Friday, June 16, 2006


Jet lagged and awake in the dark hours of the night -- of which we do not have very many here at the latitude of Ireland in summer. But for the last few nights, a special treat. There, outside our bedroom window, a waning gibbous Moon shining on Dingle Bay, only a dozen degrees above the horizon. The whole parish glows in its light.

Our exceptionally large Moon is an ornament for insomniacs, but an embarrassment for theoretical astronomers. Theories of the Moon’s origin have traditionally stumbled on the Moon’s outlandish size compared to Earth.

When I was a kid, three kinds of theories were in the works. They can be characterized by calling the Moon the "sibling," the "child," or the "spouse" of Earth.

The "sibling" theory assumed that Earth and Moon condensed together from an eddy in the larger whirlpool of gathering dust and gases that became the solar system.

The "child" theory proposed that the Moon's material was spun off from the outer layers of a rapidly spinning Earth, early in Earth's history when the planet was still mostly molten.

The "spouse" theory had the Moon form somewhere else in the solar system and subsequently captured by Earth's gravity.

Each of these theories had dynamical problems that were not easily resolved.

In recent years, a fourth theory of the Moon's origin has gained general acceptance, based on computer simulations and physical evidence. This theory assumes that a very young Earth suffered a grazing impact by a Mars-sized object. The collision blasted into Earth orbit a mass of molten materials, partly from the Earth, partly from the colliding object, which subsequently solidified to become the Moon.

The impact theory agrees with current ideas about the formation of the solar system, which apparently began as a whirlpool of gas and dust around a new star, the Sun, and condensed in stages. First, the gas and dust collected gravitationally into pea-sized objects. Then the "peas" gathered into chunks the size of buildings. The "buildings" collided to make bigger bodies, and so on until the present planets and their satellites came into being.

In the last stages of this process, a few very large impacts can be expected. One of these massive impacts apparently splashed the Moon into being. And there it is, outside the window, that splash of molten rock, now chilled, a child of Earth, but one born in awful violence.