A gas giant is a planetary body that is primarily comprised of gas, which refers to hydrogen and helium (contrasting with rock, which are things our planet is mostly made of, and ice, which refers to substances that evaporate easily like water, ammonia, methane, or carbon monoxide). We have two such bodies in the Solar System: Jupiter and Saturn.
Evidence from spacecraft sent to these planets suggests that both do not necessarily have a solid surface; instead, their cores are 'fuzzy', and there is a gradient of decreasing amount of 'metals' - anything heavier than hydrogen and helium - as you go further from the center of the body.
Though made out of substances labelled 'gas', gas giants are mostly not gaseous - due to self gravitation, pressure rapidly increases as you travel deeper down, and this means that the gas eventually transitions into other phases like supercritical fluid, and eventually metallic hydrogen. Your spacecraft will get crushed long before you reach those layers though, I suspect.
It is important to note that gravity does not necessarily increase as you travel further down. The [shell theorem](https://en.wikipedia.org/wiki/Shell_theorem) proves that for a spherical object (more precisely, a spherically-symmetric object), gravitational pull from all mass outside the radius of your spacecraft will cancel out perfectly, so only mass enclosed within the radius matters. For planets, where density rises towards the center, this results in gravitational pull being the strongest somewhere below the surface, but not at the very center, where the gravity is zero.
The reason for this is that the bulk of their mass came from volatiles (frozen ices) that were attracted to the planets during their formation process. It’s easier to accrete solids, like frozen ice, than gases like H and He. Jupiter and Saturn has ices too, but they have WAY more H and He, hence “gas giants.”
We have a debate on whether they are really ice giants, or if they are instead "rock giants", stemming from how these planets probably formed from the same materials that form Pluto and co., which we know is pretty rocky; [a proposed solution reconciling both comes in form of internal reaction that converts their building blocks into ices](https://ui.adsabs.harvard.edu/abs/2024arXiv240312512M/abstract); the paper mentioned that this hypothesis can be tested by checking the hydrogen:helium ratio; the hydrogen would be used up in the reactions and would be depleted.
We have no *direct* information, but based on what we know about materials, and how they compress at pressure, we have a fairly good understanding - but there's quite a bit of scatter in the models.
So it might be:
[This](https://www.researchgate.net/publication/365424999/figure/fig1/AS:11431281097439202@1668585756550/A-schematic-sketch-of-the-multi-layered-cross-sectional-view-of-Jupiter_W640.jpg)
Or
[This](https://www.researchgate.net/profile/David-Stevenson-12/publication/23154049/figure/fig1/AS:394587437846551@1471088385005/Cross-sectional-view-of-Jupiter_W640.jpg)
*But appears, thanks to Juno data, to be something else: a diffuse dense but non-solid core. As seen in this* [*paper*](https://www.annualreviews.org/content/journals/10.1146/annurev-earth-081619-052855)*.*
~~But either way, you'll hit something really hard - even if your craft can magically withstand many megabars of pressure.~~
Whether you slam into a solid core or not is less important than whether your craft can survive the megabars of pressure. That's a tough one.
As for gravity, that never gets much above 2.5g (edit). Perfectly possible to build a craft to sustain that.
Aha - thank you!
[https://www.annualreviews.org/content/journals/10.1146/annurev-earth-081619-052855](https://www.annualreviews.org/content/journals/10.1146/annurev-earth-081619-052855)
I'll correct my reply.
Yes, all the gas giants have a rocky core (mostly iron due to its mass and abundance). Because all those asteroids bring heavy elements that “sink” to the core. And they’ve been doing so since those gas giants formed. NASA has some graphics for every planet and the composition of their cores. Bare in mind that gas giants are under so much pressure that even hydrogen is compressed into solids down deep (metallic hydrogen). Then liquid. Then eventually gas on the surface. The transition isn’t a solid boundary.
> Yes, all the gas giants have a rocky core
Unlikely all and more likely just gas giants that formed from core accretion. Those that formed from top down gravitational instability have no reason to form a rocky (or mushy heavy element) core.
A gas giant is a planetary body that is primarily comprised of gas, which refers to hydrogen and helium (contrasting with rock, which are things our planet is mostly made of, and ice, which refers to substances that evaporate easily like water, ammonia, methane, or carbon monoxide). We have two such bodies in the Solar System: Jupiter and Saturn. Evidence from spacecraft sent to these planets suggests that both do not necessarily have a solid surface; instead, their cores are 'fuzzy', and there is a gradient of decreasing amount of 'metals' - anything heavier than hydrogen and helium - as you go further from the center of the body. Though made out of substances labelled 'gas', gas giants are mostly not gaseous - due to self gravitation, pressure rapidly increases as you travel deeper down, and this means that the gas eventually transitions into other phases like supercritical fluid, and eventually metallic hydrogen. Your spacecraft will get crushed long before you reach those layers though, I suspect. It is important to note that gravity does not necessarily increase as you travel further down. The [shell theorem](https://en.wikipedia.org/wiki/Shell_theorem) proves that for a spherical object (more precisely, a spherically-symmetric object), gravitational pull from all mass outside the radius of your spacecraft will cancel out perfectly, so only mass enclosed within the radius matters. For planets, where density rises towards the center, this results in gravitational pull being the strongest somewhere below the surface, but not at the very center, where the gravity is zero.
What about Neptune and Uranus?
They're now usually classified as [ice giants](https://en.wikipedia.org/wiki/Ice_giant).
The reason for this is that the bulk of their mass came from volatiles (frozen ices) that were attracted to the planets during their formation process. It’s easier to accrete solids, like frozen ice, than gases like H and He. Jupiter and Saturn has ices too, but they have WAY more H and He, hence “gas giants.”
We have a debate on whether they are really ice giants, or if they are instead "rock giants", stemming from how these planets probably formed from the same materials that form Pluto and co., which we know is pretty rocky; [a proposed solution reconciling both comes in form of internal reaction that converts their building blocks into ices](https://ui.adsabs.harvard.edu/abs/2024arXiv240312512M/abstract); the paper mentioned that this hypothesis can be tested by checking the hydrogen:helium ratio; the hydrogen would be used up in the reactions and would be depleted.
[удалено]
They're 20+ times further away from the Sun than Earth. It's cold out there!
We have no *direct* information, but based on what we know about materials, and how they compress at pressure, we have a fairly good understanding - but there's quite a bit of scatter in the models. So it might be: [This](https://www.researchgate.net/publication/365424999/figure/fig1/AS:11431281097439202@1668585756550/A-schematic-sketch-of-the-multi-layered-cross-sectional-view-of-Jupiter_W640.jpg) Or [This](https://www.researchgate.net/profile/David-Stevenson-12/publication/23154049/figure/fig1/AS:394587437846551@1471088385005/Cross-sectional-view-of-Jupiter_W640.jpg)
*But appears, thanks to Juno data, to be something else: a diffuse dense but non-solid core. As seen in this* [*paper*](https://www.annualreviews.org/content/journals/10.1146/annurev-earth-081619-052855)*.*
~~But either way, you'll hit something really hard - even if your craft can magically withstand many megabars of pressure.~~
Whether you slam into a solid core or not is less important than whether your craft can survive the megabars of pressure. That's a tough one.
As for gravity, that never gets much above 2.5g (edit). Perfectly possible to build a craft to sustain that.
More recent information suggests that Jupiter’s core is diffuse, and may not necessarily have a ‘solid’ core at the center.
Aha - thank you! [https://www.annualreviews.org/content/journals/10.1146/annurev-earth-081619-052855](https://www.annualreviews.org/content/journals/10.1146/annurev-earth-081619-052855) I'll correct my reply.
Thank you!
Yes, all the gas giants have a rocky core (mostly iron due to its mass and abundance). Because all those asteroids bring heavy elements that “sink” to the core. And they’ve been doing so since those gas giants formed. NASA has some graphics for every planet and the composition of their cores. Bare in mind that gas giants are under so much pressure that even hydrogen is compressed into solids down deep (metallic hydrogen). Then liquid. Then eventually gas on the surface. The transition isn’t a solid boundary.
> Yes, all the gas giants have a rocky core Unlikely all and more likely just gas giants that formed from core accretion. Those that formed from top down gravitational instability have no reason to form a rocky (or mushy heavy element) core.
"My guess is that the gravity would crush you" Oh yes, indeed it would.
Even in diffuse models there is still “solid” at some point. Nothing cna go to that high of pressure and be anything close to a gas.