Part II: Solar System

Audio update: 2014Feb23

The Moon

The Moon

A. Orbital Stuff

B. The Surface

C. Composition and Interior

D. Formation

E. Notes

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A. Orbital Motion

1. Center of Mass

2. Tides slowing earth down

3. Distance to Moon Increasing

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Earth-Moon is a “Binary Planet”

From Voyager 1 (1979) probe. Moon is 60 earth radii away

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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

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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!

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3. Distance to Moon

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

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B. Surface of Moon

1. Crustal Dichotomy

2. Craters

3. Other features

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• 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

1. Crustal Dichotomy 10

The Moon’sother side

There arevery fewmaria onthe Moon’sfar side.

We’re notsure why.

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• Virtually all lunar craters were caused by space debris striking the surface

• There is no evidence of plate tectonic activity on the Moon

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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

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Formation of an impact crater

Impacts occurat velocitiessome 10-20times the speedof a rifle bullet.

This produces apowerfulexplosion.

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May 17, 1990

Impactenergy~1.5 tons of TNT

A recent impact on Earth

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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

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Sinuous rilles

Collapsed lava tubes.

Rille Hadley

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C. Composition of Moon

1. Moon Rocks

2. Timescale

3. Composition interior

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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

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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

• 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”)

All of the lunar rock samples are igneous rocks formed largely of minerals found in terrestrial rocks

• 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

Mare basalt

High-lands

anorth-osite

Impact breccia

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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

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• 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

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

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D. Origin of Moon

1. Formation Theories

2. Evolution of the Moon

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1. Theories of Origin

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.

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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.

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28Giant Impact Theory

• 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 crustmaterial wasdepleted in metalbecause differentiationwas in progress.

29Giant Impact Theory

2. Evolution of the Moon

• 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.

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The cratering historyof the Moon (simplified)

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Formation of the great basins

Episodes of volcanic floodingOf the basins and other lowland regions

Billions of years ago

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

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The Great Lunar BasinsGigantic impacts blasted out huge circular holes

in the crust and upper mantle of the Moon

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The ImbriumBasin

(Mare Imbrium)

Flooded byvolcanic lava

flows, and laterlightly cratered.

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The ImbriumBasin

(Mare Imbrium)

Flooded byvolcanic lava

flows, and laterlightly cratered.

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The great Eastern Basin, Mare Orientale

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E. References

• http://spiff.rit.edu/classes/phys235/no_moon/no_moon.html

• http://lroc.sese.asu.edu/

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