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 few maria.

    • 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 planet.

    • More than thirty years ago, twelve humans roamed the surface of the Moon and brought back nearly half a ton of rocks.

    • 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 future.

    • 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 molten zone.

    • 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