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Old 07-21-2009, 10:02 PM   #1
Agemegos
 
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Default [Space]: Habitability modifiers for tidelocked and resonant worlds

In his classic (but now dated) Habitable Planets for Man, Stephen Dole argued that no tidelocked planet, nor any planet with an apparent solar day longer than about 96 hours, is likely to be habitable to Mankind. Dole's view was that in the case of tidelocked worlds the oceans and maybe the air would all freeze out into an ice cap on the dark side, leaving the dayside and the crepuscular band arid, and that in the case of slow-rotating worlds the daily temperature variations would at some stage become so great that no latitude would be warm enough at night and cool enough during the day for crop plants to survive.

Current views are a little less pessimistic. Atmospheric modelling of one-face worlds suggests that the atmosphere can transport enough heat from day-side to night-side to prevent the air and water from freezing out. And thinking of planets in spin-orbit resonance, with apparent days perhaps thousands of hours long, we perceive that the polar regions may be stable enough for a permanent growing season, while in the lower latitudes morning and evening can be growing seasons, with deciduous or "annual" plants surviving the day and night as seeds or in a dormant state. In the case of human crop plants that would require genetic engineering, but that seems less problematic than it did when Dole wrote, in 1971.

Nevertheless, it does not seem likely that tide-locked worlds, or worlds with thousand-hour-long days and nights would be quite as salubrious for human life as planets with a more homelike alternation of night and day. In the case of tide-locked worlds the dark side will be unsuitable for agriculture because of the perpetual darkness, while at least a large part of the middle of the day side will be unappealingly hot and perhaps dry. Plant growth will be confined to a narrow belt just on the sunny side of the terminator, and even there crop growth and photosynthesis will be restricted by the low light levels.

The Star System Generation Sequence in GURPS Space is remarkable and admirable in that it treats tidal locking and orbital resonance at all. It estimates the dayside and nightside temperatures of tidelocked worlds and the reduction in their hydrographic cover and atmospheric pressure produced by any freezing-out of air and water: with a bit of initiative the same tables can be used to estimate the "daily" temperature range of spin:orbit resonant worlds. And with a similar exercise of initiative the world's Habitability score can be calculated after temperature, hydrographics, and atmospheric pressure are adjusted.

That said, it seems clear that even if parts of its surface are watered, lit, and at least a large part of the time at an equable temperature, a tide-locked planet will not be able to support as many people as a freely-rotating one with the same average temperature--agriculture &c. will be confined to a small proportion of the surface--and a spin:orbit resonant one will be less attractive to settlers because of the extreme climatic variations of its long diurnal cycle. In short, there ought to be a negative modifier to the Habitability score of a world with a long or infinite day. This is important because, in a recent sample of 1,000 randomly generated systems, I found that 65% of planets with Habitability scores of 4 or above either were tide-locked to their star or had an apparent day longer than 96 hours. (In 1,000 systems, 69 had 'habitable worlds', of which 24 had apparent days less than 96 hours.)

How much of a tide-locked planet's surface is typically well-lit and a suitable temperature for plant growth? I find it hard to imagine that the figure would be 50%, so at least a -1 to Habitability for a tide-locked world must be called for. I would think that a -2 or even -3 might be appropriate (each -1 to Habitability halves carrying capacity).

How much should the ~ 160°F "daily" ranges of temperature in the equatorial regions of spin:orbit resonant planets affect Habitability, taking into account 1,000+ hour nights?
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Old 07-21-2009, 10:41 PM   #2
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Default Re: [Space]: Habitability modifiers for tidelocked and resonant worlds

Well, the resonant world isn't so badly off as all that. While the non-coastal land surface is going to experience radical variations of temperature, the place where I live experiences a range of about 130 degrees fahrenheit and the plants cope just fine simply by waiting for the sun to shine again and taking advantage of long days to grow twice as fast as more southerly plants. As for the tide-locked world, it's not so much that the habitability is lowered, as it is that the carrying capacity is lowered. However since both the west pole and the east pole will probably be concentrations of water, with the sea level lower in the twilight zone it won't be so bad as all that.
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Old 07-21-2009, 11:05 PM   #3
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Default Re: [Space]: Habitability modifiers for tidelocked and resonant worlds

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Originally Posted by David Johnston2 View Post
Well, the resonant world isn't so badly off as all that. While the non-coastal land surface is going to experience radical variations of temperature, the place where I live experiences a range of about 130 degrees fahrenheit and the plants cope just fine simply by waiting for the sun to shine again and taking advantage of long days to grow twice as fast as more southerly plants.
What is annual primary production per unit area around there? How does it compare with global average? The ocean off Alaska is absurdly productive, isn't it?

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As for the tide-locked world, it's not so much that the habitability is lowered, as it is that the carrying capacity is lowered.
True enough, though it comes to the same thing at TL8 and above and at Habitability 4 and above.

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However since both the west pole and the east pole will probably be concentrations of water, with the sea level lower in the twilight zone it won't be so bad as all that.
Why do you single out the east and west "poles"? I wouldn't have thought there would be a lot of difference among the points around the terminator.

Plants will grow between 0 C and 40 C, whereas the temperature range between hot pole and cold pole is likely on the close order of 100 K. A nice broad band around the twilight zone might possibly be of an equable temperature, but unless the planet on the whole is rather cool it's likely to be rather dimly lit, a constraint on photosynthesis/primary production. What effect on carrying capacity would you suggest?
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Old 07-21-2009, 11:31 PM   #4
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Default Re: [Space]: Habitability modifiers for tidelocked and resonant worlds

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What is annual primary production per unit area around there? How does it compare with global average? The ocean off Alaska is absurdly productive, isn't it?
I don't have the statistics right to hand, but I do live in a farming region

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True enough, though it comes to the same thing at TL8 and above and at Habitability 4 and above.
Not for the first couple of hundred years.


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Why do you single out the east and west "poles"? I wouldn't have thought there would be a lot of difference among the points around the terminator.
That's how tidal bulges work. If the atmospheric pressure is low enough that you lose that 25% of the hydrosphere, you'll find that water frozen into a thick oval ice cap collected around the west pole, while at the east pole you'll find the water collected into permanent high tide, which is often very high indeed since the gravitic attraction is greater than that of the moon.

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Plants will grow between 0 C and 40 C, whereas the temperature range between hot pole and cold pole is likely on the close order of 100 K. A nice broad band around the twilight zone might possibly be of an equable temperature, but unless the planet on the whole is rather cool it's likely to be rather dimly lit, a constraint on photosynthesis/primary production. What effect on carrying capacity would you suggest?
More often that not, the tidelocked worlds will be comparatively cool because it's difficult to fit a world into the red or even orange dwarf habitable zone. However another consideration is that quantity has a quality all it's own. The light may be dim but it never turns off. 24 hours of continuous sunlight is pretty respectable even if it isn't the best sunlight.
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Old 07-21-2009, 11:57 PM   #5
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Default Re: [Space]: Habitability modifiers for tidelocked and resonant worlds

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That's how tidal bulges work. If the atmospheric pressure is low enough that you lose that 25% of the hydrosphere, you'll find that water frozen into a thick oval ice cap collected around the west pole, while at the east pole you'll find the water collected into permanent high tide, which is often very high indeed since the gravitic attraction is greater than that of the moon.
Aah! When you say "east pole" and "west pole" you mean the hot and cold poles. What I call the east and west poles are the points where the terminator intersects the equator.

No, that isn't how tidal bulges work. The mantle material conforms to the same geoid as liquid water over geological time. The ice cap at the cold pole is assembled by precipitation, not tidal flow, and the hot pole is bound to have a depressed sea level because water must continually flow towards it to make up for evaporation.

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More often that not, the tidelocked worlds will be comparatively cool because it's difficult to fit a world into the red or even orange dwarf habitable zone.
And it will be comparatively dim because the spectrum of K and M stars is dominated by the IR. But as you say, that's a different issue. I am referring to the fact that in the crepuscular zone sunlight is incident to the surface at a low angle, resulting in low surface insolation. As you say that is offset by its being continuous, but the oblique incidence is more significant than constancy within 30° of the terminator.
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Old 07-22-2009, 12:38 AM   #6
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Default Re: [Space]: Habitability modifiers for tidelocked and resonant worlds

I (somewhat) recently saw a show about "green" technologies, and one of the devices shown was an A/C unit that worked during nonpeak hours by freezing a reservoir of water at night. During the day, it circulated a coolant fluid that would essentially absorb the heat in the building and dump it into the ice (slowly melting it as the day went by).

If we're talking about a civilization advanced enough to colonize alien worlds, they may have the means (although it would certainly take some time) to use a similar scheme on a tidal-locked or even resonant worlds. Creating a massive A/C system might work to more efficiently transport heat, thus warming up the dark regions and cooling down the bright ones. This would certainly improve the habitability of such planets up to around the level of those with more Earth-like days.

Failing such massive terraforming (terraforming would be the proper term for the A/C "irrigation" thing, right?) projects, of course, I heartily agree with Brett's recommendation for a Habitability modifier for such planets.
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Old 07-22-2009, 01:25 AM   #7
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Default Re: [Space]: Habitability modifiers for tidelocked and resonant worlds

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If we're talking about a civilization advanced enough to colonize alien worlds, they may have the means (although it would certainly take some time) to use a similar scheme on a tidal-locked or even resonant worlds. Creating a massive A/C system might work to more efficiently transport heat, thus warming up the dark regions and cooling down the bright ones. This would certainly improve the habitability of such planets up to around the level of those with more Earth-like days.
Ouch! What a truly brutal use of technology! I can't help feeling that it would be easier and more scalable to build orbital habitats. Once we can air-condition worlds we won't need planets.
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Old 07-22-2009, 01:29 AM   #8
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Default Re: [Space]: Habitability modifiers for tidelocked and resonant worlds

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Ouch! What a truly brutal use of technology! I can't help feeling that it would be easier and more scalable to build orbital habitats. Once we can air-condition worlds we won't need planets.
I still say that gengineering humans to be at home in space might not be as easy as everyone thinks.
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Old 07-22-2009, 01:34 AM   #9
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Default Re: [Space]: Habitability modifiers for tidelocked and resonant worlds

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I still say that gengineering humans to be at home in space might not be as easy as everyone thinks.
I was thinking of living in Stanfords, Bernals, and O'Neills rather than living in the vacuum of Space. Not much gengineering of mankind ought to be necessary, unless there is something I'm overlooking.
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Old 07-22-2009, 01:41 AM   #10
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Default Re: [Space]: Habitability modifiers for tidelocked and resonant worlds

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I was thinking of living in Stanfords, Bernals, and O'Neills rather than living in the vacuum of Space. Not much gengineering of mankind ought to be necessary, unless there is something I'm overlooking.
Radiation?

Enough mass to stop the radiation will be very heavy, enough mass to stop the radiation during solar storms will be even heavier. This will add considerably to your engineering costs. Building some Radiation Tolerance into your inhabitants would probably be cheaper and easier.
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