Bruce Dorminey reports in COSMOS on a fascinating paper that suggests the early Earth may have had three moons, only for the orbits of the two smaller (~100km or so) moons to become unstable over a billion years or so. The paper by Jack Lissauer and John Chambers is an excellent example of the sort of science that relies more on imagination and mathematical modelling than empirical evidence (and is therefore something of a thorn in Popper‘s side). Here’s what the Earth-moon system might have looked like from orbit:
I’m not entirely convinced yet, but it’s an interesting possibility, and in fact if the prevailing view that the Earth and Moon are the products of a major collision is correct, then I think it’s fair to say that one would expect debris from the impact to collect at the Lagrange points. Whether the debris would have enough mass to become a single solid mass I don’t know. I think it’s more likely the “moons” were actually clusters of rocky debris held loosely together by their own gravity and the funneling effect of the Lagrangian gravitational hollow. The other question this raises is: what happened to those moons when their orbits destabilised? If they ended up hitting the Moon then there may be little sign of it now, but if either one hit the Earth around 3-3.5 billion years ago, the results would have been catastrophic (the Chicxulub impact that seems to have wiped out the dinosaurs and ammonites was an asteroid of about 10km across, these moons may have been 100km across, although the impact velocity would have been much lower). If, on the other hand, the “moons” were just clusters of debris, then the impact may have taken place in small instalments with little effect on the history of life.