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Thread: Tidal heating and moon habitability.

  1. #1
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    Tidal heating and moon habitability.

    I recall a few months ago, there was research indicating that significant tidal forces and tidal heating would gradually strip away a world's oceans, through runaway greenhouses, and basically sterilize it.

    I keep thinking that a world can't have a day length longer than one or two Earth days in order to be habitable, and thus, if it were a moon, would need to be quite close to the parent planet, (I think i remember that some 24 hour orbits would be just barely outside the roche limits), but at that distance, the tidal forces might indeed be strong enough to trigger the runaway greenhouses.

    My question is, would this prevent Earth-sized moons of giant planets from being habitable? If said moon could become habitable, what might be the upper limit for rotation/orbit period (which is, of course, dependent on parent planet mass and orbital distance from said parent planet)?

  2. #2
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    Welcome to the board, PlutonianEmpire.

    An interesting question, but I'm wondering, why the limit on the length of day? If tidal heating is the main source of warmth, the frequency of sunny days is less important. Besides, reflected light from other moons could stisfy any sight-related needs. I can imagine a creature that goes out to forage only when the major moons are crescent or new.

  3. #3
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    Welcome to BAUT PlutoniumEmpire.

    The only place were I can envision tidal heating running havoc is at Io.

    For the rest you might want to give a link to what exactly you have read, because part of the first sentence does not make sense to me.
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  4. #4
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    Thank you for the welcome and the replies.

    The article I read was this: http://www.space.com/14535-alien-pla...al-forces.html

    And here is the arxiv paper on the subject: http://arxiv.org/abs/1203.5104

    The reason I was concerned about day length was due to the fact that I was under the assumption that after a certain point, the days would be too hot and the nights too cold as a result of the long orbit/rotation period. And since the moon would be heated from tidal heating, the too-long day would just wind up contributing to making the whole moon too hot.

  5. #5
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    A high for the day near Earth's equator of 50 c = 122 f would be fatal for unprotected humans if the humidity was over 99%, but possibly some plants, anphibians and insects could survive assuming cooler at night. I think the water loss in the upper atmosphere would still be negligible. Micro organisms that make compost frequently survive higher temperatures. as do sea vent and gyser pool creatures. At present I believe ocean surface temperatures rarely drop 5 c = 9 f just before sun rise, except near the North Pole where day and night are 6 months long.
    I think Io is the only planet where tidal heating accounts for even 0.5 c = 0.9 f of the average surface temperature. Neil

  6. #6
    Quote Originally Posted by PlutonianEmpire View Post
    I keep thinking that a world can't have a day length longer than one or two Earth days in order to be habitable, and thus, if it were a moon, would need to be quite close to the parent planet, (I think i remember that some 24 hour orbits would be just barely outside the roche limits), but at that distance, the tidal forces might indeed be strong enough to trigger the runaway greenhouses.
    I may be misinterpreting, but assuming that you mean solar day by "day," wouldn't it be the opposite, that a satellite would have to be far away from its parent planet to have a short day? It is true that you can get a short day by having a tidally locked satellite orbit very closely, but I think you can also have a satellite that is quite far away and not tidally locked.
    As above, so below

  7. #7
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    Quote Originally Posted by Jens View Post
    I may be misinterpreting, but assuming that you mean solar day by "day," wouldn't it be the opposite, that a satellite would have to be far away from its parent planet to have a short day? It is true that you can get a short day by having a tidally locked satellite orbit very closely, but I think you can also have a satellite that is quite far away and not tidally locked.
    Yes, you're absolutely correct. I've been forgetting that satellites far out enough from the parent planet can have individual rotations, but I'm unsure at what point they can, although that's probably a different discussion entirely.

  8. #8
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    In case of free rotation of secondary, the tidal acceleration is simply proportional to inverse square of orbital period.

    For the same reasons, Roche limit depends on nothing else than orbital period and density of secondary.

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