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Thread: Venusian moon

  1. #1 Venusian moon 
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    What is the probability that Venus did once have a moon or moons, but tidal forces from the slowly rotating planet's dense atmosphere made it quickly fall inwards?


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    Zero.


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    Fell inward? It will be zero Ƒor Me too.
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    Quote Originally Posted by PetTastic View Post
    What is the probability that Venus did once have a moon or moons, but tidal forces from the slowly rotating planet's dense atmosphere made it quickly fall inwards?
    Define, "Moon."
    With the right definition of the word, you could claim that Venus (As well as Earth) have hundreds of thousands of "Moons" today.
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    Quote Originally Posted by PetTastic View Post
    What is the probability that Venus did once have a moon or moons, but tidal forces from the slowly rotating planet's dense atmosphere made it quickly fall inwards?
    Tidal forces are causing our moon to slowly increase it's orbit. Why would a Venusian moon work backwards?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    The Earth is pushing the moon away because the moon orbits slower than the planet rotates.
    Ocean tides transfer the Earths rotational momentum to the moon speeding it up, while lengthening the day.

    However, Venus rotates slower than any moon would orbit, so tidal forces would move angular momentum to other way, speeding the planet's rotation and slowing the moon.

    On Earth, the major tidal fluid is water, an incompressible fluid that is only raised and lowered by gravitational action a few feet.
    On Venus, the fluid is compressable carbon dioxide gas.
    If the Sun raises multi-km tides on that Venusian atmosphere, then a moon like ours could distort the atmosphere by hundreds of km.
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    Quote Originally Posted by PetTastic View Post
    The Earth is pushing the moon away because the moon orbits slower than the planet rotates.
    Ocean tides transfer the Earths rotational momentum to the moon speeding it up, while lengthening the day.

    However, Venus rotates slower than any moon would orbit, so tidal forces would move angular momentum to other way, speeding the planet's rotation and slowing the moon.

    On Earth, the major tidal fluid is water, an incompressible fluid that is only raised and lowered by gravitational action a few feet.
    On Venus, the fluid is compressable carbon dioxide gas.
    If the Sun raises multi-km tides on that Venusian atmosphere, then a moon like ours could distort the atmosphere by hundreds of km.
    Two factors work against Venus dragging in a hypothetical moon. It's atmosphere, while thick, still only masses ~1/3 that of Earth's ocean's which reduces the gravitational tug its tidal bulge would have on said moon. Secondly, the atmosphere doesn't have any equivalent to the continents which increase the tendency of the Earth to drag the tidal bulge with it. Without that Venus' tidal bulge would be offset a lot less, also decreasing its effect on the moon.
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    Your understanding of the effects of continents is the reverse of mine.
    My understanding is the continents hold 90+% of the earth's water immobile, and hense like immobile mountains causes no tidal drag.

    In ocean basins, the effects of the moon are limited to too smaller scale.
    When water is pulled to one side, the water level one side goes up, but the other side of the basin goes down.
    As the size of the basin is small compared with the distance to the moon, the gravitational effects of the two sides cancel.

    On Venus, all of the working fluid is free to move, and also expand towards a moon.
    As CO2 is not a perfect working fluid, any expansion or compression is not fully reversible, causing lag in motion.
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    On Venus, all of the working fluid is free to move, and also expand towards a moon.
    Surface temperature on Venus is about 700 K. What fluid are you thinking of, molten lead?
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    The atmosphere is a fluid.
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    Quote Originally Posted by AlexG View Post
    On Venus, all of the working fluid is free to move, and also expand towards a moon.
    Surface temperature on Venus is about 700 K. What fluid are you thinking of, molten lead?
    Fluid does not mean the same as liquid.
    Fluid - Wikipedia, the free encyclopedia
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    fluid can be either gas or liquid hence the study of fluid dynamics. one of the major basics for airplanes and other crafts.
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    There are several theories that Venus underwent not one, but two major impacts in it's history, reversing it's direction of rotation, but I don't think either was from an orbiting body, i.e. a moon.
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    Quote Originally Posted by AlexG View Post
    There are several theories that Venus underwent not one, but two major impacts in it's history, reversing it's direction of rotation, but I don't think either was from an orbiting body, i.e. a moon.
    My interest was purely in what would happen to a moon of Venus if it did exist, for accurate Sci-Fi purposes.
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    An your interest was clear for us to see!well done.
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    Quote Originally Posted by AlexG View Post
    There are several theories that Venus underwent not one, but two major impacts in it's history, reversing it's direction of rotation, but I don't think either was from an orbiting body, i.e. a moon.
    Without a moon it's tilt could have changed quite a bit. It doesn't' need to reverse direction...it just needs to tilt more than 90 degree.
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    I was just looking at the tidal effects for the Sun on Venus, and the argument that it is tidally locked at near that rotation rate.
    When I noticed that the tidal forces on the atmosphere could create an effective heat pump.

    As the Sun pulls up the bulge in the CO2, it should expand and cool, and then heat up again as collapses.
    This effectively cools the side of Venus facing the Sun, so that it absorbs more solar energy, pumping the heat back into the atmosphere elsewhere.
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    I'm not clear why the presence of a fluid has impact on the orbital dynamics of tidal forces. Tidal forces generate friction that turns rotaional energy into heat within the smaller of two bodies in question . The friction is actually a deformational force straining the elasticity of the orbiting bodies. There does not need to be any liquid involved. Once the smaller body becomes tidally locked the point of action of the deformational forces shifts to the center of mass of the system vrs the center if mass of the non tidally locked body. That is why we see "tides" on Earth not on the moon. The Earth's rotational energy is still being changed into heat and motion.
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    Quote Originally Posted by Sealeaf View Post
    I'm not clear why the presence of a fluid has impact on the orbital dynamics of tidal forces. Tidal forces generate friction that turns rotaional energy into heat within the smaller of two bodies in question . The friction is actually a deformational force straining the elasticity of the orbiting bodies. There does not need to be any liquid involved. Once the smaller body becomes tidally locked the point of action of the deformational forces shifts to the center of mass of the system vrs the center if mass of the non tidally locked body. That is why we see "tides" on Earth not on the moon. The Earth's rotational energy is still being changed into heat and motion.
    Please see posts 10, 11 and 12 "liquid"

    The crust on Venus is too stiff for plate tectonics, so should not deform that much.
    Last edited by PetTastic; July 26th, 2013 at 04:05 AM. Reason: stuff after head bang
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    Quote Originally Posted by PetTastic View Post
    What is the probability that Venus did once have a moon or moons, but tidal forces from the slowly rotating planet's dense atmosphere made it quickly fall inwards?
    Do you mean 'recently' fell into the planet? The prevailing theory of planetary formation guarantees that every planet had one or more moon(s) at least during the early part of their creation process. Mercury is the only planet that we know of without a moon - though the MESSENGER mission is still including a search for one in its program - so Venus almost certainly had moons. There are clearly very large impact craters on Venus that had to be made by something. . . Whether it was an orbiting moon or an incoming asteroid is difficult to prove either way. As for how an orbiting moon might be slowed down to impact, atmospheric drag from a too-low orbit would be a much more likely scenario. Tidal forces might apply to a large molten core if a moon was large enough/close enough to cause the core to 'bulge' from gravitational attraction. That could explain Venus' slow (actually backward) rotation. Sorry, but you will have to consult someone else for a mathematical treatment. Oops, My bad!! Mercury AND VENUS are the only planets known to not have moons. The tiny outer planets (Sedna, Eris etc.) are still an open question on this. . .
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    Pet, you are missing my point, tidal forces do not have to result in any actual movement of fluids to still be there and opperating. The energy involved does not go away if there is no ocean or atmospher to be deformed. The tidal stresses are just felt in the rock of the world in that case. They are attempting to stretch and compress the rock. This attempted motion transfers energy as heat.
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    Quote Originally Posted by Sealeaf View Post
    Pet, you are missing my point, tidal forces do not have to result in any actual movement of fluids to still be there and opperating. The energy involved does not go away if there is no ocean or atmospher to be deformed. The tidal stresses are just felt in the rock of the world in that case. They are attempting to stretch and compress the rock. This attempted motion transfers energy as heat.
    Sorry but this statement is just wrong.
    Energy is only transferred between two objects orbiting each other if one or both objects deform.
    Heat is only generated if an object does deform.
    The attempt to deform does not generate tidal forces or cause heating by its self.
    Tidal force - Wikipedia, the free encyclopedia
    Energy is transfered by gravitational action on the buldge, no bulge no energy transfered.

    In the case of Venus, the fluid is the atmosphere, that has massive volume and mass, compared to the oceans on Earth.
    Last edited by PetTastic; September 9th, 2013 at 07:29 AM. Reason: it -> is
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    Pet, I disagree because all solids are in fact elastic substances. Some may be less elastic than others but all have the quality of elasticity. The tidal forces acting on a solid "may be thought of as acting on the center of mass" but they don't act just on the center of mass. That is just a bit of mental shorthand we do. Forces act on the atoms that compose the solid mass individually. We can't handle that level of complexity so we lump it all together, but that is us making a convienent generalization, not a statement of reality. Think of a steel beam under a varying load. Eventually the beam breaks, did the energy transfer happen only in the instant of the breakage or had it been accumulating as "metal fatigue"?
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    Quote Originally Posted by Sealeaf View Post
    Pet, I disagree because all solids are in fact elastic substances. Some may be less elastic than others but all have the quality of elasticity.
    I never used the words solid or solids. I only referd to objects deforming, and it makes no difference if the deformation is elastic or not, energy will still be transfered & heat generated. Energy = force X distance.

    Quote Originally Posted by Sealeaf View Post
    The tidal forces acting on a solid "may be thought of as acting on the center of mass" but they don't act just on the center of mass. That is just a bit of mental shorthand we do. Forces act on the atoms that compose the solid mass individually. We can't handle that level of complexity so we lump it all together, but that is us making a convienent generalization, not a statement of reality. Think of a steel beam under a varying load. Eventually the beam breaks, did the energy transfer happen only in the instant of the breakage or had it been accumulating as "metal fatigue"?
    In the case of Venus almost all the tidal forces would come from the atmosphere.
    The planet only rotates very slowly. A moon would pull up a large bulge in the atmosphere that would lag behind the moons overhead position.
    Gravitational attraction between the moon and the bulge would slow the moon until it falls in.
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