Does Every Rocky Body Have a Core?

[t@nn]

Does every rocky body in the solar system have a rocky/metallic core, a mantle, and a crust? Or are there objects without well-defined cores, or that are nothing but homogenous material throughout? And, if so, how did those coreless objects form?

[Ken G]

Your question is about the process of "differentiation", whereby denser materials sink to the center of a molten body and make a metallic (not rocky) core. But the key word here is molten-- the body has to be large enough to get hot enough to melt at some point (it can have solidified later as it cooled, of course). Large bodies get hot because of gravity and because of radioactivity, and roughly speaking, the "rocks" that are big enough to be spherical (due to gravity) are big enough to have differentiated. The really small asteroids with craggy exteriors would not have cores, and there's kind of a gray area in between that I don't know exactly where you would draw the line for having cores (but all the major planets and big moons do).

[t@nn]

Your question is about the process of "differentiation", whereby denser materials sink to the center of a molten body and make a metallic (not rocky) core. But the key word here is molten-- the body has to be large enough to get hot enough to melt at some point (it can have solidified later as it cooled, of course). Large bodies get hot because of gravity and because of radioactivity, and roughly speaking, the "rocks" that are big enough to be spherical (due to gravity) are big enough to have differentiated. The really small asteroids with craggy exteriors would not have cores, and there's kind of a gray area in between that I don't know exactly where you would draw the line for having cores (but all the major planets and big moons do).

There also the heat left over from formation, and also from impacts.

[hhEb09'1]

there's kind of a gray area in between that I don't know exactly where you would draw the line for having cores (but all the major planets and big moons do).

The core of Luna is tiny, almost nonexistent, right?

[Van Rijn]

The core of Luna is tiny, almost nonexistent, right?

The inner core is thought to be ~500 miles in diameter. Perhaps relatively small, but I wouldn't characterize it as almost nonexistent.

See here, for example:

http://www.britannica.com/eb/article-54212

http://www.britannica.com/eb/art-3478

[cjbirch]

The core of Luna is tiny, almost nonexistent, right?

The moons core is of a very small volume, possibly only a fifth the size of the moon itself thus it would be one hundredth the Moons volume, I would not say however that it was almost nonexistent. The Moons core may have played a significant part in its early development to be deemed that just because it's tiny, it may as well not exist!

[Ken G]

Yes, there are formation details that make anything tricky to state categorically, but the Moon is thought to have a core and I know of no similarly large rocky body that doesn't. Thus the short answer to the OP, which I assume is what is desired, is that large bodies have cores, small ones don't, and in between you need more details about the history of the object.

[t@nn]

Yes, there are formation details that make anything tricky to state categorically, but the Moon is thought to have a core and I know of no similarly large rocky body that doesn't. Thus the short answer to the OP, which I assume is what is desired, is that large bodies have cores, small ones don't, and in between you need more details about the history of the object.

Thats right, and you've been very helpful. I guess it's time to do some research on actual solar system objects.

[Lord Jubjub]

Ceres is thought to have differentiated and its a fraction of Luna's size.

[Kaptain K]

Ceres is thought to have differentiated and its a fraction of Luna's size.

Also, Vesta.

[aurora]

Also, Vesta.

I suspect we'll find, if we ever get a mission to Vesta, that Vesta is a remnant of a once larger asteroid that was broken apart in collision(s).

Vesta may have a large chunk of the core of the original object.

http://en.wikipedia.org/wiki/Vesta_family

[Ken G]

Yes, objects formed by collisions should have different makeups, but the rule still holds that they need to be big to be differentiated, because they need to be big to include both core and mantle of the parent body, and they need to be big to come from a violent enough collision to melt, and big enough for their own gravity to differentiate the molten result. But they don't necessarily have to be big enough to be spherical, there's a gray area in the middle that I don't know the details about. It also seems possible that Vesta-sized objects, or maybe a bit smaller, will not have central cores, and when the term "differentiated" is applied to them, it might refer to the parent body, so be careful about that issue.

[neilzero]

Likely bodies with a mass of 100 trillion tons rarely have a core nor mantle. Even smaller bodies that did a gravity assist maneuver too close to a very hot white dwarf, neutron star and/or a very hot star, might be differentiated, especially, if they suffered multiple collisions in the accreation disk. Very fast collisions produce much more melting. Neil

[Kaptain K]

I suspect we'll find, if we ever get a mission to Vesta, that Vesta is a remnant of a once larger asteroid that was broken apart in collision(s).

That is the general consensus. There are several families of asteroids that have been traced back to Vesta. Some are metallic (core), some are stony (mantle) and some are chondrites (crust).