Articles: Make Up of the Moon

By Tom Walsh

Over time as the true extent and make up of the solar system was discovered, our understanding of how it formed has been speculated upon.

The evolutionary model of the solar system was finally understood when the planetismal model cracked the puzzle of how the sun's planets rotate. Most, the earth included, spin on their axis in the same direction - counter clockwise as viewed from the north. But Venus spins clockwise. As does Pluto. And Uranus is unique; its north pole is tilted more than 90 degrees, right through the plane of its orbit. The planetismal theory accounts for these anomalies by proposing that Venus, Pluto, and Uranus were struck, during the late bombardment epoch, by objects that knocked them over or reversed their directions of rotation. This portrayal gains more support from the strange case of Miranda, a satellite of Uranus. Its surface, imaged by Voyager 2 in 1986, is a remarkable mix of old cratered terrain, young smooth terrain, and several other features unlike anything else seen in the solar system. Theory has it that Miranda was hit so hard that it was nearly destroyed. Its remains flew apart, and then fell back together to form a strangely jumbled world. If so, Miranda is late bombardment road kill.

Earth's moon is a mystery. The moon is too big: it's more than a quarter of the diameter of the Earth. Only Pluto has a satellite (Charon) so large in relation to the parent planet. The distinction is sufficiently striking that planetologists classify earth-Moon and Pluto-Charon as double planets. Moreover, the moon is made of the wrong stuff. While Earth has a massive iron core, the moon contains virtually no iron. Its density, about 3.3 grams per cubic cenitmeter, resembles that of Earth's mantle, but not the core. Nor does the moon have much in the way of volatiles - like water - with which Earth is well endowed. And yet, to further confuse mattes, there are similarities between the two objects; many minerals are found on both, and their relative abundance of various isotopes, such as those of oxygen (which on the moon is bound up in rock), is much the same. Finally, the moons orbit is all wrong. Every other major satellite in the solar system orbits above its planets' equator. This is true even for satellites of tipped-over Uranus. But while Earth's axis is tipped relative to the orbital plane, by 23 degrees, the moon orbits along Earth's orbital plane, not its equator.

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Taken together, these considerations seem to rule out two otherwise promising theories of how the Earth-Moon system originated. The first of these proposed that they formed together, from an eddy in the vast disk of material that surrounded the newborn sun. But if this were the case, both bodies would have about the same chemical composition, so that the moon would have as much iron as earth does. Yet the moon is poor in iron. The other theory held that the moon formed elsewhere but was captured by Earth. But studies of orbital dynamics show that such capture would have required the unlikely intervention of a third planetary body. This is a long odds proposition, and no such object has been found. And capture theory fails to account or the similarities in composition between Earth and the moon revealed in their isotopic ratios.

Recently a new account, based on the planetismal model and dubbed "The Big Splash", has provided possible answers to this riddle. According to this account the young Earth, while still red hot beneath its thin new crust, was struck by a massive, contending planet approximately the size of Mars. Most of the material of the wrecked invading planet was incorporated into the liquefied Earth, adding to the mass of the home planet. But vapour generated by the intense heat when the two objects collided was squirted out into space in a matter of minutes. There, the vapour settled into orbit and condensed as the moon. Because the material came mostly from Earth's crust rather than its core, the moon today lacks iron and so is lower in density than Earth. But because the two objects shared a commingled origin, some chemical simulations suggest that the infant Earth was indeed hit by at least one planet as big as Mars, and also two or three lesser but still hefty planetismal's.

A similar scenario could explain why Mercury, the innermost planet, has a much larger iron core than would be expected from so small a planet. A smaller planet hitting Mercury could have vaporized much of its rocky crust, leaving behind only the core of what was a larger planet - and perhaps, one further from the sun, the crash having knocked Mercury down into its present orbit. Plenty of work remains to be done before these questions are fully resolved.

Meanwhile it is important to keep in mind, as we talk knowingly of the origin of the Universe and of the far-flung galaxies, that we remain uncertain about the origin of our closest neighbour to our home planet.

From " The Whole Shebang" by Timothy Feffis.

This article is Copyright © to the author named at the top, and Midlands Astronomy Club. If you wish to reproduce this article, please seek permission from the author through the Club. Your consideration is appreciated.