The Universe and Galaxies: The Moon

Discovery Space: The Moon

No other celestial body is as close to Earth as the moon. It has been comprehensively explored, but some of its secrets have still not been revealed.

The surface of the moon, scarred with craters, provides a glimpse of what the Earth might have looked some four billion years ago. At that time, the newly formed planet was exposed to a hail storm of asteroids. Since then, the craters on Earth have been eroded by wind and water or filled in by terrestrial changes in the Earth’s crust. In contrast, the appearance of the moon has barely changed since the
formation of its lowlands.

Lunar seas and craters Before much was known about the moon, its dark lowlands were thought to be seas. It was on the Mare Tranquilitatis (Sea of Tranquility) that the first human visitor left his footprint in 1969. Today we know that these lunar “seas” are solidified lava flows that have filled impact craters and depressions.

The moon does not have a protective atmosphere, meaning that large and small meteorites are not prevented from impacting it. These collisions have pulverized the rocks on the surface, creating a layer of rubble and dust called the regolith. The bright highlands of the moon were formerly thought to be continents and were called terrae. They are geologically older than the lowlands and covered with substantially more craters.

Most of these craters come from asteroid impacts during the early life of the moon. They are named after astronomers, philosophers, and other scholars. Because it lacks a gaseous envelope, temperatures on the moon can vary extremely. In full solar radiation the temperature reaches approximately 265°F (130°C) and drops again to about -255°F(-160°C) during the lunar night.

Water ice

The moon is an extremely dry celestial body, especially in comparison with its nearest neighbor, the water planet Earth. Nevertheless, space probes have found evidence of possible water ice in the polar regions of the moon.

For instance, ice from impacting comets could lie at the bottom of deep polar craters, where it would remain out of reach of heating by solar radiation (and thus would be protected from evaporation and escape). Such water ice could be a valuable resource for any future space stations or human settlements on the moon.