Starship Asterisk*

Recently I asked a question about Titan's atmosphere in the "Ask questions" folder. It was a slightly stupid question, so I'm not surprised that I have not received an answer.

I have another question about Titan's atmosphere here, and I would be happy if someone would discuss it with me. I guess that I really have more than one question.

The way I see it, Titan's atmosphere resembles both the Earth's atmosphere, Venus' atmosphere and Neptune's (and Uranus') atmosphere.

It resembles the Earth's atmosphere, because the Earth and Titan are the only two bodies in the solar system which have atmospheres strongly dominated by nitrogen. So the question is, where did the Earth and Titan get their nitrogen atmospheres?

Titan's atmosphere also resembles Venus' atmosphere in that Titan and Venus are the only two bodies in the solar system whose atmospheres seem far too thick for the parent bodies' mass. Where did Venus and Titan get their very thick atmospheres?

Titan's atmosphere also resembles Neptune's atmosphere, in that both contain between 3 and 4% methane. Where did Titan and Neptune (and Uranus) get all their methane?

I would be happy if someone would discuss this with me!

Ann

I Googled for "formation of methane in stars" and came up with several references which seem to be discussing planetary formation as well as element formation within stars in the same reference. Which makes sense, since it's the elements from exploded stars which go into the next round of star and planet formation.

I am working my way through a book called "Astrophysics of Planetary Formation" by Philip J. Armitage which I thought might have something to say on this, but it barely touches on atmospheres, and then only deals with the rate at which a planet can lose its atmosphere. I haven't (so far) seen in it any discussion of the different atmospheres that may be part of any particular planet's formation.

I wonder if simple random chance is enough to explain the differences we see today? (Bearing in mind that the Solar System is still evolving, and no planet's atmosphere is in a final steady state.)

Rob

Ann wrote:It resembles the Earth's atmosphere, because the Earth and Titan are the only two bodies in the solar system which have atmospheres strongly dominated by nitrogen. So the question is, where did the Earth and Titan get their nitrogen atmospheres?

I don't know about Titan, but I assume that the process is similar to the situation with Earth. Earth's (and other planets') atmospheres are the product of geologic outgassing. Nitrogen was abundant in the presolar nebula, and has several features that allow it to easily end up in the atmosphere- it is very stable in the atmosphere, not being involved in many chemical reactions there, and it doesn't easily integrate itself into crystals and minerals, so not much is captured inside the Earth (unlike oxygen, for instance). Of course, on Earth the atmosphere is further modified by biological processes (which modulate nitrogen levels, as well), something probably not a factor on any other planets in the Solar system.

Titan's atmosphere also resembles Venus' atmosphere in that Titan and Venus are the only two bodies in the solar system whose atmospheres seem far too thick for the parent bodies' mass. Where did Venus and Titan get their very thick atmospheres?

I don't think this is well understood. I'd quibble with your suggestion that the atmospheres seem too thick for the mass of the planets. Couldn't you as easily say that Earth's atmosphere is too thin for its mass? Atmospheric density is determined not simply by gravity, but by atmospheric composition. Venus (and Titan) have atmospheres made up of much heavier gases than Earth. To understand atmospheric density you have to understand atmospheric composition. Venus has more nitrogen than Earth, but it has outgased or otherwise produced a huge amount of carbon dioxide- something that hasn't happened (or hasn't happened yet) on Earth.

Titan's atmosphere also resembles Neptune's atmosphere, in that both contain between 3 and 4% methane. Where did Titan and Neptune (and Uranus) get all their methane?

I also think this is poorly understood. Methane is very reactive, so most people seem to think that it is necessary to replenish atmospheric methane from some internal source like volcanism, or some chemical process like life.

One thing to be careful about when looking at planetary atmospheres is to consider total elemental abundance. Titan, for instance, has a strong altitude dependence on composition. Most of the methane is near the surface- total methane is therefore less. Total abundance is also not seen looking at percentage concentrations. If you look at the percentage abundances for Earth and Venus, you would think Earth has far more nitrogen, for instance, when in reality it only has one-quarter has much (in the atmosphere).

Chris wrote:

I don't think this is well understood. I'd quibble with your suggestion that the atmospheres seem too thick for the mass of the planets. Couldn't you as easily say that Earth's atmosphere is too thin for its mass?

Indeed, I've been thinking that myself - namely, that the Earth's atmosphere is too thin for its mass. But there is a possible explanation for this, I think. Most astronomers seem to believe that the early Earth collided with a Mars-sized body. Couldn't the pre-collisional Earth have had a thick atmosphere that was mostly lost in that enormous crash? Isn't it possible to think that Earth might have had an atmosphere as thick as that of Venus if that primoridal super-collion hadn't happened?

Of course I realize that all this is pure speculation. I don't think that all astronomers are even convinced that the early Earth ever collided with a Mars-sized body, for example. And I don't know what, if anything, is known about the atmosphere of the very early Earth.

Ann

But then, when you are comparing vastly different planetary bodies I don't think you can really find a standard mass vs atmospheric thickness. There are just too many local variables to deal with. The mix of gasses that are present, the state of volcanic activity, the average temperatures of the atmosphere and surface, the ambient radiation levels, magnetosphere, ect, ect.

Orca, I'm sure you're right about that.

On the other hand, the fact that planetary atmospheres would seem to be so unpredictable makes it very hard for us to guess at which extrasolar planets might be habitable. Imagine that Earthly astronomers managed to find a planet orbiting an all-but-perfect solar twin at about 0.8 AU in a beautifully circular orbit. Imagine that the planet was found to have an atmosphere, too. Is this planet habitable, or is it another Venus?

Ann

Ann wrote:On the other hand, the fact that planetary atmospheres would seem to be so unpredictable makes it very hard for us to guess at which extrasolar planets might be habitable. Imagine that Earthly astronomers managed to find a planet orbiting an all-but-perfect solar twin at about 0.8 AU in a beautifully circular orbit. Imagine that the planet was found to have an atmosphere, too. Is this planet habitable, or is it another Venus?

What is "habitable"? I see no reason to think that a planet with an atmosphere like Venus couldn't support life (assuming it were a bit farther from its star), nor a body like Titan. They have liquid solvents and they have energy. I doubt anything else is required.

Very true, the term life doesn't have to mean like us and the term habitable doesn't have to mean to suit us. As our own planet has shown us, in even the most inhospitable places on our planet, Life Will.

BMAONE23 wrote:Very true, the term life doesn't have to mean like us and the term habitable doesn't have to mean to suit us. As our own planet has shown us, in even the most inhospitable places on our planet, Life Will.

But I'm not talking about life that is necessarily like us. I'm talking about any life.Today most astronomers agree that Venus is inhabitable to any life forms. Should we disagree? Can life exist on present-day Venus? Is there, in fact, life on present-day Venus?

Ann

Ann wrote:But I'm not talking about life that is necessarily like us. I'm talking about any life.Today most astronomers agree that Venus is inhabitable to any life forms. Should we disagree? Can life exist on present-day Venus? Is there, in fact, life on present-day Venus?

Venus is likely hostile to life because of its temperature, though, not necessarily its atmosphere. Since we were discussing atmosphere, my point was that there is no reason to think that life can't exist over a very wide range of possible atmospheric conditions, from near vacuum to Venus-dense, and over many atmospheric compositions.