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How to get really deep depresion on an exoplanet?

  1. Dec 19, 2014 #1
    I want an exoplanet/exomoon with really deep depression. It would be the only place on that planet that's really habitable for humans without breating aid.

    -lower gravity (somewhere around 0.8 g) so the mountains should be able to be a bit taller
    -one supercontinent that's slowly spliting,
    -in the middle of the supercontinent near a tropic (dry place) a depression, which get's water from a river starting in mountains
    -a continental lake (like Baikal, but one size bigger) that keeps climate survivable in the middle of a supercontinent

    Any suggestions how to improve this idea? Any idea how deep I could make it before I make geologists outraged?
  2. jcsd
  3. Dec 19, 2014 #2

    Simon Bridge

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    You mean something like Canyon (Niven: n-space)?
  4. Dec 19, 2014 #3


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    First thing I thought of too.

    The entire population of the planet lives in a crevasse - the lowest part of the planet, where all the atmosphere of the planet has gathered.

    Best not ask how that crevasse got there. Especially not in a Kzinti Bar.
  5. Dec 20, 2014 #4

    Damn, someone already thought about it.

    My idea was continental rift - continent is slowly splitting, as Pangea did it, or we see now in East African Rift. Plausible mechanism or not specially?

    Anyway, it makes the story even a more thrilling because of impending doom - in maybe 10 mln years the split would be big enough to be connected with ocean :D
  6. Dec 20, 2014 #5


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    Well, the trouble with it happening over a long timeline is that erosion will tend to level things out.

    Things that have created crevasses in a shorter timeline:

    - planetary collision - Mars sustained a gigantic hit sometime in its past that created a huge depression on one side, the biggest volcano in the solar system on the other side, and the biggest canyon in the solar system between them.

    - Mars aquifers (underground bodies of water) have erupted and carved great riverbeds in its ancient history

    This last one is pretty plausible on an exoplanet. Doesn't really require a planetary collision, and doesn't require millions of years for cooling. Aquifer erupts, huge volumes of water carve a deep canyon, then evaporates.
    Last edited: Dec 20, 2014
  7. Dec 20, 2014 #6
    Think about how far you are away from home.
  8. Dec 21, 2014 #7
    It would indeed tend to, but it not prevent the lake to be quite deep:

    Anyway, how to keep sedimentation to minimum? High mountains that block most of winds in the valley? Water not coming from a river, but seeping through aquifers?

    Shouldn't a geologically active planet like Earth react with lots of lava coming out of the crater?

    Interesting, that's something that have not thought about. How deep can they be?
  9. Dec 21, 2014 #8
    One more thing that started to bug me - would such depression be much, much hotter than the surroundings? I mean on Earth it is 6C/1000m. Would rule like that be also applicable in case of a deep depression? (because you know 7000 m would give something like 42 C more than on the sea level) Or its enough to make it somewhat hotter, but with no excess?
  10. Dec 21, 2014 #9
    I started to toy with asteroid idea.

    On Mars something like that worked - Hellas Planitia, 7.1 km deep, 2300 km wide. Sounds quite satisfying.

    The only minor drawback:
    Cretaceous–Paleogene extinction event left Chicxulub crater, which is 180 km wide.

    In optimistic scenario - back to bacteria?
  11. Dec 25, 2014 #10


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    Lower gravity allows higher mountains but also smaller pressure gradients for the atmosphere, that does not help I think. Colder temperatures would help, but that leads to other problems. You can reduce the oxygen content of the atmosphere.

    Subduction zones can give depressions - the Mariana trench is one example, several kilometers below the surrounding ocean floor.
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