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Thread: Solar system rocky/icy bodiesí atmospheres: why the way they are?

  1. #1
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    Solar system rocky/icy bodiesí atmospheres: why the way they are?

    Start with Mercury (none), the Moon (ditto), Mars (wimpy), Earth (just right), and Venus (really thick).

    If the high speed tail of the Maxwell-Boltzmann distribution of a component at the top of an atmosphere is greater than the escape velocity (actually a speed), then without a source, itís only a matter of time before that species is lost to space. And if that time is less than ~4 billion years, say, all gone. As daytime surface temperatures are high enough, and gravity weak, hereís a top level explanation for Mercury and the Moon.

    Hence also why Marsí is wimpy, and has no H or He (there is, to be sure, a huge potential replenishment source, in the form of underground water/ice, but conversion to atmospheric gas is veeeery slow).

    Earth - unlike Mars or Venus - is blessed with a magnetic shield, which greatly slows down erosion due to the solar wind and solar flares (these can convert neutral species to ionized ones, which may speed them up and/or entice them away via electric or magnetic fields). And the H reservoir is vast, in the form of water.

    How, then, to explain Venusí thick atmosphere? Yes, the H is gone, ditto He (surface and sub-surface reservoirs exist, but are tiny) ... are neutral C, N, O, and S too cold (at the top of the atmosphere)? CO_2 say, not dissociated? What prevents solar wind/flares erosion (no magnetic shield)?

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    Quote Originally Posted by Jean Tate View Post
    What prevents solar wind/flares erosion (no magnetic shield)?
    If I understand it right, the ionosphere of Venus along with the planet's motion through the Sun's magnetic field induces a magnetic pressure that can balance the ram pressure of the solar wind, at least for weaker solar winds. Erosion does take place but it seems Venus is able to bring a lot of its volatiles to its surface, though its much greater heat and lack of water to absorb gases are big contributors, no doubt, so it can maintain(?) its thick atmosphere.
    We know time flies, we just can't see its wings.

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    Well, I'm no expert on atmospheres, but using some basic properties, let's see what happens in Venus' atmosphere.

    The escape velocity from Venus' surface is v(esc) = 10,360 m/s.

    The surface temperature on Venus is about T = 465 Kelvin. This temperature changes throughout the atmosphere, but let's use it for this very rough calculation.

    The typical speed of CO2 molecules at a temperature of T = 465 Kelvin is about v(CO2) = 510 m/s. A small tail of molecules will have 3 or 5 or 10 times this typical speed, and it is the fast-moving molecules in that tail that are at risk of escaping from the planet if they happen to be located near the top of the atmosphere.

    Using these rough numbers, we find a ratio of the required escape speed to the typical molecular speed of

    v(esc) / v(CO2) = approx 20

    The papers I've read suggest that when this ratio is somewhere between 6 and 10, there's a good chance that a significant fraction of the atmosphere will escape within 5 billion years. The ratio derived here -- rough as it is -- is much larger than this "danger" ratio, which suggests that Venus should be able to retain its atmosphere for a long time.

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    Quote Originally Posted by StupendousMan View Post
    Well, I'm no expert on atmospheres, but using some basic properties, let's see what happens in Venus' atmosphere.

    The escape velocity from Venus' surface is v(esc) = 10,360 m/s.

    The surface temperature on Venus is about T = 465 Kelvin. This temperature changes throughout the atmosphere, but let's use it for this very rough calculation.

    The typical speed of CO2 molecules at a temperature of T = 465 Kelvin is about v(CO2) = 510 m/s. A small tail of molecules will have 3 or 5 or 10 times this typical speed, and it is the fast-moving molecules in that tail that are at risk of escaping from the planet if they happen to be located near the top of the atmosphere.

    Using these rough numbers, we find a ratio of the required escape speed to the typical molecular speed of

    v(esc) / v(CO2) = approx 20

    The papers I've read suggest that when this ratio is somewhere between 6 and 10, there's a good chance that a significant fraction of the atmosphere will escape within 5 billion years. The ratio derived here -- rough as it is -- is much larger than this "danger" ratio, which suggests that Venus should be able to retain its atmosphere for a long time.
    Niced. Thank you.

    Regards, John M.

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    Quote Originally Posted by StupendousMan View Post
    Well, I'm no expert on atmospheres, but using some basic properties, let's see what happens in Venus' atmosphere.

    The escape velocity from Venus' surface is v(esc) = 10,360 m/s.

    The surface temperature on Venus is about T = 465 Kelvin. This temperature changes throughout the atmosphere, but let's use it for this very rough calculation.

    The typical speed of CO2 molecules at a temperature of T = 465 Kelvin is about v(CO2) = 510 m/s. A small tail of molecules will have 3 or 5 or 10 times this typical speed, and it is the fast-moving molecules in that tail that are at risk of escaping from the planet if they happen to be located near the top of the atmosphere.

    Using these rough numbers, we find a ratio of the required escape speed to the typical molecular speed of

    v(esc) / v(CO2) = approx 20

    The papers I've read suggest that when this ratio is somewhere between 6 and 10, there's a good chance that a significant fraction of the atmosphere will escape within 5 billion years. The ratio derived here -- rough as it is -- is much larger than this "danger" ratio, which suggests that Venus should be able to retain its atmosphere for a long time.
    Thanks!

    I will do similar calculations, for CO and N2. If CO2 is retained, SO2 will be too.

    I also wonder how much of the upper Venusian atmosphere CO2 is dissociated? After all, there is an ionosphere, right?

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    Quote Originally Posted by Jean Tate View Post
    After all, there is an ionosphere, right?
    Yes, this paper should help.
    We know time flies, we just can't see its wings.

  7. #7
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    I expect that the temperature at the top of Venus' atmosphere
    has no connection with the temperature at the bottom of the
    atmosphere. I also expect that atmospheric loss is dominated
    by direct impact of solar wind ions, not by atmospheric heat.

    -- Jeff, in Minneapolis

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    One can find measurements of the temperature of Venus' atmosphere at different altitudes above the surface in many places. For example,

    https://ntrs.nasa.gov/archive/nasa/c...9900013957.pdf
    https://en.wikipedia.org/wiki/Atmosphere_of_Venus

    Measurements indicate that the temperature in the upper reaches of the atmosphere is considerably lower than that near the surface, so the loss of molecules due to thermal motions would be considerably slower than the results in my quick calculations above.

    Comparing the effects of thermal motions to collisions with particles in the solar wind is too complicated for me to do right now. Perhaps someone else can perform the exercise.

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