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I'm designing a lower gravity planet that's got enough mass to hold onto an atmosphere but around 75% of Earth's gravity. I read that lower-gravity planets will have more pronounced geological features - so, higher mountains, for example. Does this also mean the crust would be more uneven, with higher and lower elevations and deeper oceans?

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    $\begingroup$ Like Mars for example? Olympus Mons and Valles Marineris? $\endgroup$
    – Gillgamesh
    May 16 at 17:51
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    $\begingroup$ There are great answers for the mountain heights, but I just wanted to point out that in a way the ocean depth and measured mountain height depend on how much water is on the planet; if there's barely any water, then ocean depth will be low and mountain height will be measured higher (based on 'sea-level'). The opposite is true as well, where having more water on the planet means deeper oceans and a higher sea-level, so mountains will be measured 'shorter' than the planet without as much water. $\endgroup$
    – Daevin
    May 17 at 15:01
  • $\begingroup$ Other major contributing factors would be planet age, planet diameter, plate tectonics (presence or absence, volatility, etc.), atmospheric pressures (high winds and erosion), land to water ratio, lunar influences. The highest mountain in the solar system is actually volcanic in nature which likely can allow it to grow higher in lower gravity. $\endgroup$
    – CitizenRon
    May 17 at 16:28

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Let's use our Solar System as sample to verify if planets with lower gravity have higher mountains, looking at the highest mountains in it.

Name Height Location (surface gravity $m/s^2$)
Olympus Mons 72,000 ft (22,000 m) Mars (3.69)
Equatorial Ridge 65,617 ft (20,000 m) Iapetus (0.22)
Boösaule Mons 59,711 ft (18,200 m) Io (1.79)
Ascraeus Mons 49,000 ft (15,000 m) Mars (3.69)
Ionian Mons 41,667 ft (12,700 m) Io (1.79)
Elysium Mons 41,338 ft (12,600 m) Mars (3.69)
Arsia Mons 38,386 ft (11,700 m) Mars (3.69)
Limb Mountain 36,089 ft (11,000 m) Oberon (0.35)
Skadi Mons 35,105 ft (10,700 m) Venus (8.87)
Euboea Montes 34,449 ft (10,500 m) Io (1.79)
Mauna Kea 33,464 ft (10,200 m) Earth (9.81)
Haleakala 29,856 ft (9,100 m) Earth (9.81)
Mount Everest 29,029 ft (8,848 m) Earth (9.81)

As you can see Earth is out of the top-10, so it looks like indeed gravity affect the elevation of the relieves on a celestial body: the lower the first, the higher can the second get, as expected from the mountains having to "fight" against the gravity to stay in place and not crumble.

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    $\begingroup$ Good data, +1, (other than that surely nobody has measured the height of the Equatorial Ridg on Iapetus to the accuracy of 1ft). But some figures can surely be contested because the points of reference don't seem to be uniform. For example the peak of Mauna Kea is apparently compared to the level of the surrounding ocean floor. That is fair enough. But then some peaks on the Andes might make the list due to the elevation difference from the bottom of the Peru-Chile trench. $\endgroup$ May 17 at 5:52
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    $\begingroup$ (cont'd) I do concede that it is problematic to define the height of a mountain in the absence of an interplanetary point of reference. Topographic prominence may come close, but is not without problems, because on Earth it is (taken literally) undefined at Everest unless we use ocean level as a cut off. See this table. $\endgroup$ May 17 at 5:59
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    $\begingroup$ @JyrkiLahtonen, I guess the measures are taken with a precision of 100 m, it's the conversion to imperial units which adds the single ft. $\endgroup$
    – L.Dutch
    May 17 at 6:24
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    $\begingroup$ One reason earth doesn't have high mountains: glaciers. High mountains tend to grow glaciers and those erode incredibly fast. So mountains on earth can only grow above glacier-growing elevation with fast tectonic uplift or volcanism. See for example d.umn.edu/~kgran/Geol4550/Egholm%202009.pdf $\endgroup$
    – Borgh
    May 17 at 12:47
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    $\begingroup$ @Borgh, Mount Everest is about as tall as a mountain can get here on Earth. The limiting factor appears to be the strength of the rock rather than the presence of glaciers; further compression of the India-Asia boundary just causes the Himalayan Plateau to spread laterally rather than get taller: earthscience.stackexchange.com/q/2586/2643 $\endgroup$
    – Mark
    May 18 at 1:45
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Looking into your question, I found this at the Astronomy Stack Exchange: Does a planet's mass or gravity affect the height of it's mountains?.

There's some mathematical and scientific things in there, if that's what you're searching for. But it seems the general answer is yes. :) Hope that link helps!

(Edit: I think this would be better as a comment, but I don't have the rep yet, sorry)

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    $\begingroup$ Posting comments as answers is not going to help you getting enough reputation to be able to properly comment. $\endgroup$
    – L.Dutch
    May 16 at 18:02
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    $\begingroup$ Welcome to the site! $\endgroup$
    – IronEagle
    May 17 at 2:08
  • $\begingroup$ You could turn your comment-answer into a proper answer by paraphrasing or quoting what you think is most valuable from the source you're linking to. Posting links to relevant source material is definitely encouraged - it's just the links shouldn't stand alone. I think this could be a great answer! $\endgroup$ May 17 at 18:09
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Looking at Mars, which has lower gravity than Earth, the crust does seem to be more uneven. It's home to both Valles Marineris, the deepest canyon in the solar system, and Olympus Mons, the highest mountain. Io's mountains are higher than Earth's as well. In general, lower gravity allows for the crust to be more uneven and have more extreme features. So, short answer, yes. Higher elevations will be higher, and lower elevations will be lower.

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It can do. There are a couple of other variables you may want to consider as well as gravity: temperature and level of volcanic activity.

Anything large enough to be classed as a dwarf planet by definition has enough gravity to bend the matter it is made of approximately into a sphere. How imperfect that is depends on whether the planet is actively working against gravity: Volcanic activity. This comes in two main forms: volcanoes, which give you peaks in their own right (including the mighty Olypmpus Mons on Mars or e.g. Iceland or Gran Canaria on Earth), and tectonic activity, which whilst very slow, is sufficiently powerful to overcome gravity and creates mountain ranges. Apart from volcanic activity, the other way, the other possibility for creating "peaks" is craters formed by bombardment from space, but since you world has oceans I'm assuming it has a fairly dense atmosphere protecting it so only very large asteroids would get through.

The second thing to bear in mind is the temperature. A planet's ability to hold on to water at all is determined by a combination of how strong the gravity is and how warm it is. Given that you want oceans at lower gravity than Earth, it will have to be a cold planet. Notice that Io and Mars do not have oceans as they don't have enough gravity to keep hold of water, so although created by volcanic activity, it leaks away into space. Although you'll here reports of there being water ice on Mars, it is in fact far drier than the driest desert on Earth, and although Io has the strongest surface gravity of any moon, it also has the least amount of water of any known astronomical object in the Solar System.

For the relationship between gravity, temperature and the ability to hold on to water (or indeed an atmosphere) see this wikipedia diagram. To hold on to water your planet needs to have a temperature / gravity somewhere near the top of the blue band (like earth), or in the green band, to the left of Earth (not too far though or your oceans will freeze). This will also ensure you have enough gravity to hold on to oxygen etc. which I assume your inhabitants will breath :-)

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    $\begingroup$ Thanks for the detailed response! I've actually already done some research to be able to calculate the smallest world that fits my goals. It has 75% of Earth's gravity and apparently is large enough to have an internal dynamo and substantial magnetic field as well as being able to hold onto water vapor, without being too cold. $\endgroup$
    – Elhammo
    May 17 at 17:43

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