does anyone know what the thought is about (in terms of planetary formation) why some planets have largerly water content (like the earth), and others have largely liquid methane?
It all comes down to where they form in the planetary nebula. The Earth when it formed would have seriously lacked water, most of this happened later, possibly through the Late Heavy Bombardment. The issue is where the water ice line in. In our solar system, this is located at about 5AU. This means that water ice couldn't exist until it got to about 5AU from the sun. Closer in that than and it existed as water gas (not enough pressure for it to be a liquid) which caused it to be blown out towards the frost line by the solar wind.
It is expected that virtually all of the water on Earth came from comet impacts very early on in its history. As for methane planets such as Neptune and Uranus and moons like Titan... Methane would be pushed out further from the sun so it is in higher concentrations the further out in the solar system we go.
its interesting to ponder what on a planet may have existed in the original protoplanetary disk and what might have been created through geological and biological processes.
for example its believed that most of the oxygen in the atmosphere on earth was created by huge volumes of simple celluar life forms very early in the development of life here.
Definitely !!
And now Pluto has thrown the spanner in the works with all it's surprises. It's an amazing time we are privileged to be in with all the discoveries happening ...
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