Would we expect other planets to have recognizable rocks?

Submitted by Toorelad t3_ye35gl in askscience

I mean, would rocks on other planets be likely to conform to what we would know as quartz or granite? Would their lava create obsidian? Does rock default to certain matrixes?

Or are specific rock types dependent on specific situations, and while their mineral contents might be similar would they be totally unrecognizable?

Or something completely different?

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CrustalTrudger t1_itz9qne wrote

In short, yes, but with some caveats. The simple answer is based on an assumption that the bulk composition of hypothetical other planets are not that different from the rocky planets of our solar system and more broadly that the bulk composition of our solar system (which sets the available elements for all of the planets in that solar system) are not that different. If we accept that assumption, from there the rocks we observe on said planet(s) would be dictated by the underlying laws of chemistry and thermodynamics (which we would expect to function similarly regardless of location, i.e., reactions between particular elements/compounds at particular temperatures and pressures will still behave the same). In terms of tests of this assumption, we have limited data, but broadly the rocks we've been able to observe (in here I'm lumping a lot of different types of observations) on extraterrestrial bodies (primarily the Moon, Mars) and meteorites conform to and/or are similar to rocks we observe on Earth.

One thing to note though, generally, Earth has a pretty wide array of rocks compared to most other bodies we've been able to explore to some degree. This is primarily due to the existence of active and long-lived mobile lid tectonics, which is the root cause of, or at least allows for, many of the igneous differentiation processes that allow for the formation of intermediate and felsic rocks (e.g., this igneous rock classification diagram) like granite (felsic). In contrast, most of what we've found on planets that either did not have plate tectonics (or had likely short lived tectonics or something like mobile lid tectonics) are ultramafic and mafic rocks and things derived from them (e.g., sedimentary rocks, which are still very enriched in iron and magnesium and thus geochemically would "look" like ultramafic or mafic rocks).

Finally, if we circle back to our original assumption, a lot hinges on this idea that the bulk composition of our hypothetical planet(s), and by extension the solar system and star within which the hypothetical planet(s) formed is similar to both Earth and our solar system/star. The limitations of this assumption are understood and discussed by people studying expolanets, e.g., Putrika et al., 2021 discuss this directly in considering some possible differences in bulk composition (e.g., things like the relative abundance of iron, magnesium, and silicon within a given solar system) and how this might influence the composition of rocks that can and do develop. As a specific example, Putrika & Xu, 2021 highlight that you could expect some exotic compositions in extreme environments (e.g., exoplanets that develop around a "polluted white dwarf").

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svarogteuse t1_itzg4rb wrote

Earth also has a lot of rocks that are a by product of life, like limestone, that we wouldn't expect to find on other planets unless it developed similar lifeforms.

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CrustalTrudger t1_itzh3bf wrote

Yes, though it's also worth remembering that while most carbonates are biotic to some degree or another, there are abiotic sedimentary carbonates that can form in specific chemical environments and rare igenous carbonates (i.e., carbonatites), so we would want to be careful with a blanket assumption that carbonates = life and thus are wholly unique to a planet that has or had life. Though, certainly something like a bioclastic limestone would be unique to a planet that had life.

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dukesdj t1_iu0509e wrote

Not a rock person but would the theoretical carbon planets not have significantly different rocks?

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CrustalTrudger t1_iu0h170 wrote

Yeah, but I would say that falls into the last paragraph category, i.e., as already stated, if the bulk composition of the starting solar nebula / protoplanetary disk is significantly different than our own, then the possibility of exotic rocks gets much more likely.

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One-Plane7101 t1_iu57y2k wrote

I think the answer would be yes and no. The same elements we find on earth, we can find in the rest of the universe(excluding man made ones). That being said, there are a lot of temperatures and pressures that we just don’t experience, and the distribution of elements would be very different somewhere else. You’d probably recognize some things, but some formations would look otherworldly(pun intended)

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MegavirusOfDoom t1_iueryjl wrote

It depends if the pressure and chemical levels were similar ranges. Generally minerals are universals, and on mars they are 98% the same kinds as on earth, with things like limonite, sandstone, quartz. On high pressure carbon planets they may expect diamond and other exotic rocks.

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dumbass_spaceman t1_itzluij wrote

In our own solar system. Yes, perhaps.

In exoplanets, no.

This particular question has already been answered.https://www.google.com/amp/s/scitechdaily.com/unlike-anything-in-our-solar-system-rocky-exoplanets-are-even-stranger-than-we-thought/amp/.

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