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Which of the four terrestrial planets has the strongest magnetic field, Mars, Mercury, Venus, or Earth?

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    $\begingroup$ Earth. Far, far. $\endgroup$
    – peterh
    Dec 1 '20 at 15:01
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    $\begingroup$ I think this may be a better fit for Astronomy SE. $\endgroup$ Dec 2 '20 at 16:00
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    $\begingroup$ "better fit" is never a close reason. Planetary science questions are on-topic here in Space @Panzercrisis Exploration SE. Planetary magnetic fields are carefully measure space-exploring spacecraft using carefully designed space-magnetometers. The magnetic fields affect things like trapped radiation which can damage spacecraft and so they need to be designed around it. This is squarely on-topic here. $\endgroup$
    – uhoh
    Dec 2 '20 at 23:00
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Which of the four terrestrial planets has the strongest magnetic field, Mars, Mercury, Venus, or Earth?

The Earth, by far.

The four giant planets have rather strong magnetic fields. Amongst the terrestrial planets, only the Earth has a moderately strong magnetic field, but weaker than those of the giant planets. Mercury, like the Earth, has a self-sustained dipole magnetic field, but much weaker than that of the Earth. The Moon, and possibly Mars, appear to have had ancient planetary dynamos, but not anymore. Finally, Venus also does not have a self-sustained dipole magnetic field.

One prior explanation for why the Moon and Mars do not have self-sustained dipole magnetic fields has fallen by the wayside. It was long posited that the cores of the Moon and Mars have long since frozen. This is not the case. Analyses of the gravity fields of the Moon and Mars have shown that those two objects have partially molten cores consisting of iron and other elements, just like the Earth.

It appears that three key ingredients are needed for a terrestrial planet to have a significant magnetic field:

  1. A molten or partially molten core consisting of iron, nickel, and other elements. All four terrestrial planets and the Moon pass this criterion.
  2. A sufficiently high rotation rate. What that sufficiently high is subject to debate. One rotation per sidereal day is obviously sufficiently high. Mars has a very similar rotation rate but does not have a self-sustained dynamo.
  3. A sufficiently high heat flux across the core-mantle boundary. This is the one factor that distinguishes the Earth from the other four terrestrial objects. Mercury, the Moon, and Mars have static lid tectonics: their surfaces are frozen. Venus appears to have alternated between static lid tectonics (which is what it has now) and extreme volcanic tectonics (the most efficient way to cool a planet).
    The Earth is the only one of those five objects to have plate tectonics. While not as efficient as extreme volcanism with regard to cooling a planet, it apparently is good enough to maintain the heat flux across the core-mantle boundary that is needed to maintain a self-sustaining dynamo.
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    $\begingroup$ “What that sufficietntly high [rotation rate] is subject to debate” – it is in particular also dependent on the size. The smaller the MHD region, the higher the rotation rate needs to be. (Other properties like density will matter as well.) — I would rephrase point 1.: “consisting of iron, nickel...” suggests that it needs to be ferromagnetic metals. That is not the case, the only important thing is that it's conductive fluids. The sun and the gas giants have hydrogen dynamos. $\endgroup$ Dec 1 '20 at 22:32
  • $\begingroup$ Looks like another "Goldilocks condition": Surface life may need a magnetic field to shield it from charged particle storms, but too much core-mantle convection probably regularly covers large swathes of the surface in magma, not conducive to life either. Geological activity needs to be "just right". Isn't it astonishing just how well Earth is suited to our development ;-). $\endgroup$ Dec 2 '20 at 14:24
  • $\begingroup$ What is a "dynamo"? $\endgroup$
    – TylerH
    Dec 2 '20 at 17:02
  • $\begingroup$ @TylerH en.wikipedia.org/wiki/Dynamo_theory Cf. also my answer here: physics.stackexchange.com/questions/373449/… $\endgroup$ Dec 2 '20 at 18:57
  • $\begingroup$ @leftaroundabout After some reading it seems the better link is en.wikipedia.org/wiki/Dynamo, e.g. "A dynamo is an electrical generator that creates direct current". So here, a (self-sustaining) dynamo is just a self-sustaining electromagnetic field around a celestial body. $\endgroup$
    – TylerH
    Dec 2 '20 at 19:16
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Mercury has a weak magnetic field (scientists were surprised it has any, as it was thought to be cold and have no molten core to provide a magnetic field) Venus and Mars don't have any significant magnetic field either.

Of the 4, only Earth has a sufficiently large molten iron core for rotation and convection to provide our strong magnetic field.

Interestingly, the gas giants have much stronger magnetic fields than Earth, and they must generate it in a different way to the four inner planets. It is expected it may be liquid hydrogen at the core which is conductive, allowing convection to create magnetic fields.

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  • $\begingroup$ Re Of the 4, only Earth has a large molten iron core which provides our strong magnetic field. This is incorrect. Mars and Venus almost certainly have a large molten iron core, and Mercury also appears to have one. $\endgroup$ Dec 1 '20 at 14:17
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    $\begingroup$ hmmm - bad wording on my part. Lemme fix. How's that? $\endgroup$
    – Rory Alsop
    Dec 1 '20 at 14:52
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    $\begingroup$ The Earth is the only one of the five terrestrial objects that has plate tectonics. The current thinking is that the other four currently have static lids, thereby trapping heat inside the planet. $\endgroup$ Dec 1 '20 at 15:22
  • $\begingroup$ @DavidHammen five terrestrial objects? $\endgroup$
    – Drake P
    Dec 4 '20 at 0:05
  • $\begingroup$ @DrakeP I spelled them out in my answer: the four terrestrial planets and the Moon. The Moon, like the four terrestrial planets, is significantly more than large to have drawn itself into a more or less spheroidal shape and has an iron-rich core that is partially molten, a dense rocky mantle, and a less dense rocky crust. Ceres might or might not qualify as a terrestrial object. It formed very close to the "ice line", so it most likely is a mix of rocky and icy material. $\endgroup$ Dec 4 '20 at 0:58

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