Hi Board

Is the Earth-Moon system rare?

you wrote...

Nick,
Your views parallel my own concerning the influence of the Moon upon the Earth. I'm convinced that Earth-like worlds without large moons will become waterworlds.

Adam - here I tend to agree with Nick's assessment. Venus shows rather low relief compared with Earth, but there are considerable uncertainties. Is Venus' crust really that much thicker?

What is not clear to me is whether these waterworlds will become hospitable for life or go into a thermal runaway that turns them into hell worlds like Venus. People have been kicking around the theory that Venus once had an ocean. However I don't know if that theory is based upon respectable science.

Adam - I would say so. It's based on hydrogen/deuterium ratios in the atmosphere of Venus. Quite a lot of water must've been lost on Venus via photodissociation and escape of the hydrogen. Grinspoon believes it was due to comet importation, but others suggest original water.

But as for the moon being rare, well dynamical modelling of late stage accretion suggests earth-moon systems occur fairly readily. Big, late impacts typify the terrestrials - mercury lost a lot of crust to one, earth has a moon, and Mars' rotation probably comes from another. There was a Space.com report that mentioned a 1/3 chance of earth-moon formation, but about 1/12 giving a low inclination to the planet. The rest turned the planet over or whatever.

So moons are probably NOT rare, but low inclinations aren't guaranteed either.

Adam