The Earth's Moon
- Unlike most planets and their satellites, the Earth and Moon
are similar in size.
- Due to tides raised on the Moon by the Earth's gravity, the
Moon's spin has been slowed to the point that it always keeps
the same hemisphere pointed toward the Earth.
- This process, called ``tidal locking'', has happened to almost
every satellite in the solar system.
- The Earth's spin is also slowing down as a result of
the Moon's tides. The day is getting longer.
- The Lunar ``near-side'' shows two distinct types of terrain.
- Heavily cratered highlands ("old'' surface -- typically 4.2 billion years old)
- Smooth ``maria'' ("young'' surface -- typically 3.5
billion years old)
- The maria are huge impact craters (basins) that subsequently filled with lava.
- The Moon began hot and largely molten. As it cooled it was able to form a thin and fragile crust.
- Before the maria-forming impacts this original lunar crust was
heavily cratered due to the infall of debris from last stages of the formation of the planets.
- The maria-forming impacts themselves obliterated the old craters and created
"basins" - depressions in the lunar surface.
- The impacts fractured the crust which, sometime later,
permitted lava to reach the surface.
- The lava flows erased any craters which subsequently formed
in the basins.
- The lava flows were very dark, resembling basalt lava flows on Earth, thus the darker appearance
of the maria (although this material has been pulverized to rubble and dust -- see below).
- New craters began to accumulate on the newly smoothed surface.
- The highlands represent the oldest surviving crust on the Moon.
- These regions have been accumulating craters since nearly
the time of the formation of the Solar System.
- The highlands (4.2 billion years old) are substantially more
heavily cratered than the maria (3.5 billion years old) because
the rate of cratering tapered off rapidly following
the formation of the Solar System.
- The early period of intense cratering perserved in
the highlands is called the "heavy bombardment."
- The Lunar ``far-side'' only has highland type terrain. There are
few maria-like features.
- The lunar crust is thicker on the far-side than on the near side
for unknown reasons.
- Overall, the Moon's surface and topography has largely been shaped
by impacts and subsequent lava flows.
- Even the Moon's "mountains" are either surviving highland terrain or rubble piles from the giant impacts which shaped the basins.
- The Moon's surface is covered with rubble and fine dust from the ongoing impacts.
- This fractured surface is called a "regolith"
- Many Moon rocks are "breccias" -- agglomerations of regolith debris fused by subsequent impacts.
- A dozen humans have walked on the Moon during the Apollo program, returning hundreds of kilograms of lunar samples from
six landing sites.
- The availability of these samples transformed lunar scientific understanding.
Of the other terrestrial planets...
- the Moon enjoyed about 1 billion years of geologic activity (i.e. molten rock could reach and modify the surface)
- during that period impacts and lava flows shaped the Moon's surface into the configuration we see today
- since that time (infrequent) impacts have made minor modification to the Lunar surface.
- the Moon's appearance has not changed significantly in the last 3.5 billion years.
- The Moon had a brief period of geologic/volcanic activity because
it is small and cooled quickly
in comparison to, say, the Earth and Venus.
- Despite being small, and thus geologically dead for the last 3.5 billion years, the Moon still had an interesting geologic
history compared with some other Solar System bodies.
- The Earth and Venus are large and still retain hot interiors today.
- Mercury, not much bigger than the Moon, is geologically long dead.
- The case for Mars is more questionable.
Revised October 25, 2006