperatures • Summer: • Day 240 K (-33 C) • Night 190 K (-83 C) • Coldest 173 K (-100 C) • Water frost deposits • Surface winds mostly moderate • but giant dust storms can occur

  27. MGS Images of Opportunity Landing Site

  28. MGS Images of Opportunity Landing Site

  29. Mars Atmosphere & Climate • Composition: • 95% carbon dioxide (CO2) • 3% nitrogen (N2) • similar to Venus! • Atmospheric pressure • 100x smaller than Earth’s • equiv. to 30 km above Earth surf. • Clouds • dust clouds • water ice clouds • dry ice (CO2) crystals

  30. Polar Caps • At both N and S poles • change with seasons • seasonal ice caps • composed of dry ice (frozen CO2) • permanent ice caps • composed of water ice

  31. Seasonal Changes

  32. Channels and Flood Plains • Evidence liquid water existed on Mars • highlands runoff channels • from ancient rainstorms? • older than 3.9 million yrs • outflow channels • much larger • carved by huge floods

  33. Climate Change • Evidence indicates • Mars had liquid water (warmer) • much denser atmosphere • What happened? • “Runaway refrigerator effect” • atmosphere began to escape into space (low gravity) • less dense atmosphere -> less greenhouse • surface cools • water freezes -> less greenhouse • more cooling • carbon dioxide freezes -> less greenhouse • happened over 3 billion yrs ago

  34. Planetary Evolution • Earth, Venus, & Mars: • very different results from planetary evolution • why so different when they started similarly? • Runaway greenhouse (Venus) • Runaway refrigerator(Mars) • Could Earth change drastically? • what triggered changes? • Could Mars be made habitable? • terraforming

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