Why No Gaseous Moons?

Palomar · 2004-03-27 13:04:58

*Gennaro's "Terraforming Titan" thread has gotten me wondering again (although, IIRC, this is the first time I've posted about it here) why the Solar System possesses no gaseous moons? Of all the satellites which the gas giants possess, only Titan has a thick atmosphere encloaking it. Even more, "Alone of all the satellites in the solar system, Titan has a significant atmosphere."

Considering how humongous Jupiter, Saturn, Uranus and Neptune are, isn't it amazing that none of them spawned a gaseous moon somehow? Maybe gaseous celestial objects must necessarily be very large, and therefore a natural satellite couldn't possibly form?

Hmmmmmm.

--Cindy

[http://www.nineplanets.org/titan.html]Text quote source

Mad Grad Student · 2004-03-27 13:19:30

It's an interesting question, one that requires more research to answer properly. For that matter, why does Venus have an atmosphere 90 times as thick as Earth's, and Mars and atmosphere one hundredth that of Earth? It's something we don't have enough info on yet. However, if someone does know why some planets have thick atmospheres, others thin ones, and some with none at all, feel free to prove me wrong.

Gennaro · 2004-04-05 03:57:24

It's not like I master anything about the subject, but I'll have a go at it anyway. In fact, I'm a little amazed no one has tried to address this issue as yet, considering the vast learning of many board members and the generally interesting question. Or maybe, because Cindy refers to my Titan post, they thought I should have a chance to kick off the discussion when I came back. Thanks guys, you're so considerate.

Anyhow, this is what I think. The reason Titan's methane atmosphere can exist is simply the low liquid range of the gasses composing it. Methane has a melting point of -182.5 degrees C and a boiling point of -161.6 degrees C. Compared to similar Moons like Ganymede, Callisto and Europa which are also rich in water and hydrogen (but have no methane) these elements have simply freezed out at the range of Jupiter and Saturn, forming the characteristic iceballs we are familiar with. In fact, below its methane atmosphere, Titan is an iceball too. Though of course this does not answer the question why Titan is so rich in methane and other outer moons are not, which after all was your actual question.

Simultaneously, in the inner solar system, methane worlds don't exist because at that range atmospheric CH4 (like ammonia - NH3) is easily photodissasociated by Solar UV radiation and lost to space.

Another basic factor to be reckoned with is that a body must have enough mass to hold an atmosphere at all or at least keep it over geologic time. This could be why Mercury and our Moon have practically none and Mars has lost most of its. Conversevily, it's certainly the reason why giant planets are almost only atmosphere around a molten super pressure core, being massive enough to have accumulated vast amounts of hydrogen.
Compared to CO2 on the other hand, methane is a heavy element, therefore on Titan the atmosphere stays!

Then particularly in reference to Mad Grad, besides mass, at the range of up to about Venus, photodissasociation is again the major culprit for life bearing atmospheres. Around Mercury the sun always was and at the range of Venus it grew too hot to sustain what is called a "cold trap" which at Earth hinders atmospheric water to dissasociate and become lost. Water vapour rises, but as it reaches the higher atmosphere it freezes and returns towards the surface before the Sun's ultraviolet rays can do much harm.
As Venus began to lose all its water when the sun grew too hot to sustain the cold trap, the CO2 hitherto mainly locked in limestone (like on Earth) began to be released. To lock CO2 in limestone (calcium carbonate - CaCO3) you need lots of water, see. The consequence of ever increasing CO2 levels in turn set off a runaway greenhouse effect, effectivily boiling away what water remained on Venus, increasing temperature and pressure to the infernal levels we see today.
If Mercury was big enough, the Venus scenario would have repeated itself.

Palomar · 2004-04-06 13:58:09

Anyhow, this is what I think. The reason Titan's methane atmosphere can exist is simply the low liquid range of the gasses composing it. Methane has a melting point of -182.5 degrees C and a boiling point of -161.6 degrees C. Compared to similar Moons like Ganymede, Callisto and Europa which are also rich in water and hydrogen (but have no methane) these elements have simply freezed out at the range of Jupiter and Saturn, forming the characteristic iceballs we are familiar with. In fact, below its methane atmosphere, Titan is an iceball too. Though of course this does not answer the question why Titan is so rich in methane and other outer moons are not, which after all was your actual question...
Compared to CO2 on the other hand, methane is a heavy element, therefore on Titan the atmosphere stays!

*Thanks, Gennaro.

The variations within the Solar System never cease to amaze me. I still am surprised, though, that none of the gas giants spawned a genuinely gaseous moon...like a minature version of one of themselves. And then contrast Callisto, Europa, Triton, etc., to Centaurs and Kuiper Belt objects pulled in by Neptune (and Uranus?).

Damn, I wish I could get the math and physics; I would love to be a professional astronomer, working to figure this stuff out.

By the way, I did Google for an answer prior to my initial post; I didn't see anything relative to this question (thought I should mention that now).

--Cindy

Shaun Barrett · 2004-04-08 00:47:21

I'm only paying flying visits to New Mars at present, mainly just to try to keep up with any blockbuster news that breaks.
I don't want to seem picky and pedantic but I do have a minor problem with one aspect of Gennaro's post. Carbon dioxide (CO2) has a molecular mass of about 44, while methane (CH4) has a molecular mass of only about 16. It should be easier for a small planet to retain carbon dioxide than methane.
A small point, I know, but it could lead to confusion if allowed to stand.

I'll be back to check on Spirit's and Opportunity's progress when I can. (I hate being away from my own computer at a time like this, with so much going on at the fourth rock from the Sun! )

Palomar · 2004-04-08 06:26:06

I'm only paying flying visits to New Mars at present, mainly just to try to keep up with any blockbuster news that breaks.
... I'll be back to check on Spirit's and Opportunity's progress when I can. (I hate being away from my own computer at a time like this, with so much going on at the fourth rock from the Sun! )

*You're on vacation (or holiday), right? We've been missing you! (Hurry back to your own computer!)

Perhaps [http://www-ssc.igpp.ucla.edu/dawn/]data from "Dawn", once it's underway, will answer some of my questions as well. Check out the info relative to Ceres, Vista, and Jupiter. "Ceres and Vesta are the right two bodies with which to address this question, as they are the most massive of the protoplanets, baby planets whose growth was interrupted by the formation of Jupiter."

-and- "Dawn determines the nature of the building blocks from which the terrestrial planets formed, improving our understanding of this formation. Finally, it contrasts the formation and evolution of two small planets that followed very different evolutionary paths so that we understand what controls that evolution."

Anyway, I thought of this thread when I read that web site's info.

--Cindy

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#1 2004-03-27 13:04:58

Re: Why No Gaseous Moons?

#2 2004-03-27 13:19:30

Re: Why No Gaseous Moons?

#3 2004-04-05 03:57:24

Re: Why No Gaseous Moons?

#4 2004-04-06 13:58:09

Re: Why No Gaseous Moons?

#5 2004-04-08 00:47:21

Re: Why No Gaseous Moons?

#6 2004-04-08 06:26:06

Re: Why No Gaseous Moons?

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