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Gas Giants and Moons, how do they work?


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#1 puenboy

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Posted 14 March 2013 - 05:40 AM

So I just got Stars of Inequity. Now, after taking a look, I see that moons can actually have their own atmosphere and its own working ecosystem. Problem is, how would this work? Say, a Paradise World-esque moon is orbiting a Titan. Sure, things are nice and sweet when you're getting direct sunlight, but eventually, it will orbit to the dark side of the Titan, meaning that the moon would no longer recieve direct sunlight, turning it into a frozen wasteland, and it may not see light again in years, due to the colossal size of the Titan. So does it not matter what you roll, as moons will always end up a freezing wasteland if it has an atmosphere?



#2 Adeptus-B

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Posted 14 March 2013 - 08:33 AM

A few possibilities:

1) The gas giant orbits it's star pole-on, like Neptune (i.e. one side always faces the sun, one side always faces away), so it's more-or-less equitorially orbiting moon has an unobstruced view of the sun.

2) The moon is heated primarily by geothermic activity (i.e. volcanos), with CO2 in the atmosphere holding the heat in.

3) The gas giant itself gives off heat; Jupiter is highly radioactive, so a gas giant that gives off radiation as heat is at least 'sci-fi' possible.



#3 Alasseo

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Posted 14 March 2013 - 09:11 AM

Actually, orbital mechanics suggest that wouldn't happen. If it's big enough to attract and keep an atmosphere, it can survive the periods of occlusion. CO2, O2, O3 and H2O are really good insulators (and black-body radiation is relatively inefficient at dumping heat), so provided this is taking place in the Primary Biosphere, it will have built up sufficient heat to prevent it becoming a frozen wasteland in the weeks-months that it's going to be in shadow (if the gas giant is large enough to leave it in eclipse for longer than that, then you run into the question of "why hasn't it become a sun?" Unless, of course it fits a special case I'm going to outline below).

The orbital period depends on the orbital velocity (and hence altitude). For a significant portion of the orbit to spent on the dark side, the semi-major axis has to be such as to create something like a 2:3 ratio between the orbital periods of the gas giant (around the sun) and the moon around the gas giant. However, given the average orbital period of a planet in the primary biosphere, this requires that the moon's orbit be so far out that it's it may be barely captured. What's more, an orbit at that altitude needs to be highly eccentric to keep the moon in the gas giant's shadow for a significant portion of the orbit (remembering that there is going to be refraction from the outer layers of the gas giant's atmosphere, some gravitational lensing, but mostly that the relative size of a gas giant and a star mean that the gas giant will not occlude as much of the sky as the star at periapse [geometry ftw]). This means that the periapse of the moon's orbit is going to be very low and fast, while the apoapse is going to be massively high- you're almost looking at a cometary style orbit.
On an astronomical timescale, these sort of orbits are not necessarily stable- look at Levy-Shoemaker 9, for a probable result. It is possible to find a relatively stable orbit, but really only when you're looking at a 3-body system (no other moons around that gas giant, no other planets in the goldilocks zone or close to it), as otherwise you are going to see sufficient orbital perturbation to crash that moon either into the gas giant or simply drop the periapse below the Roche limit (or eject it into interplanetary space), and likely before it has a chance to develop an Oxygen-Nitrogen atmosphere.


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#4 Fresnel

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Posted 14 March 2013 - 09:20 AM

The moon is only in shadow for the time it takes to pass through the Gas Giant's shadow.

Let us take the example of Titan about Saturn.

Orbital Period: 16 days

Orbital Radius: 1,222,000km

Diameter of Saturn: 120,000km

So the orbital distance of Titan is about 10 times the diameter of Saturn. Therefore when viewed from Titan, Saturn subtends 5.7 degrees. Even if a full eclipse happens, it will only last for 6 hours…

 

 

 



#5 Iku Rex

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Posted 14 March 2013 - 09:25 AM

puenboy said:

… eventually, it will orbit to the dark side of the Titan, meaning that the moon would no longer recieve direct sunlight, turning it into a frozen wasteland, and it may not see light again in years, due to the colossal size of the Titan. …

Wait, what? Why would it take "years"?

 

 

Anyway, I'm pretty sure the SoI planet generation system is far from "realistic", so I'm not that interested in the details. I blame various sufficiently advanced precursor civilizations. They've "terra"formed millions of planets all over the galaxy. Some were changed to fit alien species with alien requirements. Some were only half-finished. Others have partially reverted to their original climates since they were abandoned.  As a result you get habitable planets in odd places, and far too many planets that inexplicably have breathable atmospheres.



#6 Fresnel

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Posted 15 March 2013 - 02:31 AM

One important factor, generally ignored, is the presence of a magnetosphere. Without this on Earth, surface life would be impossible - the solar wind would also slowly strip the atmosphere.

A gas giant might have a huge magnetosphere, able to cover moons without one of their own. So if you have a gas giant in the 'goldilocks zone' it might have dozens of habital moons. They would probably all be tidally locked though.

 



#7 TiLT

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Posted 15 March 2013 - 03:29 AM

Remember, this is Warhammer 40k. Not everything is realistic. Sometimes you're perfectly allowed as a GM to say "a wizard did it". 



#8 Fresnel

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Posted 15 March 2013 - 04:39 AM

Remember, statements of obvious are rarely helpful…

For those (like the OP) who wish to maintain the barest breath of 'realism' to celestial mechanics within their game - I will assist them with my meager understanding of the topic.

Personally I don't mind when WH40K gets silly with physics, except when it is simply lazy and/or pointlessly ignorant.

For instance, the importance of a magnetosphere can be a story seed. How does this planet without a liquid iron core maintain a magnetic field? Obviously there is some archotech or xenotech at work here. Perhaps it would be worth more to the RT to salvage it, leaving the planet to its doom. Perhaps a rival RT has stolen the device from one of the PCs colonies?



#9 susanbrindle

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Posted 16 March 2013 - 10:22 AM

Presumably the moons have liquid iron cores. (Since, as I understand it, the only real difference between a planet and a moon is that one is bigger and orbits the sun and the other is smaller and orbits a planet. Gas Giants are thus big enough to afford earth-like moons, yes?)



#10 Fresnel

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Posted 16 March 2013 - 01:23 PM

Yes, that's possible. However, a moon is likely to be tidally locked to its planet, so even if it had a liquid iron core it's magnetic field might be relatively weak. My point was that a Gas Giant can have a huge magnetosphere (like Jupiter does). So a moon orbiting such a gas giant doesn't need one of its own. For instance a Mars type moon would be fine.

So a system with a dozen habitable worlds isn't so silly. No need to arm wave about wizards

Of course in WH40k many of these habitual moons might have nearly 24 hour days… Suggesting that in the Dark Age of Technology the ancient engineers set the moons spinning as they desired. This might have solved both the magnetosphere issue (if its natural one was too weak) and made the moon more terra-like for colonists.

 

 



#11 TiLT

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Posted 19 March 2013 - 01:30 AM

Sorry, I didn't mean my wizard comment to come off as obnoxious in the way it seems to have been interpreted. I just meant to say that as a GM, you're under no obligation to figure out exactly how everything works in your solar systems. If you've got a logical impossibility as a result of your die rolling and you can't find a good explanation for how it got to be like that, just accept it for what it is. It'll make a good mystery that will have your players coming up with extreme theories (some of which you may adapt) even if you have no idea what the reasons are and have no intention of ever letting the players figure it out. 



#12 Fresnel

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Posted 19 March 2013 - 01:48 AM

Perfectly reasonable. But can you give an example of the system generating something silly? I don't have the book yet.

#13 Zaltorin

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Posted 20 March 2013 - 05:23 AM

Fresnel said:

Perfectly reasonable. But can you give an example of the system generating something silly? I don't have the book yet.

How about a habitable moon of vast size orbiting a vast planet inside of the inner cauldron?  I was rolling a lot of tens, the moon ended upas a hot world with normal gravity and a pure atmosphere.  Heck it even ended up with native population, which had a primitive fuedal society.

As for the 'a wizard did it', this is more of a sci-fi setting so I think that 'an alien did it' is more applicable.






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