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Thread: atmospheric pressure of planets

  1. #1
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    atmospheric pressure of planets

    consider this a follow-up to a previous question "atmospheric composition of planets"

    Basically, I am curious as to the relationship between planetary size, temperature, and atmospheric pressure. It seems intuitive that pressure correlates positively with planetary size (big planets = thick atmosphere) and negatively with temperature (hot planets = thin atmosphere).

    However, Venus is hotter & slightly smaller than Earth, yet has a much thicker atmosphere. Here are some known atmospheric pressures for planetary bodies:

    Venus = 9300 Kp
    Earth = 101 Kp
    Mars = 0.7-0.9 Kp
    Jupiter = 20-200 Kp
    Saturn = 140 Kp
    Titan = 147 Kp

    Any insights are this topic would be appreciated.

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    There's not much to go on really.

    If a planet is big enough to hold onto a particular gas, then it can hold onto any amount of it. Other factors like volcanic activity (which adds gas) or giant impacts/solar wind (which remove gas) are generally more important. Also age, especially for smaller planets - they can have thicker atmospheres while they're still geologically active, but when they cool down (which they do faster than larger ones) their volcanoes shut down.

    Mars probably had a thicker atmosphere in the past while it was active, but since that stopped and nothing's replenishing it anymore it's been lost into space over the millions of years.

    I'd say it's probably true that - all other things being equal - larger planets tend to have thicker atmospheres and smaller ones tend to have thinner atmospheres. But that's certainly not a guarantee.

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    This topic interests me, as well. My (very) limited understanding of the matter is that atmospheric *mass* is at least as important as gravity and temperature in surface pressure.

    Case in point: Venus's gravity is less than Earth, but it has a much higher atmospheric pressure. This certainly isn't due to the greater molecular weight of CO2--the only probable explanation is that Venus has an atmosphere much more massive than Earth's, which is indeed the case. Titan is an even better example: its surface pressure is higher than Earth's, but its gravity is much weaker. Ergo, to exert the same amount of pressure there must be much more gas, and that's the scoop--IIRC, Titan's atmosphere is ten times more massive than Earth's.

    I'm sure all this has something to do with the ideal gas laws, but that's as far as I'm willing to go...

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    Quote Originally Posted by EDG_ View Post
    There's not much to go on really.

    If a planet is big enough to hold onto a particular gas, then it can hold onto any amount of it. Other factors like volcanic activity (which adds gas) or giant impacts/solar wind (which remove gas) are generally more important. Also age, especially for smaller planets - they can have thicker atmospheres while they're still geologically active, but when they cool down (which they do faster than larger ones) their volcanoes shut down.

    Mars probably had a thicker atmosphere in the past while it was active, but since that stopped and nothing's replenishing it anymore it's been lost into space over the millions of years.

    I'd say it's probably true that - all other things being equal - larger planets tend to have thicker atmospheres and smaller ones tend to have thinner atmospheres. But that's certainly not a guarantee.
    So Earth's relatively thin atmosphere for its size is an anomaly, of sorts. Maybe that's an artifact of the impact event that created our moon.

    Noticing that even the gas giants have "thinner" atmospheres than Venus, that could be explained by their lower temperature -- which would cause many gases to condensate.

    I guess knowing the condensation point for the gases would be useful as well. I can look those up

    Thanks for the replies. I'm still trying to get a handle on the dynamics involved with this.

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    planetary size, temperature, and atmospheric pressure

    This PDF may help you answer all your Qs.

    http://www.sfu.ca/~boal/390lecs/390lec9.pdf

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    Quote Originally Posted by Himanshu Raj View Post
    This PDF may help you answer all your Qs.

    http://www.sfu.ca/~boal/390lecs/390lec9.pdf
    Hey, Baric - in the section where he shows the equation for atmospheric retention, he's got his constants multiplied by 1.5, like I did! (he's also got his equations upsidedown compared to mine).
    I've corrected my post in the other thread now, based on that (thanks for the link, Himanshu)
    Last edited by EDG; 2007-Jun-19 at 05:26 AM.

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    Quote Originally Posted by baric View Post
    So Earth's relatively thin atmosphere for its size is an anomaly, of sorts. Maybe that's an artifact of the impact event that created our moon.
    Actually it's an artifact of spawning life. With CO2 split by photosynthesis, combined into solids organically, and chemically combined when forced by PH - and furthermore the resultant free oxygen combining with many exposed materials - it's little wonder that nitrogen makes up most of the atmosphere.

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    Quote Originally Posted by baric View Post
    However, Venus is hotter & slightly smaller than Earth, yet has a much thicker atmosphere. Here are some known atmospheric pressures for planetary bodies:

    Venus = 9300 Kp
    Earth = 101 Kp
    Mars = 0.7-0.9 Kp
    Jupiter = 20-200 Kp
    Saturn = 140 Kp
    Titan = 147 Kp

    Any insights are this topic would be appreciated.
    The gas giants Jupiter and Saturn don't have surfaces as such, so "surface pressure" is meaningless for them. Those figures would be for some specified depth within the atmosphere.

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." Abraham Lincoln

    I say there is an invisible elf in my backyard. How do you prove that I am wrong?

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    Quote Originally Posted by G O R T View Post
    Actually it's an artifact of spawning life. With CO2 split by photosynthesis, combined into solids organically, and chemically combined when forced by PH - and furthermore the resultant free oxygen combining with many exposed materials - it's little wonder that nitrogen makes up most of the atmosphere.
    Plus the Earth's primordial atmosphere was probably about as thick as Venus', but then most of it got blasted off when we got hit by the impactor in the event that formed the Moon.

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    Quote Originally Posted by G O R T View Post
    Actually it's an artifact of spawning life. With CO2 split by photosynthesis, combined into solids organically, and chemically combined when forced by PH - and furthermore the resultant free oxygen combining with many exposed materials - it's little wonder that nitrogen makes up most of the atmosphere.
    Sweet! I'm a lot more comfortable with that explanation than the impactor.

    So... carbon-based life sucked out the C out of the CO2, leaving the O2 to rust the surface materials.

    With that in mind... what about Mars? It has a very thin atmosphere and a heavily oxidized surface. Is it plausible that life had the same effect on the early Martian atmosphere, before the planet lost most of its atmosphere?

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    Quote Originally Posted by EDG_ View Post
    Hey, Baric - in the section where he shows the equation for atmospheric retention, he's got his constants multiplied by 1.5, like I did! (he's also got his equations upsidedown compared to mine).
    I've corrected my post in the other thread now, based on that (thanks for the link, Himanshu)
    Thanks also to Himanshu.. I'm checking out it now...

  12. 2007-Jun-19, 01:57 PM
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    superfluous

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    Quote Originally Posted by EDG_ View Post
    Hey, Baric - in the section where he shows the equation for atmospheric retention, he's got his constants multiplied by 1.5, like I did! (he's also got his equations upsidedown compared to mine).
    I've corrected my post in the other thread now, based on that (thanks for the link, Himanshu)
    Quote Originally Posted by baric View Post
    Thanks also to Himanshu.. I'm checking out it now...
    No Thanks!

    I was wandering that other factors being favourable any Earth-sized planet (with the lithospheric composition same as that of Earth) should be able to sustain life.( keeping in mind that the Earth-sized planet will have the same atmospheric composition, as Earth)

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    How can we account for the evolution of the atmosphere around a planet in the billion years of its evolution, considering the varied atmosphere types ranging from the turbulent atmosphere of Jupiter to the 'Placid Blue' of our earth.

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    Quote Originally Posted by Himanshu Raj View Post
    How can we account for the evolution of the atmosphere around a planet in the billion years of its evolution, considering the varied atmosphere types ranging from the turbulent atmosphere of Jupiter to the 'Placid Blue' of our earth.
    I was thinking about this specifically. Earth seems very anomalous -- it's atmospheric volume seems especially low considering its planetary mass and ambient temperature.

    Right now, I more focused on determining what gases would be retained, how thick the atmosphere would be, and what level of greenhouse warming would occur.

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    Quote Originally Posted by baric View Post
    I was thinking about this specifically. Earth seems very anomalous -- it's atmospheric volume seems especially low considering its planetary mass and ambient temperature.
    We don't know what "typical" should be for our kind of planet - or even if there *is* a typical thickness of atmosphere given a particular gravity and temperature.


    Right now, I more focused on determining what gases would be retained, how thick the atmosphere would be, and what level of greenhouse warming would occur.
    If you find anything, let me know . The GFX in particular has been a huge headache for me, I can't seem to find a straightforward equation that says "if you have this much CO2, in an atmosphere this thick, then the warming will be X %".

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    Quote Originally Posted by EDG_ View Post
    We don't know what "typical" should be for our kind of planet - or even if there *is* a typical thickness of atmosphere given a particular gravity and temperature.
    We do have some rough outer boundaries.

    All known giant planets have a thick atmosphere. Included are "hot Jupiters" where the H2 has been blown away by a nearby star. This indicates strongly, to me, that there is some size limit where it would be highly unreasonable to expect an airless planet.

    In addition, all "small" planetary bodies, with one notable exception (Titan), have very thin or non-existent atmospheres.

    The problem is that, while we can see general trends, we do not have a large enough sample size to explain the two key anomalies in that trend (Venus/Earth & Titan).

    Are they evidence of a systemic complexity we have yet to decipher, or are they simply the result of some peculiar process or event in the past?

    If you find anything, let me know . The GFX in particular has been a huge headache for me, I can't seem to find a straightforward equation that says "if you have this much CO2, in an atmosphere this thick, then the warming will be X %".
    lol, that's what I want! I'll try to cobble something togther as a replacement.

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    I agree with Van Rijn: Those pressures for Jupiter and Saturn are incorrect. All 4 gas giant planets have much higher pressure than Venus a thousand kilometers below the cloudtops, and no surface in sight.
    I suspect you will not find a corelation between mass and surface air pressure. Factors such as historical events, presence or absence of life and ocean, distance from the Sun, photosphere temperature, average density, surface gravity and radius all help determine the present surface air pressure, if there is a surface. Neil

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    Quote Originally Posted by EDG_ View Post
    The GFX in particular has been a huge headache for me, I can't seem to find a straightforward equation that says "if you have this much CO2, in an atmosphere this thick, then the warming will be X %".
    Good luck with that one.
    I think you'd need to factor in the black body temperature of the incoming radiation, too, as well as the black body temperature of your imaginary planet at the top of its atmosphere, and the numerical density of other gases.
    The greenhouse increment depends on the relative impact of the CO2 absorption bands on the incoming and outgoing radiation, and Doppler and collision broadening widen those bands; so all of the above will have an influence.

    Grant Hutchison

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