Australasian Science: Australia's authority on science since 1938

The Earth Had Two Moons

By Dave Rebeke

There’s evidence that the Earth once had two moons, while light pollution obscures the Milky Way from one-third of the population.

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

The Earth had two moons until one of them smacked into its big sister in what scientists from the University of California, Santa Cruz are calling “the big splat”. As a result, our planet is now overlooked by a single bulked-up and slightly lopsided Moon.

This all supposedly happened about 4.4 billion years ago, long before there was any life on Earth to appreciate the view.

The young moons had formed about 100 million years earlier when a giant proto-planet smashed into our planet. They both orbited Earth and rose in the sky together, the smaller one trailing a few steps behind like a little sister in tow.

The new model suggests that the lunar far side highlands could have been created from such a collision, and goes a long way to explaining why the near and far sides of the Moon are so puzzlingly different. The near side is relatively low and flat, while the topography of the far side is high and mountainous, with a much thicker crust.

These new studies build upon the “giant impact” model for the origin of the Moon, in which a Mars-sized object collided with Earth early in the history of the solar system and ejected debris that coalesced to form the Moon.

Advanced computer modelling shows the second moon around Earth would have been about 1200 km wide. Later, the smaller moon fell back onto the bigger Moon and coated one side with...

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