Yes, perhaps these will be reclassified as satellites and moons will have to be large enough to form spheres under their own gravity. That would mean that Mars would have no moons though.

Can you expand a bit on why a satellite would need the planet's gravity to keep a spherical shape ?

No, we're talking about a satellite forming into a sphere under its own gravity, not the planet's. Given a large enough lump of matter, gravity will overcome structural strength and force the material to flow into a roughly spherical shape. On Earth, this happens with the maximum mountain height being a few kilometers, but on Mars' moons, which are much smaller, the gravity is far too weak to overcome the rock's structure, and they are basically strange shaped rock things.

Mars' moons are really nothing more than captured asteroids. They look like "strange shaped rock things" because that's exactly what an asteroid is: just some random piece of rock.

As for the spherical shape, the term there is hydrostatic equilibrium: https://en.wikipedia.org/wiki/Hydrostatic_equilibrium.

> Mars' moons are really nothing more than captured asteroids.

Keep in mind it's not actually possible to just "capture" an asteroid. The object needs to lose speed somehow in order to be captured.

Most likely via collision - something large hitting the planet, or two other moons crashing.

So the story of the moons is much more complicated than just "random piece of rock".

Given enough time [1], all solid objects become perfect spheres, no matter their size.

[1] 10^65 https://en.wikipedia.org/wiki/Timeline_of_the_far_future

I am comfortable with a definition of "moon" that does not involve extrapolating 10^65 years into the future.

That's what they said about IPv4

Well, then you'd need to define an arbitrary time limit for how long it would take to form a sphere. In which case, you might as well have just defined a size/mass limit in the first place.

I think the definition would be that they're presently a sphere, that was formed under its gravity. That way objects could possibly become moons in the future, but the timescales are so long it's mostly irrelevant

That's technically correct (the best kind of correct!), but I think it's a misleading thing to state. The reason all objects eventually become spherical is because they eventually disintegrate into the nearest significant source of gravity, which will itself be a sphere. It's not as if, hypothetically speaking, an immortal human being floating through space without orbit would eventually morph into a sphere under the pressure of its own gravity. Rather any object is naturally going to decompose into parts which will be spherical or adjoined with an already spherical object.

Even if the conjecture that sufficiently high entropy causes quantum physical effects to dominate macro-physical spacetime is correct, long before that happens there won't be any objects left which aren't already spherical.

Actually, not always. A mass orbiting close enough to a planet is subject to tidal forces that will rip apart and eventually flatten a round body into a ring.

https://en.wikipedia.org/wiki/Roche_limit

If body is in a strange orbit taking it in and out of the roche limit, it will adopt some strange middle ground between bring a round moon and a flat ring. Inside the limit, material will be ripped away from the body and flattened towards a ring. Outside the limit, that material will fall back nearer to the equator. So you get something like a ball with a belt.

Whew, how you can possibly wrap your mind around 10^10^120 is a mystery to me.

I can't wait to be a ball!!

Is the maximum height of mountains on Earth really due to gravity or a limit imposed by geological weathering?

According to quite a lot of comparative planetary geology work, Mt. Everest, Mauna Kea, etc. are pretty close to the height limit imposed by gravity and structural strength. On some objects (like Mercury or the Moon: see https://arxiv.org/pdf/1511.04297.pdf) there aren't any mountains high enough to hit that limit, but Earth seems to have active enough tectonics to push up at that limit despite weathering.

A bit of one, a bit of the other, AFAIK. Of course, having geological weathering is also a function of gravity; small objects don't have the atmosphere for it.

The tallest planetary mountain in the solar system is Olympus Mons on Mars, about 2.5x as tall as Mount Everest (as measured from sea level). This is only possible because Mars has 1/3 the gravity of Earth.

https://en.wikipedia.org/wiki/Olympus_Mons

Aren't all moons natural satelites?

That's no moon!

Pop culture references aside, yes, a moon is defined to be a natural satellite.

It's a space station.

Nooo. Don't take away Mars' moons.

Most planets aren't even spherical, they are all elongated at right angles to the axis of rotation.

Most planets aren't even spherical in the same way that a piece of paper isn't even flat.

A fun read about this:

- [Asimov - The Relativity of Wrong](http://chem.tufts.edu/answersinscience/relativityofwrong.htm)

From the link above:

> To put it another way, on a flat surface, curvature is 0 per mile everywhere. On the earth's spherical surface, curvature is 0.000126 per mile everywhere (or 8 inches per mile). On the earth's oblate spheroidal surface, the curvature varies from 7.973 inches to the mile to 8.027 inches to the mile.

> The correction in going from spherical to oblate spheroidal is much smaller than going from flat to spherical. Therefore, although the notion of the earth as a sphere is wrong, strictly speaking, it is not as wrong as the notion of the earth as flat.

You're being unnecessarily pedantic. There's a clear difference between oblately spherical and lumpy.

You seem to read my comment as contradicting rather than supplementing.

Sorry, your use of "even" made it seem like you were trying to be contradictory.

Perhaps you meant, "No planets are perfectly spherical"?

The lay person's mental image of "moon" means something generally spherical, but it's by no means a requirement... Pretty common for moons of any of our gas Giants to be potato shaped or lumpy agglomerations of rock.

We also could probably use some adjustments on the other end too: when should a large moon be considered a second planet in the same orbit around their sun as its larger neighbor instead of a moon of that neighbor?

I've seen a couple fairly decent arguments that Earth/Moon should be considered a double planet.

One is that for every moon we know of except for Earth's, the force on that moon from the planet's gravity is more than the force on that moon from the Sun's gravity.

That's not the case for the Moon. The gravitational force on the Moon from the Sun is about twice as much as the gravitation force on it from the Earth.

I could accept a requirement for moon status being that the thing you are a moon of is the thing that has the strongest gravitational hold on you. For the Moon, that is the Sun.

Another argument is based on the shape of the paths of the bodies as seen from the Sun. Planets follow convex paths. Their moons swing them in and out as the moons go around them, but that just reduces the curvature of the planet's orbit. The Moon, for example, turns the Earth's path as seen from the Sun from essentially a circle (the eccentricity of the ellipse is low enough we can ignore it for this discussion) into a regular 12-gon with broadly rounded corners.

Same goes for other planets with moons, except their convex shape is more complicated because they have multiple moons.

When you look at those other moons from the Sun's viewpoint, they are not following convex paths. They are more like something you might have made when you first played with a Spirograph as a kid.

Except for the Moon. The Moon's path as seen from the Sun has the same basic shape as Earth's, a rounded 12-gon. In other words, it looks like a planet path, not a moon path.

(I'm not sure if that is actually another argument. It may follow from the first argument, the relative gravity strength one...I've not tried to do the math to see).

I find both these arguments pretty good, with one caveat. What happens with even bigger moons relative to their planet than the Moon?

Imagine if Earth had a moon that was much closer to Earth's mass, say 70 or 80%, and much closer to Earth?

Going by either the tug-of-war with the Sun criteria, or the convexity of orbit as seen from the Sun criteria, I think we could get a situation where each body would be classified as a moon of the other! I don't think we want to allow a system that is all moons, so we'll need some fiddling so that in this case they are both planets.

(Credits: I'm reasonably convinced I got both of the above arguments from one of Isaac Asimov's books that collected the monthly science columns he wrote for The Magazine of Fantasy and Science Fiction)

> I've seen a couple fairly decent arguments that Earth/Moon should be considered a double planet.

It should at least be considered a planet and dwarf planet pair. After all, the Moon is larger than Pluto. The relative size of the Moon compared to Earth is enough to convince me. Binary planet or double planet would make fine nomenclature.

Sounds like time poorly spent to me. A waste in fact.