Thread: Light level on a fictional moon

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Light level on a fictional moon

I have a screenplay project I'm working on that has quite a few similarities with this thread (and thanks to everyone on that thread, including poster). So it's set on a moon orbiting a Hot Jupiter. The level of research I'm doing may seem a bit excessive, but over-researching is part of my process. I've figured out the basic parameters, but the main problem I'm having is figuring out light levels. This is pretty complicated, so I'm really just trying to figure out reasonable limits and consequences.

For convenience's sake, I've so far assumed that the planet is about 1.1 AU from a G-class star (orbital period about 419d), though I might modify this. The parameters at present are (approx):

Planet mass = 2e28 kg
Moon mass = 2e24 kg
Semi-major axis (moon) = 3e6 km
Orbital period (moon) = 10.3 days

I've maxed out the gas giant size in order to have the largest possible moon, and placed it far enough away that with a thick atmosphere the radiation from the planet should hopefully not be too harmful. Despite the mass, I'm assuming the planet size is about identical to Jupiter. If I've calculated angular diameters correctly (planet = 163.86', sun = 32' for simplicity), the planet will be about 5 times the size of the sun in the sky and there will be a maximum of about a 1h53m mid-day eclipse (total darkness about 1h10m). I'm also thinking the moon will be quite small and dense, so that this might get me close to Earth gravity without Earth size. I calculate the gravity to be about 8.34 m/s^2 (I so far haven't tried to figure out if the planet's gravity would have a substantial effect on this #).

The efforts of anyone checking my math are much appreciated, as well as any suggestions anyone might have. My main question at this point, however, is about luminosity. This question came up in the Day/Night thread, but it didn't go into detail. It was merely suggested that the night-time light from the planet would be some 4000 times as bright as the sun. EDIT: MOON! Oops. My research indicates that it depends primarily on the albedo of the reflecting body and the atmosphere, right? Just taking the arbitrary 4000xfull moon number, that would create a night sky about equivalent to a very overcast day (~1080 lx), which is pretty bloody bright. With about 2h of actual darkness, this moon will have about 5 day "nights," which I'm looking to have more of an eternal twilight effect. If it helps, I'm thinking the atmosphere will be pretty thick (this is largely due to light terraforming) with a lot of particulates that give the sky an indigo hue (the particulates are important to the plot, the color to the aesthetic). What are the lower bounds I can put on my giant nightlight? I'm thinking I might not have much room to play with the albedo, b/c gas giant composition is pretty regular, right? How much can atmosphere adjust the light? What effects would necessary tweaks have? Already it is tidally locked, obviously, so I'm assuming extreme tides, pretty much impassable seas (and the moon is mostly ocean, b/c of water picked up on the Hot Jupiter's passage into the habitable zone). Possibly very little seasonal variation due to the water and atmosphere? I haven't decided on axis-tilt and various weather-related details (which will effect the exact latitude of the inhabitants).

Oh, and one other question: would the orbital radius have a large effect on weather on the moon? As compared to the planet's orbit, the moon's orbit would be like a squiggly line as it followed the planet around, and so far I've assumed this variation wouldn't have significant effects on seasons or temperatures in comparison to axial tilt and the planet's own eccentricity, but I'm not quite sure.

- William
Last edited by wmatanner; 2012-Jul-01 at 03:38 PM.

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Originally Posted by wmatanner
So it's set on a moon orbiting a Hot Jupiter.
Not really hot...
Originally Posted by wmatanner
For convenience's sake, I've so far assumed that the planet is about 1.1 AU from a G-class star (orbital period about 419d), though I might modify this. The parameters at present are (approx):

Planet mass = 2e28 kg
10,5 Jupiter masses, 3300 Earth masses
Originally Posted by wmatanner
Moon mass = 2e24 kg
1/3 Earth masses, 3,1 Mars masses. 6 Mercury masses.
Originally Posted by wmatanner
Semi-major axis (moon) = 3e6 km
0,02 AU, 8 times Earth-Moon distance, 1,6 times the distance of Callisto and 2,8 times the distance of Ganymedes.
Originally Posted by wmatanner
Orbital period (moon) = 10.3 days
1,64 times the radius of Mercury
Originally Posted by wmatanner
I've maxed out the gas giant size in order to have the largest possible moon, and placed it far enough away that with a thick atmosphere the radiation from the planet should hopefully not be too harmful. Despite the mass, I'm assuming the planet size is about identical to Jupiter. If I've calculated angular diameters correctly (planet = 163.86', sun = 32' for simplicity), the planet will be about 5 times the size of the sun in the sky and there will be a maximum of about a 1h53m mid-day eclipse (total darkness about 1h10m). I'm also thinking the moon will be quite small and dense, so that this might get me close to Earth gravity without Earth size. I calculate the gravity to be about 8.34 m/s^2 (I so far haven't tried to figure out if the planet's gravity would have a substantial effect on this #).
Certainly not.
The density, incidentally, is about 1,36 times that of Mercury. Does anyone have the Mercury composition gravitational compression numbers in handy?
Originally Posted by wmatanner
If it helps, I'm thinking the atmosphere will be pretty thick (this is largely due to light terraforming) with a lot of particulates that give the sky an indigo hue (the particulates are important to the plot, the color to the aesthetic).
Regardless of any terraforming, Titan has a fairly thick atmosphere, too. But what could make the light bluer?
Originally Posted by wmatanner
Already it is tidally locked, obviously, so I'm assuming extreme tides, pretty much impassable seas (and the moon is mostly ocean, b/c of water picked up on the Hot Jupiter's passage into the habitable zone).
No grounds to assume it. Unless something excites eccentricity, the moon can evolve to a low eccentricity orbit with little libration and little tides.
Originally Posted by wmatanner
Oh, and one other question: would the orbital radius have a large effect on weather on the moon? As compared to the planet's orbit, the moon's orbit would be like a squiggly line as it followed the planet around, and so far I've assumed this variation wouldn't have significant effects on seasons or temperatures in comparison to axial tilt and the planet's own eccentricity, but I'm not quite sure.
Earth has eccentricity 0,015 AU either way, and it has little effect. So this 0,02 AU orbit is comparable.

Now, regarding the light.

Since your planet is small in the sky - because it is very massive and therefore dense and because the moon is on a long period, remote orbit - it covers only 30 times the area of Moon in the sky - not 1000 times, and only twice the area of Earth on Moon sky. Therefore if it is as white as Venus (due to dense water clouds), it would be about 10 times brighter per area, and only 300 times as bright as Moon, right? Mere 90 lx. And that when full.

There would be phases. For the antiterran point, the planet would be full at midnight and half at dawn and dusk. Not sure what exactly the limb darkening and opposition surge look like for water clouds...

The whole far side never sees the planet, obviously. Near the edges, where the planet hangs just above the horizon, it can be a narrow crescent horns up a part of the night - namely just after the Sun has set beneath the horns or just before the Sun is to rise under the horns. Over the night the planet would wax or wane, to be gibbous just after Sun has set opposite the planet or just before Sun rises opposite the planet.

As for colour and albedo of the planet, water, ammonia and methane all absorb red light. Can you play around with the ratio of water (which condenses in habitable zone) to ammonia and methane (which do not)? This way you might get a giant with somewhat less dense and bright clouds, and more blue hue above, like Uranus.

3. I've been working on a similar arrangement for a screenplay I'll probably never finish. Good luck to you with yours.

Originally Posted by wmatanner
I have a screenplay project I'm working on that has quite a few similarities with this thread (and thanks to everyone on that thread, including poster). So it's set on a moon orbiting a Hot Jupiter. The level of research I'm doing may seem a bit excessive, but over-researching is part of my process. I've figured out the basic parameters, but the main problem I'm having is figuring out light levels. This is pretty complicated, so I'm really just trying to figure out reasonable limits and consequences.

For convenience's sake, I've so far assumed that the planet is about 1.1 AU from a G-class star (orbital period about 419d), though I might modify this. The parameters at present are (approx):

Planet mass = 2e28 kg
Moon mass = 2e24 kg
Semi-major axis (moon) = 3e6 km
Orbital period (moon) = 10.3 days
A Hot Jupiter, as normally thought of, needs to be much closer to its sun than ~1 AU.

I've maxed out the gas giant size in order to have the largest possible moon, and placed it far enough away that with a thick atmosphere the radiation from the planet should hopefully not be too harmful. Despite the mass, I'm assuming the planet size is about identical to Jupiter. If I've calculated angular diameters correctly (planet = 163.86', sun = 32' for simplicity), the planet will be about 5 times the size of the sun in the sky and there will be a maximum of about a 1h53m mid-day eclipse (total darkness about 1h10m). I'm also thinking the moon will be quite small and dense, so that this might get me close to Earth gravity without Earth size. I calculate the gravity to be about 8.34 m/s^2 (I so far haven't tried to figure out if the planet's gravity would have a substantial effect on this #).
I don't think you have to max out the planet to have a large moon. The Moon is larger compared to Earth than Jovian moons are compared to Jupiter. Double planets are plausible too.

Incidentally, the planet would not appear 5 times larger than the sun in the moon's sky. Accepting your measurements, it would appear 5 times wider than the sun, which when you calculate area of the disk (pi*r2), it comes out closer to 82 times larger than the sun.

Correction: I made a mistake and used the diameter for radius. The area of the planet's disk compared to the sun's disk is more like 26x.

The efforts of anyone checking my math are much appreciated, as well as any suggestions anyone might have. My main question at this point, however, is about luminosity. This question came up in the Day/Night thread, but it didn't go into detail. It was merely suggested that the night-time light from the planet would be some 4000 times as bright as the sun. My research indicates that it depends primarily on the albedo of the reflecting body and the atmosphere, right? Just taking the arbitrary 4000xfull moon number, that would create a night sky about equivalent to a very overcast day (~1080 lx), which is pretty bloody bright. With about 2h of actual darkness, this moon will have about 5 day "nights," which I'm looking to have more of an eternal twilight effect. If it helps, I'm thinking the atmosphere will be pretty thick (this is largely due to light terraforming) with a lot of particulates that give the sky an indigo hue (the particulates are important to the plot, the color to the aesthetic). What are the lower bounds I can put on my giant nightlight? I'm thinking I might not have much room to play with the albedo, b/c gas giant composition is pretty regular, right? How much can atmosphere adjust the light? What effects would necessary tweaks have? Already it is tidally locked, obviously, so I'm assuming extreme tides, pretty much impassable seas (and the moon is mostly ocean, b/c of water picked up on the Hot Jupiter's passage into the habitable zone). Possibly very little seasonal variation due to the water and atmosphere? I haven't decided on axis-tilt and various weather-related details (which will effect the exact latitude of the inhabitants).
I can't help much with luminosity, but your tides shouldn't be extreme at all. The water will be at equilibrium with the planet's gravity since it's tidally locked. Only the sun would cause tides, which will depend a lot on bathymetry, but I don't think it would be more than a foot or so, generally speaking.

Oh, and one other question: would the orbital radius have a large effect on weather on the moon? As compared to the planet's orbit, the moon's orbit would be like a squiggly line as it followed the planet around, and so far I've assumed this variation wouldn't have significant effects on seasons or temperatures in comparison to axial tilt and the planet's own eccentricity, but I'm not quite sure.
Well, the variation is about ±1% of the median average of the planet's solar distance of 1.1AU. This appears to be about the same variation as earth experiences between perihelion and aphelion. With a thick atmosphere and a lot of water and a fair amount of reflected/generated radiance from the planet, the variation of insolation upon the moon on such short timescales due to distance may be lost in the noise. More important, I think, will be the variation of insolation due to the the moon's slow day-night cycle. The variation will tend to be less for the near-side, but the far side will probably experience more extreme swings due to the lack of a night-time heat source, but I think weather and climate will even it out to a relatively minor fluctuation.
Last edited by Ara Pacis; 2012-Jun-29 at 04:34 AM. Reason: corrected math error

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Originally Posted by chornedsnorkack
Not really hot...

10,5 Jupiter masses, 3300 Earth masses
How old is this thing? the gas giant could be a significant source of heat, depending on its age.

Originally Posted by chornedsnorkack
1/3 Earth masses, 3,1 Mars masses. 6 Mercury masses.

1,64 times the radius of Mercury

The density, incidentally, is about 1,36 times that of Mercury. Does anyone have the Mercury composition gravitational compression numbers in handy?
No but it seems implausibly dense. If the density of the planet were normal I think the mass would be too small to retain an atmosphere.

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Originally Posted by whimsyfree
How old is this thing? the gas giant could be a significant source of heat, depending on its age.
In 2 milliard years, it should be Y class already.
Originally Posted by whimsyfree
No but it seems implausibly dense. If the density of the planet were normal I think the mass would be too small to retain an atmosphere.
There is a proven planet with density 8,8 - Kepler 10b - but it is bigger and therefore gravitationally more compressed.

Mars, 7000 km diametre and lower uncompressed density than Earth let alone Mercury almost retains an atmosphere. It seems reasonable that a planet or moon which is just slightly more massive than Mars, like 8000 km diametre, might successfully retain atmosphere.

6. Originally Posted by chornedsnorkack
In 2 milliard years, it should be Y class already.

There is a proven planet with density 8,8 - Kepler 10b - but it is bigger and therefore gravitationally more compressed.

Mars, 7000 km diametre and lower uncompressed density than Earth let alone Mercury almost retains an atmosphere. It seems reasonable that a planet or moon which is just slightly more massive than Mars, like 8000 km diametre, might successfully retain atmosphere.
The planet's magnetosphere might protect it from the solar wind, but that may depend on plasma from other moons in the magnetosphere of the planet. Ganymede is reported to have its own magnetospheric bubble inside the Jovian magnetisphere. Of course, the closed field lines are only from latitudes 30N and 30S, so perhaps atmosphere might be sputtered away from the poles. But a larger moon with a larger magnetosphere might have different results.

7. Originally Posted by wmatanner
For convenience's sake, I've so far assumed that the planet is about 1.1 AU from a G-class star (orbital period about 419d), though I might modify this. The parameters at present are (approx):

Planet mass = 2e28 kg
Moon mass = 2e24 kg
Semi-major axis (moon) = 3e6 km
Orbital period (moon) = 10.3 days
OK, so...
Planet Mass = about 11 Jupiters.

You're right, even though it's so massive, the radius of the superjovian isn't going to be much bigger than Jupiter due to self-compression. Even Brown Dwarfs up to 80 jupiter masses aren't really much bigger than this. Density is obviously WAY higher though. The jovian itself isn't quite a brown dwarf - it won't be big enough to have commenced any kind of fusion in its interior but it'd be rather warm from gravitational contraction. I doubt it'd be warm enough to affect the moon's temperature though.

Moon mass = about 0.5 Earths.
Moon radius = a bit bigger than Mars.

If your jovian is that massive, it can have a larger moon. You could comfortably have a large earth-sized moon here (there probably wouldn't be any other significant moons though - the jovian would have to be more massive for that).

Based on the Sudarsky classification, you're looking at a Class II or possibly III jovian. I'd guess that the internal heat might be enough to preclude water clouds so it may be more likely to be a Class III, which are cloudless and have low bond albedo - but there's a pretty wide range of albedo between the two types, which doesn't help so much.

If I've calculated angular diameters correctly (planet = 163.86', sun = 32' for simplicity), the planet will be about 5 times the size of the sun in the sky
Sounds about right to me, I got it to be 5.1 times bigger than our own sun/moon in our own sky.

The efforts of anyone checking my math are much appreciated, as well as any suggestions anyone might have. My main question at this point, however, is about luminosity. This question came up in the Day/Night thread, but it didn't go into detail. It was merely suggested that the night-time light from the planet would be some 4000 times as bright as the sun.
I'm guessing you mean 4000x the brightness of the full moon, not the sun!
At 1.1 AU (assuming it has luminosity equal to Sol) the star is 0.82 times the brightness of Sol as seen from Earth (due to the inverse square law). If it's a Class II Jovian, then it'll have a low albedo (about 0.12) - that said that's similar to the Moon, and look how bright that gets . Given that the area covered in the sky would be about 25 times as much as our own moon, I'd expect the full jovian to be significantly brighter in the sky than our own moon!

Already it is tidally locked, obviously, so I'm assuming extreme tides, pretty much impassable seas (and the moon is mostly ocean, b/c of water picked up on the Hot Jupiter's passage into the habitable zone). Possibly very little seasonal variation due to the water and atmosphere? I haven't decided on axis-tilt and various weather-related details (which will effect the exact latitude of the inhabitants).
The tides would actually be non-existent if the moon was tidally locked - there'd be a permanent tidal bulge aligned towards the planet, but the orbit should be circular unless something else is affecting the eccentricity of the orbit (e.g. other satellites, or solar tides). So unless that's the case, there wouldn't be much (if any) tides at all (it's also quite far from the planet too).

Oh, and one other question: would the orbital radius have a large effect on weather on the moon? As compared to the planet's orbit, the moon's orbit would be like a squiggly line as it followed the planet around, and so far I've assumed this variation wouldn't have significant effects on seasons or temperatures in comparison to axial tilt and the planet's own eccentricity, but I'm not quite sure.
Good question... the moon would range between 3 million km closer to and further from the sun relative to the jovian's orbit (assuming the jovian's orbit is circular). That's comparable to the variation in Earth's distance from the Sun due to its orbital eccentricity, but the earth's axial tilt has far more major effects on the climate of the planet than its orbital eccentricity. So... the distance variation probably wouldn't have much of an effect on the scale of a "month" (one orbit around the planet) - it's more likely that the jovian's tilt (and therefore the tilt of the moon, since it's likely to be aligned with the jovian's equatorial plane) is going to have a bigger effect. If the jovian has an eccentric orbit around the star (which I think you imply?) then that would also contribute too.

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Great responses, guys, many thanks. That clears up a lot. I somehow missed the bit about HJ's orbiting within .5 AU of their star. Since there was some suggestion that Hot Jupiters might be within the habitable zone, I figured some of them would be further out. Of course, habitable zone doesn't mean habitable for *us*, which is what I'm going for. However, the part of the moon that needs to be livable for humans can be very small, which is part of why I haven't worked out fully what sort of temperatures I'll see at what latitudes yet. I think it's obvious that I'm mainly interested in the side facing the planet, so if that side has more stable temperatures, that's handy. What variables can I play with to make a (humanly) habitable gas giant's moon? Preferably on the cold side, to be honest. A Hot Neptune could orbit at nearly 1 AU, but then I wouldn't be able to achieve the mass for a Mars-sized moon, yes?

Originally Posted by Ara Pacis
I don't think you have to max out the planet to have a large moon. The Moon is larger compared to Earth than Jovian moons are compared to Jupiter. Double planets are plausible too.
Originally Posted by EDG
Moon mass = about 0.5 Earths.
Moon radius = a bit bigger than Mars.

If your jovian is that massive, it can have a larger moon. You could comfortably have a large earth-sized moon here (there probably wouldn't be any other significant moons though - the jovian would have to be more massive for that).
As I understand it, a gas giant can only support moons up to about 1/10,000 its mass, and so I assumed my 11MJ gas giant could handle a 2e28 kg moon at most. I need near-Earth gravity, though (a little on the light side is okay); thus, I bumped up the density. I was playing with the plausibility of the idea that the density might be due to the moon originally having been a planet that was stripped to its iron core and was captured by the gas giant as it transitioned towards the star. So what's the consensus, is an Earth-sized moon possible? Because that was my original intent and would make things simple. Or can I manage a smaller, denser moon?

Originally Posted by EDG
the distance variation probably wouldn't have much of an effect on the scale of a "month" (one orbit around the planet) - it's more likely that the jovian's tilt (and therefore the tilt of the moon, since it's likely to be aligned with the jovian's equatorial plane) is going to have a bigger effect. If the jovian has an eccentric orbit around the star (which I think you imply?) then that would also contribute too.
Actually, I haven't really played with axial tilt and eccentricity so far, as I said. I'm not terribly concerned about having significant seasons. I'm mainly concerned with establishing an average local temperature, preferably somewhere between 5-14C, at a latitude that maximizes the eclipse to some degree. The tilt and latitude, then, would mainly be designed around keeping the planet and sun paths aligned without having the planet at high noon. But I haven't worked this out yet.

All very useful to know, guys, and if anyone has more thoughts on consequences of this arrangement I might not have thought of, please share. I was surprised about the tides, though. Our own moon is tidally locked, and yet is responsible for our tides, thus I assumed a large body like that would create significant tides. I’m obviously missing something.

As for the light:
Originally Posted by chornedsnorkack
if it is as white as Venus (due to dense water clouds), it would be about 10 times brighter per area, and only 300 times as bright as Moon, right? Mere 90 lx. And that when full.

As for colour and albedo of the planet, water, ammonia and methane all absorb red light. Can you play around with the ratio of water (which condenses in habitable zone) to ammonia and methane (which do not)? This way you might get a giant with somewhat less dense and bright clouds, and more blue hue above, like Uranus.
90 lx at full sounds pretty good, actually. I couldn’t figure out how to calculate that, so thank you. So what kind of light level would we be looking at for a blue gas giant rich in ammonia/methane as you propose?

Originally Posted by chornedsnorkack
Regardless of any terraforming, Titan has a fairly thick atmosphere, too. But what could make the light bluer?
The terraforming is mainly to ensure a breathable atmosphere, although the atmosphere may well be thicker than it was originally. My thought was that during this process, a large amount of particles was released into the atmosphere, which resulted in a sky that looks more indigo, rather than the blue of our sky. These particles are in fact a native organism critical to the story. I thought it might be important in thinking about light levels and greenhouse effect.

Thanks again!

9. Originally Posted by wmatanner
Great responses, guys, many thanks. That clears up a lot. I somehow missed the bit about HJ's orbiting within .5 AU of their star. Since there was some suggestion that Hot Jupiters might be within the habitable zone, I figured some of them would be further out. Of course, habitable zone doesn't mean habitable for *us*, which is what I'm going for. However, the part of the moon that needs to be livable for humans can be very small, which is part of why I haven't worked out fully what sort of temperatures I'll see at what latitudes yet. I think it's obvious that I'm mainly interested in the side facing the planet, so if that side has more stable temperatures, that's handy. What variables can I play with to make a (humanly) habitable gas giant's moon? Preferably on the cold side, to be honest. A Hot Neptune could orbit at nearly 1 AU, but then I wouldn't be able to achieve the mass for a Mars-sized moon, yes?
Have you considered paraterraforming? Which side are you defining to be the cold side and is that the side that humans will inhabit? It's not clear.

As I understand it, a gas giant can only support moons up to about 1/10,000 its mass, and so I assumed my 11MJ gas giant could handle a 2e28 kg moon at most. I need near-Earth gravity, though (a little on the light side is okay); thus, I bumped up the density. I was playing with the plausibility of the idea that the density might be due to the moon originally having been a planet that was stripped to its iron core and was captured by the gas giant as it transitioned towards the star. So what's the consensus, is an Earth-sized moon possible? Because that was my original intent and would make things simple. Or can I manage a smaller, denser moon?
I'll have to look around to see if gas giants are special due to increased fluidity or something. I might be wrong about the size of moons.

Actually, I haven't really played with axial tilt and eccentricity so far, as I said. I'm not terribly concerned about having significant seasons. I'm mainly concerned with establishing an average local temperature, preferably somewhere between 5-14C, at a latitude that maximizes the eclipse to some degree. The tilt and latitude, then, would mainly be designed around keeping the planet and sun paths aligned without having the planet at high noon. But I haven't worked this out yet.
I don't think latitude would affect the eclipse time which is based on the rate of revolution not the rate of revolution rotation, which would be negligible anyways because it's tidally locked.

All very useful to know, guys, and if anyone has more thoughts on consequences of this arrangement I might not have thought of, please share. I was surprised about the tides, though. Our own moon is tidally locked, and yet is responsible for our tides, thus I assumed a large body like that would create significant tides. I’m obviously missing something.
Yes, Earth would be equivalent to the gas giant in this arrangement. If our moon had a liquid ocean of some sort, it would not have terran tides either, just solar tides.

The terraforming is mainly to ensure a breathable atmosphere, although the atmosphere may well be thicker than it was originally. My thought was that during this process, a large amount of particles was released into the atmosphere, which resulted in a sky that looks more indigo, rather than the blue of our sky. These particles are in fact a native organism critical to the story. I thought it might be important in thinking about light levels and greenhouse effect.

Thanks again!
You'll need to not only allow indigo wavelengths to pass through, be scattered and reflected and perhaps emitted in the sky, you'd have to have the other wavelengths be absorbed without reflection in the sky or have them pass cleanly through to the surface (like on earth). A sky may look indigo with an atmosphere that doesn't scatter a lot of light as the blackness of space might be visible (similar to twilight on Earth). Perhaps you can use the magnetic field lines to support some sort of particulate or organic matter that fulfills the coloration role.
Last edited by Ara Pacis; 2012-Jul-02 at 03:35 AM. Reason: typo repeated "revolution" for "rotation"

10. Originally Posted by wmatanner
Great responses, guys, many thanks. That clears up a lot. I somehow missed the bit about HJ's orbiting within .5 AU of their star. Since there was some suggestion that Hot Jupiters might be within the habitable zone, I figured some of them would be further out. Of course, habitable zone doesn't mean habitable for *us*, which is what I'm going for. However, the part of the moon that needs to be livable for humans can be very small, which is part of why I haven't worked out fully what sort of temperatures I'll see at what latitudes yet. I think it's obvious that I'm mainly interested in the side facing the planet, so if that side has more stable temperatures, that's handy. What variables can I play with to make a (humanly) habitable gas giant's moon? Preferably on the cold side, to be honest. A Hot Neptune could orbit at nearly 1 AU, but then I wouldn't be able to achieve the mass for a Mars-sized moon, yes?

As I understand it, a gas giant can only support moons up to about 1/10,000 its mass, and so I assumed my 11MJ gas giant could handle a 2e28 kg moon at most. I need near-Earth gravity, though (a little on the light side is okay); thus, I bumped up the density. I was playing with the plausibility of the idea that the density might be due to the moon originally having been a planet that was stripped to its iron core and was captured by the gas giant as it transitioned towards the star. So what's the consensus, is an Earth-sized moon possible? Because that was my original intent and would make things simple. Or can I manage a smaller, denser moon?
At 10 MJ, an earthsized moon should be fine according to that 1/10,000th "rule" (assuming it is indeed a universal rule here). And it doesn't have to be a "HOT" Jupiter, it could just as easily be a superjovian that migrated into the habitable zone.

All very useful to know, guys, and if anyone has more thoughts on consequences of this arrangement I might not have thought of, please share. I was surprised about the tides, though. Our own moon is tidally locked, and yet is responsible for our tides, thus I assumed a large body like that would create significant tides. I’m obviously missing something.
The moon has an eccentric orbit for one thing (I'd guess that the eccentricity is maintained by solar tides? It's also inclined relative to the Earth, and actually orbits close to the plane of the solar system). The eccentricities of the major jovian moons in our solar system are generally much smaller (and when it isn't, it's because it's being forced higher by resonances).

In the absence of any other influences, the moon would be tidally locked to the planet very early in its history. A few billion years of subsequent orbital evolution would act to circularise the orbit significantly, so there shouldn't be much in the way of tides as a result. BUT... if you have solar tides or another body influencing the orbit via secular perturbations or a resonance, then you can make the orbit eccentric enough for there to be tides. At 3 million km, the tidal effects shouldn't be too bad with a slightly eccentric orbit.

90 lx at full sounds pretty good, actually. I couldn’t figure out how to calculate that, so thank you. So what kind of light level would we be looking at for a blue gas giant rich in ammonia/methane as you propose?
I dunno how bright 90 lux is, but picture something 5 times wider than the full moon in the sky, but just as bright (per full moon area). That's going to light up the sky quite nicely - a normal full moon on its own washes out the night sky pretty well, especially on the side of the sky containing the moon.

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Apologies. By "on the cold side" I merely meant a lower average temperature than Earth, not a side of the moon itself. Latitude in conjunction with axial tilt would affect the eclipse length insofar as it would determine how close to the full radius of the planet the star passes behind during the occlusion, yes? So if there were no tilt, then the equator would get the longest possible eclipses, since the star would pass through the precise middle of the planet, while moving north or south would steadily decrease their length. Correct?

Originally Posted by Ara Pacis
I don't think latitude would affect the eclipse time which is based on the rate of revolution not the rate of revolution, which would be negligible anyways because it's tidally locked.
Come again?

As far as paraterraforming, I did consider this, but for plot reasons, I think making the moon basically habitable for human life works better. Also, the terraforming, which happened millennia before the actual plot, can have been itself done millennia after our present time. Thus I can rely on significant technological advances and not necessarily the most currently plausible situation.

I can't find where I originally found this number, but this article, despite its annoying font, gives the 1/10000 ratio as well as a potential explanation:

Dr. Robin Canup and Dr. William Ward of the SwRI Space Studies Department propose that it was the presence of gas, primarily hydrogen, during the formation of these satellites that limited their growth and selected for a common satellite system mass fraction.
With this ratio, it seems that an Earth-sized moon would be impossible, even though other articles imagine they could support Earth-size satellites, because the planet size would make it a brown dwarf. Thus I maxed out the gas giant size and calculated backwards to figure out how I could make it as close as possible to Earth conditions. Though if the planet had trapped an Earth-sized planet from its solar system as it transitioned into closer orbit, then perhaps it could maintain that larger-than-usual moon...? I'm not sure it's possible for a planet to be captured that way, just throwing it out there. Also, another article (which seems to think Earth-sized moons *are* possible) mentions the possibility of a gas giant at 1 AU:

Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1 astronomical unit from a Sun-like star.
So, even if it didn't count as precisely a "Hot" Jupiter, could a massive gas giant potentially settle into an orbit of approx. 1 AU?

Originally Posted by Ara Pacis
Perhaps you can use the magnetic field lines to support some sort of particulate or organic matter that fulfills the coloration role.
I'll look into that, or perhaps others will have suggestions. That the particles are in the atmosphere is a given; I'd like them also to cause the coloration, though I did also look at different classes of stars. I found a number of articles on the subject, but the one I found most suggestive was this one. The final picture imagines a fictional world where the atmosphere is greenish primarily because of airborne phytoplankton, which gave me the idea for having my organisms likewise responsible for the coloration.

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Sorry, EDG, you actually were answering my question as I was sending the last reply, so didn't see it. I did the math on this, though: 10 MJ = about 1.9e28 kg, divide by 10,000 = 1.9e24, while Earth is almost 6e24 kg. Yes? That's where I came up with the planet and moon masses in the first place. As for light levels, I was using this chart as a guide. That would indicate that 90 lx would light up the sky more than most internal lighting, almost the level of a "very overcast day." You just made me realize...on the planet side, the moon inhabitants will only see the stars at the full eclipse, won't they?

13. Originally Posted by wmatanner
Apologies. By "on the cold side" I merely meant a lower average temperature than Earth, not a side of the moon itself. Latitude in conjunction with axial tilt would affect the eclipse length insofar as it would determine how close to the full radius of the planet the star passes behind during the occlusion, yes? So if there were no tilt, then the equator would get the longest possible eclipses, since the star would pass through the precise middle of the planet, while moving north or south would steadily decrease their length. Correct?
I don't think so, if the moon passes through the middle of the planet's shadow the edge will appear roughly parallel to the planet's poles since the planet is around 18 times greater in diameter. Add to this the fuzziness in perception of the shadow's edge due to the penumbra and planetary clouds and the difference may not be notable. Do you want there to be differences in latitude? If the planet grazes the top or bottom of the shadow, there may be a noticeable difference in shadow duration, but the eclipse duration will be much less and it may not even be complete due to the penumbra. In either case, the sky may not be entirely dark because you'll probably have a halo of light due to cloud scattering at the planet's limbs, similar to what happens on Venus. If the planet has a rings of dist, that may also increase light due to scattering.

Come again?
Sorry, typo. I fixed it. I meant rotation when I wrote revolution the second time. The latitudes on the moon will transit the shadow at the same speed. The leading side of the planet will enter first, but it will also exit first. I don't see that it would increase the actual duration of darkness, especially as the size and fuzziness will make fine distinctions in time difficult.

I can't find where I originally found this number, but this article, despite its annoying font, gives the 1/10000 ratio as well as a potential explanation:

With this ratio, it seems that an Earth-sized moon would be impossible, even though other articles imagine they could support Earth-size satellites, because the planet size would make it a brown dwarf. Thus I maxed out the gas giant size and calculated backwards to figure out how I could make it as close as possible to Earth conditions. Though if the planet had trapped an Earth-sized planet from its solar system as it transitioned into closer orbit, then perhaps it could maintain that larger-than-usual moon...? I'm not sure it's possible for a planet to be captured that way, just throwing it out there. Also, another article (which seems to think Earth-sized moons *are* possible) mentions the possibility of a gas giant at 1 AU:
If that's the case then you might claim that the moon was captured instead. Also, since it's closer in than Jupiter, there might be a higher ratio of heavier matter than at 5+ AU which may be lighter (like water ice) to affect that ratio. I'm not sure if perturbations of dust can be equivalent to large asteroids banging about if the super-jovian planet started farther out and migrated in after large planets, planetesimals and asteroids had already formed. Unless it's important to the story or unless it's way unlikely, I say something like that is allowable for fiction. After all, the jury is still out on whether a Giant Impact for Earth was highly unlikely or is common. And evidence suggests that Triton was captured by Neptune and Phobos and Deimos were captured by Mars.

So, even if it didn't count as precisely a "Hot" Jupiter, could a massive gas giant potentially settle into an orbit of approx. 1 AU?
Through in-migration.

I'll look into that, or perhaps others will have suggestions. That the particles are in the atmosphere is a given; I'd like them also to cause the coloration, though I did also look at different classes of stars. I found a number of articles on the subject, but the one I found most suggestive was this one. The final picture imagines a fictional world where the atmosphere is greenish primarily because of airborne phytoplankton, which gave me the idea for having my organisms likewise responsible for the coloration.
One posibility is that the light is polarized due to the chirality of the reflecting pigment and polarizing filters used by the humans reduce the visible light. Another possibility is that the organism uses bioluminescence.

I'm interested to know how the organisms get to and stay in the atmosphere and survive. Is there no precipitation or is there a way they can avoid being washed out (lift gasses?) or is there a method for re-establishing their population in the sky? That's one reason I suggested magnetic field lines, thinking perhaps they can use magnetic minerals that carry them aloft.

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Originally Posted by Ara Pacis
If that's the case then you might claim that the moon was captured instead.
I wouldn't have thought that was controversial, as a possibility.

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So I was thinking about the halo as well, which suggests to me that even at the darkest time on the planet, during the eclipse, there are probably no stars. I'm thinking this society may likely have no use for artificial lighting.

Originally Posted by Ara Pacis
One posibility is that the light is polarized due to the chirality of the reflecting pigment and polarizing filters used by the humans reduce the visible light. Another possibility is that the organism uses bioluminescence.

I'm interested to know how the organisms get to and stay in the atmosphere and survive. Is there no precipitation or is there a way they can avoid being washed out (lift gasses?) or is there a method for re-establishing their population in the sky? That's one reason I suggested magnetic field lines, thinking perhaps they can use magnetic minerals that carry them aloft.
I hadn't put much thought into it, but I'm now finding this very suggestive. I'd actually already intended to make the organism bioluminescent. I had kind of assumed they would just get tossed up by the winds and rained back down, like these bacteria. I really like this concept of the magnetic field lines, though. They could be something like magnetotactic bacteria. This would work well, because I intended them to be present in large concentrations in the moon's minerals. I intend them to glow brighter in proximity to other concentrations, so perhaps they get caught at some level in the magnetic field and create a sort of thin layer around the planet that lights up. Also, it might make sense to have the major native life on the moon highly iron-dependent if I go with the idea that the moon is extremely dense because it is actually a planet stripped mostly to its iron core. I can't find anything that talks about magnetic particles getting pulled into the atmosphere from the surface, though. Is that possible? And I'm wondering if this would have some odd effects, such as on the auroras at the poles. The organisms would end up being highly concentrated there, right?

16. Originally Posted by wmatanner
So I was thinking about the halo as well, which suggests to me that even at the darkest time on the planet, during the eclipse, there are probably no stars. I'm thinking this society may likely have no use for artificial lighting.
Well, it can depend on other things, like shadows. If there is a tree canopy, it might be dark underneath. If there is a dangerous radiation environment, they might need shield walls without windows. If the atmosphere absorbs a fair amount of light or reflects it with thick clouds, it could be generally darker beneath or get dark depending on the weather.

I hadn't put much thought into it, but I'm now finding this very suggestive. I'd actually already intended to make the organism bioluminescent. I had kind of assumed they would just get tossed up by the winds and rained back down, like these bacteria. I really like this concept of the magnetic field lines, though. They could be something like magnetotactic bacteria. This would work well, because I intended them to be present in large concentrations in the moon's minerals. I intend them to glow brighter in proximity to other concentrations, so perhaps they get caught at some level in the magnetic field and create a sort of thin layer around the planet that lights up. Also, it might make sense to have the major native life on the moon highly iron-dependent if I go with the idea that the moon is extremely dense because it is actually a planet stripped mostly to its iron core. I can't find anything that talks about magnetic particles getting pulled into the atmosphere from the surface, though. Is that possible? And I'm wondering if this would have some odd effects, such as on the auroras at the poles. The organisms would end up being highly concentrated there, right?
I can't tell you for certain how the moon's magnetic field would interact with the planet, since a lot will depend upon factors of the fictional setup. Here's an image of the field lines of Ganymede in Jupiter's magnetosphere. But it's just one possibility to play with. A Magnetic Sail effect might be plausible, although the motive force tends to be perpendicular and needs a source of energy (perhaps the organism's luminescence can also provide an electrical potential under certain circumstances). As part of a climate system, Hadley cells might carry them through the lower atmosphere towards the poles where the magnetic motion occurs. Or maybe there are different types of organisms at different latitudes. Perhaps the magnetism isn't propulsive but orientation along field lines at altitude works in conjunction with atmospheric updrafts and winds aloft and the organism's shape to help bring it back over the lower latitudes. Or perhaps the microorganism doesn't do this itself but hitches a ride on larger objects, such as air-dispersal plant seeds that distribute themselves this way. It's a web of life!

The aerosol method may work too, but with a thicker atmosphere the velocity of movement may be less even though the applied force would be stronger. Alternately, they might simply live above the active weather layer and are not precipitated out. If they are precipitated out then there's the possibility that they might glow on the ground and surface structures or aircraft. If you don't want that you can make a reason why they only glow at high altitude. Perhaps they only luminsce when they sense certain frequencies of light which the atmosphere filters out by the time light reaches the ground (not to be confused with fluorescence or phosphorescence which are alternatives to bioluminescence). This might be useful for inhabitants who could use special lights that reactivate the bioluminescence such as when hiking under the tree canopy or in caves. Alternately, the bioluminescence might be limited to something else, such as pressure or a chemical signal or a chemical inhibitor, making it useful for marking trails or forensics. Tweak as desired for effect: perhaps thunderstorms cause the organisms to react, or maybe fast aircraft leave a glowing wake, but balloons don't.

Earth life is highly dependent on iron, look at our blood. There's a reason the key chemical is called hemoglobin.

17. Originally Posted by wmatanner
Sorry, EDG, you actually were answering my question as I was sending the last reply, so didn't see it. I did the math on this, though: 10 MJ = about 1.9e28 kg, divide by 10,000 = 1.9e24, while Earth is almost 6e24 kg. Yes? That's where I came up with the planet and moon masses in the first place.
There's a bit of wiggle room though - not all the ratios in our own solar system are 1/10,000 (0.0001):

Jupiter: 0.00021
Saturn: 0.000247
Uranus: 0.0001
Neptune: 0.00021

I'm guessing that a ratio up to 0.0003 would be acceptable, which coincidentally is almost exactly one earth mass for your 10MJ planet. That would mean that it's the only major moon orbiting the planet. If you wanted more moons (e.g. a couple of mars-sized or luna-sized ones) then you'd need a more massive jovian (but then you're getting to the point of it being a small brown dwarf at around 13-15 MJ).

As for light levels, I was using this chart as a guide. That would indicate that 90 lx would light up the sky more than most internal lighting, almost the level of a "very overcast day." You just made me realize...on the planet side, the moon inhabitants will only see the stars at the full eclipse, won't they?
Yep, though I think the light refracted through the planet's atmosphere would turn everything on the moon's surface red too (like our own moon in a lunar eclipse) - whether that ring of red light around the planet is enough to drown out the stars near the planet or not, I don't know.

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Originally Posted by EDG

Yep, though I think the light refracted through the planet's atmosphere would turn everything on the moon's surface red too (like our own moon in a lunar eclipse) - whether that ring of red light around the planet is enough to drown out the stars near the planet or not, I don't know.
A Jupiter sized brown dwarf at 3 million km spans only slightly more (maybe one quarter more) than Earth on the sky of Moon.

Moon at an ordinary, Danjon 3 eclipse has been described as magnitude -3. Which is about 10 000 times dimmer than full Moon - meaning that an observer on Moon during a Danjon 3 eclipse would see the ring of red light shining at perhaps 50 times brighter than full moon on Earth.

But then there are lower Danjon number eclipses. Like the Danjon 0 eclipses - none seem to have happened since 1982, and there were none between 1913 to 1963.

Danjon 0 eclipses are not equal to each other either. The 1963 eclipse was reliably estimated at magnitude +4,1, whereas the 1982 eclipse only fell to +3,0.

Where is the zero point of Danjon scale - i. e. visual magnitude below which the eclipse is characterized by negative Danjon numbers?

At a Danjon 0 eclipse like 1963, the Moon is dimmed by 17 magnitudes - meaning that for an observer on the Moon, the ring of Earth is about 10 times dimmer than full Moon.

Now, the dawn and dusk on a as giant is different from a dawn on Earth.

Say, Callisto is +5,65 when in opposition and fully sunlit. So abbout +6 when out of opposition.

How bright is Callisto when in Jupiter´s umbra?

+16? +20? +23?

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Hey OP!

Great to know others are writing scripts set on other planets. I'm setting a story on a unique alien world, as well. Here's to more and better SF on the big screen!
Last edited by DuaneW; 2012-Jul-05 at 09:16 PM. Reason: typos

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Originally Posted by DuaneW
Here's to more and better SF on the big screen!
Absolutely. The state of movies in general is pretty bad. Haven't seen Prometheus yet, but heard it was disappointing, despite its potential.

Originally Posted by EDG
I'm guessing that a ratio up to 0.0003 would be acceptable, which coincidentally is almost exactly one earth mass for your 10MJ planet.
Good to know. Makes the gravity simple.

Originally Posted by Ara Pacis
Earth life is highly dependent on iron, look at our blood. There's a reason the key chemical is called hemoglobin.
Fair enough.

Everyone's comments have been extremely useful. Your suggestions on the organism especially, Ara Pacis. Chornedsnorkack: I have no idea what you're talking about, to be honest, but I'll look into these Danjon numbers when I get a chance. I think I've got enough to go on, though if the conversation continues, I'll still be tuned in. Thank you all for your input, you've definitely given the story a more solid foundation than it would have had with just my own research! Now I've got to focus on the story structure and hopefully finish the planning stages by the end of the month.

-wm

21. Good luck.

Always hoping to see a good SF story that's been thought through. I liked "Avatar", despite some of it's shortcomings, because the ideas seemed consistent.

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If you want abundant aeroplankton and easy flying, you can have the moon with appreciably weaker gravitational acceleration and yet denser atmosphere than Earth. I think 0,5 g and 4 bar atmosphere are both feasible and consistent with human life.

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Originally Posted by chornedsnorkack
If you want abundant aeroplankton and easy flying, you can have the moon with appreciably weaker gravitational acceleration and yet denser atmosphere than Earth. I think 0,5 g and 4 bar atmosphere are both feasible and consistent with human life.
Hi everybody. If anyone is interested in continuing this discussion a bit, I want to follow up on these ideas about flight and the magnetosphere. As it happens, having simple flying vehicles would be quite useful to my story as I'm now outlining it. I originally avoided this idea, as this story is about a technologically regressive society, which over the last 3000 years has almost completely lost the capabilities that allowed the colony to be established. However, they would perhaps be capable of maintaining some simple technologies left behind. They have mining and mineral processing technologies, water purification, etc.

I want the planet to be basically habitable without support systems. I'm a bit hesitant to lower the gravity more than slightly, because it would have some extreme physiological effects, and would make movement unnatural. However, increased atmospheric pressure might be feasible. I've decided it makes more sense to move the moon's semi-major axis to about 1.5e6 km. A denser atmosphere along with aeroplankton might help protect the surface from the increased radiation, which was my original reason for the distance. I've done the corresponding recalculations for orbital period and such. Would light levels then double? I'm still concerned with the brightness of the nights (which would now be about 44 hours). Would increasing the atmospheric pressure also have significant climate effects? I ran across one board like this one, but more focused on sci-fi, that was discussing the possibility of a 2 bar atmosphere. A couple members suggested this might cause massive storms and hurricanes, winds like the Roaring Forties. However, perhaps being a tidally locked moon would reduce such effects?

Originally Posted by Ara Pacis
I can't tell you for certain how the moon's magnetic field would interact with the planet, since a lot will depend upon factors of the fictional setup. Here's an image of the field lines of Ganymede in Jupiter's magnetosphere. But it's just one possibility to play with. A Magnetic Sail effect might be plausible, although the motive force tends to be perpendicular and needs a source of energy (perhaps the organism's luminescence can also provide an electrical potential under certain circumstances).
Obviously, moving the moon in closer makes the interaction with the planet's magnetic field even more intense. I'm still playing with how the organism works, though currently I have two divergent but related organisms. In fact, there may be thousands of microorganisms, but basically life did not get beyond bacterial before human colonization, all flora and fauna are imported, and we're really only interested in these two related bioluminescent microorganisms: one that lives in the sediment and is found concentrated in iron deposits and one that lives at a certain altitude along the magnetic field lines. (Also an ancestor living at a certain depth in the oceans, but this one is unimportant to the story.) I'm thinking that like earth's magnetotactic bacteria, the organism prefers a certain oxygen level. When the organism was forced into the atmosphere during terraforming, some of the organisms were able to float at a hospitable altitude, the energy they produce turning them into tiny electromagnets; thus they partially depend on the magnetic sail effect Ara Pacis suggested (the hospitable altitude would change, of course, stabilizing after the terraforming of the atmosphere was complete...meanwhile the organism continues to change and adapt to a new environment and lifecycle). They also work in concert, producing more energy together than they would apart, so perhaps they form large networks like sheets that augment the sail effect. Brainstorming, so please give me your feedback. One problem I see with the branch staying in the atmosphere is there may be no feasible way for them to reproduce and continue to produce BMPs (bacterial magnetic particles). Although they have the oxygen required to create magnetite, I doubt they have sufficient iron in the atmosphere. Unless the magnetic field is strong enough to suspend ferrous particles alone? This seems unlikely.

Originally Posted by Ara Pacis
Tweak as desired for effect: perhaps thunderstorms cause the organisms to react, or maybe fast aircraft leave a glowing wake, but balloons don't.
Since I'd like to follow up on the idea of aircraft, I do want to think about how the organism and the craft might interact. I'm thinking the aircraft themselves might use magnetic sails; perhaps a hybrid between magsails and helium balloons? Since the magsail concept is mainly designed around the idea of interstellar travel, I can't find much info on how this would work, except that the strong fields of a gas giant would be particularly well suited to such a form of travel. From what I've read, though, it would seem one can only travel in one direction? If they can only travel north/south, that's not such a problem, I can work that into the story. And perhaps they can supplement with wind sails to cut at angles? However, how exactly would this work, with such a strange magnetic field as a gas giant's moon? Using the Ganymede diagram as an example, it would seem the lines would be quite bizarre, with huge ballooning fields at the north and south poles of the moon, and small closed fields at the equator. Since I'm mostly interested in the equatorial region, the ships and organisms would depend on these closed fields, but these would also be the weakest, right? The airships can be of very limited use, slow and bulky perhaps. I am thinking that perhaps they use the organisms themselves as fuel, rising to their altitude and then burning their way along, leaving the kind of trail Ara Pacis was talking about.

Can't tell you how much help this thread has been already. Hope there are still some people out there with more good ideas!

24. Originally Posted by wmatanner
Obviously, moving the moon in closer makes the interaction with the planet's magnetic field even more intense. I'm still playing with how the organism works, though currently I have two divergent but related organisms. In fact, there may be thousands of microorganisms, but basically life did not get beyond bacterial before human colonization, all flora and fauna are imported, and we're really only interested in these two related bioluminescent microorganisms: one that lives in the sediment and is found concentrated in iron deposits and one that lives at a certain altitude along the magnetic field lines. (Also an ancestor living at a certain depth in the oceans, but this one is unimportant to the story.) I'm thinking that like earth's magnetotactic bacteria, the organism prefers a certain oxygen level. When the organism was forced into the atmosphere during terraforming, some of the organisms were able to float at a hospitable altitude, the energy they produce turning them into tiny electromagnets; thus they partially depend on the magnetic sail effect Ara Pacis suggested (the hospitable altitude would change, of course, stabilizing after the terraforming of the atmosphere was complete...meanwhile the organism continues to change and adapt to a new environment and lifecycle). They also work in concert, producing more energy together than they would apart, so perhaps they form large networks like sheets that augment the sail effect. Brainstorming, so please give me your feedback. One problem I see with the branch staying in the atmosphere is there may be no feasible way for them to reproduce and continue to produce BMPs (bacterial magnetic particles). Although they have the oxygen required to create magnetite, I doubt they have sufficient iron in the atmosphere. Unless the magnetic field is strong enough to suspend ferrous particles alone? This seems unlikely.
Which magnetic idea, the one with the seeds or the one with the sail? To create a sail, they'd have to form a structure or colony in the shape of a loop. A lot of dust gets kicked up into the atmosphere by winds and volcanism, so there can be a fair amount of material on which to grow.

Since I'd like to follow up on the idea of aircraft, I do want to think about how the organism and the craft might interact. I'm thinking the aircraft themselves might use magnetic sails; perhaps a hybrid between magsails and helium balloons? Since the magsail concept is mainly designed around the idea of interstellar travel, I can't find much info on how this would work, except that the strong fields of a gas giant would be particularly well suited to such a form of travel. From what I've read, though, it would seem one can only travel in one direction? If they can only travel north/south, that's not such a problem, I can work that into the story. And perhaps they can supplement with wind sails to cut at angles? However, how exactly would this work, with such a strange magnetic field as a gas giant's moon? Using the Ganymede diagram as an example, it would seem the lines would be quite bizarre, with huge ballooning fields at the north and south poles of the moon, and small closed fields at the equator. Since I'm mostly interested in the equatorial region, the ships and organisms would depend on these closed fields, but these would also be the weakest, right? The airships can be of very limited use, slow and bulky perhaps. I am thinking that perhaps they use the organisms themselves as fuel, rising to their altitude and then burning their way along, leaving the kind of trail Ara Pacis was talking about.
Burning them? If like most lifeforms they are mostly water, they may not readily burn. Or are you intending to give them large stores of chemical energy? Even so, they'd be fairly well dispersed or else they'd be so thick that moving through them might be difficult. So, perhaps they'd need to be gathered and compressed into a smaller volume, dryed in a preheater, compressed some more, and then burned and exhausted. I'd be afraid that you're burdening these little critters with too much importance.

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Originally Posted by Ara Pacis
Which magnetic idea, the one with the seeds or the one with the sail? To create a sail, they'd have to form a structure or colony in the shape of a loop.
Yeah, hadn't thought of that. I meant the sail idea. The seed idea would involve interacting with imported fauna. I'm open to that, but thought it would be simpler if they had their own life cycle. I want them to be interactive with their own kind, so perhaps the coil thing would work. In which case they'd be able to move from the ground to the air. I was just thinking about the sensitivity of magnetotactic bacteria on earth to oxygen, so I thought it might be better to have them capable of surviving in a certain layer of the atmosphere. I could of course have these bacteria have no such sensitivity, and perhaps they need to reach a certain altitude for another reason.

Originally Posted by Ara Pacis
A lot of dust gets kicked up into the atmosphere by winds and volcanism, so there can be a fair amount of material on which to grow.
Groovy. That might work.

Originally Posted by Ara Pacis
I'd be afraid that you're burdening these little critters with too much importance.
Could be a problem. I am writing a screenplay, though, so I need things to be tight tight tight. Using them as fuel is just a useful concept. It can be worked around. As the present outline stands, it would be convenient, however. I need a way for the humans to have airships that is simple and efficient, preferably connected to the microorganisms. However, I'm still interested in the various ramifications of having a high pressure atmosphere. Perhaps simple propellers make more sense? Looking for people's thoughts. (Also concerned about visual style, obviously, and I have a thing for the steampunk-esque.)

26. Bear in mind that a high pressure atmosphere tends to push sky colour away from blue (and indigo) and towards green and yellow. The denser an atmosphere is, the more scattering occurs; at 40 bar the sky would be nearly white.

27. Well, I guess it all depends on how hard you want the science fiction to be.

Originally Posted by wmatanner
Yeah, hadn't thought of that. I meant the sail idea. The seed idea would involve interacting with imported fauna. I'm open to that, but thought it would be simpler if they had their own life cycle. I want them to be interactive with their own kind, so perhaps the coil thing would work. In which case they'd be able to move from the ground to the air. I was just thinking about the sensitivity of magnetotactic bacteria on earth to oxygen, so I thought it might be better to have them capable of surviving in a certain layer of the atmosphere. I could of course have these bacteria have no such sensitivity, and perhaps they need to reach a certain altitude for another reason.
Well, the microorganisms might be bacteria or something a little more complex, like paramecium or maybe even more complex like a diatom. It's alien, so whatever you need it to do, we'll figure out a way. Perhaps it's temperature. Or perhaps it's a certain amount of sunlight they need or prefer, because too high and they get burned by UV and too low and the light gets blocked.

And perhaps human activity has resulted in more dust in space that creates a radiation hazard in orbit, similar to how Io puts material into Jupiter's magnetosphere. This might occur because of humans delving into volcanoes to extract heat or minerals or something.

The magnetotactic bacteria on Earth tend to favor anaerobic or microaerophilic environments, which also helps in the creation of the magnetic particles, so they may not be exactly like earth bacteria. But aliens microbes may have different proteins. Or maybe that level of the atmosphere will have less oxygen

Groovy. That might work.
Also, perhaps a lot of dust gets kicked up by human activity which is another reason they adapted to a higher altitude, perhaps also because of human flight debris.

Could be a problem. I am writing a screenplay, though, so I need things to be tight tight tight. Using them as fuel is just a useful concept. It can be worked around. As the present outline stands, it would be convenient, however. I need a way for the humans to have airships that is simple and efficient, preferably connected to the microorganisms. However, I'm still interested in the various ramifications of having a high pressure atmosphere. Perhaps simple propellers make more sense? Looking for people's thoughts. (Also concerned about visual style, obviously, and I have a thing for the steampunk-esque.)
Instead of fuel, I wonder if they could be used as a medium. Instead of burned perhaps the airship gathers them with a magnetic funnel and somehow the microbes all activate at the same time. This might allow the balloons to pull and push itself through the air in a manner similar to magnetohydrodynamics. The humans may need to dust the microbes with food to get the reaction. Maybe it's also a response to a strong magnetic field and they might emit light as a method of quorum sensing. So, you might get a huge steam-punk-esque boat-like balloon with a horn at either end with a light trail being pulled into it and a trail of darkness coming out of it from the food dust, and a lack of light caused by tired microbes. If it's a road, they may have to feed the microbes so the they'll be ready for the next ship to pass through. The food dust is sprinkled into the reaction chamber/Magnetic Alignment Chamber (MAC) to give the microbes a burst of energy and then some extra is allow to pass out with them to replenish the GlowBug Skyway. Perhaps some of the microbes also attach to winglets and streamers and help lift the craft upwards against the magnetic field of the moon.

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Originally Posted by eburacum45
Bear in mind that a high pressure atmosphere tends to push sky colour away from blue (and indigo) and towards green and yellow. The denser an atmosphere is, the more scattering occurs; at 40 bar the sky would be nearly white.
Thanks, good point. Perhaps a higher pressure atmosphere isn't really necessary if we nail down a good airship concept.

Originally Posted by Ara Pacis
Well, I guess it all depends on how hard you want the science fiction to be.
As hard as possible, while taking advantage of the wiggle room of all the stuff we don't know. I'm actually deliberately blurring sci-fi and fantasy in the story, but I want the actual concepts behind the scenes to be solid sci-fi. Even if the answer's not obvious in the movie, if anyone were to ask, I want to be ready with an answer.

Originally Posted by Ara Pacis
Well, the microorganisms might be bacteria or something a little more complex, like paramecium or maybe even more complex like a diatom.
I've been thinking of them as something more or less analogous to bacteria, but yeah, something a bit more complex like a diatom might work. Here's the basic concept behind this organism, so you can see what I'm going for: it's single-celled but does form colonies, and though not "sentient" in any human sense, in larger concentrations it functions rather like a neural network and as if it were a single organism. The really important part is that it's found human bodies also habitable, and the organisms and humans have a sort of developing symbiotic relationship which, unbeknownst to the humans, is effecting physiological and neurological changes. These changes are both increasing the humans' adaptation to their new environment as well as changing the way they perceive and function in the world.

Thinking about it, I might want to avoid them being dependent on a certain oxygen level altogether. I was originally thinking this way because, as you point out Ara Pacis, our magnetotactic bacteria are microaerophilic to anaerobic. So let me try this out and see what people think: originally, before the moon had the full Earth-level atmosphere it has now, the moon had developed an ecosystem based around chemotrophic life not unlike Earth. Because of the moon's conditions, however, life continued in this direction and merely created much more complex chemotrophic organisms and never developing anything resembling our flora and fauna, even in the ocean. When terraforming the atmosphere, humans set up generators that used the moon's vulcanism as power, as well as evaporating some of the ocean water, and released large quantities of these organisms into the atmosphere. Being durable, efficient, and rather clever in large groups, the most advanced strand of chemoautotroph developed floating colonies in the new atmosphere, using the infrared radiation from the sun to catalyze their reactions (thus perhaps they have a tendency to follow the moon's orbit?). To keep this organism and its close family members straight, we'll call them "licanth," which is their name in my story. The ancestors of this organism along with related strands are found in the moon's iron, going indefinitely dormant when without a sufficient power/food source (this may require them to be physiologically simpler than diatoms, more like bacteria?). The humans are exposed constantly to low concentrations of these various organisms in the surface air and the water, though most of them are purified out of the drinking water, which is otherwise poisonous to most of the humans. Thus other members of the licanth family, rather than heading up, found that the warmth and iron of a human body was a sufficiently hospitable home, and set up shop there (apparently we were prone to many iron-consuming parasites in our hunter-gather days, which is why our bodies generally tend towards producing an overabundance of iron, thus I think we'd make a good home for these creatures, though in extreme cases hemophilia anemia might result).

There are still a few holes to be filled in here, but this is the basic life cycle I've been groping towards.

Originally Posted by Ara Pacis
Instead of fuel, I wonder if they could be used as a medium. Instead of burned perhaps the airship gathers them with a magnetic funnel and somehow the microbes all activate at the same time. This might allow the balloons to pull and push itself through the air in a manner similar to magnetohydrodynamics. The humans may need to dust the microbes with food to get the reaction. Maybe it's also a response to a strong magnetic field and they might emit light as a method of quorum sensing. So, you might get a huge steam-punk-esque boat-like balloon with a horn at either end with a light trail being pulled into it and a trail of darkness coming out of it from the food dust, and a lack of light caused by tired microbes. If it's a road, they may have to feed the microbes so the they'll be ready for the next ship to pass through. The food dust is sprinkled into the reaction chamber/Magnetic Alignment Chamber (MAC) to give the microbes a burst of energy and then some extra is allow to pass out with them to replenish the GlowBug Skyway. Perhaps some of the microbes also attach to winglets and streamers and help lift the craft upwards against the magnetic field of the moon.
Quorum sensing! That's brilliant. That's exactly what I've been trying to imagine. A very advanced version of quorum sensing that acts not unlike thinking. I'm also liking this idea of roadways...If the organisms know that they get fed in a certain area, they might tend to concentrate around this area. (According to the life cycle sketched above, perhaps travel will only be possible in the day, though; I may want to tweak that bit, 'cause this would look more fun at night.) I'm thinking that since their biochemistry revolves around oxidizing iron, perhaps this is the "food dust" that the ships use to seed the path, supplementing the iron particles in the atmosphere due to volcanic activity. This iron dust would also already contain more organisms, which would further stimulate the colony. Perhaps the organism already uses magnetic fields to trigger their group motility, and so basically the large magnetic field confuses the organism, making it think the colony is massive and generating enough of a "current" for the airship to push itself along, funneling the poor, exhausted creatures out the other end, using the magnetohydrodynamic principle you were talking about. Something like this is exactly what I'm looking for: a way to tie the organism into the airship concept, making the screenplay that much tighter, as well as more unique. The idea of having the organism also help with lift is interesting also, though I don't immediately see how it would work.

Fantastic ideas, Ara Pacis, thank you. I'm quite serious about this project, so with some luck perhaps you'll get to see the fruits of your brainstorming someday (hopefully interpreted by a very good art director). I certainly hope so myself.
Last edited by wmatanner; 2012-Aug-02 at 01:59 PM. Reason: typo

29. Originally Posted by wmatanner
Thanks, good point. Perhaps a higher pressure atmosphere isn't really necessary if we nail down a good airship concept.
Altough, if the organisms put out blue light at a low enough altitude, perhaps even a dense atmosphere would look blue-ish. This might also depend upon the cloudscape.

As hard as possible, while taking advantage of the wiggle room of all the stuff we don't know. I'm actually deliberately blurring sci-fi and fantasy in the story, but I want the actual concepts behind the scenes to be solid sci-fi. Even if the answer's not obvious in the movie, if anyone were to ask, I want to be ready with an answer.
Sounds good. That's what I prefer to do, I just always get distracted from writing my stories because it starts to feel like work. That and the whole "I need an actual job still?" thing.

I've been thinking of them as something more or less analogous to bacteria, but yeah, something a bit more complex like a diatom might work. Here's the basic concept behind this organism, so you can see what I'm going for: it's single-celled but does form colonies, and though not "sentient" in any human sense, in larger concentrations it functions rather like a neural network and as if it were a single organism. The really important part is that it's found human bodies also habitable, and the organisms and humans have a sort of developing symbiotic relationship which, unbeknownst to the humans, is effecting physiological and neurological changes. These changes are both increasing the humans' adaptation to their new environment as well as changing the way they perceive and function in the world.
I'd be worried about how fast they adapt as background plausibility. The theory is that Earth life stayed single-celled for a long time before exploding into multicelled organisms. I'm not a biochemist or biologist, but it seems plausible to me that a lot of adaptation occurs not as just as random genetic mutation of germ cells to create a wholly new protein but also mutations that happen to cause a cell to turn on an existing protein at a different time or for a different reason. That can also happen from environmental factors if something like energy or heat or food availability causes a difference in gene expression. This might allow for rapid mutation but assumed the proteins existed already, which might suggest that there might have been a more diverse biosphere at one point, that may have declined for some reason. That's just one possibility.

Another possibility is from exchanges in genetic information. Genetic coding can be exchanged by horizontal gene transfer such as microbe sex, from plasmid exchange, from an infection by a prion, a viroid, a bacteriophage (or lysogenic conversion), or perhaps from an infection by another microorganism. The cool thing about this is that you might posit that the microbes adapted to human physiology by "communicating" with human bacterial flora.

If you want to get more exotic, then perhaps you can posit a mechanism whereby the microbes can exchange information electrically, magnetically or by light. it doesn't have to be super advanced, Perhaps it's just due to how a magnetite particle itself or the magnetosome organelle twists other structures differently in relationship to other organelles or to magnetosomes constructed of different minerals. According to that wikipedia link, at least one bacterium has more than one kind of magnetosome. Moreover, some magnetic particles exhibit different magnetic proerties at certain temperatures (see link above), so this may influence their preference for certain altitudes and other locations. Perhaps it's also part of the reason they congregate at altitude. Now, if you posit that the bacteria is more complex, then perhaps it can also have extensions like pseudopods or axons and dendrites (like human nerve cells) that allow it to actually act like a super-organism with actual intelligence, if you want that function. Perhaps this would help explain why, if the microbes were "learning" how to infect humans, that they became symbiotic instead of pathologic.

Thinking about it, I might want to avoid them being dependent on a certain oxygen level altogether. I was originally thinking this way because, as you point out Ara Pacis, our magnetotactic bacteria are microaerophilic to anaerobic. So let me try this out and see what people think: originally, before the moon had the full Earth-level atmosphere it has now, the moon had developed an ecosystem based around chemotrophic life not unlike Earth. Because of the moon's conditions, however, life continued in this direction and merely created much more complex chemotrophic organisms and never developing anything resembling our flora and fauna, even in the ocean. When terraforming the atmosphere, humans set up generators that used the moon's vulcanism as power, as well as evaporating some of the ocean water, and released large quantities of these organisms into the atmosphere. Being durable, efficient, and rather clever in large groups, the most advanced strand of chemoautotroph developed floating colonies in the new atmosphere, using the infrared radiation from the sun to catalyze their reactions (thus perhaps they have a tendency to follow the moon's orbit?).
Sounds good, but I'm not sure I understand what you mean by "following the moon's orbit."

To keep this organism and its close family members straight, we'll call them "licanth," which is their name in my story.
Sounds a little wolfish. ;-)

The ancestors of this organism along with related strands are found in the moon's iron, going indefinitely dormant when without a sufficient power/food source (this may require them to be physiologically simpler than diatoms, more like bacteria?).
Or they could convert into a spore form: endospore and exospore (cyst), which might be more applicable to complex microbes and even microscopic multicelled life.

The humans are exposed constantly to low concentrations of these various organisms in the surface air and the water, though most of them are purified out of the drinking water, which is otherwise poisonous to most of the humans. Thus other members of the licanth family, rather than heading up, found that the warmth and iron of a human body was a sufficiently hospitable home, and set up shop there (apparently we were prone to many iron-consuming parasites in our hunter-gather days, which is why our bodies generally tend towards producing an overabundance of iron, thus I think we'd make a good home for these creatures, though in extreme cases hemophilia might result).

There are still a few holes to be filled in here, but this is the basic life cycle I've been groping towards.
Hemophilia is a clotting disorder. Are you thinking of Anemia for low iron? I wonder if this scenario would also tend to select for colonists who have a genetic predisposition to Hereditary Hemochromatosis (which is more common in a few different ethnicities). On the other hand, this scenario might select against women, who are more likely to be anemic due to their monthly cycle. Maybe the microbes like spinach or make microbe-colonized humans want to eat more spinach.

Quorum sensing! That's brilliant. That's exactly what I've been trying to imagine. A very advanced version of quorum sensing that acts not unlike thinking. I'm also liking this idea of roadways...If the organisms know that they get fed in a certain area, they might tend to concentrate around this area. (According to the life cycle sketched above, perhaps travel will only be possible in the day, though; I may want to tweak that bit, 'cause this would look more fun at night.) I'm thinking that since their biochemistry revolves around oxidizing iron, perhaps this is the "food dust" that the ships use to seed the path, supplementing the iron particles in the atmosphere due to volcanic activity. This iron dust would also already contain more organisms, which would further stimulate the colony. Perhaps the organism already uses magnetic fields to trigger their group motility, and so basically the large magnetic field confuses the organism, making it think the colony is massive and generating enough of a "current" for the airship to push itself along, funneling the poor, exhausted creatures out the other end, using the magnetohydrodynamic principle you were talking about. Something like this is exactly what I'm looking for: a way to tie the organism into the airship concept, making the screenplay that much tighter, as well as more unique. The idea of having the organism also help with lift is interesting also, though I don't immediately see how it would work.
Here's a TED Talks video by Bonnie Bassler on Quorum Sensing, and her initial microbe subject uses it for bio luminescence like yours. I wrote above about a plausible(?), more advanced communication possibility above. If the microbes need light, perhaps the airship uses lights, or force-feeds light and food to "captured" microbes in some sort of engine module to pre-signal the free-floating airborne microbes to get ready for a burst of energy. If the microbes have created a super-organism intelligence then this may or may not be a good thing. It could be a plot point. Furthermore, different skyways might have different separate intelligences and thoughts on the matter.

Fantastic ideas, Ara Pacis, thank you. I'm quite serious about this project, so with some luck perhaps you'll get to see the fruits of your brainstorming someday (hopefully interpreted by a very good art director). I certainly hope so myself.
That'd be cool. Let us know if and when. :-)

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Originally Posted by Ara Pacis
I just always get distracted from writing my stories because it starts to feel like work. That and the whole "I need an actual job still?" thing.
Yeah, it is work. This project is kind of a professionalization exercise as well as building up enough material to approach agents. The real job thing is tough. I taught English in Japan for several years, which gave me enough time to teach myself how to write. Now I'm in an English PhD program which both feeds and inspires my fiction, while leaving me little time to actually practice it, except for this one summer. So hopefully having started the writing this month, hopefully over the year I'll be able to plug away at it bit by bit and have something to start sending out next summer.

Originally Posted by Ara Pacis
I'd be worried about how fast they adapt as background plausibility. ... Another possibility is from exchanges in genetic information. ... The cool thing about this is that you might posit that the microbes adapted to human physiology by "communicating" with human bacterial flora.
I've focused primarily on the ~3000 years the moon has been colonized, but I had been trying to think of some way that perhaps the moon is terraformed for several millennia before the colonizers get there. I toyed with time dilation effects, but the only thought I have is that we are able to develop near light speed travel (say .99c or greater), and we send the ships in deliberately timed waves at *different* velocities. (I ignore acceleration/deceleration in the following calcs, and assume all ships leave at the same time.) Let's say the moon is 3000 LY from Earth. Thus the first wave of ships are huge generators that automatically land at power sources and begin the terraforming process, landing at (from 0) year 3030. These ships also carry microorganisms to seed the planet. Then a wave at say .9c carrying plant life, landing Y3333, when the atmosphere is sufficiently prepared. Ships with animals at .7c = Y4286, and finally human ships at .5c arrive Y6000. Apparently, we must've invented some really incredible stasis technology--I realize that's a problem. But anyway, assuming everything worked out as planned, colonists step onto a planet with an ecosystem already well underway; if not, they find a desert world and perish.

This is based on terraforming ideas I've read about before where life is staged in steps. I might consider creating another thread to see if people have some thoughts on this side of it. I hadn't thought of the imported bacterial life interacting with this organism, however. I think that's a fantastic idea. They could borrow genetic information from the newly established biosphere first to help them adjust to the atmosphere, then to adapt to our physiology.

Originally Posted by Ara Pacis
If you want to get more exotic, then perhaps you can posit a mechanism whereby the microbes can exchange information electrically, magnetically or by light. ... then perhaps it can also have extensions like pseudopods or axons and dendrites (like human nerve cells) that allow it to actually act like a super-organism with actual intelligence, if you want that function. Perhaps this would help explain why, if the microbes were "learning" how to infect humans, that they became symbiotic instead of pathologic.
I have been thinking that. I've been fascinated for a long time by the idea of a diffuse organism, like a brain but with all its cells scattered and self-sufficient. Rather than thinking of them as little neurons with axons and such, though, I was thinking of them as more like biological computers. Perhaps their bioluminescence allows their bodies to act like binary code. They might also use magnetism in some of the ways you suggest. I like the idea of preference for temperatures allowing for different magnetic properties. I do indeed want them to be "intelligent," but basically in a way that is unrecognizable to humans. I am endlessly annoyed by the preponderance of anthropomorphic aliens in sci-fi, as if life on Earth were the blueprint for the galaxy (though perhaps the smartest example of this would be Larry Niven's Ringworld). I think it's very unlikely we'll ever find life that resembles humans very much at all, and sci-fi seems to show a lack of imagination by continuing over these old ruts. I like to try to think of how life and intelligence, if there are even things out there we might be able to recognize as life and intelligence, might look nothing like what we have on Earth. I've been toying with the idea of a single-celled version like this since high school.

Originally Posted by Ara Pacis
Sounds good, but I'm not sure I understand what you mean by "following the moon's orbit."
I just meant that as the moon turned around the planet, the organism might find it necessary to follow the light side around in order to maintain a constant source of energy. Or perhaps they go dormant at night. Or maybe they can even store enough energy to power themselves through the night like little solar cells.

Originally Posted by Ara Pacis
Sounds a little wolfish. ;-)
You're quite right, actually. I won't go into that side of it, though.

Originally Posted by Ara Pacis
Or they could convert into a spore form: endospore and exospore (cyst), which might be more applicable to complex microbes and even microscopic multicelled life.
Yes, I was thinking of the endospore concept, the bacteria that could be viable for millions of years, but I didn't have a word for it. Thanks. Perhaps the cyst idea is even better, though.

Originally Posted by Ara Pacis
Hemophilia is a clotting disorder. Are you thinking of Anemia for low iron? I wonder if this scenario would also tend to select for colonists who have a genetic predisposition to Hereditary Hemochromatosis (which is more common in a few different ethnicities). On the other hand, this scenario might select against women, who are more likely to be anemic due to their monthly cycle. Maybe the microbes like spinach or make microbe-colonized humans want to eat more spinach.
Sorry, typo corrected. Yes, I meant anemia. I'm imagining a soil considerably more saturated with iron than ours, which would be absorbed into the plant life, so that might help. But yes, their bodies might also be inclined to make up for the iron deficiency by desiring iron-rich foods, good idea. I didn't know about Hemochromatosis...That is rather serendipitous, as my colonists all happen to be descendants of the British, and so the fact that 10% of Celtic people are carriers is rather awesome (I guess I shouldn't be so excited about a disease, especially being almost half Irish myself...). The environment could definitely end up selecting for these people and causing the gene to express more, especially in the smaller gene pool. As for women, I considered the fact that they would be more prone, but as humans adjust to the environment, especially with these other considerations you've brought up, this should level out.

Thanks for the video, I'll take a look.