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Noob question. Aren't the moons of Jupiter and Saturn also tectonically active? With the immense solar pull, Mercury would have some heavy sheer forces causin

by sidcool 10y ago

Noob question. Aren't the moons of Jupiter and Saturn also tectonically active? With the immense solar pull, Mercury would have some heavy sheer forces causing plates to move. I am sure I am missing something. Please enlighten me.

mafribe 10y ago
Io, Jupiter's innermost moon and the most volcanically active body in the solar system, is interesting because its source of heat is believed to be tidal heating: Jupiter's gravity pulls Io inwards while Jupiter's other moons pull Io outwards. This leads to a weird, eccentric orbit, which in turn means changing levels and directions of gravity leading to movement of Io's mass, e.g. molten rock under the surface. This causes friction that heats Io, whence volcanism. https://en.wikipedia.org/wiki/Tidal_heating_of_Io https://en.wikipedia.org/wiki/Tidal_heating_of_Io
- parr0t 10y ago
  Fascinating. Thanks for the link.
- yathern 10y ago
  I'm no physicist, but I'm curious where the heat comes from, thermondynamically speaking. Does this changing level of gravity gradually sap the speed of Io?
  - T-hawk 10y ago
    Yes. It comes from the orbital kinetic energy of Io and the other moons. The same happens in the Earth-Moon system. The Moon causes tides on Earth's oceans, which increase friction between water and land and therefore produce heat. Thermodynamically speaking, that heat comes from the Moon's orbital momentum, as it recedes from Earth and slows in its orbit.
    - mafribe 10y ago
      Earth has also got a small bit of tidal movement of the crust, but not remotely as much as Io. There there is a theory that tidal movement of the Earth's crust is one (of many) factors that can trigger earthquakes.
      - dspillett 10y ago
        >(of many) factors that can trigger earthquakes. Or slowly feed in energy that is stored and released when something else pushes the system far enough out of equilibrium that a quake happens.
        T-hawk 10y ago
        "Feed in energy" - that energy has to come from somewhere. The point of this subthread is where does that energy come from thermodynamically? The answer is orbital or rotational kinetic energy via tidal forces, which of course do affect both crust and water.
        mafribe 10y ago
        Yes, but different celestial bodies get there energy from different sources. For the earth it's mostly radioactive decay in the core of the planet. In others, like Io, it's believed to be mostly tidal friction.
Udo 10y ago
To answer the other half of your question (mafribe already said why the moons are tectonically active): what's needed for heating due to tidal friction is not only strong gravity, but a strong gravity gradient that changes over time. I was doing some back-of-the-envelope calculations just now to show how Io experiences much more tidal stress than Mercury due to a stronger change in gradient, when I discovered my assumptions were wrong, but I'm posting the results anyway: Mercury's orbit has a huge eccentricity, and its distance to the sun varies between 0.47 AU at Aphelion and 0.307 AU at Perihelion. Taking Mercury's size into account, that should work out to a difference in solar gravity between its near and far side of about 0.000004 m/s^2 at Aphelion and 0.000013 m/s^2 at Perihelion! That's actually quite the squeeze! Now comparing that to Io, which has often been reported as having this wild orbit, upon looking up the actual numbers I saw that Io's orbit around Jupiter is actually nearly circular: 420000km at Periapsis and 423400km at Apoapsis. The moon itself has a radius of 1800km, that means its orbit is so circular its variation falls within the size of the actual moon. Furthermore, plugging in the numbers, Io's gravity gradient between its near and far side remains pretty much stable at 0.000014 m/s^2 no matter where it is, varying only by a factor of about -1.37x10^-08. So the gradient change itself is not dramatic in Io, but reasonably pronounced in Mercury - didn't expect that. Taking a look at the orbital period of both objects, Io completes about 50 orbits in the same time it takes Mercury to orbit once, but even a factor of 50 isn't enough to blow up Io's tidal action to make up for the observed difference in tidal heating. My only conclusion at this point is that I either made a careless mistake somewhere (which is likely) or that Io's squeezing action must come almost entirely from its interaction with the other Jupiter moons. In the end, a heat source is not required for Mercury to experience tectonic activity though. As described in the article, the cooling and shrinking of the planet itself are sufficient to cause disturbances on the surface.
- mcguire 10y ago
  Shouldn't Mercury have reached thermal equilibrium already?
  - Arnt 10y ago
    If Mercury is in an eccentric orbit, then the sun's gravity changes depending on where in its orbit is is. Adding to that, when it'a closer, the difference between the gravity on the bright side and dark side is more pronounced than when it's farther away.
mcguire 10y ago
Mercury is tidally locked to face the sun; I don't know about the various moons. If they're rotating, tidal flexing from rotation would also heat them up.
- Eerie 10y ago
  Mercury is NOT tidally locked to face the sun. Jeez, are you banned from Google?
  - mcguire 10y ago
    "As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years." Apparently, my memory is faulty.
    - dsp1234 10y ago
      There is a novel by Kim Stanley Robinson whose plot revolves around a city that moves along tracks that circle mercury. With the heating and cooling of the tracks, due to the sun, being the force the drives the city, rather than electricity. It also depicts people walking on the dark side staying just shy of the sun side. https://en.wikipedia.org/wiki/2312_(novel) https://en.wikipedia.org/wiki/2312_(novel)
      - astrange 10y ago
        That's a very classical approach to scifi, like those Larry Niven stories (Neutron Star etc) where a lone hero man with no friends, but a spaceship, defeats a high school physics class problem. It turns out reading only books about lone men with no friends doesn't prepare you for real life.
    - Steuard 10y ago
      My understanding is that scientists believed it was tidally locked for quite some time (until the mid-1960s, and not confirmed until Mariner 10 in 1974-5): the resonance between its rotation and its orbit (together with limitations of where in its orbit Earth had a good view of its surface) meant that observers routinely saw the same surface features in the same places. Some of that history is discussed here: https://en.wikipedia.org/wiki/Mercury_(planet)#Ground-based_telescopic_research https://en.wikipedia.org/wiki/Mercury_(planet)#Ground-based_...

by sidcool 10y ago

mafribe 10y ago

Io, Jupiter's innermost moon and the most volcanically active body in the solar system, is interesting because its source of heat is believed to be tidal heating: Jupiter's gravity pulls Io inwards while Jupiter's other moons pull Io outwards. This leads to a weird, eccentric orbit, which in turn means changing levels and directions of gravity leading to movement of Io's mass, e.g. molten rock under the surface. This causes friction that heats Io, whence volcanism. https://en.wikipedia.org/wiki/Tidal_heating_of_Io https://en.wikipedia.org/wiki/Tidal_heating_of_Io

parr0t 10y ago
Fascinating. Thanks for the link.
yathern 10y ago
I'm no physicist, but I'm curious where the heat comes from, thermondynamically speaking. Does this changing level of gravity gradually sap the speed of Io?
- T-hawk 10y ago
  Yes. It comes from the orbital kinetic energy of Io and the other moons. The same happens in the Earth-Moon system. The Moon causes tides on Earth's oceans, which increase friction between water and land and therefore produce heat. Thermodynamically speaking, that heat comes from the Moon's orbital momentum, as it recedes from Earth and slows in its orbit.
  - mafribe 10y ago
    Earth has also got a small bit of tidal movement of the crust, but not remotely as much as Io. There there is a theory that tidal movement of the Earth's crust is one (of many) factors that can trigger earthquakes.
    - dspillett 10y ago
      >(of many) factors that can trigger earthquakes. Or slowly feed in energy that is stored and released when something else pushes the system far enough out of equilibrium that a quake happens.
      - T-hawk 10y ago
        "Feed in energy" - that energy has to come from somewhere. The point of this subthread is where does that energy come from thermodynamically? The answer is orbital or rotational kinetic energy via tidal forces, which of course do affect both crust and water.
        mafribe 10y ago
        Yes, but different celestial bodies get there energy from different sources. For the earth it's mostly radioactive decay in the core of the planet. In others, like Io, it's believed to be mostly tidal friction.

Udo 10y ago

To answer the other half of your question (mafribe already said why the moons are tectonically active): what's needed for heating due to tidal friction is not only strong gravity, but a strong gravity gradient that changes over time. I was doing some back-of-the-envelope calculations just now to show how Io experiences much more tidal stress than Mercury due to a stronger change in gradient, when I discovered my assumptions were wrong, but I'm posting the results anyway: Mercury's orbit has a huge eccentricity, and its distance to the sun varies between 0.47 AU at Aphelion and 0.307 AU at Perihelion. Taking Mercury's size into account, that should work out to a difference in solar gravity between its near and far side of about 0.000004 m/s^2 at Aphelion and 0.000013 m/s^2 at Perihelion! That's actually quite the squeeze! Now comparing that to Io, which has often been reported as having this wild orbit, upon looking up the actual numbers I saw that Io's orbit around Jupiter is actually nearly circular: 420000km at Periapsis and 423400km at Apoapsis. The moon itself has a radius of 1800km, that means its orbit is so circular its variation falls within the size of the actual moon. Furthermore, plugging in the numbers, Io's gravity gradient between its near and far side remains pretty much stable at 0.000014 m/s^2 no matter where it is, varying only by a factor of about -1.37x10^-08. So the gradient change itself is not dramatic in Io, but reasonably pronounced in Mercury - didn't expect that. Taking a look at the orbital period of both objects, Io completes about 50 orbits in the same time it takes Mercury to orbit once, but even a factor of 50 isn't enough to blow up Io's tidal action to make up for the observed difference in tidal heating. My only conclusion at this point is that I either made a careless mistake somewhere (which is likely) or that Io's squeezing action must come almost entirely from its interaction with the other Jupiter moons. In the end, a heat source is not required for Mercury to experience tectonic activity though. As described in the article, the cooling and shrinking of the planet itself are sufficient to cause disturbances on the surface.

mcguire 10y ago
Shouldn't Mercury have reached thermal equilibrium already?
- Arnt 10y ago
  If Mercury is in an eccentric orbit, then the sun's gravity changes depending on where in its orbit is is. Adding to that, when it'a closer, the difference between the gravity on the bright side and dark side is more pronounced than when it's farther away.

mcguire 10y ago

Mercury is tidally locked to face the sun; I don't know about the various moons. If they're rotating, tidal flexing from rotation would also heat them up.

Eerie 10y ago
Mercury is NOT tidally locked to face the sun. Jeez, are you banned from Google?
- mcguire 10y ago
  "As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years." Apparently, my memory is faulty.
  - dsp1234 10y ago
    There is a novel by Kim Stanley Robinson whose plot revolves around a city that moves along tracks that circle mercury. With the heating and cooling of the tracks, due to the sun, being the force the drives the city, rather than electricity. It also depicts people walking on the dark side staying just shy of the sun side. https://en.wikipedia.org/wiki/2312_(novel) https://en.wikipedia.org/wiki/2312_(novel)
    - astrange 10y ago
      That's a very classical approach to scifi, like those Larry Niven stories (Neutron Star etc) where a lone hero man with no friends, but a spaceship, defeats a high school physics class problem. It turns out reading only books about lone men with no friends doesn't prepare you for real life.
  - Steuard 10y ago
    My understanding is that scientists believed it was tidally locked for quite some time (until the mid-1960s, and not confirmed until Mariner 10 in 1974-5): the resonance between its rotation and its orbit (together with limitations of where in its orbit Earth had a good view of its surface) meant that observers routinely saw the same surface features in the same places. Some of that history is discussed here: https://en.wikipedia.org/wiki/Mercury_(planet)#Ground-based_telescopic_research https://en.wikipedia.org/wiki/Mercury_(planet)#Ground-based_...