There is considerable evidence that the birth of our solar system was attended by many violent collisions between planets and other smaller but still massive bodies. These birth pangs neatly explain several features of our solar system, including how our Moon was formed.
The solar system began to emerge about five billion years ago. Prior to that the material destined to become the solar system was dispersed in a vast rotating thin cloud of gas and dust. The cloud began to collapse under its own weight. The material at the centre of the cloud became very hot and eventually ignited as a furnace of nuclear fusion - our sun.
Because the original cloud was rotating, material remained in orbit around the sun in a disc-shaped structure called the solar nebula. After a while the heavier grains of dust settled into a thin layer along the central plane. Gradually, the grains bumped into each other and stuck together to form increasingly larger conglomerates. After about 10,000 years the conglomerates were about one km in diameter. These rocky objects, called planetesimals, were the building blocks of the planets.
Until the 1980s, the planets were thought to have been formed when planetesimals rotating around the sun gradually separated into nine "feeding zones" associated with the orbits of the nine planets.
In each feeding zone, one body outgrew the others and then gobbled up the remaining smaller aggregates to become a planet. These larger bodies stayed inside their feeding zones and did not collide with larger bodies in other zones.
The aggregation of planetesimals to form planets can be studied by computer simulation. This method has been intensively used by Willy Benz at Harvard University, and others, to study the formation of the four inner planets of the solar system, Mercury, Venus, Earth and Mars. The model results suggest that the final stages of planet formation were considerably more violent than the picture associated with the separate feeding zone model.
Computer simulation shows that the merging planetesimals tug on each other by the force of gravity, making their original circular orbits more elliptical. Orbits of planetesimal aggregates begin to cross, leading to violent collisions.
Material from different parts of the inner solar nebula begins to mingle, which contradicts the earlier idea that separate feeding zones might be maintained. The models show that late in the process of planet formation the bodies that are destined to become the major planets are still colliding with other bodies of at least one tenth their size.
The energy released in the largest collisions is enough to fragment or melt a planet. These large late collisions therefore probably played a significant role in sculpting our solar system.
Analysis of Moon rocks returned to Earth by the Apollo programme show that the moon was formed about the same time as the Earth. Moon rocks resemble Earth in some respects, but not in others, e.g. Moon rocks are very low in volatile substances that are relatively easy to boil off, such as lead, zinc and water. The Moon has an iron core, as does the Earth, but it is very much smaller relative to the Earth's core.
The question of how our Moon was formed has long perplexed astronomers. Apart from Pluto, no other planet in the solar system has a satellite as large relative to the planet it orbits as our moon is relative to the Earth. The origin of most moons in the solar system is easily explained in terms of debris left over from the formation of planets.
Three possibilities have suggested themselves to explain the origin of the moon: (a) it is a rocky chunk of Earth flung into orbit by the rapid spinning of the Earth; (b) it is a small companion planet to the Earth formed from the same primordial planetesimals; (c) it wandered in from elsewhere in the solar system and was captured by the Earth's gravity.
EACH of these three explanations runs into serious objections. First, if the early Earth was rotating fast enough to throw off such a huge chunk as the Moon, the Earth-Moon combination would today retain much more spin than it displays. If the second explanation were correct, the Earth-Moon combination would display less spin than it does. And, finally, if the Moon was originally an inter-planetary wanderer, the Earth's gravity would not slow it down sufficiently to prevent it escaping again.
Computer simulations of a collision between Earth and a planet the size of Mars have shown how the Moon probably arose. The energy of impact melts the Earth's mantle, the part of the Earth between outer crust and central core which constitutes about 82 per cent of Earth's volume. The dense iron core of the Mars-sized body sinks and merges with the iron core of the Earth.
A large amount of rocky rubble, from Earth and from the impacting body, is thrown into orbit around Earth. The rubble settles into a disc orbiting the Earth, from which the Moon arises.
The impact theory can account for many Moon features that are otherwise very difficult to explain. The shortage of volatile materials on the Moon is explained by the fierce heat generated by the impact which would easily evaporate these materials.
Other chemical differences between the Moon and Earth are explained by differences between the Earth and the impacting body which contributed some or much of the lunar material. The small iron core of the moon can be explained by assuming that the impact mainly threw rock and not iron into orbit. And the current spin of the Earth-Moon combination, which is either too slow or too fast for other theories, can be explained by assuming that the impacting body hit Earth with a glancing blow rather than a head-on collision.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC.