Part II: Solar System The Moon Audio update: 2014Feb23 The Moon A. Orbital Stuff B. The Surface C. Composition and Interior D. Formation E. Notes 2 A. Orbital Motion 3 Earth-Moon is a Binary Planet 4 1. Center of Mass 2. Tides slowing earth down 3. Distance to Moon Increasing From Voyager 1 (1979) probe. Moon is 60 earth radii away The Moon s Orbit 5 6 The Moon and Earth both orbit around a point between their centers called the center of mass of the Earth-Moon system The center of mass then follows an elliptical orbit around the Sun
2(a) Tides slow earth down 7 Distance to moon measured by timing Laser beams from earth bounced back by retroreflectors left on the moon! Distance increasing 2 cm per year! Tidal bulge is carried forward by rotation (due to friction of ocean with ocean floor). This allows the moon to exert a torque on the earth, slowing it down. So 900 million years ago the day was only 18 hours long! 9 B. Surface of Moon 11 There are very few maria on the Moon s far side. We re not sure why. 1. Crustal Dichotomy 10 The Moon is onefourth the size of the Earth The light gray areas are called highlands they are heavily cratered and mountainous The dark black/gray areas are called maria they are lightly cratered and relatively smooth 1. Crustal Dichotomy 2. Craters 3. Other features The Moon s other side 8 3. Distance to Moon 12 Virtually all lunar craters were caused by space debris striking the surface There is no evidence of plate tectonic activity on the Moon
The maria formed after the surrounding light-colored terrain, so they have not been exposed to meteoritic bombardment for as long and have fewer craters 13 Formation of an impact crater Impacts occur at velocities some 10-20 times the speed of a rifle bullet. This produces a powerful explosion. 14 A recent impact on Earth 15 Impact energy ~1.5 tons of TNT 3. Straight Rilles Rima Ariadaeus as photographed from Apollo 10. This straight rille is probably a fault line, showing moon was geologically active in the past 16 May 17, 1990 Sinuous rilles 17 C. Composition of Moon 18 Collapsed lava tubes. Rille Hadley 1. Moon Rocks 2. Timescale 3. Composition interior
19 Moon Landing Results 20 Six Apollo missions to the Moon were successful; 12 astronauts have walked on the Moon s surface They brought back over 800 pounds of rocks and soil Much of our knowledge about the Moon has come from human exploration in the 1960s and early 1970s and from more recent observations by unmanned spacecraft From those rocks we have learned that The highland rocks solidified more than 4 billion years ago The maria rocks are volcanic lavas, and they are younger; they solidified about 3.2 to 3.9 billion years ago Craters ARE due to impacts (not gas bubbles ) 21 All of the lunar rock samples are igneous rocks formed largely of minerals found in terrestrial rocks 2. Lunar rocks reveal a geologic history quite unlike that of Earth 22 Mare basalt The lunar rocks contain no water They differ from terrestrial rocks in being relatively enriched in the refractory elements and depleted in the volatile elements Highlands anorthosite Impact breccia The anorthositic crust exposed in the highlands was formed between 4.0 and 4.3 billion years ago The mare basalts solidified between 3.1 and 3.8 billion years ago The Moon s surface has undergone very little change over the past 3 billion years 23 3. The Moon has no global magnetic field but has Meteoroid a small core beneath a thick mantle impacts have been the only significant weathering agent on the Moon The Moon s regolith, or surface layer of powdered and fractured rock, was formed by meteoritic action 24
D. Origin of Moon 25 1. Theories of Origin 26 1. Formation Theories 2. Evolution of the Moon Traditional Theories: (a) Capture Theory ( orphan ) (b) Sibling Theory: moon condensed at same time as earth out of same material (c) Daughter Theory: moon was drawn out of earth by tidal forces from another body. None of these seemed to match the evidence brought back from the moon. Difficulties with Formation Theories The Moon s density is much smaller than the Earth (3.3 compared to 5.5 g/cm 3 ). In particular it has very little Iron. This contradicts the sibling (condensation) theory It would have been difficult for the earth to capture such a large body (how does moon slow down and go into orbit?). The moon s orbit would have started big and then gotten smaller. Evidence suggests instead that it was closer in the past. Tidal fission theories require the action of a large body passing close by to pull the moon out. Computations now show that it would just pull the entire planet apart. 27 Giant Impact Theory 28 An object about the size of Mars (about half the diameter of Earth) collided with Earth, blasting a large quantity of the mantle and crust into space. The mantle and crust material was depleted in metal because differentiation was in progress. Giant Impact Theory 29 2. Evolution of the Moon 30 As the Moon s crust cooled, it was heavily bombarded by remaining debris After the heavy bombardment began to subside, volcanic activity arose from pockets of molten material in the mantle and crust This volcanic activity flooded the basins and low-lying parts of the Moon s surface with lava, covering the craters and forming smooth, fresh surfaces (the maria). Similar activity was occurring on the Earth impacts, further melting, volcanic activity, etc.
31 The cratering history of the Moon (simplified) Let s look more closely at a critical time in the History of the Moon 32 Formation of the great basins Episodes of volcanic flooding Of the basins and other lowland regions Billions of years ago 33 The Great Lunar Basins Gigantic impacts blasted out huge circular holes in the crust and upper mantle of the Moon 34 The Imbrium Basin (Mare Imbrium) Flooded by volcanic lava flows, and later lightly cratered. 35 The Imbrium Basin (Mare Imbrium) Flooded by volcanic lava flows, and later lightly cratered. 36 The great Eastern Basin, Mare Orientale
E. References 37 http://spiff.rit.edu/classes/phys235/no_moo n/no_moon.html http://lroc.sese.asu.edu/