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Once we have granted that any physical theory is essentially only a model for the world of experience, we must renounce all hope of finding anything like the correct theory... simply because the totality of experience is never accessible to us.
Hugh Everett

The truest acts of scientific brilliance come from those who push the envelope of conventional wisdom

I think it's a very simple explanation: Venus is rotating very slowly .

2 things are thought to generate a magnetic field (a) liquid iron core (b) rapid rotation rate.

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'Just be a good team player in life', Andrew Evans

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Once we have granted that any physical theory is essentially only a model for the world of experience, we must renounce all hope of finding anything like the correct theory... simply because the totality of experience is never accessible to us.
Hugh Everett

The truest acts of scientific brilliance come from those who push the envelope of conventional wisdom

As I understand it, electrons get free in the molten metal. Then when they move in circular orbits around the earth's axis, they are basically forming an electric current. The magnetic field "B" is proportional to the electric current "I". Also, as the rotation rate increases, "I" increases also. So slower rotations have smaller electric currents (like Venus) and hence lower magnetic fields.

I think they are still researching the details, but that is a pretty good explanation.

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'Just be a good team player in life', Andrew Evans

The Sun's magnetic field is a hundred times larger than Earth's.

The Sun's radius is a hundred times larger than the Earth's radius, and the Sun rotates much slower than the Earth, about once a month or so.Do you mean the circular orbits formed by the rotation of the earth?

That motion does not form the magnetic field--since the magnetic field rotates with the earth.

Clearly dynamo effect has not only to do with the speed of rotation, but also the mass of the rotators. You can spin a microscopic magnet between coils as fast as you want, you won't get much EM field produced compared to doing the same thing at a tenth of the speed with a much heavier one

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The impossible often has a kind of integrity which the merely improbable lacks.

Well,

I would distinguish three objects (a) stars (b) Jovian planets (c) terrestrial planets. The mechanism details are different for these 3. I was commenting on (c) only.

I don't think the rotation of magnetic field with earth negates the idea of motion within the iron core. Geologists think that the convection in the mantle combined with earth's rotation creates this net current and magnetic field.

So I disagree that Venus rotation rate is not a factor. Venus probably has a core similar to earth's and so I think it's rotation rate is the key factor.

But I concede Mercury's field is a puzzle. It probably has a molten core, but it also rotates slowly. I don't think planetary scientists have figured this one out yet.

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'Just be a good team player in life', Andrew Evans

The molten iron core is in constant convection. This convection is able to drive a dynamo mechanism. Karl-Heinz Glassmeier can explain it a lot better in this paper, for Mercury. I will quote a little part from the paper:

and then he goes on with equations galore, great paper.

So basically you need convection in the core(mantle), which can be influenced by the rotation of the planet. But another important part in this convective motion (at Earth) is the sulphur content in the iron. I am not exactly sure about how this process works, but it plays a major role.

Basically, the mechanism for all three objects that you mention are the same, it is always the alpha-omega dynamo which is working. The driving forced for these two aspects of the dynamo can be different, an iron-sulphur core in terrestrial planets, a metallic hydrogen core in gas giants, a fusion core (simplified view) at stars.

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善數, 不用籌策 (shàn shù, bù yòng chóu cè)
He who is good at counting, uses no counting tools
“A good scientist has freed himself of concepts and keeps his mind open to what is”
道德經, 二十七 (dào dé jīng, 27)

Excellent. This illustrates that these geophysics issues are still being researched and debated by the scientific community.

Earth v. Venus field: noone will know for sure until they have a good convection model for Venus' interior and if that is very comparable to earth, then the major difference is the rotation rate. I would think the convections are similar . Venus is not thought to have plate tectonics because it has a thicker more plastic crust, and that will cause some difference.

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'Just be a good team player in life', Andrew Evans

The quoted text says it is the differential rotation. The differential rotation of the earth core is small, but present--if Venus's core were not rotating at all, it might have the same differential rotation. Once the dynamo started, it might be self-sustaining.No, the motion in the core is almost certainly the source of the magnetic field. We know it's liquid, and Gauss showed that it is the origin of the earth's field.
There ya go!

Just to nitpick, the text also says that for the alpha dynamo you need convective motion.

The two processes can work separately, but in combination they are the hammer!

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善數, 不用籌策 (shàn shù, bù yòng chóu cè)
He who is good at counting, uses no counting tools
“A good scientist has freed himself of concepts and keeps his mind open to what is”
道德經, 二十七 (dào dé jīng, 27)

I don't disagree with that. Clearly, you need an energy source, which is apt to be capable of producing convectionI guess the only question I have is whether or not the planet must be in strong rotation. Didn't HypothesisTesting mean that Mercury was a counterexample?

Yes, and that is the whole purpose of the paper by Glassmeier et al. to which I linked. Mercury has different as expected magnetic field and Glassmeier shows in his paper how this can be explained. His abstract reads:

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Any comments in glorious red are to be considered in ModeratorMode.


善數, 不用籌策 (shàn shù, bù yòng chóu cè)
He who is good at counting, uses no counting tools
“A good scientist has freed himself of concepts and keeps his mind open to what is”
道德經, 二十七 (dào dé jīng, 27)

Tusenfem (The website for some reason isn’t allowing me to quote the correct line, every day a different problem, but…)

"The driving forced for these two aspects of the dynamo can be different, an iron-sulphur core in terrestrial planets, a metallic hydrogen core in gas giants, a fusion core (simplified view) at stars."

Well, if what I’m gathering is correct, sulfur plays (with relation to an iron-sulfur core in terrestrial planets) some role in the production of a magnetic field. If this is true than wouldn’t one expect Jupiter’s moon Io to have a stronger magnetic field? It seems to have one but like Venus it’s also very weak.

A paper I’ve read recently suggests that it might stem from “the inertial wave resonances generated by the eventual precession of these planets or by possible tidal distortions of their liquid cores.”

http://www.iop.org/EJ/article/1742-6...3BGYU5G92wi7Kg

__________________
Once we have granted that any physical theory is essentially only a model for the world of experience, we must renounce all hope of finding anything like the correct theory... simply because the totality of experience is never accessible to us.
Hugh Everett

The truest acts of scientific brilliance come from those who push the envelope of conventional wisdom

Isn't that saying that the Chapman-Ferraro external field is opposing Mercury's strong internally-generated field?

My main point is:

(a) Mercury is quite different from Venus/Earth internally, and so the mechanism would be different in magnitude depending on the internal convection details. I'm not surprised in Mercury's case that a slow rotater can have a moderate field because the internal structure is unusual for a planet of its size
(b) but Earth and Venus are "twin" planets and the interiors are no doubt very similar, at least in the cores. So if the interiors are similar , then convection in the core is similar. So the only difference in their magnetic fields must be primarily due to the difference in the rotation rate.
(c) forget Mars, it has a remnant field, maybe from its past history but its core is probably solid now.
(d) in Jupiter's case, the rapid rotation rate clearly contributes to its big field

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'Just be a good team player in life', Andrew Evans

No, the sulphur is only important for the convection in the iron core. I will have to look up the details somewhere.

As far as our investigations have shown, there is no internal magnetic field of Io (if there is any has an equatorial strength less then 50 nT, Kivelson et al., Jupiter book).

the Le Gal et al paper seems interesting, but I don't have time right now to read it.

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善數, 不用籌策 (shàn shù, bù yòng chóu cè)
He who is good at counting, uses no counting tools
“A good scientist has freed himself of concepts and keeps his mind open to what is”
道德經, 二十七 (dào dé jīng, 27)

I think its because of the slow rotation and the fact it rotates in the opposite direction to all the other planets, giving the impression of a low/negative magnetic field.

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Venus may not have as large an iron core as Earth. The Mars-sized object that made the Moon left most of its core here.

Some interesting stuff on Gary Glatzmaier's webpage.

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Yes, this is a good point.

My view is that current research is still being done on the magnetic fields of terrestrial planets. Scientists have just recently been able to get a detailed analysis of the earth's interior ; and it may take decades before they get a detailed view of the Venus/Mercury interiors. So this debate may go on for decades, kind of like the plate tectonics theory until the 1960s.

I think some combination of interior convection, rotation rate, and maybe the interior composition and dimensions, may all be involved in more or less degrees in all the planets.

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'Just be a good team player in life', Andrew Evans

The density of Venus is very close to the density of earth.
Also it is thought, by some, that the slow rotation of Venus might be the result of a similar collision.

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It is thought that the magnetic field is the result of the difference between the mantal speed and the core speed. Perhaps Earth's moon causes part of the difference.
Mercury with no moon does have 1/50 th ? the magnetic field of Earth/ perhaps partly due to Mercury ecentric orbit about the Sun, while the Venus orbit is almost circular. Neil

Based on theoretical dynamo model simulations, Venus could, if it cooled sufficiently, have a magnetic field. The limitation is not rotation, based on the theoretical geodynamo simulation. (See below.)

The following discusses the limitations of current geodynamo simulations for simulations of the earth.

Properties and Evolution of the Earth’s Core and Geodynamo by Nimmo & Alfe

Comment:
It should be noted that the dynamo mechanism (for planets) is only a hypothesis at this time. As noted above the geodynamo simulations only resemble the geomagnetic field.

There are a number of basic geomagnetic observations (such as, how to explain the changing time duration between geomagnetic reversals, the long period in which there were no reversals, and so forth) which to date do not have an explanation.

Also in the last 780 kyrs, there is an unexplained tracking of geomagnetic field intensity with planetary temperature which to date has not been explained. (There were theories that the ice sheets changed the earth's rotational rate which some how effect the dynamo and so forth.)

This is a link to a paper where it is argued that because Venus’ liquid core is not cooled by plate tectonics, that there is not the necessary temperature differential, across Venus’ liquid core to Venus' mantle, to drive convection in Venus’ liquid core, to generate a magnetic field.

Comments:
1) It is expected Nimmo’s paper is based on the Magellan satellite data which showed that Venus’ crust is roughly 500 to 600 million years old.

2) For some unknown reason (there are different hypotheses), Venus’ crust separated roughly 700 million years ago and the entire planet was resurfaced with magna.

3) The conclusion that the Venus’ surface is 500 million to 600 million years old was based on a statistical analysis of the current number of impact craters on Venus' surface compared to the estimate expected rate of impacts per 100 million years. Using similar reasoning, it is argued that the resurfacing of Venus’ surface with magna, is estimated to have taken no longer than 100 million years.

The following is an excerpt from Nimmo’s paper “Why does Venus lack a magnetic field?”.

http://geology.geoscienceworld.org/c...ract/30/11/987