Chapter 8: Cratered Worlds: The Moon and Mercury

8.1 General Properties of the Moon

Our most detailed knowledge of the Moon comes from the manned exploration carried out by the Apollo astronauts. (For most of you this is history, but for me this is my childhood, and what originally sparked my interest in science.)

We sent 9 Apollo spacecraft to the Moon (see Talbe 6.2). Six of them landed on the Moon. The first two, Apollo 9 and 10, were tests, Apollo 9 was the first manned craft to fly to the Moon and return to Earth, and Apollo 10 tested the docking procedure with the lunar excursion module (LEM) while in orbit about the Moon.

Apollo 11 is the craft that carried Neil Armstrong, Buzz Aldrin, and (pilot) to the Moon where Armstrong and Aldrin became the first humans to walk on the Moon.

The other missions that successfully landed on the Moon were Apollo 12, 14, 15, 16, and 17. Apollo 13 was the ill-fated flight where the landing had to be abandoned. (While many consider Apollo 13 a failure, returning the crew alive to Earth is possibly the greatest success of NASA's engineers.)

The Apollo astronauts performed experiments on the Moon, did alot of PR, and returned with samples of lunar rocks. The lunar rocks have been subjected to a great amount of analysis on Earth to understand their compostion and age. Rrom the age of the rocks, we've determined that the Moon is about 4.5 billion years old.

The surface of the Moon has flat basins known as Mare. These seem to be the result of volcanic activity in the first billion years of the Moon's history. This activity ceased about 3 billion years ago, and the Moon has been inactive since.

Studies of the craters on the Moon has led to the conclusion that the rate of impacts was much higher early in the history of the solar system. The rate then dropped until about 3.5 billion years ago, and has been roughtly steady since.

8.4 The Origin of the Moon

There are four general ideas for the origin of the Moon:

There's no conclusive evidence to allow us to select from among the numerous versions of these four categories, but the impact theory is somewhat preferred by astronomers at this time.

8.5 Mercury

Mercury is the closest planet to the Sun. In accordance with Kepler's law (P2 = D3) it has the shortest period (year), about 88 Earth days. It also has the highest speed, about 48km/s. Due to its high speed, Mercury undergoes an orbital precession in accordance with Einstein's predictions from General Relativity. This was one of the first successes of Einstein's theory which we will discuss more in a later chapter.

The composition of Mercury is dominated by a metallic iron-nickel core. The proportion of metal is much higher than in other planets. This is the main problem in understanding the origin of Mercury. It is believed that a large impact on Mercury tore away the mantle and crust, leaving behind the core.

Mercury has an odd rotation period equal to 2/3 of its period of revolution. That is, 1 Mercury day = 2/3 Mercury year, or 1 Mercury year = 1.5 Mercury days.

Determining the rotation period of Mercury was tricky.

Mercury has no appreciable atmosphere. The surface temperature on the sunlit side is 700K, and on the dark side it is only about 100K (water freezes at 273K).

© Robert Harr 2001