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neonsurge
2010-May-08, 08:51 PM
I have an embarrasingly elementary question that I've been unable to answer through Googling and searching this (and other) sites. I hope that someone out there has the patience to educate an enthusiastic amateur who's recently developed an interest in astronomy and cosmology but currently lacks the background knowledge and experience to even get started:

Planets both in our own solar system and many of those observed/hypothesised extrasolar bodies are described as "gas giants" - what does this mean? Is Jupiter literally a massive ball of gas with no "solid" mass? Surely there's a solid "core" around which a gravitational field is formed?

Once again, sorry for the dumb (but honest) question.

Elukka
2010-May-08, 09:18 PM
A large ball of gas will form a gravitational field just the same as a large ball of rock, it just tends to be less dense.
As I understand Jupiter might have a rocky core somewhere deep within but it's not a given. Regardless, hydrogen itself will turn solid and metallic under great pressure, so the deeper parts of a gas giant will be solid.

Peter B
2010-May-08, 11:45 PM
I have an embarrasingly elementary question that I've been unable to answer through Googling and searching this (and other) sites. I hope that someone out there has the patience to educate an enthusiastic amateur who's recently developed an interest in astronomy and cosmology but currently lacks the background knowledge and experience to even get started:
Hey, far better to acknowledge your ignorance and seek an answer than to celebrate your ignorance as some people like to do.


Planets both in our own solar system and many of those observed/hypothesised extrasolar bodies are described as "gas giants" - what does this mean? Is Jupiter literally a massive ball of gas with no "solid" mass? Surely there's a solid "core" around which a gravitational field is formed?
It's probably better to think of the term as a comparative rather than an absolute. Jupiter and the other gas giants are gaseous in comparison with rocky planets like the Earth or even Venus (which has a thick atmosphere). That is, the mass and height of atmosphere on the rocky planets is small, sometimes negligibly small, in comparison with the mass and radius of the rocky part of the planet.

Also, with the rocky planets there's a very obvious divide between the rocky bit and the gaseous bit (that divide is, of course, the ground you're standing on), and the two parts are made of very different materials. With gas giants, the solid bit of the planet emerges almost imperceptibly out of the atmosphere, is (I understand) made of pretty much the same stuff as the atmosphere, and is largely a consequence of the large mass of the planet. That is, even if you had a really well-built Jupiter truck, there wouldn't be an obvious surface for it to drive on.

chornedsnorkack
2010-May-09, 08:41 AM
Regardless, hydrogen itself will turn solid and metallic under great pressure, so the deeper parts of a gas giant will be solid.

Hydrogen would turn solid and metallic under great pressure and low temperature. But the interior of Jupiter is hot. At high temperatures hydrogen will not be solid even at high temperature - it would be molten metal or plasma.

Solid metal, whether iron or hydrogen, would not support dynamo and would not have magnetic field (except ferromagnetic).

Shaula
2010-May-09, 05:03 PM
Worth pointing out that the state of hydrogen is a function of temperature and pressure. For high temperatures there is a pressure above which metallic hydrogen will form. Just because it is hot doesn't mean metallic hydrogen is not going to form. Having said that Jupiter's core has a predominance of liquid metallic hydrogen with maybe a bit of solid stuff at the centre according to current models.

neonsurge
2010-May-09, 07:19 PM
Thanks for the learnin' guys. Much appreciated.

neilzero
2010-May-09, 10:35 PM
I don't think we are absolutely sure what is near the middle of gas giant planets, but metallic hydrogen is likely for Jupiter. A probe did penetrate 100? kilometers below the cloud tops and sent back data, but our understanding of the next 23,000 miles is mostly computer modeling, which involves quite a few assumptions. The pressure is likely not high enough at the center of Saturn, Uranus nor Neptune to produce metallic hydrogen. The cloud tops of all 4 of our gas giant planets are not as cold as calculations suggest, so the centers are likely hotter than the center of Earth. One estimate was 25,000 degrees for Jupiter. If so, highly ionized plasma may fill 1/2 of the volume. Since comets and asteroids fall into the gas giant planets occasionally, most of the elements are likely plus at least 80% hydrogen near the center. A solid surface is unlikely due to the very high temperature. Neil

Hungry4info
2010-May-10, 08:41 AM
The core of a gas giant should be comprised of elements heavier than H and He simply because those heavy elements would sink into the centre of the planet.

chornedsnorkack
2010-May-10, 09:15 AM
The core of a gas giant should be comprised of elements heavier than H and He simply because those heavy elements would sink into the centre of the planet.

Those elements would not appear in the photosphere of the gas giant, because the photosphere is cooler than Sun, and the elements would condense and sink out of photosphere. But why should they sink all the way to the centre? The core of a gas giant is hot. Is it hot enough to evaporate/dissolve the heavy element in hot dense metal/plasma hydrogen and mix them uniformly over large volume of interior?

Hungry4info
2010-May-12, 07:22 AM
Good question. I suspect the answer to all this lies in discussions about the equation of state of gas giant planets, and I don't know nearly enough to go further.

Schneibster
2010-May-12, 09:45 AM
Food for thought (https://www.llnl.gov/str/Nellis.html). Enjoy.

Robert Tulip
2010-May-13, 01:52 PM
Those elements would not appear in the photosphere of the gas giant, because the photosphere is cooler than Sun, and the elements would condense and sink out of photosphere. But why should they sink all the way to the centre? The core of a gas giant is hot. Is it hot enough to evaporate/dissolve the heavy element in hot dense metal/plasma hydrogen and mix them uniformly over large volume of interior?With Jupiter vacuuming up asteroids and comets over billions of years, surely it would have a substantial non-gas core? Or is all that debris like a fly on an elephant? I remember reading a science fiction story about aliens who extract gold from the core of planets on the theory that the heaviest metals fall to the middle.

chornedsnorkack
2010-May-13, 03:02 PM
With Jupiter vacuuming up asteroids and comets over billions of years, surely it would have a substantial non-gas core? Or is all that debris like a fly on an elephant? I remember reading a science fiction story about aliens who extract gold from the core of planets on the theory that the heaviest metals fall to the middle.

Heaviness? Thatīs fiction.

Carbon dioxide is heavier than air. Yet it does not fall to Earth surface. Salt is heavier than water. Yet it does not sink to the bottom of the sea.

BigDon
2010-May-13, 04:10 PM
Could a newer more modern probe descend deeper than 60 miles? Was it heat or pressure that killed the last probe? And what are the odds of a balloon rover?

EDG
2010-May-13, 04:30 PM
Carbon dioxide is heavier than air. Yet it does not fall to Earth surface.

Uh, it does actually. Remember lake Nyos in Cameroon? All those people died there because the CO2 release from the lake hugged the ground and suffocated them in their houses.



Salt is heavier than water. Yet it does not sink to the bottom of the sea.

Salt's dissolved in the water though. Sediments on the other hand (which certainly are denser than water) do sink to the bottom of the sea.

neilzero
2010-May-13, 10:49 PM
So some elements and compounds sink below the cloud tops of Jupiter, but convection currents and change of state due to increasing pressure and temperature likely keep most from sinking all the way to the core center. The interactions are quite complex and largely untested. Neil

Schneibster
2010-May-14, 02:24 AM
Jupiter is an environment with the volume of 108 Earths most of which exists in conditions we cannot reproduce in the laboratory for more than a millionth of a second or so. On the other hand, there is no sign of extensive activity that violates known scientific law.

Shaula
2010-May-14, 03:02 PM
Jupiter has more than a thousand times the volume of Earth.
Re = 6370km
Rj = 71490km
Rj/Re = 11.22
so Vj/Ve = 1400 (roughly).

Hornblower
2010-May-14, 06:18 PM
Uh, it does actually. Remember lake Nyos in Cameroon? All those people died there because the CO2 release from the lake hugged the ground and suffocated them in their houses.
That carbon dioxide did not settle out of the air. It emerged from under the water and simply took a while to disperse.


Salt's dissolved in the water though. Sediments on the other hand (which certainly are denser than water) do sink to the bottom of the sea.

Schneibster
2010-May-16, 10:32 AM
Jupiter has more than a thousand times the volume of Earth.
Re = 6370km
Rj = 71490km
Rj/Re = 11.22
so Vj/Ve = 1400 (roughly).You're correct. My bad.

After research: That's supposed to be surface area, and it's an old figure. The better, current figure is 121 Earths. Thanks for letting me know.

Robert Tulip
2010-May-19, 11:09 PM
Heaviness? Thatīs fiction.

Carbon dioxide is heavier than air. Yet it does not fall to Earth surface. Salt is heavier than water. Yet it does not sink to the bottom of the sea.Heaviness is not fiction. Lumps of iron and lead do fall to the earth surface or the bottom of the sea. I would expect that gases and liquids would mix in Jupiter's atmosphere, but all the lumps of heavy solids that have hit it would have made a solid core. Yes, if an iron meteor burned up in Jupiter's atmosphere it might not fall to the middle, but surely an asteroid big enough to make it through the outer atmosphere would eventually fall to rest on a solid core? Surely there would have been very many such large metal asteroid meteors vacuumed up as Jupiter cleared its orbit?

Jeff Root
2010-May-20, 03:13 AM
All the metal and even rock would melt from the high temperatures.
However, the pressures are high enough to resolidify many substances
at great depth.

The visible banded clouds are all in the top 100 km. Below them is an
atmosphere 17,000 km deep consisting mainly of molecular hydrogen.
Below that is a 44,000 km deep ocean of liquid metallic hydrogen. At
the center is a rocky core probably about 20,000 km in diameter -- not
that much bigger than Earth (12,800 km diameter) -- even though its
mass is 13 times the mass of Earth. It is crushed by the enormous
weight of liquid and gas on top of it to a density of 20 g/cm^3 at the
center. The pressure at the center is about 80 million atmospheres,
and the temperature is probably about 25,000 kelvins.

Summarized from Kaufmann's 'Discovering the Universe', 1987.

Don, I forget whether heat or pressure killed the Galileo atmosphere
probe. How about you look it up (if you haven't already) and tell us?

-- Jeff, in Minneapolis

chornedsnorkack
2010-May-20, 03:55 PM
All the metal and even rock would melt from the high temperatures.
However, the pressures are high enough to resolidify many substances
at great depth.
Depends on temperature.

Inside Earth, the rock is solid almost to the bottom. Iron of inner core is solid, but iron of outer core is firmly molten.

Precisely which layers of Jupiterīs interior transmit shear waves, and at which speeds?

EDG
2010-May-20, 04:23 PM
That carbon dioxide did not settle out of the air. It emerged from under the water and simply took a while to disperse.

That doesn't change my point. It didn't disperse immediately, because it was heavier than air - that caused it to remain at ground level and suffocate everyone.

Jeff Root
2010-May-21, 12:26 PM
The fact that carbon dioxide is heavier than air doesn't have much,
if anything, to do with it not dispersing immediately. It takes time
for two volumes of gas to mix together nomatter what their relative
densities.

-- Jeff, in Minneapolis

mugaliens
2010-May-22, 03:10 AM
The fact that carbon dioxide is heavier than air doesn't have much,
if anything, to do with it not dispersing immediately. It takes time
for two volumes of gas to mix together nomatter what their relative
densities.

-- Jeff, in Minneapolis

Tell that to the few thousand survivors of the CO2 eruptions over the last few hundred years who survived by clamboring up the hillsides in a mad fury while their countrymen died below.

neilzero
2010-May-22, 05:03 AM
Hi Shaula: The radius you used for Jupiter is, I believe the diameter, so 1400 is way too many Earths. Many elements combine to form compounds at the high temperatures 1/3 of the way to the center, then disassociate at the still higher temperatures 2/3 (or there abouts) to the center. ie Carbon dioxide above it's critical temperature and pressure dissolves many substances. The chemistry and physics are somewhat different for plasma than for hot gas and hot vapor. Neil

eburacum45
2010-May-22, 06:40 AM
It is not as simple as just saying 'the heaviest elements sink to the bottom' when considering planetary composition.
Here is the Goldschmidt classification
http://en.wikipedia.org/wiki/Goldschmidt_classification
certain elements bind best with the stony crust, while others bind with the iron core.
Note that gold is a siderophile element, so would indeed be found in the iron core of a typical planet, but uranium, with a very similar density, is found in the crust.
Elements like nitrogen, hydrogen and helium are generally found in atmospheres, but strangely enough oxygen prefers to be incorporated into rocks (the amount of oxygen in our crust greatly exceeds the oxygen in the atmosphere).

Shaula
2010-May-22, 07:04 AM
Hi Shaula: The radius you used for Jupiter is, I believe the diameter, so 1400 is way too many Earths.
I will admit I got those numbers from Wikipedia for convenience but checking back with my well worn copy of the Guinness book of Astronomy (1992, 4th edition) I get diameter Jupiter = 143884km equatorial, 133708km polar. The Earth is given as 12756km equatorial, 12714km polar (diameters again). Handily it also gives me some direct figures for relative volumes and so on: Mass of Jupiter = 317.89 Earths, Volume Jupiter = 1318.7 Earths. 1400 is not way too many Earths (wrong by just over 5%) but I am guessing it did ignore the considerable polar flattening of Jupiter due to rapid rotation. And was rounded up.

Please check and provide figures before you tell me I am probably wrong. Saves confusion!

chornedsnorkack
2010-May-24, 06:49 PM
Tell that to the few thousand survivors of the CO2 eruptions over the last few hundred years who survived by clamboring up the hillsides in a mad fury while their countrymen died below.

So what? Mixing carbon dioxide with air, or salt water with fresh water, takes time. But it is irreversible process - once mixed, carbon dioxide will not settle out of air, nor salt out of water.

EDG
2010-May-24, 08:25 PM
So what? Mixing carbon dioxide with air, or salt water with fresh water, takes time. But it is irreversible process - once mixed, carbon dioxide will not settle out of air, nor salt out of water.

You do understand that salt is dissolved in water, whereas CO2 is not dissolved in air, right? There's nothing preventing CO2 from settling out of air if it's in an enclosed environment - outside, it's more likely to be dispersed and mixed into the surrounding atmosphere by winds, but the fact remains that it is still a denser gas that the air around it.

Jeff Root
2010-May-24, 11:09 PM
EDG,

CO2 cannot settle out of air at any temperature that naturally
exists at Earth's surface. It does the opposite: It mixes with air.

Put pure CO2 into the bottom of a closed tank containing air, and
the air and CO2 will mix completely in a short period of time.
The mixing occurs by diffusion.

-- Jeff, in Minneapolis

EDG
2010-May-24, 11:33 PM
If CO2 supposedly "diffuses in a short period of time" then what happened at Lake Nyos?

from: http://en.wikipedia.org/wiki/Lake_Nyos

Although a sudden outgassing of CO2 had occurred at Lake Monoun in 1984, killing 37 local residents, a similar threat from Lake Nyos was not anticipated. However, on August 21, 1986, a limnic eruption occurred at Lake Nyos which triggered the sudden release of about 1.6 million tonnes of CO2; this cloud rose at nearly 100 kilometres (62 mi) per hour.[4] The gas spilled over the northern lip of the lake into a valley running roughly east-west from Cha to Subum, and then rushed down two valleys branching off it to the north, displacing all the air and suffocating some 1,700 people within 25 kilometres (16 mi) of the lake, mostly rural villagers, as well as 3,500 livestock. The worst affected villages were Cha, Nyos, and Subum.[8] Scientists concluded from evidence that a 300-foot (91 m) fountain of water and foam formed at the surface of the lake. The huge amount of water rising suddenly caused much turbulence in the water, spawning a wave of at least 80 feet (24 m) that would scour the shore of one side.[9]

[...]

Carbon dioxide, being about 1.5 times as dense as air, caused the cloud to "hug" the ground and descend down the valleys where various villages were located. The mass was about 50 metres (164 ft) thick and it traveled downward at a rate of 20–50 kilometres (12–31 mi) per hour. For roughly 23 kilometres (14 mi) the cloud remained condensed and dangerous, suffocating many of the people sleeping in Nyos, Kam, Cha, and Subum.[4] About 4,000 inhabitants fled the area, and many of these developed respiratory problems, lesions, and paralysis as a result of the gases.[12]

I remember very clearly that the news reports were saying that the people died because the gases erupted from the lake were denser than air and hugged the ground too. Was it just that the sheer volume of gas emitted took long enough to disperse and diffise that it hung around long enough to kill people?

Hop_David
2010-May-24, 11:40 PM
Tell that to the few thousand survivors of the CO2 eruptions over the last few hundred years who survived by clamboring up the hillsides in a mad fury while their countrymen died below.

If the heavier gases tended to settle out, the bottom twelve meters of our atmosphere would be CO2. Then O2. Then nitrogen.

So people on the beach would aphyxiate. Most of the structures above twelve meters would ignite in a pure oxygen atmosphere. Folks high on a mountainside would aphyxiate in a pure nitrogen atmosphere.

But this doesn't happen. Obviously, the gases mix even though they're different densities.

A number of people have explained that it takes time for volcanic CO2 to mix once it's spewed into the atmosphere. The cases of volcanic CO2 smothering people doesn't indicate that gases tend to stratify by density.

Hornblower
2010-May-24, 11:43 PM
You do understand that salt is dissolved in water, whereas CO2 is not dissolved in air, right? There's nothing preventing CO2 from settling out of air if it's in an enclosed environment - outside, it's more likely to be dispersed and mixed into the surrounding atmosphere by winds, but the fact remains that it is still a denser gas that the air around it.

In the absence of any wind or turbulence to stir the mixture, the carbon dioxide would settle somewhat because of the greater molecular weight, but its partial pressure would stabilize it vastly short of forming a suffocating layer at ground level. From what I have studied about the scale height of atmospheric gases, I estimated that the concentration at ground level would increase by about 50%, to about 500 parts per million. That settling is so slow that ordinary atmospheric motion prevents it from settling except at extremely high altitudes.

Once again, that disaster was caused by a large amount of carbon dioxide gushing out of the lake and displacing the overlying air. In the absence of wind or updrafts, the gases mixed by diffusion very slowly, so there was plenty of time to suffocate the victims.

I refreshed my 40-year-old memory from college by googling "scale height" and browsing from there. If anyone has further questions about my reasoning, please ask.

grant hutchison
2010-May-24, 11:57 PM
Yes, "scale height" is the answer.
If utterly undisturbed, the various atmospheric gases would settle, but only to their own independent scale heights. Carbon dioxide would behave like a pure carbon dioxide atmosphere, with a scale height about 0.7 times that of oxygen and 0.6 times that of nitrogen. It would have a slightly higher pressure at ground level than it currently has, and a slightly lower pressure at altitude. But it doesn't take much stirring to ensure that the lower atmosphere assumes a single characteristic scale height determined by its mean molecular mass.
As others have said, it just takes time for gases to mix by diffusion. If there's a continuous source of carbon dioxide at ground level, an equilibrium is achieved between the rate of addition and the rate of diffusion. In places like the Grotto del Cane, that equilibrium puts a toxic level of carbon dioxide below knee height (killing small dogs, but not people). At Lake Nyos, the equilibrium level while the lake was overturning was well above head-height for people living at the lakeside and downhill from that. There's a Swahili word, mazuku, which designates pockets of carbon dioxide in low-lying ground, sufficient to kill people who enter that area: people have died as a result of sleeping in ditches near a volcanic lake, for instance.

Grant Hutchison

EDG
2010-May-25, 01:38 AM
Oh right. Well that makes things a lot clearer.

As a related point - what would happen if you had a massive rocky world that could hold onto a lot of helium (but not hydrogen) in its atmosphere as well as nitrogen and oxygen and other gases? Would it all be mixed up as well, or would the helium tend to congregate at the top of the atmosphere because it's so much lighter than the other gases? It sounds like it'd be mixed up, but if you had say 10 times as much helium as the other gases combined then it wouldn't really be breathable would it?

Hornblower
2010-May-25, 02:19 AM
Oh right. Well that makes things a lot clearer.

As a related point - what would happen if you had a massive rocky world that could hold onto a lot of helium (but not hydrogen) in its atmosphere as well as nitrogen and oxygen and other gases? Would it all be mixed up as well, or would the helium tend to congregate at the top of the atmosphere because it's so much lighter than the other gases? It sounds like it'd be mixed up, but if you had say 10 times as much helium as the other gases combined then it wouldn't really be breathable would it?

That mixture looks like what a scuba diver would be breathing at extreme depths, where they use lots of helium to prevent nitrogen narcosis and oxygen toxicity. It appears that it would be breathable at several times our sea level pressure, but not at our familiar pressure.

The scale height of helium will be about 7 times that of regular air. Suppose we start with a mixture of 100 parts helium and 10 parts air, eliminate all further mixing and let it differentiate. If my reasoning is correct we will end up with about 14 parts helium for every 10 parts of air at ground level, and almost pure helium at extreme heights. At our sea level pressure we would be hurting for oxygen, but merely doubling the pressure would put us in good shape.

In this thought experiment I would enlarge the planet and make the mass proportional to the square of the radius. That will keep the surface gravity equal to Earth's, but increase the escape velocity in proportion to the radius for the purpose of holding onto the helium.