Australasian Science: Australia's authority on science since 1938

Cosmic Time Machine

Moon craters

Craters reveal the Moon's turbulent history.

By Marc Norman and Tim Wetherell

Precise dating of impacts on the Moon might contribute to a better understanding of life on Earth.

Marc Norman is a Senior Fellow with the Research School of Earth Sciences at the Australian National University, where Tim Wetherell is Science Editor.

The full text of this article can be purchased from Informit.

The planets of our solar system formed about 4.5 billion years ago from a collapsing disk of dust and gas rotating around the axis of an infant star. Within this cosmic colosseum, violent events of enormous magnitude played out. The early Sun flared hot magnetic jets deep into outer space; supersonic shock waves swept across the disk; and portions of the disk were heated to temperatures in excess of 1500°C.

As the particle density inside the disk increased, clumps of debris grew rapidly into kilometre-sized boulders, or planetesimals. Like speeding cars in a roundabout, these planetary embryos began to collide, collecting more mass and clearing their orbits of smaller bodies.

Within a few million years, protoplanets the size of Mars and the Moon had formed, and the consequences of collisions among these bodies had increased in scale proportionately. Rather than simply breaking apart, as happens when asteroids collide, close encounters between protoplanets released enough energy to cause wholesale melting, creating oceans of magma that took millions of years to cool. In one of these late-stage collisions, a body about the size of Mars is thought to have smashed into the proto-Earth, melting the Earth and ejecting the material that formed the Moon.

Once this early stage of planet formation was complete, relatively stable orbits were established and the...

The full text of this article can be purchased from Informit.