Sponge – What is a synchronous orbit?

Surface Features - Maria - oceans or seas. (Galileo thought they were oceans when he saw them through his telescope.) Maria are large, dark, flat areas that are probably the result of lava flows in the Moon’s past.

There are 14 maria, all roughly circular. Most of the maria are on the side of the moon that faces the earth. The “far side” has almost no maria.

Terrae - highlands. (Galileo thought these were the land masses of the moon.) Terrae are elevated several kilometers above the maria.

Craters – bowl-shaped depressions, mostly due to meteoric impact.

The largest craters are 100’s of km in diameter, the smallest are microscopic. Craters are found all over the Moon’s surface, but are more prevalent in the highlands.

The far side of the Moon is almost entirely highlands, no maria. This fact affects theories of the Moon’s formation.

The Moon does undergo erosion despite the lack of an atmosphere. This erosion is due to collisions with interplanetary debris.

These particles, large or small, all collide with the surface since there is no atmosphere to cause them to burn up.

These particles strike at several km/sec. Even small particles carry tremendous energy at these speeds.

A 1 kg meteorite hitting at 10 km/s releases as much energy as 10 kg of TNT.

This blasts out a crater with a diameter about 10 times that of the meteorite itself.

The depth of the crater produced is about twice that of the meteorite’s diameter.

A 10 cm meteorite would produce a crater 1 m across and 20 cm deep.

The material thrown out surrounds the crater in an ejecta blanket.

Objects hit the Moon all the time, from meteorites to protons from the solar wind.

The larger the crater, the less often they are formed. There just are not many large pieces still floating randomly around the solar system. Most of the large objects have already collided with something.

The larger craters are mostly all older, while smaller craters are relatively young.

The Orientale crater

If the Orientale crater was facing Earth, we would think a big eye was looking at us. Creepy!

The Moon’s average erosion rate has been calculated to be about 10,000 times slower than Earth’s.

This is primarily due to the absence of running water and atmospheric wind.

The crater density is greater in the highlands. This leads scientists to conclude that the highlands are older than the maria. (We will use this standard to judge the oldest parts of a planet or moon thoughout the solar system.)

Moon rocks brought back from the Apollo missions have allowed the different areas of the moon to be dated.

Highlands are typically more than 4 billion years old; maria are from 3.2 to 3.9 billion years.

This information on age combined with the crater density indicates that the Moon experienced a sudden drop in meteoric bombardment about 3.9 billion years ago and that this slower rate of collisions has been constant since that time.

The highlands solidified and received most of their cratering before 3.9 billion years ago, the maria solidified afterward.