Part II: Solar System The Moon Audio update: 2014Feb23

The Moon Orbital Stuff The Surface Composition and Interior Formation 2 The Moon Orbital Stuff The Surface Composition and Interior Formation Notes

A. Orbital Motion Center of Mass Tides slowing earth down 3 A. Orbital Motion Center of Mass Tides slowing earth down Distance to Moon Increasing

Earth-Moon is a “Binary Planet” 4 From Voyager 1 (1979) probe. Moon is 60 earth radii away

5 The Moon’s Orbit 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

6

2(a) Tides slow earth down 7 2(a) Tides slow earth down 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!

3. Distance to Moon 8 Distance to moon measured by timing Laser beams from earth bounced back by retroreflectors left on the moon! Distance increasing 2 cm per year!

9 B. Surface of Moon Crustal Dichotomy Craters Other features

1. Crustal Dichotomy The Moon is one-fourth the size of the Earth 10 The Moon is one-fourth 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

The Moon’s other side There are very few maria on the Moon’s far side. 11 The Moon’s other side There are very few maria on the Moon’s far side. We’re not sure why.

12 Virtually all lunar craters were caused by space debris striking the surface There is no evidence of plate tectonic activity on the Moon

13 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

Formation of an impact crater 14 Formation of an impact crater Impacts occur at velocities some 10-20 times the speed of a rifle bullet. This produces a powerful explosion.

Impact energy ~1.5 tons of TNT A recent impact on Earth 15 May 17, 1990

3. Straight Rilles 16 Rima Ariadaeus as photographed from Apollo 10. This straight rille is probably a fault line, showing moon was geologically active in the past

Sinuous rilles 17 Collapsed lava tubes. Rille Hadley

C. Composition of Moon 18 Moon Rocks Timescale Composition interior

19 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

Moon Landing Results From those rocks we have learned that 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 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”)

High-lands anorth-osite 21 All of the lunar rock samples are igneous rocks formed largely of minerals found in terrestrial rocks High-lands anorth-osite 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 Impact breccia

2. Lunar rocks reveal a geologic history quite unlike that of Earth 22 2. Lunar rocks reveal a geologic history quite unlike that of Earth 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 Meteoroid 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

3. The Moon has no global magnetic field but has a small core beneath a thick mantle 24

25 D. Origin of Moon Formation Theories Evolution of the Moon

1. Theories of Origin Traditional Theories: Capture Theory (“orphan”) 26 1. Theories of Origin Traditional Theories: Capture Theory (“orphan”) “Sibling” Theory: moon condensed at same time as earth out of same material “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 27 Difficulties with Formation Theories The Moon’s density is much smaller than the Earth (3.3 compared to 5.5 g/cm3). 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.

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.

29 Giant Impact Theory

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.

of the Moon (simplified) 31 The cratering history of the Moon (simplified)

Let’s look more closely at a critical time in the 32 Let’s look more closely at a critical time in the History of the Moon 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

Flooded by volcanic lava flows, and later lightly cratered. 34 The Imbrium Basin (Mare Imbrium) Flooded by volcanic lava flows, and later lightly cratered.

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

37 E. References http://spiff.rit.edu/classes/phys235/no_moon/no_moon.html http://lroc.sese.asu.edu/