On Earth the magnetic poles are near the geographic poles.
- Does Earth have this situation by pure luck?
- What is the situation on other planets?
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1$\begingroup$ Probably a better question for Astronomy SE. Actually, it might be put on hold here as off-topic. Is there a space exploration connection somewhere here? $\endgroup$– HDE 226868Sep 7, 2014 at 15:49
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4$\begingroup$ This could be slightly reworded to fit our scope (planetary-science is on-topic), otherwise, if asking directly about the Earth's magnetic field's axial symmetry (though that boils down to naming conventions, polarity is actually currently nearly reversed, with magnetic South pointing close towards geographic North, but we do have evidence that it can flip in paleomagnetic records), this would be a better fit on Earth Science. (Not on Astronomy, there Earth science is equally off-topic). $\endgroup$– TildalWaveSep 7, 2014 at 16:12
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1$\begingroup$ @TildalWave Good points, but I had meant that the extension to other planets would fit better on Astronomy, if one were to find information regarding how the evolution of other planets' poles was determined by astronomical processes (Wish I could find specifics!). $\endgroup$– HDE 226868Sep 7, 2014 at 16:17
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$\begingroup$ @HDE226868 Astronomy is in this case merely means of providing data on other celestials, though most detailed data on planetary magnetism and remnant magnetism actually comes from probes, orbiters and even ground based hardware. Interpreting these data isn't necessarily a task fit for their scope there either. We've closed several Geoscience questions on Astronomy before as off-topic. They also don't have a planetary-science tag, which we do. My only nitpick about this question so far is that it seems Earth-centered (geophysics). $\endgroup$– TildalWaveSep 7, 2014 at 16:25
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1$\begingroup$ I think this fits better on astronomy than earth science. The underlying theory is not unique to Earth, and the Earth is far from the only object in the solar system to have a magnetic field; see my answer. Our solar system is not unique. We see evidence of starspots (and hence magnetic fields) on many other stars. $\endgroup$– David HammenSep 8, 2014 at 3:04
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1 Answer
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It is not coincidence and it does not apply to just the Earth.
The Sun, Earth, Jupiter, Saturn, Uranus, and Neptune all have sizable magnetic fields. Mercury and Ganymede have smaller but still noticeable magnetic fields. All of these bodies have one thing in common: They have a sizable amount of rotating, electrically conductive fluid somewhere beneath the surface.
The underlying theory of why those objects that do have a magnetic field do have them is magnetic dynamo theory. This theory suggests that the object's rotational and magnetic axes need to be a bit offset from one another.
There are some enigmas that dynamo theory still needs to explain. Saturn's magnetic field is almost perfectly aligned with it's rotation axis. Uranus and Neptune have magnetic fields that are tilted by a bit too much, and the center of their fields are offset from the center of the planet by a considerable amount.
On the other hand, Venus, the Earth's Moon, Mars have negligible magnetic fields, despite all three apparently having partially liquid cores. What they most likely don't have is a convective liquid core. These three bodies have a stagnant lid: no active vulcanism, no plate tectonics. This keeps heat bottled up inside and keeps the liquid core from roiling.
answered Sep 8, 2014 at 2:55
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1$\begingroup$ There seems to be some new thoughts about this. 1) The most important thing for planetary magnetic fields could be the cooling and convection rate. 2) Low (but non-zero) electric conductivity is better for magnetic fields because electric and thermal conductivity go hand in hand and would cool the planet quickly and kill the dynamo. 3) Rotation is unimportant because, even Venus rotates fast relative to mantel convection flows. KISSCaltech: Dave Stevenson youtube.com/watch?v=F488kOmhu9Q Gerald Schubert youtube.com/watch?v=QvY-QldNkJU&list $\endgroup$ Sep 8, 2014 at 8:57
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$\begingroup$ I like your answer, but Mars is thought to have a liquid core, at least partially, possibly entirely. sciencedaily.com/releases/2003/03/030307071457.htm and newscientist.com/article/… both by studying how much it stretches by the Sun's tides by studying the expected temperature and material of it's core indicate a mostly liquid core. $\endgroup$– userLTKDec 6, 2016 at 17:14
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$\begingroup$ @userLTK -- You are correct. So does our Moon. I updated the answer. $\endgroup$ Dec 6, 2016 at 18:40
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$\begingroup$ You should probably also mention geomagnetic reversals. $\endgroup$ Dec 7, 2016 at 1:30