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July 29, 2011 8:47 AM   Subscribe

Does Saturn have a solid core?

Ok, this is a friendly, but ongoing argument my SO and I have been having. I contend that Saturn must have some sort of rocky core because what else kept all the gases in place to create the planet? I will accept that the core may be in a molten state, but I think there must be something other than just a big ball of gas. My SO disagrees. :) He insists that it is a gas planet and that there are different types of gases and some strong enough to create a sort of gravity field that holds it all together.

As you can see, neither of us are scientists! And although I concede that he probably knows more about this type of thing than I do, I still think I could be right. But although I thought this would be easy enough to confirm from actual scientific data online, the only thing I've found so far is this from the Wikipedia posting: The interior of Saturn is probably composed of a core of iron, nickel, silicon and oxygen compounds. Of course, my SO argues that this is one person's theory on Wikipedia, which doesn't count; and says that even if that is true, it could be the gas form of those compounds.

Surely there is a mefite with astronomical knowledge that could decide this argument for us. So which is it? Does Saturn have some type of rocky core or is it entirely made of gas?
posted by Eicats to Science & Nature (21 answers total) 1 user marked this as a favorite
 
Gas giants may or may not have rocky cores... Jupiter, for example, is thought to lack one but deep down there its gasses are compressed so much that the core may be liquid metallic hydrogen. Saturn's is probably the same, but there is currently no way to be sure.
posted by Guy_Inamonkeysuit at 8:50 AM on July 29, 2011 [1 favorite]


You're right.
posted by supercres at 8:50 AM on July 29, 2011


No one is sure, but if you would google this you would quickly find supercres' link, as well as other that have support for a small core for Saturn. We won't know for sure until we can use higher intensity radar on it to see for any solid returns.
posted by Horselover Phattie at 8:53 AM on July 29, 2011


... what else kept all the gases in place to create the planet?

A planet-sized chunk of gas would have the same gravitational pull as a planet-sized chunk of rock.

It's like that "which is heavier, a pound of feathers or a pound of rocks" thing.
posted by odinsdream at 9:00 AM on July 29, 2011 [2 favorites]


Also, the bit about Saturn's core on Wikipedia isn't just a random Wikipedia editor's theory, it's cited from a credible source.
posted by Horselover Phattie at 9:00 AM on July 29, 2011 [1 favorite]


Well, I guess my google-fu failed me, but I am happily enjoying supercres's success!

(And yes, I know that we won't know for SURE until our technology advances enough, but I'm looking for best probability here.)
posted by Eicats at 9:01 AM on July 29, 2011


I realize my comment can be misread by the sufficiently pedantic. I meant that equal masses of gas and solid would have equal gravitational pull.
posted by odinsdream at 9:02 AM on July 29, 2011 [1 favorite]


... what else kept all the gases in place to create the planet?

Well, think of clouds -- they don't have a binding solid part, but have a defined shape based on the forces around them. Temperature, moisture, and pressure control a cloud's shape, and gravity is what keeps Saturn's shape. If a cloud doesn't have enough moisture, is too warm or too cool, or the air pressure changes, it dissipates -- but as long as conditions are right, boom, you have a cloud at that spot. There's no nearby planet's gravity to pull away Saturn's gases, there's enough gases to create gravity around a common center, and it's not too hot or too cold to boil away or condense the gases that are there.

In fact, the entire planet of Saturn is less dense than water, as a whole, even if it did have a solid core. The density of an object doesn't give it gravity; its mass does. Saturn is massive enough to hold itself together, at the center of a space in Space where there aren't forces to pull it apart.

I'm not saying one answer is right or the other, but just that it's possible to have a planet without a solid core.
posted by AzraelBrown at 9:11 AM on July 29, 2011 [1 favorite]


A planet-sized chunk of gas would have the same gravitational pull as a planet-sized chunk of rock.

It's like that "which is heavier, a pound of feathers or a pound of rocks" thing.


Umm...Not really. Gravity is dependent on mass, not volume. A ball of rock will have a much greater gravitational pull than a ball of gas the same size, because it will have more mass.

But, a ball of gas still does have a gravitational pull, which is why Saturn may still not have a solid core.
posted by zachawry at 9:26 AM on July 29, 2011


zachawry; see my other comments where I clarify.
posted by odinsdream at 9:28 AM on July 29, 2011


Of course, my SO argues that this is one person's theory on Wikipedia, which doesn't count; and says that even if that is true, it could be the gas form of those compounds.
Oh, you should definitely dump him. What, this isn't that kind of question?

Okay seriously though, one nice thing about wikipedia is that most of the facts are sourced. That particular source is citation #15 which is this article on "The Astrophysics Spectator"

Here is the "About the Author" on that site:
I am Dr. Jerome James Brainerd, a professional astrophysicist who is working on theoretical problems in high-energy astrophysics. I have a B.S. in Physics from the University of Notre Dame and a Ph.D. in Astronomy from Harvard University. My primary research is on the radiative processes responsible for the light we see from x-ray pulsars and gamma-ray bursts. This research is published in the Astrophysical Journal and in numerous conference proceedings. My strongest interest, however, is in the philosophy of science, and I am particularly preoccupied with how the limits of human knowledge constrain our ability to understand the universe.
Wikipedia is actually a really, really good source when it comes to things that are scientific and uncontroversial, like what makes up the core of Saturn. The idea that someone would just post "one person's theory" about what makes up the core on Wikipedia and it would stay there is pretty unrealistic.

The other thing is the thing, and this is important is the metallic hydrogen in the core. Metallic hydrogen is a solid. It's a metal just like lithium or sodium, and it's a very powerful electrical conductor. We know for a fact that hydrogen turns into a metal at a high pressure, and we know, based on the mass, that the core of Saturn must have a pressure great enough to turn Hydrogen into a metal, and of course that Saturn has hydrogen.

So we now for sure that Saturn has a core of metallic hydrogen (around the nickle/iron sub-core). The hydrogen metal is what gives Saturn and Jupiter their strong magnetic fields.
posted by delmoi at 9:56 AM on July 29, 2011


In fact, the entire planet of Saturn is less dense than water, as a whole, even if it did have a solid core.
So is a solid block of lithium, and I would assume hydrogen metal as well.

Also, most of the pages about this from real scientists say that Saturn formed around a rocky core.
posted by delmoi at 10:07 AM on July 29, 2011


interesting tidbit: Jupiter, and quite likely Saturn, probably have Uranium-feuled nuclear reactors in their cores...
posted by sexyrobot at 10:15 AM on July 29, 2011


That much matter is bound to include some solids, but even if the planets somehow formed without a solid core, collisions with asteroids, comets, etc. would have added some solid material, which would naturally sink to the center because of its density.
posted by shponglespore at 10:20 AM on July 29, 2011 [1 favorite]


also, from this page...

The gas giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the Jovian planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements.[10] Planetesimals beyond the frost line accumulated up to four Earth masses within about 3 million years.[29] Today, the four gas giants comprise just under 99% of all the mass orbiting the Sun.[note 2] Theorists believe it is no accident that Jupiter lies just beyond the frost line. Because the frost line accumulated large amounts of water via evaporation from infalling icy material, it created a region of lower pressure that increased the speed of orbiting dust particles and halted their motion toward the Sun. In effect, the frost line acted as a barrier that caused material to accumulate rapidly at ~5 AU from the Sun. This excess material coalesced into a large embryo of about 10 Earth masses, which then began to grow rapidly by swallowing hydrogen from the surrounding disc, reaching 150 Earth masses in only another 1000 years and finally topping out at 318 Earth masses. Saturn may owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was less gas available to consume.[29]
posted by sexyrobot at 10:20 AM on July 29, 2011


I will accept that the core may be in a molten state, but I think there must be something other than just a big ball of gas. My SO disagrees. :) He insists that it is a gas planet and that there are different types of gases and some strong enough to create a sort of gravity field that holds it all together.

Well, you're both wrong! I mean that in the nicest way; this is a fun question because it seems simple but the details are actually quite complex. But you both have some of the right ideas. Gasses are perfectly capable of being gravitationally bound systems, regardless of their chemical composition. An easy, indisputable example is the sun, which definitely has no solid core even though it's made almost entirely of hydrogen, the lightest element. So your SO's reasoning is correct, but may not be precisely applicable to this situation.

Your intuition that the planet must have formed around a solid object is probably correct. We don't really know much about planet formation right now, but it's pretty clear that the initial seeds for planet growth had to be some sort of dusty/rocky/icy planetesimals, which would suggest that the planet must have a rocky core. This formation mechanism is in contrast to stars, which do not need solid "seeds" and can collapse under their own gravity while they're completely gaseous. The only problem is that the composition of the planet can change a lot over time; and whatever initial planetesimal there was could have been turned to gas by now (or not, we don't know).

So, bottom line, you each get half credit for being half right with your reasoning, but I honestly don't think we know enough to say whether or not there is any solid material at the center of Saturn. There are definitely heavy elements, and there's definitely weird stuff like metallic hydrogen, but I don't think that's enough to say it must be solid.

Side note: there terminology in this question can be misleading. Most of the time in the scientific literature when you see "core", they mean a region in the center with a different chemical composition than outer layers. This does not say anything about whether the core is solid or not. These cores tend to be rich in heavy elements (iron, silicon, oxygen), but that is the result of heavier elements "sinking" to the bottom in a gas, and doesn't mean that the core is necessarily solid. So use caution when attempting to adjudicate this debate.

This summary from Science goes into detail on some of the unknown aspects of Saturn and Jupiter, though it might take some work to decode what they're saying. The gas giants are fascinating planets.

Next side note:

Metallic hydrogen is a solid.

Not true; in Jupiter metallic hydrogen is a liquid. But it's kind of over-simplifying the issue to try and classify materials at such high temperatures and pressures as liquids or solids, since the atoms interact with each other in completely different ways than they do at room temperature. Cite 1, Cite 2. (Both of those are sort of tangential, but the last section of #2 discusses Jupiter specifically.)
posted by kiltedtaco at 10:47 AM on July 29, 2011 [1 favorite]


I will accept that the core may be in a molten state, but I think there must be something other than just a big ball of gas.

The Sun, I rest my case.
posted by DU at 11:23 AM on July 29, 2011


I'm glad I posted this because I am enjoying seeing how the debate is continuing here with a lot of the same counter arguments. (SO already haughtily- get it?- used the sun as support of the gas only theory. It's a strong point... but still doesn't disprove my side.) I hesitate to mark a "best answer", at least not yet, because I don't want to pick ones that just support my argument. :) I appreciate all the responses-- please continue!

It does seem like the concensus is supporting my theory...
posted by Eicats at 11:35 AM on July 29, 2011


The answer is that Saturn (and Jupiter) have cores made of material denser than hydrogen and helium, but they are not really "solid."

There is some debate over how Saturn (and Jupiter) formed. There are two possibilities:

1) The core accretion model: A core of solid materials (rock, metal, and ices) built up, and when its mass was large enough, it had a strong enough gravitational pull to hold on to hydrogen and helium gas that filled the forming solar system. In this case, these silicates and metals are still certainly part of Saturn. They are more dense than hydrogen gas, so they are down at the center in the planet.

2) The disk instability model: In some places in the solar system (including the center, where the Sun formed, obviously) the hydrogen gas was dense enough to for gravity to hold together under its own weight. Jupiter and Saturn may have formed this way; even if they did, Uranus and Neptune probably formed by core accretion.

Even in the disk-instability case, however, Jupiter and Saturn would still have accreted rock, metal, and ices. The early solar system was full of rocks and chunk of ice, and a very large amount of these would have impacted on the gas giants due to their strong gravitational pull. (Remember Shoemaker-Levy 9? It's safe to say that events like that happened a few trillion times over the history of the Solar System.) Again, because they are denser than the hydrogen gas, these impactor materials would have sunk to the core.

As kiltedtaco says, under the intense heats and pressures of a gas giant interior, most materials (including metallic hydrogen) are fluids (I won't go so far as to say liquids because that refers to specifically the phase between gas and solid, while fluids can be gasses, liquids, or weird states between.) because things under those conditions behave in strange ways. It is incorrect to imagine that there is a gaseous atmosphere, and under that a liquid "ocean", and under that a solid core. It's more a case that as you descend through the planet's interior, it gets hotter and hotter and denser and denser as you go down, without sharp boundaries like we have on Earth. The outer atmosphere is gaseous and made mostly of hydrogen and helium. The very center more liquid-like than gaslike, and maybe it does behave like a solid in some ways; it is almost certainly made of heavier elements than hydrogen and helium.

Nonetheless, as DU says, you don't need a non-hydrogen core to hold the planet together, as is the case with the Sun.

I have a PhD in astronomy and I am a planetary astronomer. I study Saturn's rings, not Saturn's interior. I'll defer to anyone who has more expertise than me on interiors, but I think what I've given here is a reasonable summary of the consensus (or lack thereof) in the field, and you can certainly confirm what I've said by browsing an up-to-date astronomy textbook.
posted by BrashTech at 12:26 PM on July 29, 2011 [1 favorite]


Well, if it's any consolation, we'll have answers about Jupiter's core soon enough! (Well, if July 2017 is "soon enough"). Allow me to introduce you to Juno!
posted by Grither at 12:44 PM on July 29, 2011


But it's kind of over-simplifying the issue to try and classify materials at such high temperatures and pressures as liquids or solids, since the atoms interact with each other in completely different ways than they do at room temperature.

kiltedtaco totally took the words right out of my mouth. The Solid-Liquid-Gas we all learned in 3rd grade science really doesn't apply at extreme (high or low) temperatures and pressures (like out in Space). Things get all freaky at the molecular level.
posted by Green Eyed Monster at 5:17 PM on July 29, 2011


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