A question about Small (Ice) World atmosphere

[pro100kostya]

There is a table called Pressure Factors Table on page 87 in "GURPS Space". According to these table the Pressure Factor of Small (Ice) worlds equals 10 while Standard (Garden) planets have Pressure Factor 1. Is it correct?

I don't understand how smaller planet without greenhouse effect can have factor 10.

Thanks for answers.

[Agemegos]

Originally Posted by pro100kostya

There is a table called Pressure Factors Table on page 87 in "GURPS Space". According to these table the Pressure Factor of Small (Ice) worlds equals 10 while Standard (Garden) planets have Pressure Factor 1. Is it correct?

I don't understand how smaller planet without greenhouse effect can have factor 10.

Thanks for answers.

That is correct. The figure is based on Titan, largest moon of Saturn, which is cold enough to retain a considerable atmosphere against Jeans escape despite not being as large as Ganymede or Mercury.

I can’t say that I understand your reference to the greenhouse effect.

[ericthered]

Originally Posted by pro100kostya

There is a table called Pressure Factors Table on page 87 in "GURPS Space". According to these table the Pressure Factor of Small (Ice) worlds equals 10 while Standard (Garden) planets have Pressure Factor 1. Is it correct?

I don't understand how smaller planet without greenhouse effect can have factor 10.

Thanks for answers.

Ice planets have more available ices.


"ice" in an astronomy context refers to water, ammonia, methane, carbon dioxide, and other substances that are liquid or gaseous on earth but solid out by Jupiter. They are much more abundant in the universe than the elements that make up the rocky cores of the inner planets like earth and mercury, but they don't form astronomical objects inside of the snow line (see gurps space page 106). Small Ice worlds are mostly made of ice, and thus have a lot more hydrogen and other elements that could form an atmosphere available to them. At least according to the best theories of the time.

[Pursuivant]

Originally Posted by Agemegos

I can’t say that I understand your reference to the greenhouse effect.

It could be that sort of greenhouse effect is needed to keep the planetary atmosphere warm enough to keep atmospheric elements from freezing.

That is, a cold small planet isn't going to have enough gravitational force to attract and hold gas particles. What particles do get attracted will take the form of ice on its surface - at least until they sublimate.

A bigger, slightly warmer planet will attract more material and will hang onto it a bit better even in gaseous form. If there's enough gas in the atmosphere, you might get a self-reinforcing greenhouse effect, at least for a while.

[Agemegos]

Originally Posted by Pursuivant

That is, a cold small planet isn't going to have enough gravitational force to attract and hold gas particles.

Then it would be Tiny, not Small. "Size" — which is to say, which atmospheric gasses are able to escape — is determined by the temperature at the top of the atmosphere (where molecules escape to space from), not at the surface. So it is independent of the greenhouse effect.

[pro100kostya]

Originally Posted by Agemegos

That is correct. The figure is based on Titan, largest moon of Saturn, which is cold enough to retain a considerable atmosphere against Jeans escape despite not being as large as Ganymede or Mercury.

I can’t say that I understand your reference to the greenhouse effect.

Thanks. I've read a little about Titan and it's clearer for me now.

I haven't used words "Greenhouse effect" properly and thus wasn't understood.

What I mean by saying about greenhouse effect is that it's the consequence of greenhouse gases such as carbon dioxide on Venus. Carbon dioxide is pretty heavy. That's why Venus'es atmosphere is thick and thus has pressure factor 100. I didn't know why Small (Ice) planets have pressure factor 10.

Looks like Titan has dense atmosphere because it's large enough and cold enough for it. And far enough so solar wind haven't blown the atmosphere away.

But I still don't completely understand why it's dense. But at leat now I know pressure factor 10 is not a mistake.

[RyanW]

Originally Posted by Agemegos

Then it would be Tiny, not Small. "Size" — which is to say, which atmospheric gasses are able to escape — is determined by the temperature at the top of the atmosphere (where molecules escape to space from), not at the surface. So it is independent of the greenhouse effect.

Well, there's also the opposite problem. Pluto retains nitrogen just fine despite its small size. It just retains it as a component of its crust.

[Agemegos]

Originally Posted by RyanW

Well, there's also the opposite problem. Pluto retains nitrogen just fine despite its small size. It just retains it as a component of its crust.

The “size” of a planet or moon in GURPS Space is a category of what it retains as an atmospheric gas. Nitrogen ice is not a gas. Pluto fits the description of a Tiny Ice world on p.75

[Agemegos]

Originally Posted by pro100kostya

Looks like Titan has dense atmosphere because it's large enough and cold enough for it. And far enough so solar wind haven't blown the atmosphere away.

But I still don't completely understand why it's dense. But at leat now I know pressure factor 10 is not a mistake.

Well, I’m not sure why either, in terms of the underlying physics. I think it might be as ericthered suggested — Titan formed, and other Small Ice worlds may be presumed to have formed, outside the primordial ice line, from material that was rich in volatiles because it had never been heated. Perhaps this suggests that other worlds outside the ice line ought also to have more massive atmospheres.

In terms of the design of GURPS Space, I think that pressure factor was just an empirical fit to make Titan, the only known Tiny Ice world, typical of its Type.

[RyanW]

Originally Posted by Agemegos

The “size” of a planet or moon in GURPS Space is a category of what it retains as an atmospheric gas. Nitrogen ice is not a gas. Pluto fits the description of a Tiny Ice world on p.75

I was partially making a joke and partially making a point, and tried to be so clever that I wound up doing neither.

I was trying to add a reminder that when you get cold enough, you can also have larger worlds that have no atmosphere, as it all winds up frozen on the surface. Tiny worlds have effectively no atmosphere, but not all worlds without atmospheres are tiny.