What is difference between platenary core, mantle & crust?

[swampwiz]

There seems to be a standard way of looking at the composition of planets, and am curious as to why one region is called the core and then something above that is the mantle, and then finally the crust.

It seems that the core is metallic, but what exactly defines the mantle & crust. I was reading that some outer planets (or perhaps their moons) have a mantles made of ice, whereas the Earth's mantle is rock (liquid rock?)

I can't seem to get a straight consistent answer on this.

 

[jedishrfu]

The mantle core, mantle and crust are defined by different densities of material.

https://en.m.wikipedia.org/wiki/Mantle_(geology)

Even within these three layers they distinguish between outer and inner layers to give even greater stratification ie outer and inner core, outer and inner mantle...

 

[stefan r]

There seems to be a standard way of looking at the composition of planets, and am curious as to why one region is called the core and then something above that is the mantle, and then finally the crust.

It seems that the core is metallic, but what exactly defines the mantle & crust. I was reading that some outer planets (or perhaps their moons) have a mantles made of ice, whereas the Earth's mantle is rock (liquid rock?)

I can't seem to get a straight consistent answer on this.

There is a phase boundary between the regions. Earth quakes send a wave through earth. Waves change direction when passing through phase boundaries. We have seismographs stationed around the globe and measure the time. From that information geologists calculate the size of the core. They also use upper and lower mantel because the waves travel at different velocities.

Earth's crust is a solid phase, the mantle is liquid. Earth also has an atmosphere and oceans.

Astronomers use some category words. "metals" is everything except hydrogen and helium. Sulfur is a "metal". "Ices" can include things like solid methane or ammonia. The object has a "core" and "mantle" if material separated. The core is the high density stuff in the middle the mantel is the stuff around the core.

One region is called the "core" because that region is inside of the "mantle". The solid surface is called a crust because it is there and because we can see it.

 

[swampwiz]

OK, so astronomers use the term "core" for the innermost region that has a density on the order of iron, which is very plentiful in the universe because it has the lowest binding energy and therefore is the utmost result of exothermic nuclear reaction. and "mantle" for the region that has a density on the order of a non-metallic mineral (which technically water, carbon dioxide, methane, etc. is when in the solid phase). Of course, that makes it sound like astronomers/geologists just assigned the words "core" & "mantle" to the 2 innermost regions - and begs the question that if it were ever to be discovered that a planet had 3 regions of different density (i.e., not counting the very outer region, the crust), they would have to assign another name to the 3rd region. And if the mantle is liquid, then the top of it is the crust, just like slag at a metal foundry. But if the mantle is an ice, then how could there be a crust on top? That doesn't make sense.

 

[Drakkith]

Of course, that makes it sound like astronomers/geologists just assigned the words "core" & "mantle" to the 2 innermost regions - and begs the question that if it were ever to be discovered that a planet had 3 regions of different density (i.e., not counting the very outer region, the crust), they would have to assign another name to the 3rd region.

We already do that to the Earth. The Earth's core and mantle are both subdivided into different regions. The choice of how to define these different regions is arbitrary, meaning that we can choose to define them in whatever way we want. So we typically choose to define them in ways that are convenient and meaningful, and this means that these definitions can change over time as we learn more about the planets and other bodies in the solar system and beyond. In addition, "convenient" and "meaningful" mean different things to different people. A person designing a geology illustration for a middle school or high school textbook isn't going to think that having a dozen or more different layers is either of those since that would put his illustration well over the heads of his intended audience.

In the end, who cares if we call these layers "core" and "mantle" vs "inner core", "outer core", and "mantle", or something completely different? Scientists will not be confused just because we choose to have two named layers instead of three.

And if the mantle is liquid, then the top of it is the crust, just like slag at a metal foundry. But if the mantle is an ice, then how could there be a crust on top? That doesn't make sense.

I haven't looked too much into this topic, but from the little bit that I've read it appears that geological processes are quite complicated and can form things you wouldn't expect. If I can find anything further on ice mantles I'll link it to you.

 

[stefan r]

OK, so astronomers use the term "core" for the innermost region that has a density on the order of iron, which is very plentiful in the universe because it has the lowest binding energy and therefore is the utmost result of exothermic nuclear reaction. and "mantle" for the region that has a density on the order of a non-metallic mineral (which technically water, carbon dioxide, methane, etc. is when in the solid phase). Of course, that makes it sound like astronomers/geologists just assigned the words "core" & "mantle" to the 2 innermost regions - and begs the question that if it were ever to be discovered that a planet had 3 regions of different density (i.e., not counting the very outer region, the crust), they would have to assign another name to the 3rd region. And if the mantle is liquid, then the top of it is the crust, just like slag at a metal foundry. But if the mantle is an ice, then how could there be a crust on top? That doesn't make sense.

Would be nice to get a comment from a geologist (areologist, planetary scientist) Websters has this:

e :the central part of a celestial body (such as the Earth or sun) usually having different physical properties from the surrounding parts

I did not have the impression that the core needs to be as dense as iron. A planet could have CO2 separate from methane and ammonia. It is just that the outer and inner regions have separated. The asteroid Vesta has a core and mantle even though it is smaller than planets are.

I think we already ran into the problem of finding extra layers. Earth has "inner core", "outer core", "inner mantle" and "outer mantel". They add a few other words too.

And if the mantle is liquid, then the top of it is the crust, just like slag at a metal foundry. But if the mantle is an ice, then how could there be a crust on top? That doesn't make sense.

You could have solid water ice all the way down. I went hiking on a lake in Madison Wisconsin a few times. I did not think of the snow as part of the lake. The white stuff was a crust which was sitting on the frozen lake. Glacier ice is different. Glaciers have strata and move. At higher densities water will form alternate crystal structures. If there is no (or low pressure) atmosphere then crustal water can also be an amorphous solid. The mantel of an solid ice ball planet could not have a crack or pit. The crust would have lots of pits from meteor impacts and the cracks/pits would fill in with frost.

I am confident that there are no planets that are pure water ice. If the core has some water ice it will be carbonated brine with lots of suspended dust/gravel. The mantel will have more of the light weight dissolved volatile gasses and fewer minerals.