This is actually not an answer to your question and I would like to simply comment it but it's way too complex. It is still an important thing to consider when building your walnut planet imo. Increasing Iapetus to the size of Earth, its gravity will also increase preventing the ridge to grow above a certain height.
Let's do some calculations:
So first off we have the formula to calculate the volume of Earth:
Inserting the Earths radius, which is we'll get the following:
We can calculate the mass of an object by its volume and its density:
When we insert the density of Iapetus, which is we'll get:
Now, we can take this formula to calculate the gravitational force between two objects:
and if we combine it with the formula of force, which is we can eliminate the first object and get the formula to calculate the gravitational acceleration on the 2nd object's surface:
Let's insert the values for our giant Iapetus:
Due to the lower density, the gravitational acceleration is significantly lower than on Earth (
9.81) and even smaller than the one on Mars (
3.96), which means our mountain range can indeed grow significantly larger than on Earth, assuming we have the same material. But how large exactly?
I have found this website addressing this question ultimately coming up with the following formula:
Now there are a lot of assumptions and simplifications made, the shape of the mountain is a square and the material it's made of is pure silicondioxide, which of course is not the real world case so it's just an approximation. I'm also not sure where some of the numbers come from as I'm not a physicist but anyway, let's calculate.
- is the energy required to melt a single molecule of silicondioxide
- is the number of protons and neutrons in a single molecule
- is the mass of a single proton (which is also about the mass of a single neutron)
- is the gravitational acceleration we calculated earlier.
So this comes down to:
Now, the website states that this is
the order of the height of the highest mountain
and calculates a maximum height of 4.9 km while Mt. Everest is about 9km high, but it also mentions the result for Mars would be less than 13km with Olympus Mons being 26km high, so it seems to be about the double with me assuming the difference comes from the shape and material simplification, so our giant Iapetus would have a maximum mountain range of
which is much less than your desired 120km. Now, I suppose you could increase the value with a rather sharp density drop-off, the inner and outer mantle being denser than the crust, which forms the mountain range, though I'm unsure as to whether this would be realistically feasible. Decrease your planet's density as a whole but again, no idea how low you can go and whether it depends on the size of the planet. Change the material to a lighter but harder one but also not sure whether this might be possible. I also was unable to find out what material Iapetus' crust is actually made of so I just took the siliconedioxide that was already simplified for Earth.
Now, even if you get a better value I actually don't think it's possible to create such an enormous mountain range realistically.