Serene, your posts are masterpieces of clarity. I tried to say the same thing yesterday and didn’t post, ‘cause I just couldn’t get it right to my own satisfaction. Thank you.

With regard to Margiani’s views, I get the feeling others see them as I do. M’s enthusiasm is refreshing, and the ideas will make you stop and think, but the groundwork (pun not intended) and references are a little shaky. I’m still not sure what a geotrack is. Maybe a trail left by a geo, an unknown deep sea animal?

Sea floor spreading is not, or perhaps better currently not regarded as, driven by ridge volcanism. See ‘Gravitational Sliding’ for starters in the following article: http://en.wikipedia.org/wiki/Tectonic_plates There is also a good section on other planets in this article.

Cosmogeology? Maybe planetary geology would be a better term. I think M is looking for some unifying principles of geology across a spectrum of planets. The only confirmed one I can think of offhand is that if the object is more than about 300 miles in diameter, it’s almost certainly going to be nearly spherical, allowing for rotational distortion. The rest of it – surfaces, cratering rates, atmospheres, amount of water, internal radioactive heating, cooling rates – is all at the observational stage.

A point here. Within the solar system we see the following types of planets (or objects, I threw in a few moons):

1. Rocky planets. Mercury, Venus, Earth, Mars. I personally would include the Moon, or include it with Mercury as an airless rocky planet.
2. Gas giants. Jupiter and Saturn.
3. Ice giants. Uranus and Neptune.
4. Ice planets. Pluto, Charon, Ceres (?), Titan (?), Triton (?) assorted Kuiper belt objects.


Do you suppose that planets bin out this strongly around other stars? Are there forbidden sizes/compositions?