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Cratered Worlds: The Moon & Mercury Chapter 7

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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. - PowerPoint PPT Presentation

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Page 1: Cratered Worlds:

Cratered Worlds:

The Moon & Mercury Chapter 7

Page 2: Cratered Worlds:

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)

Page 3: Cratered Worlds:

Apollo 17Dec 1972

Page 4: Cratered Worlds:

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

Page 5: Cratered Worlds:

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

Page 6: Cratered Worlds:

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)

Page 7: Cratered Worlds:

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

Page 8: Cratered Worlds:

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

Page 9: Cratered Worlds:

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.

Page 10: Cratered Worlds:

Formation of Moon

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Giant Impact Theory

Page 11: Cratered Worlds:

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

Page 12: Cratered Worlds:

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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Page 13: Cratered Worlds:

Mercury - Surface

• No evidence of tectonic activity

• Scarps (cliffs) in middle of craters– crust shrank and cracked

(after craters formed)

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Page 14: Cratered Worlds:

Earth-Like Planets:Venus and Mars

Chapter 7

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Page 15: Cratered Worlds:

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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Page 16: Cratered Worlds:

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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Page 17: Cratered Worlds:

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?

Page 18: Cratered Worlds:

Venus: General Properties• Mass

– 82% of Earth’s mass

• Radius – about same as Earth

• Density (5.3 g/cm3)– about same as Earth

Page 19: Cratered Worlds:

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

Page 20: Cratered Worlds:

Radar Map of Venus

Page 21: Cratered Worlds:

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

Page 22: Cratered Worlds:

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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Page 23: Cratered Worlds:

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

Page 24: Cratered Worlds:

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

Page 25: Cratered Worlds:

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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Page 26: Cratered Worlds:

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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Page 27: Cratered Worlds:

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MGS Images of Opportunity Landing Site

Page 28: Cratered Worlds:

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MGS Images of Opportunity Landing Site

Page 29: Cratered Worlds:

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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Page 30: Cratered Worlds:

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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Page 31: Cratered Worlds:

Seasonal Changes

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Page 32: Cratered Worlds:

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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Page 33: Cratered Worlds:

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

Page 34: Cratered Worlds:

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