An atmosphere of helium

[TransformerRobot]

If there was a planet an atmosphere of helium, what would it's overall temperature be like? Can helium trap sunlight, or would it not be able to carry out a greenhouse effect?

[swansont]

I doubt He would absorb much IR. AFAIK the first excitation is well into the UV, meaning that there will be very few atoms in excited states, where they might have an IR transition. You generally need to have a molecule to have accessible IR transitions.

[mississippichem]

I doubt He would absorb much IR. AFAIK the first excitation is well into the UV, meaning that there will be very few atoms in excited states, where they might have an IR transition. You generally need to have a molecule to have accessible IR transitions.

 

Right. To add to that, IR radiation is generally on the order of the energies between rotational/vibrational levels in molecules where, as you stated, atomic electronic transitions for the first few n levels are usually more in the UV/X-ray region.

[TransformerRobot]

So in other words an atmosphere of helium wouldn't get much in the way of heat from the sun?

[swansont]

So in other words an atmosphere of helium wouldn't get much in the way of heat from the sun?

It would tend to interact more with the sun, which has some UV, than with the re-radiation from the earth.

[John Cuthber]

Just to clarify the answer given. Helium has absolutely no IR absorptions.

It's totally transparent for all energies up to the fairly hard UV.

[TransformerRobot]

Well Oxygen does though, right?

[mississippichem]

Well Oxygen does though, right?

 

Not really. In order for a vibrational transition to be IR active, it has to result in a net change in the dipole moment of the molecule so homonuclear diatomic molecules or single atoms will not show IR absorption.

 

But look at methane. We can bend one of the C-H bonds and change the dipole moment so it's IR active.

[TransformerRobot]

Then what if there was a planet where the atmosphere was 50% Carbon Dioxide, and it was 80 million kilometers from it's sun?

[mississippichem]

Then what if there was a planet where the atmosphere was 50% Carbon Dioxide, and it was 80 million kilometers from it's sun?

 

Like Venus but worse

[TransformerRobot]

Okay, how about without the carbon dioxide?

[granpa]

every 10 degree increase in temperature doubles the amount of water vapor in the air.

this increases the number of thunderstorms which transport warm moist air from the ground to just below the stratosphere where it is above most of the infrared absorbing atmosphere.

 

so the warmer the earth becomes the more efficiently its cooling mechanism becomes.

[John Cuthber]

every 10 degree increase in temperature doubles the amount of water vapor in the air.

On this planet: however, your mileage may vary.

[TransformerRobot]

every 10 degree increase in temperature doubles the amount of water vapor in the air.

this increases the number of thunderstorms which transport warm moist air from the ground to just below the stratosphere where it is above most of the infrared absorbing atmosphere.

 

so the warmer the earth becomes the more efficiently its cooling mechanism becomes.

 

You mean this planet is self-cooling? If I knew that I wouldn't worry so much about saving electricity.

[granpa]

Not really. In order for a vibrational transition to be IR active, it has to result in a net change in the dipole moment of the molecule so homonuclear diatomic molecules or single atoms will not show IR absorption.

 

But look at methane. We can bend one of the C-H bonds and change the dipole moment so it's IR active.

 

 

so an atmosphere of hydrogen doesnt absorb much IR either?

[mississippichem]

so an atmosphere of hydrogen doesnt absorb much IR either?

 

Nope. There are some electronic transitions between high excited states that may fall in this range but at room temperature these states are not likely to be populated.

 

Of course you can cheat by looking at the quadrupole in a high pressure sample of H₂ gas. This will require a ridiculously long path length to observe IIRC.

[granpa]

I found this but I dont have access.

 

http://www.nrcresearchpress.com/doi/pdf/10.1139/p57-090

 

AN ATTEMPT TO DETECT INFRARED ABSORPTION IN LIQUID...

 

 

so just how well would a 10,000 km thick atmosphere of pure liquid helium keep a planet warm?

 

I have no idea how to set up even a 'back of the envelope' calculation.

Edited April 20, 2012 by granpa

[TransformerRobot]

I found this but I dont have access.

 

http://www.nrcresear...10.1139/p57-090

 

AN ATTEMPT TO DETECT INFRARED ABSORPTION IN LIQUID...

 

 

so just how well would a 10,000 km thick atmosphere of pure liquid helium keep a planet warm?

 

I have no idea how to set up even a 'back of the envelope' calculation.

 

How can an atmosphere be in any state other than a gas state?

[granpa]

once the gas is compressed to the density of the liquid there is no longer much difference.

Edited April 21, 2012 by granpa

[TransformerRobot]

once the gas is compressed to the density of the liquid there is no longer much difference.

 

So it's the same case with our atmosphere on Earth?

[granpa]

huh?

our atmosphere isnt compressed to the density of liquid air

[TransformerRobot]

Well an atmosphere is defined as "The gases surrounding the Earth or any astronomical body." Gases, not liquids. You can't have a liquid atmosphere, can you? You could drown in it. Or do you mean something else by "liquid air".

[granpa]

the density of liquid helium is 0.14 g/cm^3 and like a gas it is a fluid.

pretty much everything else in the universe will sink below it.

 

so yes I would call it an atmosphere.

 

you can call it an ocean if you want to but its an ocean you couldnt swim in.

 

creatures that evolved there would fly through it.

they wouldnt swim through it.

Edited April 21, 2012 by granpa

[TransformerRobot]

So you're saying that our own atmosphere of oxygen and carbon dioxide is a liquid that we can't swim in?

[granpa]

I thought we were talking about this:

 

I found this but I dont have access.

 

http://www.nrcresearchpress.com/doi/pdf/10.1139/p57-090

 

AN ATTEMPT TO DETECT INFRARED ABSORPTION IN LIQUID...

 

 

so just how well would a 10,000 km thick atmosphere of pure liquid helium keep a planet warm?

 

I have no idea how to set up even a 'back of the envelope' calculation.