Lunar Properties 

Radius: 1738 km = 0.27 Earth Radii (hence the term double planet)
Mass:   7.35x1022 kg = 0.012 Earth Masses
Rotation Period = Orbital Period = 27.3 days. The Moon is tidally locked, with the same side always facing the Earth.

Lunar Phases: changing orientation between the Earth, Moon, and Sun as the Moon moves on its orbit.

Lunar Phase Movie

What does the Moon look like? The Rotating Moon

The Near Side (at full Moon)


The Far Side (from Apollo 16)

The Lunar Surface - the highlands,maria, and craters.


Over time, the volcanic theory died and we now recognize the lunar craters to be impact sites. Does this mean there is no evidence of volcanic activity on the Moon? No! The maria are volcanic in origin.
The Lunar Maria
  The Lunar Highlands

The Lunar Interior

The Moon has a thick, solid lithosphere (unlike the Earth), and therefore has no plate tectonics -- no surface activity.

Many silicates are found in the Moon's crust -- composition similar to that in the Earth's mantle.

In the crust, many refractory elements (high melting points) and few volatile elements (low melting points) are found, suggesting the Moon's surface was originally very hot. The volatile elements ``boiled off.''

The inner portions of the Moon are somewhat denser then the crust, but any iron-rich core must be very small.

Why is the moon solid, while the Earth has a molten interior?


Cooling Time of a planet:

Rough consideration: The amount of heat production in a planet is proportional to its volume (ie more radioactive material, or more mass, etc):

The rate at which it looses heat is proportional to its surface area:

So the cooling rate is proportional to the planets radius:

Larger planets cool more slowly. Small planets or moons cool quickly.

Why are all the maria on the near side?

The center of mass of the moon is not at the moon's geometric center! The center of mass is actually about 2 km closer to the Earth than the geometric center.Why might that be?

So the near side crust is thinner; molten material (in the early history of the Moon) could more easily reach the surface to form lava basins.