Venus Magnetic Field: Dynamo Theory & Slow Rotation

[Freeman Dyson]

Venus currently does not have a magnetic field. I have come across one source that says the rotation plays a part in this, and another source that says that is a misconception.

Although the details of the dynamo effect are not known in detail, the rotation of the Earth plays a part in generating the currents which are presumed to be the source of the magnetic field. Mariner 2 found that Venus does not have such a magnetic field although its core iron content must be similar to that of the Earth. Venus's rotation period of 243 Earth days is just too slow to produce the dynamo effect.

http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/magearth.html"

and then

. It is important to note that, contrary to popular belief, dynamo theory does not credit the smallness of the magnetic moment to the slow rotation of Venus (a Venus day of ~ 243 Earth days is almost equal to the length of its year of ~ 224 days, and its sense of rotation is retrograde)

http://dawn.ucla.edu/personnel/russell/papers/venus_mag/"

Anyone clear this up? thanks.

 

[Chronos]

You have caught the essential part - venus does not have a significant magnetic field because it barely rotates.

 

[granpa]

venus is much less dense than earth.

 

[Vanadium 50]

venus is much less dense than earth.

Nonsense. Venus' density is 5.24 g/cm³ and Earth's is 5.5 g/cm³.

 

[granpa]

yes.
earth density is 5.515 g/cm^3
venus density is 5.24 g/cm^3
mercury density is 5.43 g/cm^3

mercury's density is thought to be due to its composition. its mostly iron which has a surface density of 7.874 g/cm^3
earth on the other hand is mostly rock yet its density is the same as mercury's. this is due to the fact that both the rock and iron in the Earth is much denser than it is at the surface (due to the greater pressure in the interior of the earth. when calculating the pressure bear in mind that gravity actually increases as you go down until you reach the core). Venus is somewhere in between. (and judging from its lack of a magnetic field I would guess that it lies closer to mercury)

 

[Vanadium 50]

And my cat's name is Mittens.

This (5% density difference) is relevant exactly how?

 

[qraal]

Venus currently does not have a magnetic field. I have come across one source that says the rotation plays a part in this, and another source that says that is a misconception.



http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/magearth.html"

and then




http://dawn.ucla.edu/personnel/russell/papers/venus_mag/"

Anyone clear this up? thanks.

The latter is more correct than the former. The internal dynamo is powered by core energy sources, specifically the heat-flow. Earth's rotation contributes to the overall vorticity in the core that helps convection occur, but the energy source is chiefly thermodynamic. Venus may or may not have a solid core, so I think the second source is perhaps premature about that particular item. But there is very likely some significant difference between the cores which causes Venus's field to currently be negligible. That's only for now. We don't have any data on just how recently the field died away and if Venus is merely going through a low field phase before the core kicks in again. Russell's statements are conclusions from a model of Venus's geophysics NOT from direct data other than the present-day low field. His model might be right, but it's just one amongst many.

Due to the high surface temperature what Venus doesn't have is remnant crustal magnetism, which on Earth is so important to the field's local structure and strength. It's just too hot on Venus.

 

[Andre]

Better late than never.

Elaborating a bit more on the core business.

It is hypothized (Gary Glatzmeier) that Earths magnetic field is the total vector resultant of heat convection currents of the fluid outer core of the Earth, transporting the geothermal heat from the solid inner core to the mantle.

If this is correct then it would follow logically that Venus either has no fluid outer core or not enough heat gradient in the core(s) to cause convection currents.

If this mechanism is related to the rotation of the planet then it might relate to internal friction between the several shells of the cores due to different precession tendencies as http://www.me.ucsb.edu/dept_site/vanyo.htm attempted to research. Obviously core mantle friction due to different precession of the different rotation axes is no factor for Venus.

I hope it helps

 

[CLICKHEREx]

I would like to point out that, regarding the strength of gravity, related to the depth below the surface of a planet, the gravity reduces, and the pressure increases, so that, at the centre of the planet, a body is weightless, but the pressure is at its maximum.

At the surface, gravity is at its maximum, because the whole mass of the planet is attracting it, (warping spacetime) although pressure is at its minimum.

 

[qraal]

I would like to point out that, regarding the strength of gravity, related to the depth below the surface of a planet, the gravity reduces, and the pressure increases, so that, at the centre of the planet, a body is weightless, but the pressure is at its maximum.

At the surface, gravity is at its maximum, because the whole mass of the planet is attracting it, (warping spacetime) although pressure is at its minimum.

Gravity is only maximum at the surface of an isodense body. In differentiated planets with distinct cores the density rises to a maximum near the core, then declines. Check out this table of the http://geophysics.ou.edu/solid_earth/prem.html" to see what I mean.

 

[CLICKHEREx]

I take your point, and now realize that there must be regions of gravitational anomalies under large bodies of dense ore, such as Broken Hill, in New South Wales, Australia, which was the biggest silver, lead, and zinc mine in the world. The overall force of gravity in the crust immediately below would have been lower than deeper down, which would be further away from such a large mass.

Regarding the virtually non existent magnetic field of Venus, my hypothesis is that it is as a result of the way it was formed, by the process of agglomeration, and that, despite the presence of a large amount of ferromagnetic material, as indicated by its density of 5.24 gm/cm^3, the individual dipoles are more or less unaligned, with the overall effect of cancelling each other out.

It was the impact of a Mars sized body into the proto-Earth at a shallow angle, which caused the resulting mass to melt, with most of the denser material settling into the core, due to gravitational attraction, with the large amount of lighter material which was thrown off into space initially forming a ring like structure, which eventually became our considerably less dense moon, Luna, with its density of 3.34 gm/cm^3, compared to the Earth's, at 5.52 gm/cm^3.

A dynamo requires torque, and the resulting body was rotating every 5 hours then, with a core of solid material, mostly iron, surrounded by currents of liquid ferromagnetic material, which produces our powerful magnetic field.

 

[FTL_Diesel]

It was the impact of a Mars sized body into the proto-Earth at a shallow angle, which caused the resulting mass to melt, with most of the denser material settling into the core, due to gravitational attraction, with the large amount of lighter material which was thrown off into space initially forming a ring like structure, which eventually became our considerably less dense moon, Luna, with its density of 3.34 gm/cm^3, compared to the Earth's, at 5.52 gm/cm^3.

The differentiation of the Earth didn't require the Moon-forming impact to occur - after all, the rest of the terrestrial planets (and some asteroids) are differentiated as well.

As I understand it, the exact reason for why Venus is not geologically active (or why it does not have a core capable of creating a magnetic field through the dynamo effect) is poorly understood.