Pedagoguery

The Earth is not a good place to try to investigate the earliest periods of the solar system's formation. Much of the Earth's surface is of much younger vintage, due to erosion and plate tectonics. However, nearby there is a body that provides intriguing clues to an event early in the solar system. That body is the moon and the event is called the Late Heavy Bombardment.

During the middle of last century, detailed observations of the moon noticed that cratering is significantly heavier in the lunar highlands than it is in the maria. Cratering in the highlands is about 32 times as dense as it is in the Maria – in fact the highlands are saturated with craters – any new crater will obliterate older ones. Early estimates are that the maria were about 3.5 billion years old, so that first 700 or 800 million years after the moon's formation must have been a tumultuous time.

More data was clearly needed to pin down ages more precisely. Fortunately, the Apollo missions provided the perfect opportunity. The rocks they brought back did two things – they pegged the age of the maria to about 3.9 billion years, and they also identified that at that same time there was a tremendous burst of cratering that obliterated the previous record.

All this pointed to a cataclysm at around 3.9 billion years ago. Additional evidence from meteorites and from observations of other planets and moons backs this up. The question is whether this was the tail end of the solar system's formation, or a sudden flare up. For a while, it was unclear how either scenario could be correct. Models of the solar system's formation indicated that most of the debris in and around the orbits of the planets would have been cleared out by then, and there was nothing that pointed to a mechanism to initiate a flare up.

Then, the Nice Model came along. Named for the city on the French Riviera, the Nice Model was a model of the chaotic gravitational interactions of the four gas giant planets during the early solar system. Its intent was to explain the observed orbital eccentricities of those planets, as well as to explain how Uranus and Neptune are where they are when current models of solar system formation indicated that no planets that large could have formed that far from the sun.

In the Nice Model, the planets start in different positions than they occupy today. Jupiter starts at 5.5 astronomical units (a.u.) from the sun, Saturn at 8.2, Naptune at 11.5, and Uranus at 14.2. Beyond Uranus, there is a thick belt of debris – the precursor to the modern Kuiper Belt, that extends from 15 to 35 a.u. There is still at this time a small amount of debris among the planets, and as these are scattered away, Jupiter gradually creeps inward while the other three creep outward. This happens until Jupiter and Saturn pass through a 2:1 orbital resonance, and then all hell breaks loose. Saturn's orbit becomes increasingly eccentric and its gravitational effects throw Neptune out beyond Uranus' orbit into the midst of the Kuiper Belt. Neptune scatters large numbers of icy planetesimals inward, where the other planets scatter them all over. In addition, Jupiter migrates a little further inward, partially disrupting the asteroid belt. The result is a sharp spike of impacts in the inner solar system that rapidly trails off as bodies are either scattered out of the solar system or hit planets or moons.

The Nice Model explains a great many things. In addition to explaining the orbital eccentricities and positions of the outer planets, and the observed cratering on Mars, the moon, Mercury, as well as Ganymede and Callisto, it also explains the presence of the Trojan asteroids of Jupiter and the many small eccentric and retrograde moons of Jupiter and the orbital-element distribution of the Kuiper Belt. While the Nice Model is certainly strong evidence for the Late Heavy Bombardment, proof is probably going to require sample analysis from Mars, Mercury, and Callisto.

Next time, the Multiverse.