How to get really deep depresion on an exoplanet?

[Czcibor]

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?

 

[Simon Bridge]

You mean something like Canyon (Niven: n-space)?

 

[DaveC426913]

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.

 

[Czcibor]

You mean something like Canyon (Niven: n-space)?

[googling]

Damn, someone already thought about it.

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

 

[DaveC426913]

[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.

 

[Hercuflea]

How to get really deep depresion on an exoplanet?

Think about how far you are away from home.

 

[Czcibor]

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?

 

[Czcibor]

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?

 

[Czcibor]

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?

 

[mfb]

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.