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Cratered Worlds:
The Moon
&
Chapter 7
Mercury
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
• 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
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• Composition
– mostly lighter silicates
– depleted in iron
– similar to Earth’s crust
• Mantle
– solid
– little seismic activity
Not geologically active (now)
• Core
– Small, possibly iron rich
– solid and cold
Differentiation
Moon’s interior molten in past
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)
b)
c)
d)
has no ocean to cover the craters.
orbits a large Jupiter sized planet.
has a cold, solid interior.
has no protective magnetic field.
Formation of Moon
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Giant Impact Theory
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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
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Chapter 7
View from Earth
• Venus shows phases
– otherwise featureless
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• Obscured by clouds
• Mars appears red
– due to iron oxides (rust)
– polar ice caps
– surface markings
• Seasonal changes
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Martian
Canals
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• 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
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• 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)
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
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Mars Atmosphere & Climate
• Composition:
– 95% carbon dioxide (CO2)
– 3% nitrogen (N2)
– similar to Venus!
• Atmospheric pressure
– 100x smaller than Earth’s QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture
– equiv. to 30 km above Earth surf.
• Clouds
– dust clouds
– water ice clouds
– dry ice (CO2) crystals
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