Exploration of the Solar System

Topic 6

Week 5

The Moon

http://www.oarval.org/MoonMapen.htm Here’s a map of the Moon

There are a number of reasons for giving a lengthy discussion of the Moon in a course like this

1. The Moon is one of the most obvious solar system objects. If you have not seen the Moon in a small telescope, you have really missed something.

2. The Moon is the only celestial object that has been visited by human beings. That happened for the last time 32 years ago, and the situation is unlikely to change in the foreseeable future.

3. As a result of the Apollo program of 30 – 40 years ago, we learned a great deal, not only about the Moon, but also about the origins of the entire solar system. We can view the Moon as a Rosetta Stone for studying the entire solar system.

Keep an eye on the Moon this week; it will be in ideal position for observing in the evening sky. .

Let’s start with a list of its basic properties.

1. The sidereal period of its orbit around the Earth is 27.5 days. The period of the phases, i.e. the time between successive full moons, is 29.5 days. Think about why these two periods are different. It is discussed in the textbook.

2. The semimajor axis of the Moon’s orbit is 384,000 kilometers. This is a distance equal to 60 Earth radii.

3. The eccentricity of the Moon’s orbit is 0.055, so there is a 10 % difference between the distance of closest approach and greatest distance.

4. The diameter of the Moon is 3476 kilometers, versus 12756 for the Earth. The Moon is just over one quarter the diameter of the Earth. The relative size of the Earth and Moon is illustrated in Figure 10.1 of the textbook, or the following picture of the Earth and Moon together.

http://antwrp.gsfc.nasa.gov/apod/ap011015.html

5. The mass of the Moon is only 1.23 percent that of the Earth, so it is a much less substantial object.

6. Its density is 3.34 grams per cubic centimeter, as compared with 5.52 for Earth. The mean density of the Moon is close to that of rock, where that of the Earth is intermediate between rock and metal. This simple numerical fact about the Moon indicates that it does not have a massive metallic core like the Earth.

Another attribute of interest about the Moon is that it has no atmosphere.

Surface Features of the Moon

Even observations with a small telescope reveal a lot about the surface geography of the Moon. Recognition of these terms will help a lot in appreciating the enormous discoveries of the Apollo program. Check out the Moon map above. Another good choice would be to order the “Lunar 100” card from the Sky Publishing Corporation. You can order it online at http://skyandtelescope.com/

Here are the main classes of lunar surface features. The first two are visible to the naked eye.

• Maria (plural of Mare) are smooth, dark areas visible to the naked eye from Earth.
• Terrae (plural of Terra) are the bright areas that we know are of higher elevation. They are sometimes called the lunar highlands for this reason.
• Craters are big holes in the ground. These are all over the Moon, and there are five of them that are bigger than 200 kilometers in diameter. We now know that they were caused by impacts from huge meteors and asteroids (more later). One of the arguments that helped people come to this conclusion is the fact that some of them have rays coming out of them, which correspond to the ejecta blankets of matter which was thrown up into space and then settled back to the Moon.

http://antwrp.gsfc.nasa.gov/apod/ap030312.html Picture of crater

http://antwrp.gsfc.nasa.gov/apod/ap020913.html Crater Aristarchus with rille

http://antwrp.gsfc.nasa.gov/apod/ap010513.html Copernicus, a prominent crater

• Mountains The Moon has mountain ranges, the most prominent of which is the lunar Apennines. In contrast to mountains on Earth, these are the edges of huge craters called impact basins.

http://antwrp.gsfc.nasa.gov/apod/ap001228.html The lunar Appenines

Rilles: Rilles are features on the surface of the Moon that can look like old river beds. At one time there was a great deal of interest in them. A good picture of the Hadley Rille is given in Figure 9.23 of your book.

http://antwrp.gsfc.nasa.gov/apod/ap021029.html A lunar rille

The Other Side of the Moon

As was mentioned earlier in the course, because of synchronous rotation we only see one hemisphere of the Moon. The other side was unglimpsed prior to the space age. When spacecraft went around the back side of the Moon, they found that it was geologically quite different from the hemisphere we see. This is nicely shown in Figure 9.19 of your textbook.

It is illustrated in the following video which shows the rotation of the Moon, as we would see if from a spaceship out away from the Earth-Moon system.

http://www.solarviews.com/eng/moon.htm

The different nature of the backside of the Moon is due to the thicker crust there, which stopped lava flows to the lunar surface. This is discussed in the textbook.

The Apollo Program

Most of what we know about the Moon came from the Apollo program, which culminated in several landings of manned spacecraft between 1969 and 1972. The pictures which were returned are truly spectacular, and until the arrival of the Mars Exploration Rover spacecraft at Mars in January, 2004, they were far better than those for any other astronomical object. These pictures are neat to look at, and serve as a prelude to the discussion of the lunar rock samples.

http://www.solarviews.com/eng/moon.htm This provides a few pictures of astronauts on the surface.

http://images.jsc.nasa.gov/ This contains the digital archive of photographs of the Johnson Spaceflight Center, and has many pictures of the lunar surface from the Apollo program