Magnetic field of Solar System planets

[Pedro de la Torre]

Hello. I am reading a paper which says that only Earth (between the rocky planets) keep a strong magnetic field and it arose an question on this:

Why does the magnetic field of the planets decrease with time in such planets?

Why the magnetic field of the Earth is the only one which keeps strong?

 

[davenn]

Why does the magnetic field of the planets decrease with time in such planets?

because their cores have cooled off

Why the magnetic field of the Earth is the only one which keeps strong?

because it still has an actively rotating core

 

[rootone]

Anticipating the obvious next question, why so for Earth and not the other rocky planets?
1, It is the largest of the rocky planets.
2, It has a very large Moon. which causes tidal friction; contributing to keeping the interior hot.

 

[Drakkith]

The reason a hot core is important is because the Earth's magnetic field is believed to be generated by the dynamo effect. This requires a hot, molten mantle/core.

 

[Ophiolite]

Anticipating the obvious next question, why so for Earth and not the other rocky planets?
1, It is the largest of the rocky planets.
2, It has a very large Moon. which causes tidal friction; contributing to keeping the interior hot.

This is slightly misleading in two ways.

First, Venus is almost the same as the Earth (dimensions, composition, gross structure) and is thought to have a partially molten core close to the size of the Earth's, though there are some marginally plausible core compositions that could have solidified. However, if the core is molten, as seems likely, then Earth's "large" size cannot explain the absence of a field from Venus, while it does explain it for Mercury, Mars and the moon.

Assuming there is a molten core, it seems there is no significant convection - a necessary condition to initiate and maintain a geodynamo. In that case why no dynamo? It has been suggested that Venus's slow rotation inhibits convection, but this is generally discounted. (For example, here.) The more likely explanation is the thick lithosphere. This supresses plate tectonics and thereby restricts the loss of internal heat, a necessary condition for convection.

Secondly, the Earth's internal heat is primarily a consequence of residual thermal energy from accretion, radioactive decay and core formation. The contribution from tidal flexing is insignificant. (See, for example, Chapter 9, [page 1150] Basaltic Vulcanism on Terrestrial Planets.)