# Crustal composition of an ammonia planet

I am currently building a planet that weighs about 4 earth masses and is 2.3 earth radii long. It is an ammonia planet (as the title suggests), and has ammonia oceans with some dissolved water ice, methanol and salts (sodium, chlorides, sulfates, potassium etc.). The average temperature is about 260ºK on the planet and the avg pressure at sea level is about 2.5 atm.

It has an ocean with the following composition:

• Ammonia: 80%
• Ice: 12% (which sinks under the ammonia and is mostly regular ice, but at very deep places, it may become ice III)
• Salts: 5.5%
• Ethanol: 2%
• Other: 0.5%

If you can't already tell, this solution is quite basic, so basic that a lot of common minerals that we have in our crust would simply dissolve. As a result, I would like to understand what minerals/compounds would be able to survive the fierce chemical weathering and make up the continental and oceanic crust. If you give percentages, that would be great.

• Ammonia is a solvent, just like water. Ammonia simply dissolves the dissolvable faster than water will. But it still takes a lot of time to dissolve rock. I've never found ammonia-based biology believable due to how quickly it can dissolve bonds, but ignoring the organic component, the crust of Earth would be more than suitable as a model for your ammonia planet. If you want to test the theory, fill a mason jar half full of dirt and fill the rest with ammonia. Close the lid (to simulate a non-oxygen-based atmosphere) and wait a few weeks. Drain, dry, and look at the dirt that's still there.
– JBH
Commented Mar 23 at 2:03
• Understood and agreed. The only issue with your theory is that over millions of years, all that ammonia is definitely going to dissolve at least some significant amount of stuff. That's why I asked this question, and I'm not able to find any resources on what/how much minerals dissolve in an ammonia/ice mixture (with some other stuff). Commented Mar 23 at 5:50
• I would say volcanic activities make this a non issue as generally new crust is always forming. What i think is more of a problem is that the ammonia ocean may just dry up over time. The most probable way to avoid this maybe to have some highly resistant layer but Ii have no idea how it would form naturally. Commented Mar 23 at 15:20
• @Fallenspacerock makes an excellent point about volcanism. It's why, after millions of years, water hasn't worn our world down to nothing. In other words, your world need only more volcanic to solve the problem. This lets you craft the crust based on Earth's with shifts in percentages to meet your world's needs.
– JBH
Commented Mar 23 at 17:43
• @Fallenspacerock Understood. I just need more volcanic activity to make sure that the crust doesn't fizz away and it can probably be just like earth's. What can I do about the ocean dissolving (that wouldn't be very good). Commented Mar 23 at 19:06

Well first some key points.

First, your pressure would not be 2.5 atm, it would be higher. Your world, assuming a distribution equal to that of the Earth, should have a gravity of approximately 3.3 g since you have a world 4 times more massive, although somewhat larger than the Earth (I do not want to do the algebra to calculate the gravity of your world exactly, if you are interested, use Newton's equation a = GM / R^2).

Now we need to know exactly the height, density and composition of your atmosphere, let's say your atmosphere is proportional to the earth's with your modification and the composition of the first layer is 50% diatomic nitrogen, 12% nitrogen oxides, 8% water, 4% CO2 and 26% other gases (warning here, I'm about to put up a wall of text with numbers and data for whoever cares, feel free to skip it) (note this is to one atmosphere what a spherical cow in a vacuum is to a cow) here the densities:

N2: 1.2506 g/ml

NOx: 1.58 g/ml

H2O: approx 1 g/ml

CO2: 0.001976 g/ml

(Note that ml is equivalent 1 to 1 to cm3)