9.1 Properties of the Moon
1. Basic Properties of the Moon
The Moon is significantly smaller and less massive than Earth:
Mass: only one-eightieth (\frac{1}{80}) the mass of Earth (0.0123 Earth mass).
Surface gravity: about one-sixth (\frac{1}{6}) Earth’s surface gravity (0.17 Earth average).
Diameter: 3476 km.
Density: 3.3 g/cm^3.
Escape velocity: 2.4 km/s.
Rotation period: 27.3 days.
Surface area: 0.27 Earth average.
Atmosphere: Due to its low surface gravity, the Moon cannot retain a permanent atmosphere.
Movable gas molecules easily escape into space.
Temporary atmospheres from impacting comets are quickly lost by freezing or escape.
Volatiles: The Moon is dramatically deficient in volatiles (elements and compounds that evaporate at low temperatures).
Surface Features: The hemisphere facing Earth shows several dark maria, while the far side is dominated by highlands (Figure 9.2).
2. Lunar Exploration: Achievements and Learnings
2.1 Manned Lunar Exploration: The Apollo Program
Objective: Discuss what has been learned from manned lunar exploration.
Most of our knowledge comes from the US Apollo program (1968-1972):
Sent nine piloted spacecraft to the Moon.
Landed 12 astronauts on the surface.
Historical context:
Prior to spacecraft, astronomers mapped the Earth-facing side with ~1 km resolution; lunar geology was nascent.
Russia initially led with Luna 3 (1959, first photos of far side) and Luna 9 (1966, first soft landing, transmitted data).
Apollo 11 (July 20, 1969): First American astronaut on the Moon.
Key Apollo Missions and Accomplishments (Table 9.2):
Apollo 8 (Dec. 1968): First humans to fly around the Moon.
Apollo 10 (May 1969): First spacecraft rendezvous in lunar orbit.
Apollo 11 (July 1969): First human landing (Mare Tranquillitatis); 22 kg of samples returned.
Apollo 12 (Nov. 1969): First Apollo Lunar Surface Experiment Package (ALSEP); visited Surveyor 3 lander.
Apollo 13 (Apr. 1970): Landing aborted due to service module explosion.
Apollo 14 (Jan. 1971): First “rickshaw” on the Moon (Mare Nubium).
Apollo 15 (July 1971): First “rover”; visited Hadley Rille; astronauts traveled 24 km.
Apollo 16 (Apr. 1972): First landing in highlands (Descartes); 95 kg of samples returned.
Apollo 17 (Dec. 1972): Geologist (Jack Schmitt) among the crew (Taurus-Littrow highlands); 111 kg of samples returned.
Scientific Objectives Achieved by Apollo Missions:
Sample Collection: Astronauts collected nearly 400 kg of lunar samples for laboratory analysis on Earth, providing immense insights into the Moon's history and composition.
ALSEP Deployment: Each landing after Apollo 11 deployed an ALSEP, operating for years to collect scientific data.
Orbital Instrumentation: Apollo command modules carried instruments to photograph and analyze the lunar surface from orbit.
Cessation and Future of Manned Exploration:
The last human left the Moon in December 1972, ending the program due to political and economic pressures.
Cost: approximately 100 per American, spread over 10 years.
Future Plans: Renewed international interest in human lunar flights.
NASA’s Artemis program plans to place astronauts in lunar orbit by mid-2020s, focusing on polar landings and including diverse crews.
China has also expressed interest and completed its own space station.
2.2 Robotic Lunar Exploration
Objective: Discuss what has been learned from robotic lunar exploration.
Alongside manned missions, numerous robotic spacecraft have greatly advanced our understanding.
Early Robotic Efforts: The USSR sent many robotic spacecraft in the 1960s, including robot sample return missions.
Modern International Enterprise: Lunar exploration is now a global effort with contributions from NASA, ESA, Japan, India, and China.
Recent Robotic Missions (Table 9.3 highlights some):
Orbiters: Clementine (USAF/NASA, 1994), Lunar Prospector (NASA, 1998), SMART-1 (ESA, 2003), SELENE 1 (JAXA, 2007), Chang’e 1 & 2 (CNSA, 2007, 2010), Chandrayaan-1 & 2 (ISRO, 2008, 2019), LRO (NASA, 2009), GRAIL (NASA, 2011), LADEE (NASA, 2013), Danuri (KARI, 2022), CAPSTONE (NASA, 2022), Quegiao-2 (CNSA, 2024).
Landers/Rovers: Chang’e 3 (CNSA, 2013), Chang-e 4 (CNSA, 2019, far side), SLIM (JAXA, 2023), Chandrayaan-3 (ISRO, 2023).
Impactors: LCROSS (NASA, 2009).
Sample Return: Chang-e 5 (CNSA, 2020), Chang-e 6 (CNSA, 2024).
Key Focus of Robotic Missions: A significant area of interest is the search for accessible water ice, particularly near the lunar South Pole.
3. Composition and Structure of the Moon
Objective: Describe the composition and structure of the Moon.
Density and Composition: The Moon’s average density of only 3.3 g/cm^3 indicates it is made almost entirely of silicate rock.
Compared to Earth, the Moon is depleted in iron, other metals, and volatiles.
This suggests the Moon is composed of similar silicates as Earth’s mantle and crust, but with metals and volatiles selectively removed.
Internal Structure: Studies confirm the absence of a large metal core.
Seismometers deployed by the Apollo program provided initial insights.
Twin GRAIL spacecraft (2011) offered more precise tracking of its shallow interior structure.
Water on the Moon:
Depletion: Lunar samples show depletion of water and other volatiles from the lunar crust.
Chemically Bound Water: Scientists have concluded that some chemically bound water is present in lunar rocks.
Water Ice at Poles: Most dramatically, water ice has been detected in permanently shadowed craters near the lunar poles.
LCROSS Mission (2009): Successfully crashed into Cabeus crater near the South Pole at 9,000 km/h, releasing a plume of water vapor and chemicals.
LRO Mission: Measured very low temperatures inside lunar craters and imaged their interiors by starlight.
Estimated Quantity: Hundreds of billions of tons of water ice, enough to fill a lake 100 miles (160 km) across, a remarkable amount for the dry lunar crust.
Origin: This polar water was likely carried to the Moon by impacting comets and asteroids.
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