Geology on low density planets

Question

I am currently working on a piece that has a planet (about double the size of Earth). It is worth noting that this is a low density planet. Without any other factors (Earth-like atmosphere, same pressure and temperature range) what, if any, differences would we see geologically?

Answer 1

what, if any, differences would we see geologically?

Just about everything.

If you want your planet to be a terrestrial planet (i.e. something that you can walk on), it has to be made out of rock, and rock is dense. The larger the planet, the more rock you have, and the more dense it is.

Let's start close to home. Earth has a very dense metallic core, surrounded by a less dense (yet still very dense) mantle. You can remove the core, which will lower the density. The implications of not having a core are profound - no magnetic field, probably no atmosphere, and no heat source to drive plate tectonics. This is going to be a dead planet.

However, because your planet is still double the size of Earth, you are going to have a lot of rock and that will increase your gravitational pull. You need something less dense to balance it. Your best option is water or ice - but this is not a tiny layer of water or ice, you need it to be hundreds of kilometres deep to balance the dense rock. And then it's an ice planet.

Answer 2

Let's run this through an analysis. From this source we find:

• 35% of the Earth is iron.
• 30% of the Earth is oxygen.
• 15% of the Earth is silicon.
• 13% of the Earth is magnesium.

That's 78%. The rest of the mass is a mix of everything else. My goal is to determine what we'd need to do to the density to achieve the results you want. But first, I need to make an assumption. You said, "double the size." As I mentioned in my comment, there's three ways to look at that. Let's assume you meant, "double the surface area." This makes the math simpler.

Twice the surface area means we need twice the volume of atmosphere and twice the gravity to hold it in place. Ouch. Our planet has 2G gravity representing twice the mass of Earth.

But it doesn't have twice the volume...

The surface of a sphere is calculated as A = 4𝛑r². Earth's surface is 510e6 Km², we want 1.02e9 Km². That's r=9,009 Km (Earth is 6,371Km) and a volume of 3.06e12 Km³ or about 3X the volume of Earth.

Now we're cooking with gas. We have 2X the mass and 3X the volume so the planet's density is 66.7% that of earth or 3.67 g/cm³.

So, I've achieved part of your request. Without changing anything other than surface gravity, we have a lower-density planet capable of holding an Earth-like atmosphere in place.

How does this affect your geology?

• Simply upsizing the Earth suggests a thicker mantle.
  1. Between a thicker mantle and higher gravity, I predict your overall volcanism to be lower and your mountain building to be less. You still have tectonic plates with fissures, but less of that warmth gets to the surface. I'm thinking smoother landscape.
  2. However, if we assume the same mantle thickness as Earth, the higher gravity would produce a greater tendency to shatter, meaning far more fissures and faults. Perhaps less tectonic mountain building, but potentially much more volcanic mountain building.
• A lower density means less mass-per-square-meter to do anything with. Just because you have a larger core (whether case #1 or #2 above) doesn't mean that mass is available to punch holes through the mantle or move plates around. Just as deep sea currents tend to be independent of surface sea currents, a deeper core will not translate to anything at the surface. Therefore, the physically larger core will ultimately have a lower effect on surface geology. I predict far fewer tectonic-based mountain building.
• Finally, if you really want to ratchet down the density (we need to stick with a transition metal, so our best case is scandium at 80% density of iron, which is only 35% of the mass of Earth, so the overall change is only about 7% lower density....) then we run into the problem of a lower gravity, meaning less (thinner) atmosphere and less mass to punch holes through the mantle or move plates around. It would lower the geology even more.

Please note that I'm ignoring calculations for pressure. The higher gravity means higher pressure in the outer core which means a greater chance for all kinds of mountain building. That, itself, may normalize everything such that your geology on your planet is basically identical to Earth.

Of course, your rain, wind, etc, is still eroding your mountains, so rivers, plains, gorges, canyons, they're all basically built the same way as before. Most of your actual geological shaping mechanisms won't substantially change (methinks) with the larger planet as defined.

All things being equal, I think I'm going to vote for more volcanic mountain building, less tectonic mountain building, and everything else stays the same.

Answer 3

With a lower density but the same size, you would have trouble maintaining an atmosphere on that world.

For one thing, the gravity would be lower, allowing gasses to escape faster.

For another, you might not have as much iron in the core. The lack of an iron core could leave the planet without a strong magnetic field. This would allow the solar winds to strip the atmosphere.

Look at Mars for a close approximation to what you can expect on the planet.

You can mitigate this by having the planet be bigger. It's surface gravity could be similar or even higher than Earths if it is big enough. That will help hold in the atmosphere. Also, the bigger size will keep the core molten longer (even if it might be smaller). Then you can have everything like earth conditions just with a bigger sized planet.

Then we get to another issue: is there life on the planet? If not, then there is no appreciable oxygen in the atmosphere and the air will be generally nitrogen and methane. If there is life on the world and it developed as ours did with photosynthesis, any iron on the surface will be rusted. However, there will probably be less iron so you may not have the red deserts that we see on Mars.

There will be more silicates. As far as the other chemistry bits, that's not my expertise.

However, you will generally lack the heavier elements. This will result in a less radioactive core and the core will cool faster unless it is bigger.