Jump to content

moon of a moon


TheNewTeddy
 Share

Recommended Posts

And the evidence for this is... what, exactly?

Opacity of a water vapor cloud at different wavelengths can be directly measured here on earth. And from there, the net effect on energy flow is quite straightforward to compute.

Link to comment
Share on other sites

Opacity of a water vapor cloud at different wavelengths can be directly measured here on earth. And from there, the net effect on energy flow is quite straightforward to compute.

On Earth. The net effect on energy flow depends on the makeup of the atmosphere, not merely the altitude of the clouds. Atmospheres are complicated, poorly understood things, and quite frankly no one knows what a water dominated atmosphere averaging 85 degrees C would look like, nor whether or not it would be stable.

Link to comment
Share on other sites

On Earth. The net effect on energy flow depends on the makeup of the atmosphere, not merely the altitude of the clouds. Atmospheres are complicated, poorly understood things, and quite frankly no one knows what a water dominated atmosphere averaging 85 degrees C would look like, nor whether or not it would be stable.

The physics of water condensation is hardly the frontier of human knowledge. Most of it is several decades old. I think that our theories about it are robust enough to warrant a little extrapolation.

Link to comment
Share on other sites

The physics of water condensation is hardly the frontier of human knowledge. Most of it is several decades old. I think that our theories about it are robust enough to warrant a little extrapolation.

True but that is compared to the atmospheric conditions on earth. On other planets there are many factors that come into play f.x. what the atmosphere is made of, the atmospheric pressure, the gravity and even the wavelength emited from its parent star. In that regard we only have limited knowledge to how water behaves under these conditions (We can't even assume that water is the main liquid on the planet).

Link to comment
Share on other sites

True but that is compared to the atmospheric conditions on earth.

Extrapolation that works only within the original conditions where the theory was developed is no extrapolation.

On other planets there are many factors that come into play f.x. what the atmosphere is made of, the atmospheric pressure, the gravity and even the wavelength emited from its parent star. In that regard we only have limited knowledge to how water behaves under these conditions (We can't even assume that water is the main liquid on the planet).

Which are all factors well within current physics knowledge, and their effect can be quantitatively calculated from already decades old basic principles.

Link to comment
Share on other sites

If you have a blue supergiant star, let's say like Bellatrix, with 8.4 times the mass of the Sun and 6400 times the Sun's luminosity, its habitable zone would be about 80 AU from the star. If you have a brown dwarf 20 times Jupiter's mass at this distance, it would orbit the star once every 247 years. This brown dwarf would have a sphere of influence of ~7 AU.

Let's say the ocean super-Earth has a mass of 5 Earths and orbits at 0.2 AU from the brown dwarf (about the closest it can be without being tidally locked). Then it would orbit the brown dwarf once every 240 days. This ocean super-Earth would have a sphere of influence of 0.01 AU or 1.7 million km.

Now let's say the Earthlike moon has a mass of 1/2 that of Earth and orbits at 500,000 km from the ocean super-Earth. Then it would be tidally locked with a period of 17 Earth days. The ocean super-Earth might or might not be tidally locked to its moon.

The Earthlike moon would be 0.8 times the radius of the Earth and the ocean super-Earth would be 1.8 times the radius of the Earth, assuming the same density as Earth. At the same density, surface gravity depends only on radius, so the Earthlike moon would have a surface gravity of 0.8 g's and the ocean super-Earth would have a surface gravity of 1.8 g's.

(Brown dwarfs are all about the same size as Jupiter, but packed in tighter with much higher density, so this brown dwarf would have about 50 g's at its surface.)

From the surface of the Earthlike moon, the ocean super-Earth would have an apparent size of 5 times the size of a full moon on Earth. The brown dwarf would have an apparent size of 0.5 times the size of a full moon on Earth. And the star would have an apparent size of 0.1 times the size of the Sun on Earth.

This is just an example. I used wolfram|alpha along with the SOI equation and Kepler's third law. You can change the parameters to figure out what you want the system to look like.

Also, blue supergiants usually burn their fuel much faster than normal stars, with lifetimes of less than 100 million years. So this alien planet would need to evolve intelligence pretty fast, and wouldn't have long to survive.

Edit: oops, totally didn't see that this thread had more than one page

Edited by metaphor
Link to comment
Share on other sites

Which are all factors well within current physics knowledge, and their effect can be quantitatively calculated from already decades old basic principles.

Which I don't deny. But we only have the theoretical weather systems in place. The point were you have to calculate, there are so many factors just on earth you have to take into consideration. When calculating local conditions over a few days you usually get it right, but here we talk about a global system over millions of years on a planet with different physical conditions. Even our most advanced computers have a hard time predicting what weather conditions are like on earth in a few centuries.

Link to comment
Share on other sites

Which are all factors well within current physics knowledge, and their effect can be quantitatively calculated from already decades old basic principles.

Really? I defy you to produce a predictive climate model based on atmosphere composition, density, and solar radiation.

We can't even do that for the Earth; how do you expect to be able to do it for some unknown world? And how then can you categorically rule out the existence of a water-cycle dominated atmosphere with a stable average temperature of 85 degrees C?

Link to comment
Share on other sites

Which I don't deny. But we only have the theoretical weather systems in place. The point were you have to calculate, there are so many factors just on earth you have to take into consideration. When calculating local conditions over a few days you usually get it right, but here we talk about a global system over millions of years on a planet with different physical conditions. Even our most advanced computers have a hard time predicting what weather conditions are like on earth in a few centuries.

Sorry to butt in, since I don't really know much about meteorology, but for the purposes of science fiction, doesn't there always have to be some sort of unreliable extrapolation? I mean I totally get the desire to make things as accurate as possible, but do they need to be more accurate? I think you kind of have to go with things based on what we know and not completely give up on major ideas because our understanding is not complete enough to confirm that it would work 100%. So long as it is a logical extrapolation of our knowledge to the best of our understanding.

Even with alien life, we might theorise that life is probably not all that uncommon in the universe, to the best of our knowledge, but then there's always that possibility that it really was a crazy fluke and there's perhaps not a single other form of life in our galaxy, or at least the only macroscopic life but that doesn't stop us from creating speculative fiction containing alien life.

Link to comment
Share on other sites

here you are.

http://www.guardian.co.uk/environment/2013/mar/27/climate-change-model-global-warming

.

And I've made some predictions about your answer too. We shall see how accurate they turn out to be :wink:

Not hard to predict at all. After all, I baited a trap, you walked into it, and now I get to gloat.

EDIT: That said, this thread is derailed enough. I suspected from the moment you said "wet runaway greenhouse effect" that this was the nonsense you were on about, but I honestly don't want to get into a full-blown climate change discussion (at least, not in this thread; if you want to make another thread about it, be my guest). We've already gotten too far afield from the original point; I'll not respond on this topic further.

Edited by Stochasty
Link to comment
Share on other sites

Is this the Moonception thread?

Why not read the title of the thread, or the OP?

Aside from that, I if the mun was say, only a few dozen meters wide, and it was orbiting a rather large moon, which was orbiting far from it's parent body, I see no reason as to why this wouldn't work.

Link to comment
Share on other sites

One thing to point out: You could not have the super-Earth moon tidally locked to both the grandmoon and the grandplanet (that's what I'm calling them because it's adorable :) ).

Setting the orbital periods of the grandmoon around the moon and the moon around the grandplanet equal, you can see that the semi-major axis of the grandmoon's orbit is equal to the Hill-Sphere radius of the moon, so it would not be stable no matter how far away the moon is from the grandplanet. The moon can be locked to the grandmoon (likely), the grandplanet (very unlikely), or neither (unlikely), but never both (impossible).

I think the most likely situation is that you have a very small grandmoon relatively close to the moon, which is way the hell away from the grandplanet but still comfortably in the sphere of influence. That means that we would like a huge star to orbit around, but you wouldn't want to be orbiting a blue supergiant, either, because those last only millions of years.

Actually you must have a star very close to the Sun's mass or smaller, because the lifetime of a star drops off as the -5/2 power of the mass, and the Sun is probably only about twice as long-lived as it needs to be to develop complex life. That means tidal effects from the star will be hard to avoid if we have to stay in the habitable zone, unfortunately. Now we're talking about something approaching a binary star system, with the Sun and a big brown dwarf for the near-partner of the star at roughly Mars' orbit (that's about as far out as you can go and still keep warm with the greenhouse effect). Take an object on the border between Super-Jupiters and brown dwarfs (13 Jupiter masses) and put it at Mars' orbit, and you get a Hill Sphere of about 0.35 AU, or a SoI of about 0.26 AU. Put a 5 Earth mass moon at 0.15 AU and it will have an SoI of something like 4 times the Earth-Moon distance. You could have a Mars-sized grandmoon hiding in there, I think, but just barely, because the Roche limit (distance below which gravitational tide will disrupt the grandmoon) for a 5 Earth mass moon is about 1/4 the Earth-Moon distance. It may seem like a lot of room but it isn't. The fact that these numbers are in the same ballpark means we'll probably have to deal with tidal heating and the surface will be very... interesting.

My potentially feasible (I think?) system:

1 solar mass star - call it Lol

13 Jupiter mass grandplanet/brown dwarf at Mars orbital radius - call it Jupiderp

5 Earth mass moon at 0.15 AU from the planet - call it Herpth

Mars-sized grandmoon at the Earth-Moon distance - call it Goldilocks

I would imagine that Goldilocks and Herpth would be mutually tidally locked in nearly circular orbit, just because if they weren't, Goldilocks' porridge would be too hot (tidal heating = lotsa volcanoes), and also the drag on the rotation of the bodies would affect the orbital radius like our Earth's fast rotation is pushing the Moon farther away from us. Since we have a very narrow window for a stable orbit, that can't be going on very much for very long. This gives an orbital period of about 12 to 13 of our Earth days, which would also be the day/night cycle on Goldilocks.

You would want a slightly elliptic orbit in order to encourage some tidal heating to keep Goldilocks' porridge hot enough to maintain plate tectonics and an atmosphere thick enough to keep the surface habitable.

All of these bodies are going to be pretty unique. There won't be a system of large, Herpth-sized moons around Jupiderp, because they would perturb Goldilocks. Similarly, Jupiderp will be the only large planet anywhere near its orbit, since Herpth and Goldilocks are quite far out in the SoI.

How did this form? I think a not-ludicrous story would be that Herpth and Jupiderp formed around Lol, Herpth was captured by Jupiderp and smashed into a large moon in the process of clearing out most of the other moons of Jupiderp. Think Triton's arrival at Neptune. The impact carved off Goldilocks into the perfect region for stability and also happened to keep Herpth's rotation slow enough that Goldilock didn't slowly spiral out of its SoI before tidal locking was mutual.

Well that was fun! I apologize if this isn't the most well-organized thing I've ever written, but it's late and I'm kind of riffing. I hope this helps! There are other configurations you could use, but this is kind of a middle-of the road approach. For example, you could use a much bigger star and therefore give yourself a lot more room to play with if you don't need life to have evolved on this planet independently. If it's a colony of an advanced race, the the few million years that it'll be around would be plenty of time to establish a permanent base provided something could be done about the wild volcanic activity on the young grandmoon.

Edited by Horn Brain
Link to comment
Share on other sites

One thing to point out: You could not have the super-Earth moon tidally locked to both the grandmoon and the grandplanet (that's what I'm calling them because it's adorable :) ).

Wait, if they're supposed to be tidally locked to each other and the gas giant, that means the moon wouldn't really be a moon-moon, it would be another moon at a lagrange point between the gas giant and the larger moon, wouldn't it?

Ah, ignore me, I don't know what I'm talking about.

Link to comment
Share on other sites

Not hard to predict at all. After all, I baited a trap, you walked into it, and now I get to gloat.

.

Omg. I've made some predictions about your behavior, and, bingo ! You fulfilled not only one, but two I considered the most probable.

.

I had the suspicion from your first post where you essentially repeated the denialist talking points against modern climatology. Thank you for confirming that you are objecting against what I wrote basically because you've just an ideological ax to grind.

.

TheNewTeddy can at this point summarize, and I think that he was interested in writing hard science fiction, not hardcore ANTIscience fiction :wink:

Link to comment
Share on other sites

Wait, if they're supposed to be tidally locked to each other and the gas giant, that means the moon wouldn't really be a moon-moon, it would be another moon at a lagrange point between the gas giant and the larger moon, wouldn't it?

Ah, ignore me, I don't know what I'm talking about.

No, you're actually spot on. I made a small error in the Hill Sphere radius calculations of my post (it shouldn't affect the final numbers because of the tolerances I used), but if I hadn't made that error I would have noticed that this orbit passes through the L1 and L2 points roughly, so you wouldn't even expect to see a single orbit completed in that configuration.

Link to comment
Share on other sites

In all likelihood, a satellite of large enough size to be worth mentioning wouldn't be dynamically stable over long time scales. You could maybe play around in Universe Sandbox or a similar program to get a feel for what configurations would or would not be stable. Not sure if it models tidal effects though.

I tried this before in Universe sandbox, it COULD work however think about Io around Jupiter, it literally gets stretched by gravity, now if there were to to be a moon of moon, the stress of gravity on the moon would be so great, i seriously doubt that life could survive due to stress on the core would literally make the moon a volcanic wasteland. :\ however, lets not let facts ruin a good story however shall we? :)

Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...