LUNAR GEOLOGY

A Brief History of the Moon:

Both the Moon and Earth (and the rest of the solar system) formed 4.6 billion years ago. There used to be three basic theories on how we got our Moon:

1. The Moon formed in orbit around the Earth when the Earth formed.

2. The Moon formed in orbit around the Sun but was captured by Earth early in its history.

3. The Moon split off from Earth shortly after the Earth formed.

The problem was, none of these theories explained the fact that the Moon has such a tiny iron core, the fact that the Moon's orbital plane is close to the ecliptic plane, and the similarities in the composition of Moon and Earth rocks.  The only plausible way to explain all of these three facts is if the Moon formed from a grazing collision between an object the size of Mars and the Earth, shortly after the Earth formed.  That’s right, the Earth got smashed by another planet and the Moon represents the leftovers from the collision. How is that for wild! 

Both bodies were heavily bombarded by large meteoroids after they formed. Some of the meteoroids were hundreds of miles across, traveling at about 20 miles per second! These impacts left huge craters on both the Earth and the Moon. Plate tectonics and erosion have nearly erased the evidence on Earth. The Moon has almost no erosional processes and the scars remain to this day; many are visible with the naked eye.

The cratering barrage had ended by 3.1 billion years ago. As the crust of the Moon solidified and the Moon synchronized its rotation so that it kept the same face towards the Earth at all times,  the denser, then-molten mantle "sank" closer to Earth.  This means that the Moon's crust (which is not as dense) was thinner on the near side than the far side. This allowed the magma from the interior to reach the surface of the near side through cracks in the crust that had formed from large impacts. All of the large impact basins on the near side were thus flooded with basalt-like lava that cooled and solidified. The basalt is darker than the rest of the crust and can be seen on the nearside as "maria" or "seas" of dark material. You can see them anytime you look up at the Moon. Some of the craters that were in the floors of the impact basins were only partially flooded and can still be seen protruding through the giant frozen sea of basalt. Some are just below the surface, and can be seen faintly as "ghost craters" when the Sun shines low on them, casting long shadows. Some craters were half flooded and appear as "bays" along the coastline. Sinus Iridium is a good example.

Mountain ranges still exist on the Moon that were created from the early, huge impacts. The Haemus Mts. and the Caucasus Mts. that border the impact basin that is now Mare Serenitatis, and the Apennines that border Mare Imbrium are excellent examples. Some ranges are half buried in the solidified basalt, such as the Straight Range, and some ranges are almost entirely buried, with only the highest peaks still visible above the Mare surface, such as Mt. Piton and Mt. Pico.

Impact craters still formed after the maria solidified, but they were much less frequent and smaller. When a crater is made today, it's excavation during impact overturns the surface which then appears very bright, especially when the Sun is high in the lunar sky. As millions of years wear on, the particles from the solar wind slowly darken this ejected material (called "ejecta") and the crater slowly fades to match its surrounding environs. Thus, the most recent craters are the brightest spots on the Moon. Excellent examples of recent impacts are Aristarchus (the brightest spot on the Moon), Tycho (whose ejecta "rays" can be seen for thousands of miles across the surface), Copernicus, and Kepler.

There are other geologic features on the Moon, including faults (such as the Straight Wall), rilles, and rift valleys (such as Alpine Valley).

Objectives: 1. To familiarize yourself with the appearance of the Moon through a telescope.

2. To learn to interpret geologic features that you see in terms of the history of the object.

3. To become aware of the Moon and its history.

Requirements: For two different phases:

1. A sketch of the entire Moon as seen through the telescope. Just include the largest and most

obvious features (identify and label them!). This should take about 20 minutes for each sketch.

2. Detailed sketches of (each should take 5-10 minutes)

A. A mare (or a portion of a large one). You pick it.

B. A recent impact crater.

C. A mountain range.

D. A partially submerged crater.

E. A fault, rille system, or rift valley.

Directions:

1. Set up a telescope and align it with the north celestial pole. Approximate alignment is all that is necessary.

2. Find the Moon and begin getting your eyes adapted to it at medium power. Pick out all of the important dark and light areas, impact craters, mountain ranges, etc. that you want to sketch.

3. Sketch the important features on the Moon.

4. Locate a feature that you want to sketch in detail.

5. Move the feature to the center of the eyepiece.

6. Switch to high power and focus.

7. Make a detailed sketch of the feature.

8. Go back to medium power, and back to step 4 and repeat.

9. After you have observed the SAME features near first quarter and during full moon, then answer the questions below and include these answers along with your sketches to receive credit for this project.

Questions: 1. What is different about the lighting conditions on the visible part of the Moon at first quarter phase compared to conditions at full moon?

2. Does the Moon rotate on its axis?  Why could you see the same maria during the full Moon as during first quarter?  How can you see the far side of the Moon?

3. How much bigger is the Earth than the Moon?  Which one has been hit by more big asteroids and comets?  Why?

4.  The Moon shows many more craters on its surface than the Earth does.  What does this tell you about conditions on the Moon compared to Earth?  Does it mean that the Moon is much older than the Earth?

5.  Where in your sketches are the Southern Highlands on the Moon?  What about them tells you that they are older than the maria?

6.  What shape are most of the maria?  Why are they shaped that way?

7.  What features are mountain ranges on the Moon located next to?  What does this have to do with their formation?  By comparison, how are mountains formed on Earth?

8.  What are the brightest spots that you observed on the Moon?  Why are they so bright?

9.  What are the darkest areas that you observed on the Moon?  Why are they so dark?

10.  Which features appeared most prominent at the first quarter phase?  Why did they look prominent then?

11.  Which features appeared most prominent at the full moon phase?  Why did they look prominent?

12.  Which features were invisible at one phase but seen at the other phase?  Why?