I have been teasing several people here in various threads with mysterious phenomena on Earth as well as on Venus. I have suggested that both may be caused by one single mechanism, that we call the Rapid True Polar Wander. Following the scientific method, we try to explain observations with a hypothetical cause. But that causal mechanism needs to follow the normal concrete physical laws, then we test it to see by experiments if possible and if it can do predictions and explain more phenomena. Now the Rapid True polar Wander could explain the "emergency braking" of Venus and the shifting climate zones in the ice age. But how does it work? We know about the characteristix of spinning bodies (gyroscopes) and their rigidness of the spin axis. But is the body itself rigidly tied to that spin axis? First experiment with the egg. Just take a raw egg and put it on the table. -It will wobble around a bit until it finds it stable lying position. -Now turn it upside down in the most unstable position and mark the top, or north pole, with a marker stift. If you let go of the egg it should rotate and wobble around until the mark is underneath again, touching the table. -If you spin a top it remains upright and rotates only along one axis. -What happens to the egg if you let it spin, starting initially with the heavy marked side up? That's homework for tomorrow
On Earth or in orbit of Earth? Would the experiment matter if being correlated to the spin of a body (albeit much more massive) that is not nearly as influenced by the gravity of another body as the egg is by the Earth?
Hi SW Long time no see. Well the objective of this little eggsperiment was to show that however stabil a spin axis is, it is not automatically stabilizing the body itself. You should noticed that the X dissaperead quickly to the underside of the egg despite it's spinning. So if the X was not tied rigidly to the spin axis of the egg, why would the Earth geographic poles be rigidly tied to it's spin axis as well. Should there be some instability in spinning then one could expect that the Earth would readjusts it's geography, like the egg did, to stabilize the spinning again. The orginal concept of the True Polar Wander is based on that concept whereas the natural inertia tensor of the Earth is assumed not to be aligned with the spin axis. http://www.gps.caltech.edu/~devans/iitpw/science.html The rapid true polar is based on a misalignment of the Mantle spin axis with the solid inner core spin axis. More next time.
So why is the Rapid True Polar Wander important? Because it explains the ice ages in great detail. But that would require a few dozen threads to explain So let's continue with the proposed physical mechanism. First we look at the structure of terrestrial planets On planet Earth, from the outside in we encounter the lithosphere first, the outer shell with relatively little mass, compared to the mantle. The mantle is the next shell which extends down to about the halfway mark on the radius of the Earth. The mantle is comprised of rocky material that can deform more or less elastically, depending on high temperatures and extremely high pressures. The core of the planet is beneath the mantle, and it is thought to consist mainly of iron. It is liquid iron surrounding a solid sphere at the center of the Earth. . The iron is molten due to the high temperatures, on Earth the core temperature is believed to be around 5500 degrees Kelvin. As the pressure towards the center of the Earth continues to increase, it becomes so high that the molten iron is compressed to a solid. Pressure, not cooling, makes the inner core solid. This whole system is spinning and consequently has gyroscopic characteristics. Terrestrial planets move in many cycles, They are orbiting the sun in a more or less elliptical trajectory according to Keppler’s first law of planetary motion, and they spin around their own axis. A spinning planet can be compared with a gyroscope. A gyroscope maintains the direction of its spin axis, regardless of angular or lateral movements. Only external torque forces can cause a deviation of the spin axis, perpendicular to the torque force. This is called precession, Furthermore mechanical pulse forces can cause wobbling movements or nutations Planets and sun interact with gravity forces, this causes several small secondary perturbations, like precession cycles, obliquity cycles, eccentricity cycles, inclination cycles and nutations all exerting torque forces on the planet as gyroscope. However due to the distinctions between the different shells we actually have to deal with a set of three different coupled gyroscopes, the lithosphere-mantle, the fluid outer core and the solid inner core. We will examine the effects of torque forces on this system next
One of the most distinct pertubations that act on earth is the precession of the equinoxes. This is the main driver of the Rapid True Polar Wander This is a motion of the equinoxes along the ecliptic, or a motion of the axis of a planet to describe a cone in somewhat the same fashion as a spinning top. This causes the seasons to shift in time. After half a precession period, the orientation of the spin axis of the Earth is opposite to the original orientation. For Earth this motion was first noted by Hipparchus c.120 B.C. The precession is caused by the different gravitational attraction of a moon and the sun on the near and far side of the equatorial bulge of the planet. This causes a very slight torque that produces precession on a spinning object. For Earth the period of precession is roughly 26,000 years. It was Sir Isaac Newton himself who formulated that explanation. Since Venus has no significant equatorial bulge and a very slow spinning rate, no precession is noted today. However if Venus was spinning like the Earth or Mars in the far past, interacting gravity with the Sun also would have caused a precession of the equinoxes, so the same may have been true for Venus' past. Again as the torque force is acting on the equatorial bulge, it in fact acts on the Crust-Mantle gyroscope, not on both core gyroscopes. The solid inner core is following the physical same mechanisms and the shape of the inner core would determine its own individual precession tendency. This shape however, is not only a function of spinning. It’s also dependent on the temperature and pressure at its surface. These are not easily determined. Moreover, the local gravity differences of the planet’s mantle would exert much strongere torque force on irregularities of the planet’s solid inner core much more than due to the presence of sun and moon. Due to all these complications, it’s highly unlikely that the precession tendency of the solid inner core gyroscope equals the precession tendency of the crust- mantle gyroscope. Consequently, the spin axes of the solid inner core and mantle are not automatically aligned. So do we have a problem here?
So how is the mantle forcing the solid inner core to keep it's spinning axis aligned? Interaction between the core and the mantle of a planet has been explored by several sources. S Aoki (1969) [Friction between Mantle and Core of the Earth as a Cause of the Secular Change in Obliquity - The astronomic Journal 74 nr 2284 -290 March 1969] proposes that a difference between the theoretical change in obliquity of the Earth and the observed changes is caused by the assumption that Earth was a solid mass. Of course the Earth is not a rigid body but has internal motion within it. So he assumes that the friction between mantle and core is responsible for the discrepancy. This logic predicts a westward drift of the core in general and it would give a deceleration of Earth spinning that is much higher than actual. Consequently it would imply that the friction coefficient between the core and mantle, parallel to the spinning axis is smaller than in other directions, allowing for some slip between the core and the mantle. The implication of this hypothesis is that the core should have some freedom of movement. Yet, he also assumes without elaboration that the spinning axis of the inner core gyroscope is always following the spinning axis of the mantle gyroscope. the stabilization of the inner core is a function of properties of inner and outer core. The stabilizing properties being rotational convection cells, and likely magnetism and viscosity. The viscosity of an outer core is uncertain and could deviate within 10 orders of magnitude. However, assuming that a planet’s magnetism is a function of the inner core, collapses of the magnetic field on Earth are believed to be related to chaotic processes in the outer core. This would certainly have its implications on the stabilizing properties of the outer core. The other item of interest is that the momentum of the inner core is a direct function of its size. During the life cycle of a planet, continuous changes take place. It may be assumed that the initial protoplanet was too hot to have a massive solid inner core, and perhaps the whole core may have been probably molten. However, as the compression occurred, the pressure steadily increased in the core, and as the planet continually cooled down, the solid inner core may have formed slowly. The size of the inner core is a function of temperature and pressure. The temperature of the planet’s core is the result of all the physical functions acting upon it. The general tendency is cooling as the heat is flowing away via the mantle to the surface of the planet. However, radiogenic processes may generate heat, likely with the decay process of potassium (40K) prevailing (V. Rama Murthy et al 2003 Nature 423, 163 - 165). Also, mechanical friction between mantle, outer, and inner cores will generate heat as well. Processes may or may not be balancing, but the balance point may shift depending on the parameters. If the balance is towards cooling, decreasing temperatures near the core will tend to solidify the inner core and hence the inner core will grow. As the inner core accumulates mass by solidifying particles from the outer core, so its inertia will increase and consequently the forces to set it in motion. A bigger core will be more unwieldy. In the case of a spinning body, the angular inertia and the angular momentum are determining the forces required for attaining precession rate. As the inner core grows with cooling, the individual angular momentums of all individual particles that attach to the inner core are also transferred to the core. Since the individual radii of the particles decrease slightly in that process, moving inwards to the core, their angular speed will increase slightly. As a consequence the solid inner core --while growing-- should also spin up slightly like the spinning figure skater pulling in her arms in to speed up the spinning. And as mentioned, the Earth’s inner core is indeed spinning faster, although there are alternative explanations for that such as electromagnetic interaction (Glatzmaier 1999) Meanwhile, the gain of angular inertia of the solid inner core at the expense of the fluid outer core can be quantified. The angular momentum for a sphere is proportional to the product of its angular inertia, mass, and angular speed the angular momentum. The mass is proportional to the volume, which is proportional to the third power of the radius, while the angular inertia is proportional to the square of the radius. Consequently the angular momentum of the inner core is proportional to the fifth power of its radius and a small increase in inner core radius has considerable consequences for its momentum and hence for the outer core to correct its precession tendency. Now a certain critical combination of increased size of the inner core and reduced stability perhaps due to collapsing convection flows in the outer core (Glatzmaier 1999) may induce a drifting precession tendency of the inner core gyroscope that will not be corrected by the fluid outer core. Now we do have a problem for sure.
Oops that was a though one, that previous post. So No discussion just apathy I guess. Just remember, the inner core spin axis is not automatically aligned with the main Earth (mantle) spin axis and the same goes for Venus of course. (the design flaw that I mentioned and that got major objections) Talking about Venus, the major difference with Earth is our moon. The lack thereof may have caused Venus to enter the terminal stage, where it is right now, a couple of billion years earlier than Earth. Now, instead of digesting another long post of dull geophysics, why not do another simple experiment. Take a glass and fill it with a fluid. I suggest beer but water will do. Now, stir the fluid to make it spin and then drop a couple of sand grains in it. Where would those sand grains end up and why?
well, for the sake of reviving this very interesating topic, IMO; they will be pulled down the center of the vortex, inline with the pull of gravity. They will then follow the current up the sides of the glass to the surface again; if the current is strong enough.
Really? just try it but don't spoil the beer. The result of that little experiment is essential for understanding the physics of the rapid true polar wander.
it's St Patty's day! I'm not wasting beer with this! Please Register or Log in to view the hidden image! Why is beer better than water? I did this last night w/ Ice tea mix; basically sand - most rose up from the bottom, being pulled up by the vortex (backwards of what I said yesterday). Those grains which did not go into the vortex swirled around in a horizontal eddie which ran around the glass like a donut, roughly halfway up the glass.
Using the system of philosophy taught us by our ancient Greek predecessors, (i.e.foregoing experimentation, but relying upon deduction) I suggest a bimodal distribution, with concentrations at the margin and in the centre of the glass.
Due to the centrifugal forces, the water is pressed to the outside of the glas. So the pressure is higher at the sides. At the bottom of the glass, the friction in the boundary layer reduces the speed of the water, concequently reducing the pressure. This means that the higher pressure at the sides pushes the bottom boundary layer water to the inside, so a secundary flow is generated from the inside out and via the bottom back to the inside. Now as soon as the sand grains hit the bottom, this inside flow carries the grains towards the centre of the glass and with quite some force, which you would notice if you really would bother to try it.
so how does this then effect the earth? because we would be dealing with a sphere w/o a major external gravity source, how does the sand in a glass analogy apply? as we spin around the liquid, and it spins, centripital force tries top get it going in a straigt line. Therefore, it hits the inside of the earth's crust/mantle and pressure builds up there. So then you would see multiple vortecies of magma coming out from the center of the earth, then swirling back towards the center (due to both pressure and gravity); most likely only reaching roughly halfway before momentum send them back out again. All this would be occuring inside, around the equator @ the point of fastest speed. The north and south poles, as well as the very center of the sphere, whould have much less active movement (though I'm not sure if it would be a double-ended cyclone or something). edit: wait, the center is solid, isn't it? so the double cyclone thing would be nullified, I was imagining the earth filled with swirling beer. Please Register or Log in to view the hidden image! we have the mantle w/ is fairly dense, hot stuff, then the outercore which is viscous liquid, and very hot+dense and then the core, which is lopsided and solid, and very, very dense, most likely made of nickle and iron, etc. it is solid because of the pressure @ that "depth" preventing it from expanding out into a molten state, like how bubbles don't form really deep under water (hot liquid->gas). http://www.answers.com/main/content/wp/en/d/dc/Phase-diag.png
Good thinking Please Register or Log in to view the hidden image! Were getting somewhere. Now keep that swirling beer image in mind. That's important. The specific mass of the mantle is much less though than the liquid outer core and the solid inner corre. Some 7 g/cm^3 vs 12-13 /cm^3. Now try to image what would change that swirling when the solid inner core is spinning at an oblique angle. Specifically in the triangular area between the two spinning axes in the north and the south.
Well, I have used a spinner in chemistry; place a plastic (non-reactive) covered magnet into a beaker w/ some liquid, and place it on a device which spins the magnet. Assuming the core is spinning faster than the rest of the planet, then the outcome should be similar. The spinner would stir the mixture from inside, creating a similar situation to the spoon-stirred example, a single vortex to the surface would form. However, I wouldn't expect the core to spin faster than the outside of the planet; the spin of the earth (I assume, not knowing for sure) is residual from the spin of loose debris in space as it condensed around a gravity well. The earth should be over-all slowing down it's rotation (which is supported by measurements), with occational speedups due to earthquakes and stuff. So I'd expect the core to spin slower than the outer core or the mantle or the crust, pushed along somewhat by the faster moving (but less dense) outer material. Honestly, I'm having a tough time visualising, because all analogies would take place on a desktop, with gravity pulling down. Most of those examples don't work when gravity pulls in a 360 degree "downward" angle. edit:as far as the core spinning at an angle to the rotational angle of the rest of the planet, I'd say that the rotational axis would tend to align (due to directions of force), and the more dense core would be the director of any direction change. So if the planet was reotating at a complete verticle, and the top-heavy core began slumping in it's direction of spin, that the earth would wobble as well - first in reaction, but then in a stablising way; the mass of the earth shifting in relation to the pull of the core would then subduct the core's rotational change, bringing it back to verticle.
Okay, let's visualise what is happening. An oblique spinning inner core. Think about the swirling in the liquid outer core, especially in the blue dotted area. Please Register or Log in to view the hidden image! and the trick with the spining beer had a message. BTW the core spins faster indeed, one revolution in 400 years but that's not relevant right now.
wouldn't that be that the core spins slower than the crust? or do you mean that the core spins faster than the crust, and gains an extra rotation cycle every 400 years? a Guess: around the equator, the magma in the inner core would swirl in a horizontal corckscrew, being pulled in toward the center while being "pushed" out by the overall rotation. Above and belowthat midline flow would be a north and south whirlpools, pulling into the center, then shooting up to the roational poles in a vortex pattern. The stuff in the mantle is only semi-liquid, so I really don't know how that would act under spin - I'd expect the equator to remain sort of pressed against the crust, then slide out under the pressure either north or south, creating pretty much what we have w/ air currents; two horizontal whorls on either side of the equator that ring the globe. Total guess based on how water currents work.
The core definitely spins faster I could think of three possible explanations. But it's not relevant right now, yet The effect I was trying to suggest, is hidden in the arrow heads. Look at the second big arrow from the top, the rotation direction of the mantle spin axis, now look at the third arrow from the top, the spinning direction of the solid inner core. Now shift both second and third arrows towards each other and see that the arrowheads point towards each other. What kind of effect would that have on the fluid in the (orange colored) liquid outer core?
