Chapter 7

Our Barren Moon

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Chapter 7Our Barren Moon

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The seismograph placed on the Moon by the Astronauts of the Apollo Program

Does the moon rotate?

What if the Moon did not rotate?

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Chapter 8Our Barren Moon

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Synchronous Rotation

One rotation for one revolution

Lunar Phases as described by Aristotle

We only see the lighted portion of the Moon

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Craters on the Moon and the Earth

ImpactMaria, Basins, Rays

Impact and volcanismRills and Domes

Volcanism

Surface Features

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Craters, Maria and Basins are all due to meteorite impact

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Moon has large dark flat areas, due to impact and lava flow, called maria or basins (early observers thought the basins were oceans or seas)

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Craters have round bottoms, flat bottoms and flat bottoms with a hump depending on the energy deposited.

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Far side of Moon has some large craters, but no large maria. The reason for this difference is still not fully understood.

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Meteoroid strikes Moon, ejecting material; explosion ejects more material; leaving crater, lips (rim) and ejecta

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Regolith: The surface of the moon is heavily cratered. One of the results of the infalling bodies is to pulverize the surface, thus creating a layer debris, much of it is very fine dust. The surfaced is layered with debris on top of pulverized rock.

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Most lunar craters date to at least 3.9

billion years ago; much less

bombardment since then.

Craters are typically about 10 times as

wide as the meteoroid creating them, and

twice as deep as they are wide.

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Early Intense Bombardment

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Moon is still being bombarded by very small objects called “micrometeorites” which trends to round the edges of craters and leave a layer of dust.

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Meteorites also hit Earth; this crater is in Arizona

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The great “lakes “ of liquid rock that filled the large craters are greatest evidence of vulcanism on the moon. Vulcanism ceased when the moon cooled.

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This is an edge of a mare. The sooth appearance is due to the lava that flowed up through cracks , smoothed out then cooled.

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Hadley Rill seems to be an extended lava tube whose roof has fallen in. There are other caved in lava tubes but they are much shorter,

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Field of Domes

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Domes can not be explained by meteorite impact

Moon’s density is relatively low, and it has no magnetic field – cannot have sizeable iron/nickel core. Due to cooling over time the crust has thickened.

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Why Air Sticks AroundAir molecules have high speeds due to thermal motion. If the average molecular speed is well below the escape velocity, few molecules will escape.

Escape becomes more probable:

• for lighter molecules (higher speed for same kinetic energy)

• at higher temperatures

• for smaller planets (escape speed is less)

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Early theories of the origin of the MoonChapter 7

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The currant, post Apollo, theory is the Collision Ejection Theory. Two large planetismals collide. Their crusts splash, their interiors merge. The merged interiors become the Earth. The splashed crustal material becomes the Moon.

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This theory accounts for the difference in density of the Moon and Earth, the reason for the high percentage of iron in the Earth, and the strange orbital placement.

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Time Event4.6 billion yr (?) Formation of Moon; heavy

bombardment 3.9 billion yr Bombardment much less

intense; lunar volcanism fills maria

3.2 billion yr Volcanic activity ceases

The early history of the moon includes the Early Intense Bombardment which is the last step in the accretion process.

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End Chapter 7