Chapter 21 The Moon and Mercury: Comparing Airless Worlds
Outline I. The Moon A. The View From Earth B. The Apollo Missions C. Moon Rocks D. The History of the Moon E. The Origin of Earth's Moon II. Mercury A. Rotation and Revolution B. The Surface of Mercury C. The Plains of Mercury D. The Interior of Mercury E. A History of Mercury
Lunar Surface Features Two dramatically different kinds of terrain: Highlands: Mountainous terrain, scarred by craters Lowlands: ~ 3 km lower than highlands; smooth surfaces: Maria (pl. of mare): Basins flooded by lava flows
Highlands and Lowlands Sinuous rilles = remains of ancient lava flows May have been lava tubes which later collapsed due to meteorite bombardment. Apollo 15 landing site
What do we know? (cont) Post-1960 s: Apollo Missions take us to the Moon Seven manned missions between 1969 and 1972. Six successfully land Leave seismometers Leave reflectors Bring back 842 lbs of rock! 2,415 samples. Study of Lunar Geology Begins! 5
The Apollo Missions
The Blue Marble Apollo 17 December 7, 1972 7
Apollo Landing Sites
Apollo Landing Sites First Apollo missions landed on safe, smooth terrain. Apollo 11: Mare Tranquilitatis; lunar lowlands
Apollo Landing Sites First Apollo missions landed on safe, smooth terrain. Apollo 11: Mare Tranquilitatis; lunar lowlands Later missions explored more varied terrains. Apollo 17: Taurus-Littrow; lunar highlands
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Scenes From Apollo: Mare Tranquilitatis: Landing site of 1 st Apollo mission. Reflector deployed by Apollo 14 astronauts. 10
Reflectors: Moon is moving away from Earth about 3.8 cm per year! Was much closer to us in the past! 11
12 Buzz Aldrin placing lunar seismometer. (1969) Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Seismometers: data radioed back to Earth turned off in 1977 4 Types of Moon quakes: Deep quakes (700km) due to tidal forces Shaking due to meteor impacts Thermal expansion of crust Shallow (30km), long (up to 10 min) quakes 13
Seismometers: data radioed back to Earth turned off in 1977 4 Types of Moon quakes: Deep quakes (700km) due to tidal forces Shaking due to meteor impacts Thermal expansion of crust Shallow (30km), long (up to 10 min) quakes 28 detected in 8 years (3 w/ mag 5.5) Source unknown; landslides in crater walls? 13
The Clues in the Rocks: Basaltic Rock (Mare) Age: 3.1-3.8 billion years Anorthositic Rock (Highlands) Age: 4.3 billion years The oldest rocks are comparable to age of Earth. Mare rocks are younger Moon lacks volatiles (elements with low boiling point). 14 Moon has weak magnetic field
Moon Rocks All moon rocks brought back to Earth are igneous (= solidified lava) No sedimentary rocks => No sign of water ever present on the moon. Different types of moon rocks: Vesicular (= containing holes from gas bubbles in the lava) basalts, typical of dark rocks found in maria Breccias (= fragments of different types of rock cemented together), also containing anorthosites (= bright, low-density rocks typical of highlands) Older rocks become pitted with small micrometeorite craters
Formation of Maria
Formation of Maria Impacts of heavy meteorites broke the crust and produced large basins that were flooded with lava
Formation of Maria (2) Major impacts forming maria might have ejected material over large distances. Apollo 14 Large rock probably ejected during the formation of Mare Imbrium (beyond the horizon!)
Origin of Mare Imbrium
Origin of Mare Imbrium
Origin of Mare Imbrium Terrain opposite to Mare Imbrium is jumbled by seismic waves from the impact.
Kaguya Orbiter Japan, 2007 http://www.youtube.com/watch?v=oy2l9jti9z4 http://www.youtube.com/watch?v=v7tzngygg-8&feature=relmfu LCROSS October 9, 2009 Cabeus Crater Water on Moon http://www.youtube.com/watch?v=mvyeoffv_lo 19
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The Origin of Earth s Moon Early (unsuccessful) hypotheses:
The Origin of Earth s Moon Fission hypothesis: Early (unsuccessful) hypotheses:
The Origin of Earth s Moon Fission hypothesis: Early (unsuccessful) hypotheses: Break-up of Earth during early period of fast rotation
The Origin of Earth s Moon Fission hypothesis: Early (unsuccessful) hypotheses: Break-up of Earth during early period of fast rotation Problems: No evidence for fast rotation; moon s orbit not in equatorial plane
The Origin of Earth s Moon Fission hypothesis: Early (unsuccessful) hypotheses: Break-up of Earth during early period of fast rotation Problems: No evidence for fast rotation; moon s orbit not in equatorial plane Condensation hypothesis:
The Origin of Earth s Moon Fission hypothesis: Early (unsuccessful) hypotheses: Break-up of Earth during early period of fast rotation Problems: No evidence for fast rotation; moon s orbit not in equatorial plane Condensation hypothesis: Condensation at time of formation of Earth
The Origin of Earth s Moon Fission hypothesis: Early (unsuccessful) hypotheses: Break-up of Earth during early period of fast rotation Problems: No evidence for fast rotation; moon s orbit not in equatorial plane Condensation hypothesis: Condensation at time of formation of Earth Problem: Different chemical compositions of Earth and moon
The Origin of Earth s Moon Fission hypothesis: Early (unsuccessful) hypotheses: Break-up of Earth during early period of fast rotation Problems: No evidence for fast rotation; moon s orbit not in equatorial plane capture hypothesis: Condensation hypothesis: Condensation at time of formation of Earth Problem: Different chemical compositions of Earth and moon
The Origin of Earth s Moon Fission hypothesis: Early (unsuccessful) hypotheses: Break-up of Earth during early period of fast rotation Problems: No evidence for fast rotation; moon s orbit not in equatorial plane capture hypothesis: Capture of moon that formed elsewhere in the solar system Condensation hypothesis: Condensation at time of formation of Earth Problem: Different chemical compositions of Earth and moon
The Origin of Earth s Moon Fission hypothesis: Early (unsuccessful) hypotheses: Break-up of Earth during early period of fast rotation Problems: No evidence for fast rotation; moon s orbit not in equatorial plane capture hypothesis: Capture of moon that formed elsewhere in the solar system Condensation hypothesis: Problem: Requires succession of very unlikely events Condensation at time of formation of Earth Problem: Different chemical compositions of Earth and moon
Fission Hypothesis: Young Earth spins so fast that material gets flung off. Material coalesces to form Moon. Appeal: density of Moon is similar to Earth s crust we know rotation rate of Earth has been decreasing Problems: energies required are too large! Moon is missing volatiles in its rocks molecules that are common in Earth s rocks. 22
Capture Hypothesis: Young Earth seizes a fully formed object that happens to wander too close. Appeal: resolves differences in composition Moon s orbital plane is close to ecliptic Problems: why are average densities so different? probability of capture is very low! (need d<50,000 km) 23
Co-Creation Hypothesis: Earth and Moon form concurrently from same parental cloud of gas and dust. Appeal: At first glance, this is the most intuitive hypothesis Problems: fails to explain different densities. fails to explain lack of volatiles. why is moon so large? 24
Large Impact Hypothesis: Young Earth suffers a collision with an object large enough to throw substantial debris into orbit. Debris coalesces into Moon. This is currently the most widely accepted paradigm for the origin of the Moon. Why? 25
Could such a collision produce a Moon with the properties we observe? Numerical simulations are powerful tools! 26
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