<p>I have a question about planetary cores and their relation to the magnetic field.</p>
<p>Around how massive (using this in the sense of mass, not size, for clarity) does a core need to be in relation to the planet in order to produce a strong magnetic field in accordance with modern theories? Does the fraction of the planet that is within the Core (the CMF, as I am going to be calling it) decrease with the mass of a planet increases.</p>
<p>So, for instance, Earth's CMF is about 33.4%, according to my math (which is supported by looking online which gives values of ~33% or &quot;above 30%&quot;), but would a larger planet require less CMF? I ask this due to the fact that, for instance, 33.4% CMF for a 1M (M for Earth Masses), is smaller than 25% CMF for a 1.5M (0.335 and 0.375 Earth Masses respectively).</p>
<p>Basically my question is, is it the proportionality that matters, or is it simply about the mass of the core independent from planetary mass/radius?</p>
<p>And it would be lovely if you could explain what the theorized relation is, as, for added context, I am just creating fictional planets and just want to be as scientifically accurate as I can be, and CMF is a pretty major factor in planetary density, and thus mass, radius, gravity, and so forth. I just thought that this Stack was better than Worldbuilding for this question as this is strictly about astronomy/astrophysics, and I feel I might find better answers here.</p>
<p>I apologize if this is not worded the best. I can add further clarification if requested.</p>

CC BY-SA 4.0
2023-12-18T00:29:00.563Z
Mercury has a very high CMF, but 1% of Earths magnetic field
James K
CC BY-SA 4.0
2023-12-18T01:15:13.447Z
@JamesK So it would be more about precise mass, so the neccessary CMF would reduce as planetary mass increases? Is what you are implying?
DanceroftheStars
CC BY-SA 4.0
2023-12-18T03:31:05.933Z
See https://en.wikipedia.org/wiki/Dynamo_theory We still aren't sure, but it's likely that a planet's magnetic field depends on core rotation and fluid circulation, not just core mass.
PM 2Ring
CC BY-SA 4.0
2023-12-18T17:55:45.157Z
Sulphur (or sulfur..) seems to be important in planetary dynamos by lowering the melting point of iron and keeping the core liquid for longer; stories about the [Mars dynamo](https://daily.jstor.org/how-mars-lost-its-magnetic-field-and-then-its-oceans/) and [Mercury dynamo](https://www.sciencenews.org/article/mercury-magnetic-field-insulating-metal-iron-sulfide-hot-core)
astrosnapper
CC BY-SA 4.0
2023-12-19T05:38:40.667Z
@PM2Ring So, question, does the influence of the coriolis effect mean a planet needs a great enough rotatation rate in order to produce a magnetic field? Is there any way of knowing what the theoretical requirements are for the angular velocity?
DanceroftheStars
<magnetic-field><density><core>
Question about the core mass fraction of planets and its relation to the magnetic field
DanceroftheStars
# Question about the core mass fraction of planets and its relation to the magnetic field

(Asked by: username_0 on 2023-12-17)

I have a question about planetary cores and their relation to the magnetic field.

Around how massive (using this in the sense of mass, not size, for clarity) does a core need to be in relation to the planet in order to produce a strong magnetic field in accordance with modern theories? Does the fraction of the planet that is within the Core (the CMF, as I am going to be calling it) decrease with the mass of a planet increases.

So, for instance, Earth's CMF is about 33.4%, according to my math (which is supported by looking online which gives values of ~33% or "above 30%"), but would a larger planet require less CMF? I ask this due to the fact that, for instance, 33.4% CMF for a 1M (M for Earth Masses), is smaller than 25% CMF for a 1.5M (0.335 and 0.375 Earth Masses respectively).

Basically my question is, is it the proportionality that matters, or is it simply about the mass of the core independent from planetary mass/radius?

And it would be lovely if you could explain what the theorized relation is, as, for added context, I am just creating fictional planets and just want to be as scientifically accurate as I can be, and CMF is a pretty major factor in planetary density, and thus mass, radius, gravity, and so forth. I just thought that this Stack was better than Worldbuilding for this question as this is strictly about astronomy/astrophysics, and I feel I might find better answers here.

I apologize if this is not worded the best. I can add further clarification if requested.

**Tags:** magnetic-field, density, core

> **Comments:**
> **username_1**: Mercury has a very high CMF, but 1% of Earths magnetic field  
> **username_0**: @username_1 So it would be more about precise mass, so the neccessary CMF would reduce as planetary mass increases? Is what you are implying?  
> **username_2**: See https://en.wikipedia.org/wiki/Dynamo_theory We still aren't sure, but it's likely that a planet's magnetic field depends on core rotation and fluid circulation, not just core mass.  
> **username_3**: Sulphur (or sulfur..) seems to be important in planetary dynamos by lowering the melting point of iron and keeping the core liquid for longer; stories about the [Mars dynamo](https://daily.jstor.org/how-mars-lost-its-magnetic-field-and-then-its-oceans/) and [Mercury dynamo](https://www.sciencenews.org/article/mercury-magnetic-field-insulating-metal-iron-sulfide-hot-core)  
> **username_0**: @username_2 So, question, does the influence of the coriolis effect mean a planet needs a great enough rotatation rate in order to produce a magnetic field? Is there any way of knowing what the theoretical requirements are for the angular velocity?  

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## 0 answers

