Cratered Worlds:
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Transcript of 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 17Dec 1972
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
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
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
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DifferentiationMoon’s interior molten in past
Not geologically active (now)
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
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 of heavy bombardment
• prior to 3.8 billion years ago
A moon covered with numerous and very old craters created by meteorite impacts likely
a) has no ocean to cover the craters. b) orbits a large Jupiter sized planet. c) has a cold, solid interior. d) has no protective magnetic field.
Formation of Moon
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Giant Impact Theory
Mercury
• Closest planet to sun• Surface similar to Moon
– heavily cratered– no atmosphere
• Only one spacecraft flyby– Mariner 10 flew by 3 times
in 1974-75– New mission Messenger
is in planning stages
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!
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Mercury - Surface
• No evidence of tectonic activity
• Scarps (cliffs) in middle of craters– crust shrank and cracked
(after craters formed)
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Earth-Like Planets:Venus and Mars
Chapter 7
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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
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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
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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?
Venus: General Properties• Mass
– 82% of Earth’s mass
• Radius – about same as Earth
• Density (5.3 g/cm3)– about same as Earth
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
Radar Map of Venus
Surface of Venus
• Surface features – produced by volcanic and
tectonic activity– but no plate motion– 75% low lava plains
• produced like lunar maria• very “young” surface (few
craters)
– 25% mountains/mountain ranges
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
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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
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
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)
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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
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MGS Images of Opportunity Landing Site
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MGS Images of Opportunity Landing Site
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
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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
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Seasonal Changes
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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
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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
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