Geology of the Solar System 6: The Moon


My notes on the Moon are hella long, so I will try to only include some of the more awesome stuff and gloss over the boring regular stuff.

Here are some of the basics on the Moon, which I will gloss over to a large extent.  Basically, the Moon is slowing down Earth’s rotation over time (theoretically), and is also gradually moving farther away from Earth (measured by shooting lasers at reflectors on the Moon to measure distance pewpew!).

The Moon rotates on its axis and orbits the Earth in exactly the same period of time, which is why we always see the same side of the Moon. Its density is 3.34 g/cm3, there is no liquid water, atmosphere, or surface pressure, and there is no magnetic field, though there might once have been.


This picture will communicate most of what you need to know about the Moon’s structure:

The density of crustal materials is 3.30 g/cm3 compared to its overall density of 3.34 g/cm3 suggests that the core is very small, if it exists at all.

Basically, the Moon is now a cold, dead object that hasn’t experienced tectonism or volcanism for over 3 billion years.

Surface Geology

There are two main divisions of lunar surface:

Terrae are characterized by bright, rugged, densely cratered areas and comprise about 60% of the near side of the Moon, and almost all of the far side. They are mostly anorthosite (remember that term from the Mercury notes?) and date to well over 4 billion years, which is consistent with how heavily cratered they are.

Mare (pronounced mar-eh) are characterized by smooth dark layers comprised of thin layers of basalt with few impact craters, and are between 4 and 3 billion years old.  They are abundant and large on the near side of the Moon, and are virtually absent from the far side.

There are some volcanic features, such as lava flows, channels and rilles formed from collapsed lava tubes or by the flow of lava over the surface, and a few lava domes and pyroclastic volcanoes (the exploding ones).

There are also some global tectonic features, like linear rilles related to thermal expansion around the time the mare basalts were formed and wrinkle ridges related to compression when the Moon cooled.

Lunar History

Like Earth, the Moon has a relative geologic time scale, pictured below so that I don’t have to type the whole thing out. I should mention before you look at the picture that it is a bit out of date, and the mare basalts should be deleted from the Eratosthenian Period.

Origin of the Moon

Now, this is the interesting stuff!

There are a number of competing theories of how the Moon may have formed, though one tends to top the rest.

Fission from Earth: Basically, the Earth was spinning so rapidly it flung some of its material into space, which was then kept in orbit by its gravitational field, becoming the Moon. There are some problems with this theory, mainly that it is highly unlikely the Earth was ever spinning this fast, as it would have had to have spun at the same frequency as the critical vibration frequency required to break a wine glass.

Capture: A body bypassing Earth was neatly caught and held in its graviational field. Also highly unlikely, as rocks that come from somewhere other than Earth have very different oxygen isotope ratios (there are different types of oxygen molecule that occur in different concentrations around the solar system), but the Moon’s oxygen isotope ratio is pretty much the same as Earth’s.  The Moon also has a coplanar orbit, suggesting a shared origin with the Earth.

Binary Accretion: The Moon and Earth accreted fromt the same material basically next to each other.  This is the most unlikely, as it would have resulted in two bodies with exactly the same density and composition (the Moon is depleted in volatiles).

Collision: A large protoplanet about the size of Mars crashed into Earth very early in its history, adding some materials to Earth and flinging others off, which would eventually accrete into the Moon, illustrated below:

Or, more artistically:

It wouldn’t have excavated deeply enough to fling off any of Earth’s core material, but would have excavated crust and mantle material instead, accounting for the Moon’s low density.  This theory also explains the similar oxygen isotope ratios, as the Moon would be made mostly of Earth’s crust and mantle, and also explains Earth’s anomalously high density, as the protoplanet would have added iron and magnesium to the system.  It can also explain how the Moon’s orbit and rotation are exactly the same.  All in all, it’s currently the most favoured theory.

Hooray for the Moon!


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