MadSci Network: Astronomy
Query:

Re: Do all planets with significant magnet fields have the angular momentum

Date: Tue Dec 12 22:58:38 2000
Posted By: John W. Weiss, Grad Student in Planetary Science
Area of science: Astronomy
ID: 976379912.As
Message:

To put it simply, no. But the story is a bit more interesting than that, so let me explain.

Recall the Earth's magnetic field is tilted about 11 degrees relative to our rotation axis (which is itself titled 23.5 degrees from perpendicular to our orbit). Below, I've provided the tilts of the other planets with magnetic fields of their own (Mercury, which does have a surprisingly strong field, is poorly studied and consequently, no tilt data appears to be available):

Planet Magnetic Axis Tilt (relative to rotation axis)
Earth 10.8
Jupiter 9.7
Saturn less than 1
Uranus 59
Neptune 47
(all tilts are in degrees; data from Kivelson and Russell's Introduction to Space Physics, 1997, page 508)

Now, you'll notice that while Earth, Jupiter and Saturn all have small tilts in their fields, Uranus and Neptune most definitely do not. But also Uranus is an odd planet that rotates "on its side" relative to its orbit. This is believed to have been caused by a large impact on Uranus, so it isn't clear what effect that this might have had on the magnetic field.

So, while not all planets have a magnetic field aligned with their spins, 3 of the 5 above do (and one of the others may not be indicative of any kind of trend). So there seems to be some trend towards aligning the two axes, but not an overwelming one.

This is made more interesting when we consider that these planets can have fields point in either direction (that is, north pole of the magnetic field in the northern hemisphere or the southern). Recall that the north pole of Earth's field is in the southern hemisphere (which is why compasses point north - their north poles are attracted to the south pole of the Earth field). This is a bit confusing, I know. Jupiter, however, is the opposite: the north pole of its field is in its northern hemisphere. So that is another way other planets differ from Earth.

Are these differences that surprising? Not really. Recall that Earth's magnetic poles wander around over geological times scales, and also periodically flip. If humans had evolved a million years later, our field may look more like Jupiters. All in all, it is hard to see what the patterns are, here. It's a field in which a great deal of work needs to be done, so watch for more results!


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