Astronomy

A. Dayle Hancock

adhancock@wm.eduSmall 239

Office hours: MTWR 10-11am

http:// physics.wm.edu/~hancock/171/

The Moon ● The Moon's surface● Humans on the Moon● The Moon's interior● The difference between Moon and Earth rocks

● The collision that formed the Moon

Moon Data

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Comparing the Earth and Moon

The average distance from the Earth to the Moon is 384,400 km. The Moon's diameter is about ¼ the Earth's diameter. The mass of the Moon is only 1.2% the mass of the Earth

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Center of Mass of the Earth-Moon System

The Earth and Moon both orbit around their center of mass. It is this center of mass point that follows the elliptical orbit around the Sun.

The Moon Rotation

The Moon's synchronous rotation results in our seeing only one side of the Moon. We never see the far or back side. It takes exactly the same time for the Moon to rotate on its axis as it takes to complete one orbit around the Earth. The synchronous rotation is because of 'tidal locking'

Actually, you can see slightly more of than ½ the Moon's surface (59%). This is because the Moon's orbit is slightly elliptical (e=0.0549). The orbital motion does not keep up with its rotation so it appears to 'wobble' back and forth. Because the rotation axis is not exactly perpendicular to the plane of the orbit it appears to 'nod' up and down. These effects are know as libration and result in 59% of the surface being visible from earth at some time during its orbit. 5

Moon Craters

Tens of thousands of impact crates are visible from Earth through a telescope. They range from size from 1 – 100s km. They are traditionally named after philosopher, mathematicians and scientist.

From lunar orbit millions of impact crates can be seen. All of the impact crates on the Moon are the result of asteroids and meteoroid strikes.

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Moon Craters

Nearly all lunar features are the result of impacts by solid bodies early in the Moon’s historyCraters forms when a high-velocity projectile impact

– A cloud of vaporized rock and fragments result in a hole in the surface

There is no weather or current tectonic activity on the moon

– We see the scars of a long history of impacts

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Highlands and Maria

Surface divided into two major regions➢ Highlands –Rugged areas composed mainly of a rock rich in

calcium and aluminum silicates and pitted with craters. Lighter in color and ancient

➢ Maria – Large, smooth areas surrounded by highlands and composed primarily of congealed lava (more dense than the highlands). Younger and darker in color

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Surface Features

• Astronauts brought back rock samples for radiometric dating> Rocks from the highlands are as old as 4.4 billion years> Maria rocks are 3-3.9 billion years old> The very few craters in the maria tell us that the intense

bombardment ended by about 4 billion years ago

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Origin of Lunar Surface Features

• The highlands are the result of the very intense bombardment by solar system bodies soon after the Moon formed and created a solid surface

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Origin of Lunar Surface Features

A mare formed when early in the Moon’s history, a few large bodies (over 100 km) strike the Moon. Molten material floods the newly formed lunar depression and eventually congeals

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Near Side Far Side

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The nearside of the moon has large areas of Maria. There are only a few craters in the Maria areas

The far side of the Moon is mainly highlands which are heavily cratered.

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The central peak in a crater

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Rilles

Rilles are ancient cooled lava flows that once flowed like a river. Note the impact craters that happened after the lava solidified.

Scarps

Scarps are long cliff structures which were carved by ancient lava flows or crustal cracking (… think uncovered pudding in the fridge cracking).

Small impact craters that cut across this scarp indicate the Moon is still shrinking and cracking.

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Humans on the Moon

• How many men have been to the moon?> 12 have walked on the surface> 24 different men have been

near the moon> There have been no women

on the moon• Apollo 11 landed in 1969• Apollo 17 was the last in

1972

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Recent Robotic Spacecraft

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In 1994, the spacecraft Clementine mapped the lunar surface. The upper image shows the iron content of the Moons surface. It also suggested there could be ice at the south pole.

The lower image show the permanently shadowed crater that was impacted by the LCROSS probe. Spectra observed from Earth showed water. However, the amounts of water are very small.

The Moon Interior

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• How do we learn about the interior of the moon? > Hint: How did we learn

about the interior of the earth?

• Moonquakes (seriously!)

Moonquakes

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Seismic detectors set up on Moon by astronauts > Essentially found to be

inactive and has simpler structure than Earth’s

Many fewer moonquakes than on earth, less intense, but last longer

Because the fluid core is so small, the Moon has no magnetic field

Moonquakes

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Moonquakes are not caused by tectonic activity. There are no tectonic plates or plate motion on the Moon.

Moonquakes are caused by the Earth's tidal forces on the Moon. These tidal forces deform and flex the solid body of the Moon.

The tidal forces are greatest when the Moon is nearest to the Earth at perigee. The moonquakes are greatest from the Apollo seismometer at perigee. The seismometers also recorded hits by meteoroids. 80-150 meteoroids hit the Moon every year and ranging in size from 0.1 – 1000 kg.

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Lunar Atmosphere• No atmosphere for

two reasons> Lack of volcanic

activity to supply source of gas

> Moon’s gravitational force not strong enough to retain gases even if there was a source

• Lack of atmosphere and plate tectonics implies that the Moon has been unchanged for billions of years

• Lack of an atmosphere means extreme changes in lunar surface temperature from night to day

Moon Dust - Regolith

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• Surface layer is shattered rock chunks and powder (from repeated impacts)

• Tens of meters thick• Regolith up to hundreds of

meters deep in some places• Regolith reflects the local rock

composition

Moon Rocks

The Apollo astronauts returned 383 kg of rock samples from the six landing sites. The Soviet unmanned landers luna 16, 20 and 24 returned 0.3 kg from different sites. Together they give a geological history of the Moon. This dark moon

rock is a mare basalt that was originally lava with dissolved gases. When the material reached the surface, the gases expanded. When the lava cooled, the gas bubbles formed the holes you see in the surface. From radioactive dating, basalt rocks are typically 3.1 – 3.8 billion years old.

Moon Rocks

This light colored anorthosite lunar sample is from the lunar highlands. This material is thought to be from the original lunar crust. It is less dense than basalt and rose to the surface when the surface was molten. The color can vary from dark gray to white. Since anorthosites are from the original crust, they are 4.0 – 4.3 billion years old.

Moon Rocks

Most of the lunar highlands are anorthosite but some of the lunar samples are impact Breccias. It is a mixture of various types of rocks that have been mixed and fused together by a series of meteoritic impacts. Because they are common in the highland area, they are evidence that the highlands have been bombarded by meteorites for eons.

The Moon

Studying the crater density and age of moon rocks, the history of the bombardment of the Moon can be determined. The early heavy bombardment cratered the entire moon. The bombardment 4.0-3.8 billion years ago created the maria regions from giant impacts. The bombardment has been smaller and more constant over the last 3.5 billion years.

The Moon is Receding

The tides caused by the moon elongate the Earth's oceans. Since the earth is spinning, the friction between Earth and the ocean is pulled slightly ahead of the line between the Moon and Earth. This pulls the Moon forward and slightly increase its speed. This is causing the Moon to slowly spiral outward. The rate is only 3.8 cm per year!

There are three discredited ideas of how the moon formed.

The fission theory: The early Earth was spinning so fast a large amount of material tore away which became the Moon.The capture theory: The moon was formed elsewhere in the solar system and captured by the Earth's gravitational field.The co-creation theory: The Moon and Earth formed at the same time but separately.

The Moon Formation

The current best theory of how the Moon formed is called the Collision Ejection Theory

The Moon's Formation

The Moon

This Collision Ejection theory explains:The impact would vaporize low-melting-point materials (e.g., water) and disperse them explaining their lack in the MoonOnly surface rock blasted out of Earth leaving Earth’s core intact and little iron in the MoonEasily explains composition difference with the EarthThe splashed-out rocks that would make the Moon would more naturally lie near the ecliptic than the Earth’s equatorial plane Earth’s tilted rotation axis is explained