What can my moons be made of?
So I asked a question before about having two moons. A wise patron suggested my moon be made out of something like blown glass, in order to have a big bright moon that isn't too dense. That has me thinking... Would that make them glow, all the way through, so there would necessarily be a dark side? Could it be gemstones, to make it even brighter? I've heard there are planets made of "precious" minerals.
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1 answer
Carbon planets
We typically expect a moon's composition to reflect the part of the protoplanetary disk it came from. If it's orbiting a planet close to the star, we'd expect it to be composed largely of silicates, like Earth or the Moon. If it orbits farther out, it would be reasonable to expect there to be plenty of volatiles, too, like water ice or ammonia. Gaseous exomoons are less likely, of course, as the bodies are almost certainly too small to retain gaseous envelopes.
The more exotic compositions you're indicating are reminiscent of several particular carbon planets. These are planets originating in a system with an unusually high amount of carbon, including possible carbon-to-oxygen ratios greater than one (see Delgado Mena et al. 2010). Some of these planets contain so much carbon that graphite or diamond may be their primary constituents - hence the popular term "diamond planet". The name has been applied to several carbon-rich exoplanets:
- 55 Cancri e, about 8 Earth masses
- WASP-12b, about 1.4 Jupiter masses
I should note that the composition of these planets is not totally certain, and some groups have argued the graphite is the primary constituent, not diamond (Teske et al. 2013).
Why am I talking about planets, though, when you asked about moons? Well, as I said before, the composition of a moon should reflect the composition of the protoplanetary disk it came from. If planets with significant diamond content can form, then so too could moons.
Structure
We don't have detailed models of carbon moons, though work has been done on the structure of carbon planets of varying masses (Hakim et al. 2019). It seems that a low-mass carbon planet would likely have the following structure, due to stratification:
- An iron inner core depleted of sulfur
- An iron outer core with sulfur
- A substantially larger mantle made of silicates
- An outer layer of carbon, likely graphite.
Some worlds will have a layer of diamond underneath the graphite; moreover, convection may transport some of that diamond into the mantle, enriching it with diamond.
The only way you can produce a planet that is entirely diamond would be if there were no silicates, iron, or similar compounds or elements in the protoplanetary nebula, and that's just not likely.
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