Astronomy 100 -- Our Moon in Detail

THE MOON AS A MOON
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Basic Properties of the Moon

1) How big is it?
A) Diameter = 1/3 the diameter of the Earth.
(The Full Moon at Perigee and Apogee)
B) Mass = 1/100 the mass of the Earth.

2) How old it it?

4.6 billion years

3) Where is it?

1.25 light-seconds from the Earth.

4) What's it doing?

Rotation Period = 27.3 days.
Revolution Period = 27.3 days.

This is known as SYNCHRONOUS ROTATION.
This results in the same side of the Moon always facing the Earth.

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Note the very different appearance of
the near side and far side of the Moon!
Very few dark regions on the far side.

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THE MOON'S INTERNAL STRUCTURE

A) Possible small solid iron core with a radius about 300 km.
Evidence is magnetic field in lunar rocks.

B) Partially molten shell about 1000 km thick.

Evidence is weak "moonquakes".

C) Thick crust (60 km) and lithosphere (1000 km).

No plate tectonics ==> no mountains forming.

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Large Scale Features

1) Highlands - light colored, heavily cratered old surface.

2) Mare (Latin for "sea") - Dark, smooth, "newer" surface, few craters.

Small Scale Features

1) Rilles - like dry river beds, due to flowing lava.




2) Craters - Raised circular ridges due to impacts.



Some have central peaks ==> impact origin.

But, there are some volcanic cinder cones on the surface.

• SURPRISE! Water ice in polar region craters! Probably from comets.

Understanding the origin of the Moon has been a great problem.

The Simple Theories

1) Fission theory - Moon broke away from fast spinning Earth while forming.
Earth would have had to be spinning very fast!

2) Accretion (binary) theory - Moon accreted by itself when Earth formed.

Why doesn't the Moon have a large iron core like Earth?

3) Capture theory - Earth captured Moon after each had formed.

If they formed in different places, why are they so similar in composition?

Early Earth hit by Mars-sized object. The iron core is either ejected or coalesces with the Earth's core. Rock fragments coalesce to form the Moon.

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A) Formation Period

Actual formation process uncertain.
Believed to have occurred very quickly (few million years).

B) Early Period - Formed as a liquid and differentiated.

Meteors impacted solid crust up to 4 billion years ago.

C) Volcanic Period - Lava flows filled in basins caused by impacts.

Occurred between 4 and 3 billions years ago.
Created the MARIA.

D) Inactive Period - "Geologically dead" for the last 3 billions years

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AGES FROM CRATER DENSITY
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Based on Lunar regions with ages determined by radioisotopic dating, a relation between AGE and CRATER DENSITY has been established.

Steep rise in crater density for older lunar regions has lead to concept of

"PERIOD OF INTENSE BOMBARDMENT"

In the first 500 million years of the Solar System, many impacts on ALL planetary surfaces.

Based on lunar relation, AGES of other surfaces in the Solar System are estimated.

EXAMPLE

These rivers beds on Mars occur only in the heavily cratered regions on Mars, not in areas of low crater density. So, flowing water on Mars only occurred long ago as the evidence is only in the old surface regions of Mars.