How to get really deep depresion on an exoplanet?

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Discussion Overview

The discussion revolves around the conceptualization of an exoplanet or exomoon featuring a deep depression that could potentially be habitable for humans without breathing aids. Participants explore various geological and environmental factors that could contribute to the formation and characteristics of such a depression, including gravity, continental drift, and climate implications.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests a supercontinent with a deep depression fed by a river from surrounding mountains, proposing lower gravity to allow for taller mountains.
  • Another participant references the concept of a crevasse where the atmosphere gathers, questioning the geological history that could lead to such a feature.
  • There is mention of a continental rift as a plausible mechanism for the depression's formation, drawing parallels to historical geological events like Pangea and the East African Rift.
  • Participants discuss the impact of erosion over time on the depression's depth and structure, with one suggesting that planetary collisions could create significant depressions.
  • Concerns are raised about how to minimize sedimentation in the depression, with ideas including high mountains blocking winds and water seeping through aquifers.
  • Questions are posed regarding the temperature variations in deep depressions, referencing Earth's temperature gradient with depth.
  • One participant proposes the idea of an asteroid impact creating a deep depression, citing Hellas Planitia on Mars as an example.
  • Discussion includes the implications of lower gravity on atmospheric pressure and temperature, as well as the potential for subduction zones to create depressions like the Mariana Trench.

Areas of Agreement / Disagreement

Participants express a variety of ideas and hypotheses regarding the formation and characteristics of the depression, with no clear consensus reached on the best approach or model. Multiple competing views remain regarding geological mechanisms and environmental conditions.

Contextual Notes

Limitations include unresolved assumptions about geological processes, the impact of erosion over time, and the specific conditions required for habitability in such a depression.

Czcibor
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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.

Idea:
-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?
 
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You mean something like Canyon (Niven: n-space)?
 
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.
 
Simon Bridge said:
You mean something like Canyon (Niven: n-space)?

[googling]

Damn, someone already thought about it.

DaveC426913 said:
Best not ask how that crevasse got there.
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
 
Czcibor said:
[googling]

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
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 historyThis 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:
How to get really deep depresion on an exoplanet?

Think about how far you are away from home.
 
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DaveC426913 said:
Well, the trouble with it happening over a long timeline is that erosion will tend to level things out.

It would indeed tend to, but it not prevent the lake to be quite deep:
The bottom of the lake is 1,186.5 m (3,893 ft) below sea level, but below this lies some 7 km (4.3 mi) of sediment, placing the rift floor some 8–11 km (5.0–6.8 mi) below the surface: the deepest continental rift on Earth.
http://en.wikipedia.org/wiki/Lake_Baikal

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?

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

- Mars aquifers (underground bodies of water) have erupted and carved great riverbeds in its ancient historyThis 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.
Interesting, that's something that have not thought about. How deep can they be?
 
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?
 
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.
http://en.wikipedia.org/wiki/Hellas_Planitia

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

In optimistic scenario - back to bacteria?
 
  • #10
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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