A tale of two very different worlds.
Two worlds so close together, yet so different. One is teeming with life in its oceans, on land and in its atmosphere. The other is barren, as dry as dust, largely unchanged since its birth. But that birth profoundly altered the evolution of both worlds.
Just how our Moon — two-thirds the size of the planet Mercury — was created and came to be orbiting Earth was a mystery until the 1980s. For more than a century, astronomers had debated the merits of three scenarios: the adopted-cousin theory (the Moon was a small planet gravitationally captured by Earth), the sister theory (Earth and the Moon were born as a double planet) and the daughter theory (the Moon fissioned from a rapidly spinning primordial Earth).
Studies of the 842 pounds of lunar material returned by the Apollo astronauts failed to support any of these theories. Instead, a fourth hypothesis emerged and is now widely accepted.
The new view — we’ll call it the chip-off-the-old-block theory — is a product of modern computer simulations of the formation of the solar system. These simulations suggest that 10 million years or so after the solar nebula initially evolved into a primordial disc centred on the newborn Sun, the material in the region where Mercury, Venus, Earth and Mars eventually would orbit had built up thousands of mountain-sized planetesimals. These, in turn, collected into perhaps a dozen large bodies in the Mercury-Mars zone. Earth might have had four or five neighbours up to three times the mass of Mars, all closer than Mars is now.
One of them smashed into the nascent Earth. The enormous amount of heat generated by a collision with something that big would have completely melted the surface of our young planet.
Debris from both Earth and the impacting body splashed into nearby space. Some of the ejected material fell back to Earth, but a portion lingered in orbit around Earth and eventually coalesced into the Moon.
That, in brief, is the chip-off-the-old-block theory. Astronomers have accepted it because it is the only explanation for the Moon’s origin that fits with the Moon samples gathered by the Apollo astronauts. Those samples revealed that Moon material is different enough from Earth rock that the sister and daughter theories became untenable. However, Moon rock is enough like Earth rock that the adopted theory fails as well.
The Moon contains very little iron and volatiles (more easily vaporized substances), such as water, chlorine and potassium, which indicates that the Moon was at one time heated to incandescence, presumably caused by the giant primordial impact.
The chip-off-the-old-block theory also explains the uniqueness of the Earth-Moon duo among the planets in our solar system. No other planet has a satellite anywhere near as big as itself.
Editor Terence Dickinson invites your comments and astronomy-related observations and photos, which can be directed to him at [email protected].
Check out the March/April issue of SkyNews (on newsstands now) for an expanded version of Terence Dickinson’s Editor’s Report.