Gas Giant Problems

[Rdivine]

I have many questions about gas giants.

1. How do astronomers determine their rotational period? There is no "surface" as a frame of reference. Furthermore, the inner clouds of a gas giant rotate faster than the outer clouds of a gas giant, so where does astronomers get rotational periods for jupiter, saturn, uranus and neptune?

2. Where is sea level? How do astronomers determine the sea level of gas giants? There isn't a "sea level" on a gas giant, unless they were referring to the altitude at which the gas becomes a liquid.

3. How do scientists determine their radius? Do they take measurements from the center to the tallest cloud? Or do they measure to the karman line?

4. Why are there no gas moons? Why does the gas always end up at the main planet instead of the moons?

 

 

These are some questions that have serious implications to profound existence and life as we know it.

Just kidding, i was curious

[Steel]

8 minutes ago, Rdivine said:

4. Why are there no gas moons? Why does the gas always end up at the main planet instead of the moons?

Firstly there is a certain radius required to achieve a density high enough to collapse into a gaseous object and many moons are simple too small to have been gaseous. 

Then there's another issue. Because a moon is necessarily quite close to it's parent, tidal gravitational forces would likely tear it apart due to the weak iteration between gas molecules unless it is very far away from the parent body.

[Steel]

26 minutes ago, Rdivine said:

I have many questions about gas giants.

1. How do astronomers determine their rotational period? There is no "surface" as a frame of reference. Furthermore, the inner clouds of a gas giant rotate faster than the outer clouds of a gas giant, so where does astronomers get rotational periods for jupiter, saturn, uranus and neptune?

2. Where is sea level? How do astronomers determine the sea level of gas giants? There isn't a "sea level" on a gas giant, unless they were referring to the altitude at which the gas becomes a liquid.

These can be rolled up into one question: when does the gas stop being the planet's atmosphere?

A quick google suggests this varies even for the giants in our system. For Jupiter, it seems, the "surface" is defined as being the point where atmospheric pressure is 10 times earth's (1MPa). For Uranus it is defined as the lowest point that can be measured by remote sensing, corresponding to a point with a pressure of ~10MPa.

[justidutch]

Quote

Gas Giant Problems

 

And here I thought there was a topic about eating beans for lunch...

[problemecium]

Short answers:
1. They don't. But you can judge rather predictably, say, when the Great Red Spot will go from facing you all the way around and back to facing you. This is useful for scientists, more so than calculating the internal rotation speeds in most cases. Earth actually has the same problem: the core rotates faster than the surface, and the atmosphere moves in all kinds of directions. But the clearly visible surface features move with a fairly predictable period.
2. They don't. Gas giants don't have seas, so there is no sea level. Reference tables indicating sea level are usually referring to the average radius.
3. They don't. My hunch is that typical measurements go to whatever is the first opaque cloud layer, but this actually doesn't matter much, because compared the the size of a giant planet, the difference between radius up to the Karman line, up to "sea" level, etc. is very very small. Even a hundred miles, compared to an object 20,000 miles across, is only half a percent.
4. Our current models predict that gas moons are entirely possible, but our Solar system simply happens to not have any. We likewise don't seem to have any super-Earths or hot Jupiters, but other systems somewhere surely do.

[fredinno]

56 minutes ago, parameciumkid said:

Short answers:
1. They don't. But you can judge rather predictably, say, when the Great Red Spot will go from facing you all the way around and back to facing you. This is useful for scientists, more so than calculating the internal rotation speeds in most cases. Earth actually has the same problem: the core rotates faster than the surface, and the atmosphere moves in all kinds of directions. But the clearly visible surface features move with a fairly predictable period.
2. They don't. Gas giants don't have seas, so there is no sea level. Reference tables indicating sea level are usually referring to the average radius.
3. They don't. My hunch is that typical measurements go to whatever is the first opaque cloud layer, but this actually doesn't matter much, because compared the the size of a giant planet, the difference between radius up to the Karman line, up to "sea" level, etc. is very very small. Even a hundred miles, compared to an object 20,000 miles across, is only half a percent.
4. Our current models predict that gas moons are entirely possible, but our Solar system simply happens to not have any. We likewise don't seem to have any super-Earths or hot Jupiters, but other systems somewhere surely do.

Gas moons would likely require a host planet at least a few times larger than Jupiter, for the moons to gain enough mass to absorb so much gas to become a 'gaseous' moon- unless, of course, in the case of binaries, but it would be debatable whether those should be considered moons.

[lajoswinkler]

4 hours ago, Rdivine said:

2. Where is sea level? How do astronomers determine the sea level of gas giants? There isn't a "sea level" on a gas giant, unless they were referring to the altitude at which the gas becomes a liquid.

There isn't such altitude. Hydrogen and helium don't ever become liquid in gas giants. They turn from gas into supercritical fluid. No phase boundary like with out water oceans. It just gets hotter and denser with more depth. Deep enough, the fluid gets more and more electrically conductive.

Of course, that doesn't mean there aren't liquids in gas giants. For example, there could be neon virgas, just like there's sulfuric acid virgas on Venus.

[cantab]

1) The best handle we have on the rotation of a gas planet's interior is from observing its radio emissions, which vary depending on the rotation of the planet's magnetic field which is generated in the deep interior. For both Jupiter and Saturn this defines a respective "System III". The visible clouds undergo differential rotation.

2 and 3) An altitude corresponding to a somewhat arbitrary pressure is chosen.

[YNM]

16 hours ago, Rdivine said:

1. How do astronomers determine their rotational period ?

They use radars that bounce off some cloud or some thick site, you should know that the atmosphere can also rotate (so the rotation figures you've seen is actually, like, the few first hundred km of atmo rotation), and then some redshift / blueshift can be measured in the bounced back radar. Lately measurement with infrared / microwave / passive radio have been suggested, as most gas giants release some additional radiation from themselves, and is suggested to change along with (core/inner area) rotation. Consider the Sun - even its equatorial rotation, near-polar rotation, and core rotation all doesn't agree at one figure !

 

16 hours ago, Rdivine said:

2. Where is sea level ?

No sea level AFAIK. Can be wrong though.

 

16 hours ago, Rdivine said:

3. How do scientists determine their radius ?

Basic trigonometry ? I mean, knowing the distance between you and the Moon, you can tell it's radius roughly. I suppose they did it first from ground observation then they get better results from Voyagers mission and any subsequent mission.

 

16 hours ago, Rdivine said:

4. Why are there no gas moons ?

Think of it this way - Venus is large compared to most moons, and it only holds heavy things that thin (compared to, say, Neptune) ! Most gas are in the form of hydrogen and helium, and AFAIK Venus, Earth and Mars have a hard time holding them. There's simply not enough mass for there to exist a ball of Gas Moons. Also, this is why those gas giants are thought to hold large cores of rocky bodies, probably some super-super-Earths.

Edited January 6, 2016 by YNM

[Spaceception]

Would Titan count as a gassy moon at all?

[Rdivine]

8 minutes ago, Spaceception said:

Would Titan count as a gassy moon at all?

I don't think so. Titan is a rocky satellite with a dense atmosphere. I was wondering if natural satellites could form in such a way that it formed a mini gas giant(a gas moon).

[Spaceception]

6 minutes ago, Rdivine said:

I don't think so. Titan is a rocky satellite with a dense atmosphere. I was wondering if natural satellites could form in such a way that it formed a mini gas giant(a gas moon).

Ahhh, okay.

[SSR Kermit]

On 2016-01-05 at 7:40 PM, Rdivine said:

I have many questions about gas giants.

1. How do astronomers determine their rotational period? There is no "surface" as a frame of reference. Furthermore, the inner clouds of a gas giant rotate faster than the outer clouds of a gas giant, so where does astronomers get rotational periods for jupiter, saturn, uranus and neptune?

2. Where is sea level? How do astronomers determine the sea level of gas giants? There isn't a "sea level" on a gas giant, unless they were referring to the altitude at which the gas becomes a liquid.

3. How do scientists determine their radius? Do they take measurements from the center to the tallest cloud? Or do they measure to the karman line?

4. Why are there no gas moons? Why does the gas always end up at the main planet instead of the moons?

 

 

These are some questions that have serious implications to profound existence and life as we know it.

Just kidding, i was curious

Thought I'd fill in some details:

1. This is actually a difficult and open problem.  In the solar system it is assumed the period of gas giants radio emissions matches the rotation of the core, backed up by incidentals and implications with respect to all the other rotational moments present in a gas giant. But the exact source of the radio waves is still up for debate. Even the assumption of a core at all is not certain. On Saturn, for example, there are three rotational systems used for defining the period; System I (10 hr 14 min 00 sec), System II ( 10 hr 38 min 25.4 sec) and System III (radio period: 10 hr 39 min 22.4 sec).

2. If you see a reference to "sea-level" in planetology, it generally refers to the altitude where barometric pressure matches that at Mean Sea Level on the earth; that is also used as a reference for the "surface". Again, taking Saturn as an example, gravity reaches about 1.065 g when the pressure reaches 1 bar, while the temperature is a brisk 134K. So the "surface gravity" is given as 1.065 g.

3. Radius is determined by trigonometric calculations on the apparent size of an object, usually during occultations or eclipses in order to shine light through atmospheres. This is especially true for exoplanets. There's undoubtedly special data sets that require different measurements, but those would be more or less impossible to obtain for any exoplanet. Internal measurements, such as from center to some arbitrary cutoff point, become complicated very quickly since those measurements aren't uniform across the object. For gas giants, even determining the center is difficult. The cut off-point is also rather contentious. Indeed, defining the edge of the atmosphere even on Earth is rather ambiguous, it depends very much on what you are looking for. In that frame of reference, in a deep gravity well, you may also have to consider relativistic effects to accurately measure the height (eg, the famous GPS satellites' drifting cesium clocks). Defining the radius optically and from a somewhat distant observation is a lot easier.

4. That's not known to be the case. In fact, current models would expect at least some mini-neptunes to be captured by super-jupiters during orbital migration. I'm thinking in terms of the currently popular pebble-accretion model, but can't see why a smaller, early and runaway accretion zone couldn't orbit a larger one even during formation. So the answer is: there probably are gas moons around large planets or brown dwarfs in planetary orbits, most likely they would occur in high-mass, high-metallicity star systems.

[magnemoe]

As for an gas moon, you could have Neptune in orbit around an large gas giant, problem would be heat during planet formation, more so if you go up to brown dwarf size.

[K^2]

On 1/5/2016 at 10:40 AM, Rdivine said:

2. Where is sea level? How do astronomers determine the sea level of gas giants? There isn't a "sea level" on a gas giant, unless they were referring to the altitude at which the gas becomes a liquid.

There is no point at which gas becomes a liquid on gas giants. Their "oceans" are supercritical. If you were to descend from above, you'd encounter denser and denser gas, until there is no longer a physical distinction between gas and fluid, but you'd never hit a surface. In larger gas giants, like Jupiter, the first real surface you'd encounter would be metallic hydrogen. In a smaller one, like Neptune, you'll hit a mantle of water, ammonia, and methane ice. But in neither case is there an actual surface of the ocean, despite the fact that at the bottom, atmosphere behaves like a liquid in every way.

 

P.S. I hate this new text editor with passion. A mod that allows us to edit bbcode instead would be greatly appreciated.

Edited January 10, 2016 by K^2

[Rdivine]

9 hours ago, K^2 said:

There is no point at which gas becomes a liquid on gas giants. Their "oceans" are supercritical. If you were to descend from above, you'd encounter denser and denser gas, until there is no longer a physical distinction between gas and fluid, but you'd never hit a surface. In larger gas giants, like Jupiter, the first real surface you'd encounter would be metallic hydrogen. In a smaller one, like Neptune, you'll hit a mantle of water, ammonia, and methane ice. But in neither case is there an actual surface of the ocean, despite the fact that at the bottom, atmosphere behaves like a liquid in every way.

Whoa. I never knew that gas giants contained supercritical fluid.

Is it all possible that the supercritical fluid may form a solid with a proper point for crystallization to start? This may result in a layer of "solid" gas. (If it is possible, the layer of solid might change back to a liquid from tidal forces).

[Sandworm]

1 hour ago, Rdivine said:

Whoa. I never knew that gas giants contained supercritical fluid.

Is it all possible that the supercritical fluid may form a solid with a proper point for crystallization to start? This may result in a layer of "solid" gas. (If it is possible, the layer of solid might change back to a liquid from tidal forces).

https://en.wikipedia.org/wiki/Metallic_hydrogen

[K^2]

1 hour ago, Rdivine said:

Is it all possible that the supercritical fluid may form a solid with a proper point for crystallization to start? This may result in a layer of "solid" gas. (If it is possible, the layer of solid might change back to a liquid from tidal forces).

There is no such state as a "solid gas", or "supercritical solid". Transition between solid and liquid is far more dramatic, since solids have much higher order. Whereas liquids and gases have the same amount of disorder at the same density.

There is, however, a very interesting and very rare state of matter known as supersolid. There are some condensed matter theory groups that have suggested that metallic hydrogen in gas giants might exist in this supersolid state. However, even theoretical support for this is weak, and there is absolutely no experimental evidence for it. Of course, later doesn't say much, since we're still having trouble confirming metallic hydrogen in the lab.

Another potential property of metallic hydrogen that's of great interest is metastability. If metallic hydrogen is, indeed, a metastable solid, then like diamond, while it would require immense amount of pressure to create, it could exist at much, much lower pressure. Probably not room temperature and pressure, but perhaps in a cryogenic, pressurized container. The hype over metastable metallic hydrogen stems from two predicted properties. First, there is some indication that it would be an exceptional superconductor at fairly high temperatures. Some people blame Jupiter's magnetic field on this fact. And second, that it would be the best chemical fuel known to man, allowing for as much as a factor of two in I_SP gain over conventional LH2 fuel. On paper, at any rate. Coming up with a good design for an engine will be extremely challenging, even if we manage to produce it in sufficient quantities.

Shame we couldn't just mine it from gas giants, huh?

[lajoswinkler]

18 hours ago, K^2 said:

There is no point at which gas becomes a liquid on gas giants. Their "oceans" are supercritical. If you were to descend from above, you'd encounter denser and denser gas, until there is no longer a physical distinction between gas and fluid, but you'd never hit a surface. In larger gas giants, like Jupiter, the first real surface you'd encounter would be metallic hydrogen. In a smaller one, like Neptune, you'll hit a mantle of water, ammonia, and methane ice. But in neither case is there an actual surface of the ocean, despite the fact that at the bottom, atmosphere behaves like a liquid in every way.

 

P.S. I hate this new text editor with passion. A mod that allows us to edit bbcode instead would be greatly appreciated.

"Ice" in planetary geology means a type of material - volatiles such as methane, water, ethane, ammonia, oxides of carbon, etc., not phase. It's quite annoying, but geologists are stuck with it. "Gas" means "permanent gases - gaseous stuff we were unable to liquefy in the past" and "rock" means "stuff in the ground it's hard to melt". Pretty silly, if you ask me.

Here's a little table.

GAS	hydrogen, helium
ICE	methane, water, nitrogen, oxides of carbon, oxides of sulfur, sulfur, ammonia, ammonia salts, noble gases, various salts
ROCK	silicate salts, silicon(IV) oxide, iron, nickel

So there is no reason to say you'd hit a mantle of "water, ammonia and methane ice". Those are ices and mentioning the word "ice" just adds to confusion because people think there are solids present, or even cold solids, which is far from truth. The only thing that, in ice giants, changes relatively more abruptly than the phase, is composition. Their tops are hydrogen and helium (called "gases" in planetary geology) with some methane, and down below it's mostly methane/water/ammonia. But it's all supercritical. Scorching hot, dense hell dwarfing Venusian hell in everything.

As for Jupiter and Saturn, there is also no evidence for any metallic surface. Supercritical fluid just gets more and more conductive with increasing pressure. Looking at the hypothesized diagrams, the change seems abrupt, but that's because of the scale. Earth's lithosphere looks like a well defined line on diagrams for the same reason.

In reality, there should be no phase boundaries as we see them with hydrosphere-atmosphere on Earth and Titan.

 

I hate this text editor, too.

Edited January 11, 2016 by lajoswinkler

[Shpaget]

When does a rocky planet with dense atmosphere stop being a rocky planet and becomes a gas giant?

Imagine Earth like planet slowly accumulating atmosphere. At one point it becomes Venus like. If it continues to gather gases, eventually it starts to resemble Jupiter, doesn't it?

Surely there is some iron in the Jupiters core.

Edited January 11, 2016 by Shpaget

[K^2]

Well, yeah. If we were talking about a planet composed of just one kind of material, say, hydrogen, we'd have fairly sharp boundaries. Give or take for circulation. But with mixtures, especially of varying compositions, all you are going to get is a gradients of slush. Still, that's a lot closer to being "surface", even if very fuzzy one, than what you get in terms of transition from gas to liquid. Supercriticality means you won't notice any kind of a difference, other than stuff getting denser and denser.

As for transition from fluid hydrogen to metallic hydrogen, all theory points to a sharp phase transition. So while it might still be more of a slush than a surface, there are going to be entirely distinct liquid states and solid states.

[pincushionman]

As to whether there can be gaseous moons, consider how few solar system bodies even have permanent (as in captured gas, not simply continuous outgassing gas mostly already at escape velocity) atmospheres. It really is a matter of scale.

[lajoswinkler]

4 hours ago, Shpaget said:

When does a rocky planet with dense atmosphere stop being a rocky planet and becomes a gas giant?

Imagine Earth like planet slowly accumulating atmosphere. At one point it becomes Venus like. If it continues to gather gases, eventually it starts to resemble Jupiter, doesn't it?

Surely there is some iron in the Jupiters core.

AFAIK we don't have defined transitions.

Suface of Venus (probably ignoring the mountain tops) is also supercritical fluid, except almost entirely CO2. That makes it very similar to a tiny ice giant because carbon(IV) oxide is an ice.

 

There absolutely has to be iron in Jupiter. All planets were made from the same material, but the ones closer to the Sun lost most of their ices and all gases. The interiors are made out of rock, therefore metals/silicates.

The problem is determining the state of such rocky core. Its temperature is high enough for the gas to degenerate into plasma if it was 1 bar, but the pressure is enormous. I don't think we even have decent models for such conditions.