Juno mission to Jupiter

LordOfWizards

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#1
It is there right now. Collecting Data and taking photos.

Where is Juno?

NASA’s Juno mission to Jupiter, which has been in orbit around the gas giant since July 4, 2016, will remain in its current 53-day orbit for the remainder of the mission. This will allow Juno to accomplish its science goals, while avoiding the risk of a previously-planned engine firing that would have reduced the spacecraft’s orbital period to 14 days.

There is a cool application that you can download and fly along with the perspective of the satellite. Here: Juno's Eyes
 

Venusian Broon

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#3
I was going to berate the journalist for writing: "The mission has also produced a partial answer to the question of whether the planet has a core, showing that the inner 96% of the planet rotates “as a solid body”, even though technically it is composed of an extraordinarily dense mixture of hydrogen and helium gas."

I mean there is such a thing as solid-liquid-gas phase transitions for all matter (and we have made solid hydrogen in the lab)

But the scientist probably didn't help by saying: “There may be a small hard [solid] core very, very deep, but we’re thinking it’s just dense gas enriched in heavy elements … it’s not a solid that you can imagine,” said Kaspi. “The normal concept of gas, liquid and solid don’t really hold at these pressures.”

Possibly I think he's saying, "We have no idea what sort of atomic structure a solid hydrogen/helium core would have at these enormous pressures, 'cause we can't recreate such pressures on Earth!" :p
 

Brian G Turner

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I hate to ask a stupid question in public, but something that confuses me: in the BBC article, when talking about Jupiter's core, it states:

the predominant gases of hydrogen and helium start to transition under immense pressure into exotic fluid states
However, basic physics says that gravity is strongest at the surface of a sphere, and weakest at its core:



In which case, rather than the core being crushed by the force of gravity on Jupiter's mass ... wouldn't the core actually be a low pressure area, with its mass attracted to the surface?

Am I just having a brainfart moment? :)
 
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LordOfWizards

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I think it's partially my fault for using that graph. ;) I think that it just represents the amount of gravity you would experience at that radius. I did some cursory research and the pressure in the core of any planet is extremely high due to all of the mass above it.

I have the same confusion around cold front vs. high pressure hot air mass. Cold air is low pressure but dense (more atoms per unit volume). High pressure air is warmer but high pressure due to atoms flying around faster. :)
 

Brian G Turner

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I think that it just represents the amount of gravity you would experience at that radius.
I think that's what got me confused. Let's try this: the graph simply shows that the smaller mass at the centre generates a smaller gravitational field than the greater mass enclosed by the surface of a sphere.
 

Venusian Broon

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Firstly gravitational force and pressure are related but different concepts.

Looking at gravity inside a planet. If we just look at an object like Jupiter and ignore any interactions with other bodies (such as the Sun) the reason the gravitational force drops with depth can be conceptually seen quite simply. Any mass in the centre of Jupiter is attracted by the rest of the mass of the planet but because this mass is in an even shell all around it, all the various gravitational forces, thus also being even, cancel themselves out. The mass in the very centre is not attracted to any other part of the planet gravitationally! (It will be attracted to the Sun of course, the whole planet needs to orbit that :D)

For some reason this has really stuck in my head from first year Physics, Gauss's law (wonderful thing!) and all that.

Now pressure is defined as "force applied perpendicular to the surface of an object per unit area over which that force is distributed." Or you could say, a little bit simpler, it's the total weight of everything above an area. Now in the previous example gravity has not magically switched off, it's just on a point by point basis decreased due to the effects of large masses cancelling each other out. In the case of calculating pressures, if you were to pick an arbitrary square metre area inside Jupiter, you will see the large force of all the weight above it that is pushing in that perpendicular direction.

I suppose the way to think about it is that a huge mass will form a sphere and instigate powerful attractive forces inward symmetrically, hence all the mass in the centre will want to be compressed together. However the deeper you go, the less the gravitational force becomes so matter close to the centre has less impact on pressure than mass on the surface! (And calculating pressure with depth can be quite involved. :p)
 

LordOfWizards

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I think that's what got me confused. Let's try this: the graph simply shows that the smaller mass at the centre generates a smaller gravitational field than the greater mass enclosed by the surface of a sphere.
This is correct. I'll just add what makes it easier for me to understand it: If you have a smaller planet (or moon even) then there is less gravity at the surface than a bigger one. So I picture a smaller sphere inside a bigger sphere. VB has explained the difference between gravity and pressure quite well. Gravity is what causes the pressure, but there is less pressure at the surface and more pressure in the middle. So pressure (inside a planet) seems to be inversely related to gravity. Strange but true.
 

Venusian Broon

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#10
I knew I was having a brain fart moment. :)
Not at all! I do feel it's good to ask questions, because sometimes the answer isn't really obvious. (I had to think about it a lot, just to make sure my answer was reasonable!)

In this case, this question does seem to come up a lot, as I can see why from a 'common sense' viewpoint it feels like a zero-g environment should be really be low pressure :D
 

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