5. The Moon: stepping stone to the planets
- When the Moon moves into the
Earth's shadow, the full Moon turns blood red; when the Earth travels into the
Moon's shadow it can become dark during the day.
- The full Moon looks bigger near
the horizon than directly overhead, but its changing size is an illusion.
- The Moon spins on its axis with
the same period in which it revolves around the Earth, at 27.3 days, keeping
its far side forever hidden to Earth-bound observers.
- The near side of the Moon
contains light, rugged, cratered regions called highlands and dark smooth lava
flows dubbed maria; the far side of the Moon is mostly highlands and has very
- For more than two centuries,
lunar craters were attributed to volcanoes on the Moon, but they are now widely
known to be due to the explosive impact of interplanetary projectiles, known as
meteors when in space and meteorites upon hitting the surface of a moon or
- More than thirty years ago,
twelve humans roamed the surface of the Moon and brought back nearly half a ton
- Because the
Moon has almost no atmosphere, its sky remains pitch black in broad daylight
and there is no sound or weather on the Moon.
- Two modest spacecraft, named Clementine and Lunar Prospector, chalked up an impressive list of accomplishments
in the 1990s, including evidence for a lunar core and for water ice at the
poles of the Moon.
- Rocks returned from the Moon
contain no significant amounts of water, but there is evidence for small
quantities of water in some places such as the permanently shaded regions at the
lunar poles. Comets may have deposited the water.
- Space agencies from China,
Europe, India, Japan, and the United States have all sent spacecraft to the
Moon in the early 21st century, obtaining detailed information about
the altitude, geological, chemical and gravity characteristics of the lunar
surface and sending their spacecraft into controlled impact with the Moon.
- High-resolution maps acquired
from lunar orbit are being used to specify potential landing sites and
resources for future human exploration of the Moon.
- Humans might return to the Moon
to create unique astronomical observatories, and establish a permanent base and
way station for trips to Mars; but that is not likely to happen in the near
- Moonquakes, which are much weaker
than earthquakes, indicate that the Moon has a small dense core, probably
surrounded by a partially molten zone. The core has been confirmed by gravity
measurements from the orbiting Lunar
Prospector spacecraft, and laser- ranging measurements have confirmed the
- There is no life on the Moon, and
there apparently never was any.
- Earth rocks and Moon rocks are
similar in their mix of light and heavy oxygen isotopes, but the Moon rocks
contain relatively little iron and few volatile elements common on Earth.
- Impact basins excavated by cosmic
collision produce as much topographical relief on the Moon as there is on the
Earth due to ongoing tectonic processes.
- Vast blocks of the lunar surface
are magnetized, but they do not combine into an overall global dipole like the
Earth's magnetism. Some of the ancient lunar magnetism has been concentrated
on the other side of the Moon from large impact basins.
- Radioactive dating indicates that
the oldest rocks returned from the Moon are about 4.6 billion years ago, which is
about the same time the Earth was formed.
- During its early youth, between
4.4 and 4.6 billion years ago, a global sea of molten rock covered the Moon,
but now a layer of fine, powdery Moon dust covers it.
- A heavy bombardment cratered the
highlands until about 3.9 billion years ago, when the large impact basins were
formed; lunar volcanism subsequently filled these basins to create the maria
between 3.2 and 3.9 billion years ago.
- Most of the features we now see
on the Moon have been there for more than 3 billion years.
- The Moonís gravity draws the
Earthís oceans into the shape of an egg, causing two high tides as the planetís
rotation carries the continents past the two tidal bulges each day.
- The Moon acts as a brake on the
Earthís rotation, causing the length of the day to steadily increase and the
Moon to move away from the Earth.
- The Moon provides a steadying
influence to the Earth's seasonal climatic variation, anchoring and limiting the
tilt of the planet's rotation axis.
The Moon was most likely born during the ancient collision of a Mars-sized body
with the young Earth; the giant impact dislodged material that would become the
Moon that we know.
Copyright 2010, Professor Kenneth R. Lang, Tufts University