EPONA CONSTRUCTION COMPANY COMMENTARY

 

We received this letter from Paul Preuss on the subject of Coti Mundi's magnetic field:

 

"I have a query which I think should be forwarded to Martyn Fogg. His description of 82 Eridani III notes that it has 55% of Earth's mass and 85% of Earth's size (radius), and that, 'Its material constitution was similar, having a liquid iron core, rocky outer layers, and a watery surface.'  My question concerns the core. Given a low mass, presumably pressure‑temperature curves do not permit a solid iron core, but I don't have the expertise to compute this. Also, I wonder how large the liquid iron core is (another way of asking how much iron the planet contains). Since there are no moons, the planet presumably was denied one source of Earth's copious supply of iron ¾ Earth's moon is generally thought to have resulted from the collision of a Mars‑sized object with the primitive Earth, which object, disintegrating, contributed much of its iron core to Earth's and its stony outer layers, mixed with matter from Earth, to build our moon.

 

If there is no solid core, it seems to me the magnetic field of Coti Mundi could be a very strong and stable dipole. Earth's magnetic field is presumably generated by vortices in the liquid core, but these must be spaced around the solid inner core, which may contribute to their complexity and instability. Without such a solid inner core, a single vortex could form along the spin axis of the planet.

 

But if there's not as much iron inside Coti Mundi to begin with, this could reduce the chance of a strong magnetic field.  It has been suggested that magnetic fields are used by some organisms for foraging and navigation and may have larger effects over evolutionary time. Any guidance?"

 

Martyn Fogg replies:

 

I must admit that planetary magnetism is one of the areas of planetology that I am most ignorant of. What I do know is that nobody else fully understands how planetary magnetic fields are generated, how the reversal mechanism works and why some fields seem to be weirdly tilted to their planet's spin axis. Presumably, Mars doesn't have a magnetic field because its core is solid and may be mostly troilite (FeS); Venus may have a liquid core but perhaps its slow rotation prevents the formation of the sort of currents that generate a magnetic field.  Thus, for 82 Eridani III, I had to resort to "hand waving," reasoning that the planet has a rapid spin and probably a liquid core, and therefore it has a magnetic field. No more thought went into it than that I'm afraid.

 

Your hypothesis concerning the strength and stability of the planet's magnetic field is interesting, but I am not convinced. It may be instead that the presence of a solid center to the core may result in the liquid layer receiving shear forces from both above and below generating the flows that we require. Shear currents may not occur in a completely liquid core and, if its temperature gradient is adiabatic, there wouldn't be much convection either. (I vaguely remember talking to somebody who said this is why Venus has no field ¾ its core is completely liquid ¾ no shear. But I tried looking it up today and couldn't find anything.)

 

Nice to hear from you, Paul. If you want to take this matter further then I'd be interested to hear of any thoughts. Otherwise, for magnetic fields at least, it's back to the "hand waving."

 

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