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Day/night and eclipse brightness of a moon orbiting a gas giant?

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#1 ChrisBu

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Posted 13 December 2021 - 02:51 AM

Hi, new to the forum. Really not an expert in astronomy, science fiction or the like but I'm trying to add science fiction aspects to a fantasy-oriented world and was hoping for some input/verification of facts.

 

I know there's a lot of exotic stuff out there and an infinite number of possibilities to consider. I just really would like to have one habitable moon with an earthlike composition, with a smaller moon system and/or ring, orbiting one gas giant with one Sun-sized star. Let's say theoretically we start with a Mars-sized satellite or larger in a close orbit maybe the distance of our moon. As I understand it the planet/satellite ratio for Jupiter and Saturn is about 5000:1. Therefore the parent planet would have to be heavier than Jupiter if the moon was the size of Mars.

 

The moon would be tidally locked of course, and the planet could fill up as much as 10° of the sky (as I understand it, allegedly). It would also be very bright at night, as I understand it (allegedly) if the planet had reflective water vapor clouds. From what I've read it could be as much as a thousand times brighter than our moon, which would be like an LED-lit interior of a home – perhaps brighter than the day side.

 

Does this sound accurate? Also is it even realistic that a moon could orbit close-in to a gas giant? Would it become too vulcanized or torn apart by the Roche limit so need to be more of a distant orbit to support life, which in turn would dim things? How would radiation and magnetism affect life? Is it even a realistic question to ask if life could form on a moon orbiting a planet? I will note that Avatar had floating mountains, so don't be too harsh.

 

Lastly as I understand it there would be daily eclipses that could last for several hours during certain seasons (spring/summer). I'm just trying to get a grasp on how dark the sky would get during the eclipses. Twi-lightish or pitch black? I'm not trying to be precisely scientific about this. My life might wither up and die quickly in real life, but I want to try and get things as accurate as I can and maybe start up a discussion.

 

Thanks again.


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#2 Sleep Deprived

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Posted 13 December 2021 - 04:26 AM

The idea that there is a certain ratio of moon mass to planet mass may not be accurate.  In our solar system (thus, the only planet/moon examples that we have accurate data for) Earth's moon was likely formed due to an early impact by another planet-sized body and Neptune's large moon, Triton, is likely a captured body.  Also, it appears that Uranus and Neptune had a rough history - swapping orbits, being forced in/out due to interactions with Saturn and Jupiter, thus their moon histories are probably not well known.  That just leaves Saturn and Jupiter - moon/planet ratio fits fairly well with those two planets.  To extrapolate a sample size of 2 to the countless number of gas giants that exist in the galaxy would not be advised.  Clearly, that ratio seems to work, so if you want to use it, it shouldn't be a problem.  I think you could allow yourself some latitude there, though.  Just be aware that this is an observed ratio and planetary formation theory doesn't necessarily result in the same ratio.

 

Also, consider how close the moons are to your planet.  Quite close (like Io's orbit around Jupiter) and you would likely have a wildly geologically active body - especially if there are other moons in the system.  The volcanic activity on Io is orders of magnitude greater than that of Earth.  Much of its activity is due to the other large moons around Jupiter that are 'pulling' at it (flexing it) during every orbit.  Even though most activity is due to other moons nearby, I would bet being so close to such a large object would also reek havoc on the inside of the moon - a circular orbit and being tidally locked would reduce this effect, though.

 

Another thing to consider is the (possible?  probable??) magnetic field around the planet.  There are no requirements of such a field, it is probably likely in most, or all, large gas giants.  The most distant of Jupiter's moons, Callisto, is doused by about 10 times the radiation that bombards Earth.  The 3 more inner moons get much greater doses.  I would expect if the planetary system were closer to its star than Jupiter that the radiation bombardment would go up - the planetary magnetosphere would likely guide more solar radiation around the field - any moons would get an extra jolt.

 

One last thing to consider:  Jupiter is a big gravitational 'sucker'.  It, by far, gets more incoming bodies hitting it than any other planet in the Solar System  Because the moons are in orbit around it, they also get smashed by orders of magnitude greater than Earth.  It has been suggested that we are lucky to have Jupiter 'protecting' the inner Solar System - if it weren't there we would get more bombardment on Earth than we do.  Since we have been observing Jupiter (a few 100 years) we have observed many direct hits, big enough to be visible from Earth.  Comet Shoemaker-Levy9 was big enough to cause a 'scar' on Jupiter the size of Earth.  We can only imagine how many, and how often, other hits there have been - the moons would intercept a small ratio of those.

 

Life on a moon of a gas giant would likely not be very secure.  Stability wouldn't be in the vocabulary of the moon's inhabitants.

 

If it were a single large moon, maybe you could reduce the amount of geologic activity.  Maybe some scenario could reduce radiation.  A thick atmosphere would reduce small impacts - they may burn up, as many do on Earth, before impact.  There would be nothing to save them from the larger impacts.

 

Good luck with your project.


Edited by Sleep Deprived, 13 December 2021 - 04:30 AM.

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#3 ChrisBu

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Posted 13 December 2021 - 06:43 AM

One last thing to consider:  Jupiter is a big gravitational 'sucker'.  It, by far, gets more incoming bodies hitting it than any other planet in the Solar System  Because the moons are in orbit around it, they also get smashed by orders of magnitude greater than Earth.  It has been suggested that we are lucky to have Jupiter 'protecting' the inner Solar System - if it weren't there we would get more bombardment on Earth than we do.  Since we have been observing Jupiter (a few 100 years) we have observed many direct hits, big enough to be visible from Earth.  Comet Shoemaker-Levy9 was big enough to cause a 'scar' on Jupiter the size of Earth.  We can only imagine how many, and how often, other hits there have been - the moons would intercept a small ratio of those.

All those were great points and I will have to think on it, but since it's a fantasy world one way you could solve this issue is to say it was recently captured by the gas giant so life could have developed on it prior to it becoming a moon. Anyways appreciate the input exactly what I'm looking for. There's ways to make these things as realistic as possible, that's the game. Big puzzle.



#4 KBHornblower

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Posted 15 December 2021 - 09:32 PM

Hi, new to the forum. Really not an expert in astronomy, science fiction or the like but I'm trying to add science fiction aspects to a fantasy-oriented world and was hoping for some input/verification of facts.

 

I know there's a lot of exotic stuff out there and an infinite number of possibilities to consider. I just really would like to have one habitable moon with an earthlike composition, with a smaller moon system and/or ring, orbiting one gas giant with one Sun-sized star. Let's say theoretically we start with a Mars-sized satellite or larger in a close orbit maybe the distance of our moon. As I understand it the planet/satellite ratio for Jupiter and Saturn is about 5000:1. Therefore the parent planet would have to be heavier than Jupiter if the moon was the size of Mars.

 

The moon would be tidally locked of course, and the planet could fill up as much as 10° of the sky (as I understand it, allegedly). It would also be very bright at night, as I understand it (allegedly) if the planet had reflective water vapor clouds. From what I've read it could be as much as a thousand times brighter than our moon, which would be like an LED-lit interior of a home – perhaps brighter than the day side.

 

Does this sound accurate? Also is it even realistic that a moon could orbit close-in to a gas giant? Would it become too vulcanized or torn apart by the Roche limit so need to be more of a distant orbit to support life, which in turn would dim things? How would radiation and magnetism affect life? Is it even a realistic question to ask if life could form on a moon orbiting a planet? I will note that Avatar had floating mountains, so don't be too harsh.

 

Lastly as I understand it there would be daily eclipses that could last for several hours during certain seasons (spring/summer). I'm just trying to get a grasp on how dark the sky would get during the eclipses. Twi-lightish or pitch black? I'm not trying to be precisely scientific about this. My life might wither up and die quickly in real life, but I want to try and get things as accurate as I can and maybe start up a discussion.

 

Thanks again.

My bold.  I don't think so, based on my own observations.  I find from various sources that the total brightness of a clear blue sky is about 10% of the Sun's brightness, and my exposure meter shows a typical landscape at high noon to be pretty similar.  Even if covered with fresh snow, it will not light up the underside of an object as bright as the Sun lights up the top.  A giant planet with a high albedo might come close, but will not exceed it.


Edited by KBHornblower, 15 December 2021 - 09:33 PM.

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#5 KBHornblower

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Posted 15 December 2021 - 09:42 PM

The idea that there is a certain ratio of moon mass to planet mass may not be accurate.  In our solar system (thus, the only planet/moon examples that we have accurate data for) Earth's moon was likely formed due to an early impact by another planet-sized body and Neptune's large moon, Triton, is likely a captured body.  Also, it appears that Uranus and Neptune had a rough history - swapping orbits, being forced in/out due to interactions with Saturn and Jupiter, thus their moon histories are probably not well known.  That just leaves Saturn and Jupiter - moon/planet ratio fits fairly well with those two planets.  To extrapolate a sample size of 2 to the countless number of gas giants that exist in the galaxy would not be advised.  Clearly, that ratio seems to work, so if you want to use it, it shouldn't be a problem.  I think you could allow yourself some latitude there, though.  Just be aware that this is an observed ratio and planetary formation theory doesn't necessarily result in the same ratio.

 

Also, consider how close the moons are to your planet.  Quite close (like Io's orbit around Jupiter) and you would likely have a wildly geologically active body - especially if there are other moons in the system.  The volcanic activity on Io is orders of magnitude greater than that of Earth.  Much of its activity is due to the other large moons around Jupiter that are 'pulling' at it (flexing it) during every orbit.  Even though most activity is due to other moons nearby, I would bet being so close to such a large object would also reek havoc on the inside of the moon - a circular orbit and being tidally locked would reduce this effect, though.

 

Another thing to consider is the (possible?  probable??) magnetic field around the planet.  There are no requirements of such a field, it is probably likely in most, or all, large gas giants.  The most distant of Jupiter's moons, Callisto, is doused by about 10 times the radiation that bombards Earth.  The 3 more inner moons get much greater doses.  I would expect if the planetary system were closer to its star than Jupiter that the radiation bombardment would go up - the planetary magnetosphere would likely guide more solar radiation around the field - any moons would get an extra jolt.

 

One last thing to consider:  Jupiter is a big gravitational 'sucker'.  It, by far, gets more incoming bodies hitting it than any other planet in the Solar System  Because the moons are in orbit around it, they also get smashed by orders of magnitude greater than Earth.  It has been suggested that we are lucky to have Jupiter 'protecting' the inner Solar System - if it weren't there we would get more bombardment on Earth than we do.  Since we have been observing Jupiter (a few 100 years) we have observed many direct hits, big enough to be visible from Earth.  Comet Shoemaker-Levy9 was big enough to cause a 'scar' on Jupiter the size of Earth.  We can only imagine how many, and how often, other hits there have been - the moons would intercept a small ratio of those.

 

Life on a moon of a gas giant would likely not be very secure.  Stability wouldn't be in the vocabulary of the moon's inhabitants.

 

If it were a single large moon, maybe you could reduce the amount of geologic activity.  Maybe some scenario could reduce radiation.  A thick atmosphere would reduce small impacts - they may burn up, as many do on Earth, before impact.  There would be nothing to save them from the larger impacts.

 

Good luck with your project.

My bold.  That could be misconstrued as saying that Jupiter is currently protecting us from incoming bodies such as comets from the Oort cloud and Kuiper belt.  I think what it did was to eject a lot of primordial small stuff over the eons and make the inner solar system far less of a shooting gallery than it would have been otherwise.  Only a tiny portion of new incoming stuff is going to hit Jupiter.  For those that miss and keep coming in, Jupiter's gravity might turn what would have been hits into misses, but it could just as easily turn what could have been misses into hits.


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#6 Sleep Deprived

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Posted 15 December 2021 - 10:22 PM

My bold.  I don't think so, based on my own observations.  I find from various sources that the total brightness of a clear blue sky is about 10% of the Sun's brightness, and my exposure meter shows a typical landscape at high noon to be pretty similar.  Even if covered with fresh snow, it will not light up the underside of an object as bright as the Sun lights up the top.  A giant planet with a high albedo might come close, but will not exceed it.

Plus, having a bright (albeit not nearly as bright as the star that is illuminating it) large ball in the sky (presumably much bigger than the star) would cast a very diffuse shadow.  When the Moon casts a shadow on Earth, it looks very similar to a Sun shadow (except for the brightness/contrast) because it is about the same size (angular size, in the sky) as the Sun.  Spread out the source of light in the sky to, say, 1/4 of the sky, and shadows may be so diffuse as to not actually be recognizable as shadows, except only very close to the ground.  It might be similar to how a shadow is cast on a cloudy day - there is plenty of light, but no 'spot' source, so shadows are hard to come by.  Granted, the sun is not a 'spot' source, but it is much closer to one than something that takes up 1/4 of the sky.


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#7 DSOGabe

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Posted 16 December 2021 - 05:25 PM

Assuming that your Jovian planet is a lot like Jupiter, I would say that its gravity could have an effect on the tectonic activity of the moon and the radiation from the planet would surely have a negative effect on any life (unless its deep underground or underwater).

The concept of a habitable moon as shown in Avatar-with the gas giant so close- is not a likely scenario. 


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#8 Keith Rivich

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Posted 16 December 2021 - 07:51 PM

Take a look at this series of books: https://www.amazon.c...k/dp/B07GKPC16B

 

The author does a good job of describing what it is like on the Saturnian moon. 


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#9 ChrisBu

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Posted 18 December 2021 - 05:08 AM

I appreciate all the input and plan on digesting more this weekend. I'm still trying to visualize the daily eclipses and what they might look like. How long would they last? And how dark would it be? I mean that could be dramatic, you have this bright planet then pitch black for one or two hours everyday.


Edited by ChrisBu, 18 December 2021 - 05:08 AM.


#10 KiwiObserver

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Posted 14 January 2022 - 10:32 PM

Well here is a new finding which you may find very interesting - there are signs (at the 4.8 sigma level) of a possible moon with a radius of 2.6 Earth radii orbiting a Jupiter-sized planet in a 1.6 au orbit from the Sun-like star:

 

(open access, Nature Astronomy) https://www.nature.c...21-01539-1.pdf 

 

What an interesting world that could be. It's noted however that this potential moon will have to be confirmed with further observations 


Edited by KiwiObserver, 14 January 2022 - 10:38 PM.



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