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What kind of effect would lower-than-Earth gravity have on the geography of a smaller-than-Earth terrestrial planet? The Gravity is lower because the planet is smaller, the core must be large enough to maintain a magnetosphere. The planet should have erosion and active volcanism as well.

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    $\begingroup$ Why is the gravity lower? Is the core less dense or something? Please put more detail into your question. $\endgroup$ – Just'Existing Feb 7 at 4:10
  • $\begingroup$ Definitely lacks detail and clarity: please focus on one aspect or problem of worldbuilding in each of your questions. The question as you wrote it is too broad -- you're basically asking for a list. Check out the tour and help center so you can gain a better understanding how this forum works and how you can ask better questions! $\endgroup$ – elemtilas Feb 7 at 4:45
  • $\begingroup$ Does it have molten core? $\endgroup$ – user6760 Feb 7 at 6:40
  • $\begingroup$ Yes it has a molten core, and are there any more details needed for this question? $\endgroup$ – YLong Feb 7 at 14:48
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A really good place to start with this is to look at the Geography of Mars, also known as Areography.

Mars is a smaller terrestrial planet than Earth and has less gravity, so for those reasons alone it is worth the comparison but there are some gotchas in this which we'll cover later in this answer.

Perhaps most important to note that the geological formations of Mars are not that dissimilar to those on Earth in that both planets are round, and both have mountains and valleys, etc. The primary difference is that even though Mars is smaller and has less gravity, the geological formations on Mars are more exaggerated. Mount Olympus is higher than Everest on Earth, and Valles Marineris makes the Grand Canyon look like a ditch.

Not all of this can be attributed to gravity though. There are some theories about Valles Marineris being the result of a glancing meteor strike, and there are also theories that a large part of why Mars is so small and exhibits a larger amount of exaggerated craters and valleys could be that Jupiter destabilises matter in the asteroid belt and flings it at Mars from time to time.

Perhaps the biggest difference though is that Mars doesn't have an active core that generates new volcanoes and continental drift, and has nowhere near the water or the atmosphere that would cause some geological actions like erosion. So in some respects it is difficult to say.

I would argue however that the similarity in many of the geological formations we see on Mars to those we would expect to see in arid climates on Earth tends to indicate that gravity is not as much a factor in geological activity as active vulcanism, water and thick atmospheres would be. If I am right, it would mean that the answer to your question is 'not much' and perhaps one of the celestial bodies that might help us confirm this in the future is Titan.

Titan is the largest of Saturn's moons and we don't know a huge amount about its geography at the moment as it has a thick atmosphere and (it would seem) liquid oceans on its surface. Despite the thick atmosphere, it only has similar gravity to our own moon, so it is in many respects a closer fit for an example to your question than Mars is, except that we know a lot about the geography of Mars and very little about Titan's.

If we can land there and explore the actual surface with probes one day, we might get a clearer picture of its geography and how it is either similar or different to Earth. I know of no research or information that tells us much about whether its core is active either, so that is another consideration, but I suspect one day, when we know more about the geography of Titan, we might have a better answer to your question than I can provide here.

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  • $\begingroup$ Thanks to cloud-penetrating radar, we know a fair bit about Titan's geography. We have a full topographic map and terrain map. Also beneath-the-clouds photography showed details of a river system $\endgroup$ – dspeyer Feb 7 at 8:39
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    $\begingroup$ Looking forward to the 2047 answer including upcoming data from Titan exploration. +1. $\endgroup$ – Emilio M Bumachar Feb 7 at 13:08
  • $\begingroup$ Main difference between mars and earth is the absence of plate-tectonics on mars for the last ~3.5 billion years - if it ever had similar plate tectonics .. because the core mars was way smaller it cooled faster than earth's core $\endgroup$ – eagle275 Feb 7 at 15:31
  • $\begingroup$ When comparing Earth to Mars, you say "both planets are round." Does this mean that there are non-round planets? I always thought all planets were round, or as near as makes no difference. $\endgroup$ – Avrohom Yisroel Feb 8 at 19:15
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    $\begingroup$ @AvrohomYisroel You're right about all planets being round, but that is basically a nod to the fact that it takes a LOT less gravity than Earth's to leave you with something like the shape of Phobos, for instance, which is small enough not to be round but its surface is also too small to have what we would consider to be a geographical feature set $\endgroup$ – Tim B II Feb 8 at 23:03
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The low gravity alone wouldn't do much. Dunes and soil slopes could be steeper. Mountains might be a little higher, assuming the same forces are driving them up.

A low gravity world might have trouble holding an atmosphere, and therefore having liquids. That would make a huge difference, erosion-wise.

It might also have a more stable mantle, resulting in fewer tectonic features.

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