How large can a moon be?

[boomslang]

For captured or collision formed moons, I believe the limit is about 75-90%, at which point it is a double planet (since the baricenter is most likely going to be outside the larger.

Indeed.

There's some debate about how to handle the description of binaries with orbital eccentricity large enough to move the barycenter in and out of the larger body, but there's also a theory that such systems would not be long-term stable due to tidal effects...

 

[Thuban]

There are two figures... one is that for non-caputured bodies, which I've heard 1% primary's mass being the 'generous' upper limit and 10% the 'you have got to be kidding' line.

For captured or collision formed moons, I believe the limit is about 75-90%, at which point it is a double planet (since the baricenter is most likely going to be outside the larger.

The original question was for satellites of jovians, for which I linked the article - for terrestrial worlds, then aramis is spot-on.

 

[JAFARR]

ADF is definitely not a "hard" si-fi person, but he is a great story teller. Back to my question. I seem to be getting 2 ideas here. 1. If I use a moon of a gas giant, it can likely be no larger than say Mars. 2. If I use a captured planet with a slightly larger, orbit than a gas giant it can be larger, but it should have an orbital velocity that keeps it in a trojan position with respect to the GG. That way I could have a stable orbit and have my world say 90% the size of earth or even earth sized. Am I understanding what has been said correctly?

 

[Agemegos]

Thanks for that link to the Roche limit, Boomslang. A while back, I had an adventure idea based on the premise of a region of 'negative gravity' on the surface of a moon, inside which the primary planet 'lifted' things from the moon's surface.

Things like air, water, soil, rocks, continental plates. . . .

You are aware, I hope, that planetary material has zero tensile strength.

 

[BillDowns]

Back to the Question

IIRC, in the last year there was a report about an Earth-sized "moon" discovered around a GG in another star system. I'll see if I can find that, but I beleive it astounded all the experts. It was also close to, if not actually in, the habitable zone of the star.

Anyone else recall that, or am I hallucinating.

 

[BillDowns]

Sorry - Wrong details remembered

I was completely mistaken. I was incorrectly remembering Gliese 581c, but it is a planet not a moon.

Sorry - one of the problems of getting over 50

 

[JAFARR]

I was completely mistaken. I was incorrectly remembering Gliese 581c, but it is a planet not a moon.

Sorry - one of the problems of getting over 50

What do you mean - over 50? I was doing that in my teens, my twenties, ... you get the picture. Can't remember back to my pre - 10 years. Maybe it is linked to higher IQs instead of age.

 

[Icosahedron]

Things like air, water, soil, rocks, continental plates. . . .

You are aware, I hope, that planetary material has zero tensile strength.

It does?

I'm aware that it would be a vacuum world, and depending on how long it has been in that position and tidally locked, it may have no liquids or regolith, but I was under the impression that the solid bedrock of the planet would have sufficient tensile strength to withstand a small tidal force.

 

[Agemegos]

It does?

I'm aware that it would be a vacuum world, and depending on how long it has been in that position and tidally locked, it may have no liquids or regolith, but I was under the impression that the solid bedrock of the planet would have sufficient tensile strength to withstand a small tidal force.

No, I'm afraid not. On geological scales and timescales the mantle material is a liquid.

 

[Anthony]

I was under the impression that the solid bedrock of the planet would have sufficient tensile strength to withstand a small tidal force.

That kind of depends on your definition of 'small', but generally no. Structural strength varies with the second power of radius. Gravitational self-attraction and tides both vary with the fourth power of radius. Generally speaking, the gravity holding an object together is stronger than its raw material strength for objects in the 5-10 kilometer range, though for something exotic like a gigantic diamond it could be a few hundred kilometers. Thus, by the time you've got the equivalent of a size-1 planet, gravity is many thousands of times stronger than material bonds.

There's a reason all large objects are spheres. It's because gravity exceeds the compressive strength of the material the object is made of. Rock has lousy tensile strength compared to its compressive strength.

 

[Icosahedron]

Ok, you both seem to be confident of your data, and admittedly I'm not, so I'll take your word for it rather than spending loads of time potentially proving myself wrong.

So what are you describing, then?

IF a moon were placed such that a portion of its surface experienced more pull from the parent planet than from the moon, yet the moon had a stable orbit (assuming this configuration is possible at all) what would be the effects on the surface? You seem to be implying a 'Mr Whippy' ice cream effect, where the material of the planet would be extruded away in a mega-mountain and lost to the parent over the eons. Is that correct?

I imagine a huge bump like that would destabilise the orbit, so maybe this moon-planet configuration is just not possible at all?

No great loss, I just fancied a world where you got lighter and lighter as you walked/drove, and eventually started to float away.

 

[Anthony]

IF a moon were placed such that a portion of its surface experienced more pull from the parent planet than from the moon, yet the moon had a stable orbit (assuming this configuration is possible at all) what would be the effects on the surface?

I assume you mean that the tidal force exceeds the surface gravity. The gravity of the primary fairly regularly exceeds the surface gravity of orbiting objects; objects fall apart when the tidal force (rate of change of gravity) of the primary exceeds the rate of change of gravity (with respect to radius) of the moon.

You seem to be implying a 'Mr Whippy' ice cream effect, where the material of the planet would be extruded away in a mega-mountain and lost to the parent over the eons. Is that correct?

No. What you're describing is the Roche Limit, and a moon that comes inside of the Roche Limit will break apart very rapidly (I haven't done the math for exactly how fast, but I suspect it's less than a single orbital period).

 

[Icosahedron]

I'd understood from previous posts in this thread and elsewhere, that the Roche Limit was what defined the superiority of the primary's tidal force, and that the Roche limit varied depending on the material of the moon, such that a rocky moon would survive at a closer orbit than a liquid or gaseous body, and hence there might be a specific orbit at which gases, liquids, and free regolith (including unwary travellers) might be drawn from the surface, but the bedrock, having greater tensile strength, would remain.

Are you telling me that no such orbit can exist?

 

[aramis]

I'd understood from previous posts in this thread and elsewhere, that the Roche Limit was what defined the superiority of the primary's tidal force, and that the Roche limit varied depending on the material of the moon, such that a rocky moon would survive at a closer orbit than a liquid or gaseous body, and hence there might be a specific orbit at which gases, liquids, and free regolith (including unwary travellers) might be drawn from the surface, but the bedrock, having greater tensile strength, would remain.

Are you telling me that no such orbit can exist?

Not long term. Maybe for a few years at perigee.

 

[Anthony]

I'd understood from previous posts in this thread and elsewhere, that the Roche Limit was what defined the superiority of the primary's tidal force, and that the Roche limit varied depending on the material of the moon.

It does, but not in the way you think. The Roche limit varies depending on the density of the moon -- since rocky moons are denser, they can get slightly closer (not very much closer; it depends on the cube root of density). This is because a denser object has stronger gravity at any given distance from the center, and the roche limit occurs when tidal force beats gravity.

 

[Icosahedron]

Hmm, interesting stuff, I'll check it out someday. I might never have used the world anyway.
Thanks for the info.