Vital Statistics. The Moon. The Tides The gravitational pull between the Earth, the Moon and the Sun causes three inter-related effects: Lunar Phases

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1 Vital Statistics Orbit & tides Apollo & beyond Surface Interior Origin The Moon Vital Statistics Mean distance from Earth km Orbital period (sidereal) Rotational period days Eccentricity of orbit Inclination of orbit Diameter (at equator) Mass days (1:1 resonance or synchronous) degrees d Earth (3476 km) M earth Mean density 3340 kg m -3 Surface gravity 0.17 g Lunar Phases Synchronous orbit means we only see near-side. We actually see ~60% of the surface because of: Librations due to Sun s gravity and Earth s orbit. Parallax Phase cycle is a synodic month of days. The Tides The gravitational pull between the Earth, the Moon and the Sun causes three inter-related effects: 1. The gravitational force acting on Earth due to the Moon varies by about 3% across the Earth s diameter. This causes two tides per day as the Earth rotates. 1

2 The Tides The Sun produces about half the effect of the Moon. Largest (spring) tides are when the 3 bodies are aligned. The Tides 2. Due to friction, the line of the tidal bulge is displaced from the Earth-Moon line in the direction of Earth s spin. This tidal torque slows the Earth s by 1.4 ms per day per century (on average). To conserve angular momentum the Moon moves away from Earth at ~3.8 cm year -1. Leap seconds UT1 (GMT = mean solar time). UTC = TA1 (n x seconds) where TA1 is International Atomic Time. By international law UTC & UT1 cannot differ by more than 0.9 s. Currently: The Tides 3. The larger tidal torque acting on the Moon caused the synchronous orbit and appears to have altered the Moon s internal mass distribution probably set when the Moon was partly molten and closer to Earth. UT1-UTC TA1 is ahead of UTC by 35s (For more information see: 2

3 Project Apollo Yuri Gagarin: USSR, Vostok 1, April Alan Shepard: USA, Freedom 7, May John F. Kennedy: Speech to Congress, May Manned Lunar Exploration LEM President John F. Kennedy Rice University, Texas September 12, 1962 Neil Armstrong, Apollo 11 Sea of Tranquility July 20, 1969 For sale: $40 Million; one very, very careful driver Passive seismic station Where did Apollo go? Post-Apollo examples Clementine: SDI test-mission multi-wavelength survey. Lunar Prospector: 1998/9 NASA mission. Six landing sites, all on the near side Smart 1: ESA mission. GRAIL: NASA mission. 3

4 Lunar Reconnaissance Orbiter: imaging of Apollo 11 landing site Why you have a pilot TV camera Moon the near side and the far side The LEM (Eagle) Aitken Reflector & seismic package Impact Craters Armstrong s footprints to Little West crater Far side (80 km) The Moon s surface is dominated by craters and dark features the maria (17% of area), mostly on the near side. Maria & Rilles LM Copernicus 93 km wide Lava layers Herschel (45 km) Mare Imbrium the largest ~ 1400 km diameter Hadley Rille site for Apollo 15 4

5 Impact Cratering No evidence of plate tectonics. The lithosphere of the Moon is one plate. Most of the surface features of the Moon are impact generated. Surface features erode slowly by new bombardment (~ 5 m per 10 9 years). The shape of a crater depends on many factors (e.g. gravity, mantle depth, rock strength/density, infill ). On Moon depth:diameter ~ 1:20 Lunar gravity Interior No dipole B-field. Some remnant surface magnetism. Iron core ~300 km radius. Partial melting to ~700 km. Thick lithosphere ~900 km. Crust ~ 40km thick (varies) (all numbers approximate) Moon GRAIL (average ~160,000 mgal; 1 mgal = cm s -2 ) Crust thinner on the near-side. Possibly why more marias on the near-side? 5

6 Lunar aging using craters & rocks Use the geological law of superposition (younger features lie on top of older features) to divide the lunar meteorite bombardment period into 5 eras. Get absolute ages from radiometric dating (isotope ratios) of Apollo rocks. Pre-Nectarian Nectarian Imbrian Eratosthenian Copernican billion years ago billion years ago billion years ago billion years ago 1.0 billion years ago now Lunar Rocks See three major surface types: Regolith almost the entire surface is covered in a layer, usually 2-7 m thick, of fine dust and small rocks breccia due to impacts and melts. The underlying bedrock is fractured down to ~20 km. Breccia from highlands Lunar Rocks Highlands dominated by anorthosite, an intrusive, igneous rock. Rich in Ca/Al, poor in Ti/Fe. Impacts melted surface ~3.8-4 billion years ago. Anorthosite from highlands Maria dominated by extrusive igneous basalt. Infill is fairly shallow (<1 km) so the maria comprise ~1% of crust by mass. Most billion years old. Basalt from maria 1. Fission Earth throws off the Moon. Requires rapid Earth rotation and hard to keep Moon in orbit. 2. Capture already formed Moon captured. Low probability and why are isotope ratios similar? 3. Co-accretion form together. Why low density and orbital inclination? 6

7 Post-Apollo formation theory Giant impact: hit the young Earth with a large (~Mars size) object. Moon formed from mix of Earth-mantle material and remains of the impactor. Impact +30 mins +5 hours +10 years Impactor needs to be 10-15% Earth mass (~Mars). Hits Earth when Earth was about half formed. Earth grows significantly (gains ~2/3 of impactor). Moon forms quickly from debris field very hot. Earth and Moon continue to grow by accretion. Today The End 7