Celestia Forums

So every now and then when I'm worldbuilding I end up with a terrestrial world that is massive enough to hold onto helium (molecular weight 4) but not hydrogen (molecular weight 2).

I'm assuming that the terrestrial world - which is within the 'frost line' of the star - isn't going to snowball into a gas giant because of this. If it's in the outer zone then I assume it *does* snowball since there's more ice and gas out there.

Anyway, I digress. What I'm wondering is this - what would a world like that actually LOOK like? Say it's got 80-90% Helium in a thick atmosphere around it, with the rest being other heavier gasses (CO2, N2, CH4, etc) from planetary outgassing which would presumably settle at the bottom. But since Helium is so inert (nothing forms a compound with it in nature) would the atmosphere just be totally transparent and clear or something, since there's no hydrides or hydroxides?

Any ideas?

From what you've said I would say you'd end up with what would look like a two-tier atmosphere, with the a helium envelope extending some distance beyond the heavier compounds below. Depending on the level of outgassing, I suppose you may end up with a world rather like 'Plateau' ( Tau Ceti I) from Niven's Known Space, with possibly high plateaus like Mount LookitThat that extend above the 'seas' of smog to the clear helium layer above. However, it may be that the boundary betwen atmospheric layers may not be so clear-cut, due to weather conditions. It'd be fun if enough mixing of the atmospheric constituents went on to provide oxygen as a secondary element to the helium

For some reason I have visions of gengineered heavy-worlder colonists with very high squeaky voices living on the plateaus, with high-pressure lifeforms (perhaps) in the dense lowland regions forming a different order of life entirely.

Err... please excuse the braindump

Cormie

I figured that much... probably there would be two distinct layers, and a lot of helium (though being a very light gas, you can have a lot of it there without increasing the surface pressure too much).

What I can't figure out isn't so much the structure of the atmosphere but more the actual appearance of the planet from space. Would you get clouds in the helium atmosphere for example? What colour would it be? And so on.

Such a world could appear as a Dwarf Jovian Planet with a helium atmosphere (hydrogen-less by the way!) and forzen CO2 and rocks continents and an inferred liquid methane ocean. And we could describe a "Methanegeology" instead of a "Hydrogeology" because CH4 in outer zone of Habitable one (but closer than Titan and Saturn's) could behave like water on our Earth: evaporate from the oceans, lifts up, loose warm, gets down then falls down as a liquid methane rain and comes back to the sea. It's intriguing...

ZZ-Cephei wrote:Such a world could appear as a Dwarf Jovian Planet with a helium atmosphere (hydrogen-less by the way!) and forzen CO2 and rocks continents and an inferred liquid methane ocean. And we could describe a "Methanegeology" instead of a "Hydrogeology" because CH4 in outer zone of Habitable one (but closer than Titan and Saturn's) could behave like water on our Earth: evaporate from the oceans, lifts up, loose warm, gets down then falls down as a liquid methane rain and comes back to the sea. It's intriguing...

Well, it might look like that if it was outside the habitable zone... but these worlds aren't. I'm talking about basically a "big Earth" sitting in the habitable zone, that happens to be big enough to retain Helium in its atmosphere.

I'm figuring that it may have a really thick atmosphere... most of which just doesn't do anything. AFAIK Helium isn't a greenhouse gas so it won't affect the surface temperature, and because it's so light you can have a huge thickness of it without increasing the surface pressure by much at all. But I guess it'd be good for asteroid/meteorite protection though... and aurorae might be prettier?

But if for example the atmosphere was ten times thicker than the Earth's (but the top 90% of that was all Helium), what would the planet actually LOOK like? Would that upper 90% be totally transparent? Would you get any weird scattering or refraction or anything like that, that could hide the surface and lower atmosphere from view?

Malenfant wrote:

ZZ-Cephei wrote:Such a world could appear as a Dwarf Jovian Planet with a helium atmosphere (hydrogen-less by the way!) and forzen CO2 and rocks continents and an inferred liquid methane ocean. And we could describe a "Methanegeology" instead of a "Hydrogeology" because CH4 in outer zone of Habitable one (but closer than Titan and Saturn's) could behave like water on our Earth: evaporate from the oceans, lifts up, loose warm, gets down then falls down as a liquid methane rain and comes back to the sea. It's intriguing...

Well, it might look like that if it was outside the habitable zone... but these worlds aren't. I'm talking about basically a "big Earth" sitting in the habitable zone, that happens to be big enough to retain Helium in its atmosphere.

I'm figuring that it may have a really thick atmosphere... most of which just doesn't do anything. AFAIK Helium isn't a greenhouse gas so it won't affect the surface temperature, and because it's so light you can have a huge thickness of it without increasing the surface pressure by much at all. But I guess it'd be good for asteroid/meteorite protection though... and aurorae might be prettier?

But if for example the atmosphere was ten times thicker than the Earth's (but the top 90% of that was all Helium), what would the planet actually LOOK like? Would that upper 90% be totally transparent? Would you get any weird scattering or refraction or anything like that, that could hide the surface and lower atmosphere from view?

! Ah Ok, then! So I'd have got two models for a G2 star with circa 1 solar mass. Habitable zone is fixed to 1 AU.

Model 1 - Helium Planet

Mass: 1.5 - 3.5 Earth masses (right?)
Average separation: 1.8 AUs
Chemistry: He (90 %), CO2 (5 %), CH4 (2 %), Others (3 %)...
Average temperature: 10 - 20 C
Note: Planet lays in the outer limit of habitable zone, but average temperature is right the same of the Earth-like zone due to high CO2 and CH4 concentrations which create a greenhouse effect. Thus the planet is like an Earth in the orbit of Mars

Model 2 - Helium Planet

Mass: 1.5 - 3.5 Earth masses
Average separation: 1.1 AUs
Chemistry: He (90%), N2 (5 %), CH4 (1%), Ar (0.07 %), CO2 (0.03 %), Others (3 %)...
Average temperature: 15 C

So, tell me, Malenfant, if my models are likely

Helium forms small molecules... so you would get a lot of Rayleigh Scattering. The power of Rayleigh Scattering is inversely proportional to the fourth power of the wavelength... so with lots of scattering (far more than on Earth) I would think that the atmosphere from orbit would be purple? A bright violet? Since the smaller wavelengths are scattered more.
I think that as the helium layer got denser, it would become whiter.... with light scattered more equally.... so forming a dense layer of white helium fog I guess. Below, the lowermost layer of heavier gasses would be yellowish or orange, I believe, depending on what percentage of CO2 and CH4 there was.

Im not sure, maybe someone with better mathmatics could work out the rayleigh scattering better than me? but I hope thats pointed in the right direction Malenfant.

Hm, I think ZZ's on the right track with his first model (though I was literally thinking more along the lines of take earth's atmosphere - i.e. mostly nitrogen and oxygen - and add a load of He on top. I don't think there's any particular reason why having He in the air would result in an uninhabitable planet.

Helium won't form molecules at all, it's a (very) noble gas. So all you have are individual helium atoms. I'm not sure it'd form a "fog" either, unless you just meant that the atmosphere would just turn white with depth. But that's not a fog in the sense of "condensed water vapour rolling along the ground", which is what the word usually means.

Looking at
http://en.wikipedia.org/wiki/Rayleigh_scattering

I notice that they have a "rayleigh scattering coefficient" that is "the number of particles per unit volume N times the cross-section". So I guess with smaller particles (and a lot of them) which you'd see in a helium atmosphere, this coefficient would be greater?

I dunno though, hence why I ask . I think we need Grant for this!!!

Malenfant wrote:Helium won't form molecules at all, it's a (very) noble gas. So all you have are individual helium atoms. I'm not sure it'd form a "fog" either, unless you just meant that the atmosphere would just turn white with depth. But that's not a fog in the sense of "condensed water vapour rolling along the ground", which is what the word usually means.

Oh yeah silly mistake. If theres no molecules though, that would just result in twice the number of even smaller particles, so I guess the Scattering effect would be even greater.
I meant "fog" in the sense it would (probably) look like water fog - white, and difficult to see through

Neethis wrote:Helium forms small molecules... so you would get a lot of Rayleigh Scattering. The power of Rayleigh Scattering is inversely proportional to the fourth power of the wavelength...

Actually, no.
Helium has very little polarizability. Which means Rayleigh scattering is very weak. About 70 times weaker per molecule.

Meaning that a given weight/pressure of helium, even though it is 7 times the molecules, is 10 times weaker scatterer.