<blockquote>
  <p>I've learned that the Earth's core is hot due to decay of radioactive elements.</p>
</blockquote>

<p>This is unproven, non-standard geophysics. There are several arguments against this. One is that <strong>all</strong> of the long-lived radioactive isotopes are isotopes of uranium (two isotopes, <sup>235</sup>U and <sup>238</sup>U), thorium (<sup>232</sup>Th), and potassium (<sup>40</sup>K). The problem: Uranium, thorium, and potassium are strongly lithophilic ("rock-loving") elements. These elements dissolve very nicely in molten rock, but not so much in molten metal. The presence of long-lived radioactive isotopes is enhanced in the Earth's crust, slightly depleted in the Earth's mantle, and by all rights should be strongly depleted in the Earth's core. </p>

<p>Another problem is that any significant amounts of uranium and thorium in the Earth's core have been ruled out due to neutrino detectors. Potassium-40 has not been ruled out because the neutrinos from <sup>40</sup>K decay are too low in energy to detect, but that brings us back to problem #1.</p>

<p>The only hope for this conjecture is that potassium somehow becomes siderophilic at high pressure. There are some experimental results, most of which are highly controversial, that this <em>might</em> be the case.</p>

<p>Yet another problem is the conjecture of high radioactivity in the Earth's core was motivated by explaining the Earth's magnetic field. A number of recent papers say that there is zero reason for this motivation. The Earth's magnetic field is fully explainable without resorting to the chemically unsupported hypothesis that <sup>40</sup>K somehow becomes a siderophile at high pressure.</p>

CC BY-SA 4.0
2019-12-06T06:34:09.460Z
This is consistent with wiki [Earth's internal heat budget](https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget#Radiogenic_heat), but then [Earth's magnetic field](https://en.wikipedia.org/wiki/Earth%27s_magnetic_field#Physical_origin) says: "The Earth's field originates in its core. This is a region of iron alloys extending to about 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core, with a radius of 1220 km, and a liquid outer core.[48]   ...
Keith McClary
CC BY-SA 4.0
2019-12-06T06:40:26.463Z
... The motion of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary, which is about 3,800 K (3,530 °C; 6,380 °F).[49] The heat is generated by potential energy released by heavier materials sinking toward the core (planetary differentiation, the iron catastrophe) as well as decay of radioactive elements in the interior." It is implausible to me that that the relatively small inner core can stay so hot on remnant heat in the middle of the liquid outer core for billions of years. Should I Ask that?
Keith McClary
CC BY-SA 4.0
2019-12-06T10:05:35.143Z
@KeithMcClary - Note the lack of a citation on "as well as decay of radioactive elements in the interior" in that wikipedia article. That said, this is one of those areas where scientists agree to disagree. Geochemists say that there is very little, if any, of the four radioactive long-lived isotopes mentioned in my answer in the Earth's core due to the chemical nature of those three elements. Geophysicists argue that there has to be a good amount of at least one of those isotopes in the core to provide the heat flux they think is necessary to support the geodynamo.
David Hammen
CC BY-SA 4.0
2019-12-06T15:44:32.970Z
So what in modern geothermal models allow the Earth as a whole (not the core) to be as warm as it is vs. the pre-20th century models which calculated an obviously false age of Earth to be less than a million years or so?
Carl Witthoft
CC BY-SA 4.0
2019-12-07T04:57:27.593Z
The lack of citation seems not entirely an accurate point, as any cursory google search on something like "why is the earth's core hot" will pull up dozens of pages from scientists and scientific organizations attributing most of the matter to radiogenic heat.  And the mantle, being way, way, way more voluminous than the core, is supposed to harbor most of those radioactive elements.  Their heat is then transported in part to the core.  The core doesn't need any radioactive elements; the mantle does, and it has them as a consequence of the lithophilia you mention.
zibadawa timmy
CC BY-SA 4.0
2019-12-07T13:30:51.967Z
@CarlWitthoft - Residual heat from the formation of the Earth, and heat generated by the formation of the solid core, with the latter coming in two forms. One is that forming solid iron/nickel from molten iron/nickel is an exothermic reaction. The other is that the solid inner core has fewer impurities (oxygen, silicon, sulfur, ...) than does the liquid outer core. In addition to being an exothermic reaction in and of itself, the formation of the core changes entropy.
David Hammen
CC BY-SA 4.0
2019-12-07T13:33:14.817Z
@zibadawatimmy - Even at the core-mantle boundary, the mantle is cooler than is the core. The heat transfer is from the core to the mantle, not the other way around. That said, a warmer mantle means less heat transfer from the core to the mantle than does a cooler mantle. A warmer mantle however raises a new problem, which is that the Earth's magnetic field requires a certain amount (the exact amount is subject to debate) of heat transfer across the core-mantle boundary.
David Hammen
CC BY-SA 4.0
2020-01-28T22:49:25.217Z
I am a bit confused by this notion. Neutrino observatories have established that a dominant part of Earth's internal heat budget comes from radioactive decay. Or are you simply saying "yes that's true, but not in the core"?
AtmosphericPrisonEscape
CC BY-SA 4.0
2020-01-30T15:08:19.073Z
@AtmosphericPrisonEscape -- To the contrary. With one possible exception, neutrino observatories have established exactly the opposite: The dominant part of the heat budget of the Earth's core cannot come from radioactive decay. The one exception is potassium-40; the neutrinos emitted by the decay of potassium-40 are below the energy detection threshold of current neutrino detectors. You are perhaps conflating the Earth's internal energy budget, which includes the crust, mantle, and core, with the energy budget of the Earth's core.  (continued)
David Hammen
CC BY-SA 4.0
2020-01-30T15:13:28.943Z
Uranium, thorium, and potassium are highly lithophilic elements, so very highly lithophilic that they tend to concentrate in the Earth's crust. Geochemists argue that the Earth's core has to be highly depleted in those elements. The only possible exception might be potassium, but even that remains controversial. There are some experiments that show that potassium might switch from lithophilic to siderophilic at sufficiently high temperatures and pressures. Then again there are other experiments that say this is nonsense.
David Hammen
CC BY-SA 4.0
2020-01-30T16:07:34.630Z
@DavidHammen Yes, this was my question, thanks!
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-12-05T23:19:23.500Z
2019-12-05T23:19:23.500Z
David Hammen
<p>There are two main factors that control if planets have magnetic field. There must be a fluid conducting medium (liquid iron for Earth, liquid metallic hydrogen for Jupiter), and the faster the core rotates the stronger the field.</p>

<p>Mercury rotates slowly,, not sure how much of it's large metallic core is liquid: weak field
Venus probably similar structure as earth because of similar density and mass so will have a liquid outer core like earth, but rotates very slowly : no field
Earth liquid iron outer core rotates fast
Mars: rotates about same period as earth, but core is probably solid. Smaller mass, any heat has leaked away, so core has frozen. The Insight mission is to sort out Mars's internal structure and if they can get the mole down the current heat flow. </p>

<p>There are other  planets with magnetic fields. 
Jupiter and Saturn, liquid metallic hydrogen as the conductor and both are fast rotators.</p>

<p>Uranus and Neptune have odd magnetic fields, but possibly salty ice(water methane)/mush or liquid and both rotate faster than Earth.</p>

CC BY-SA 4.0
2019-12-11T07:18:26.503Z
Thanks for the detailed explanation. Is there a known reason why Venus rotates more slowly than Earth?
Also, comments above stated Mars' core is partially liquid, based on observations. If that is the case, it stands to reason that its magnetic field should be stronger, right? And that it would've been stronger the further back in time you go. So maybe it lost its oceans and atmosphere later than previously expected?
RonS
CC BY-SA 4.0
2019-12-12T23:41:29.283Z
Venus: there has been a recent study showing early oceans could have slowed the rotation before the oceans disappeared. Not sure if that method would result in retrograde rotation (axis tilted more than 90 deg). An old idea was to invoke a collision.
TazAstroSpacial
CC BY-SA 4.0
2019-12-10T02:59:49.480Z
2019-12-10T02:59:49.480Z
TazAstroSpacial
<p>[Edited] I've learned that the Earth's core is hot due to decay of radioactive elements, causing the liquid part of the core to stay liquid. This was stated as an explanation to why the Earth has a strong magnetic field.</p>

<p>Assuming the rocky planets were formed roughly the same way (planetoid collisions etc.), why was Earth the "lucky winner" of abundant radioactive elements, while Mars &amp; Venus were left with much less, causing their cores to solidify, their magnetic fields to weaken, their water blown into space, and their fate sealed to become arid desert and boiling inferno, respectively? </p>

CC BY-SA 4.0
2019-12-05T20:52:22.080Z
I don't think we know that much about the nucleid content of Venus. For Mars we have at least the fact that it has 10 times less Mass, which is bound to play a huge role in heat generation and isolation...
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-12-05T21:00:21.553Z
Well, we do know that Venus has a weak magnetic field, which, at least in the case of Mars & Earth is connected to the existence or lack of a molten iron core. If Venus' core is very different from Earth's, then this merely expands the scope of the original question
RonS
CC BY-SA 4.0
2019-12-05T21:26:21.850Z
My point is: You can't say that Earth's core is more radioactive than its neighbours, because we don't know that. Also please specify by what you mean with "more radioactive". Higher radioactive element content, heat fluxes, ...?
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-12-06T10:25:32.613Z
Mars most likely has a partially molten core (i.e., a solid inner core surrounded by a liquid outer core). A completely solid core is inconsistent with observations of Mars' gravitational field while a partially molten core is consistent with those observations.
David Hammen
<planet><magnetic-field><core><terrestrial-planets>
Why is the Earth's magnetic field stronger than its neighbors?
RonS
# Why is the Earth's magnetic field stronger than its neighbors?

(Asked by: username_0 on 2019-12-05)

[Edited] I've learned that the Earth's core is hot due to decay of radioactive elements, causing the liquid part of the core to stay liquid. This was stated as an explanation to why the Earth has a strong magnetic field.

Assuming the rocky planets were formed roughly the same way (planetoid collisions etc.), why was Earth the "lucky winner" of abundant radioactive elements, while Mars & Venus were left with much less, causing their cores to solidify, their magnetic fields to weaken, their water blown into space, and their fate sealed to become arid desert and boiling inferno, respectively?

**Tags:** planet, magnetic-field, core, terrestrial-planets
> **Comments:**
> **username_1**: I don't think we know that much about the nucleid content of Venus. For Mars we have at least the fact that it has 10 times less Mass, which is bound to play a huge role in heat generation and isolation...  
> **username_0**: Well, we do know that Venus has a weak magnetic field, which, at least in the case of Mars & Earth is connected to the existence or lack of a molten iron core. If Venus' core is very different from Earth's, then this merely expands the scope of the original question  
> **username_1**: My point is: You can't say that Earth's core is more radioactive than its neighbours, because we don't know that. Also please specify by what you mean with "more radioactive". Higher radioactive element content, heat fluxes, ...?  
> **username_2**: Mars most likely has a partially molten core (i.e., a solid inner core surrounded by a liquid outer core). A completely solid core is inconsistent with observations of Mars' gravitational field while a partially molten core is consistent with those observations.  

---

## 2 answers

### Answer by username_2 on 2019-12-05. Score: 8

> I've learned that the Earth's core is hot due to decay of radioactive elements.

This is unproven, non-standard geophysics. There are several arguments against this. One is that **all** of the long-lived radioactive isotopes are isotopes of uranium (two isotopes, 235U and 238U), thorium (232Th), and potassium (40K). The problem: Uranium, thorium, and potassium are strongly lithophilic ("rock-loving") elements. These elements dissolve very nicely in molten rock, but not so much in molten metal. The presence of long-lived radioactive isotopes is enhanced in the Earth's crust, slightly depleted in the Earth's mantle, and by all rights should be strongly depleted in the Earth's core.

Another problem is that any significant amounts of uranium and thorium in the Earth's core have been ruled out due to neutrino detectors. Potassium-40 has not been ruled out because the neutrinos from 40K decay are too low in energy to detect, but that brings us back to problem #1.

The only hope for this conjecture is that potassium somehow becomes siderophilic at high pressure. There are some experimental results, most of which are highly controversial, that this *might* be the case.

Yet another problem is the conjecture of high radioactivity in the Earth's core was motivated by explaining the Earth's magnetic field. A number of recent papers say that there is zero reason for this motivation. The Earth's magnetic field is fully explainable without resorting to the chemically unsupported hypothesis that 40K somehow becomes a siderophile at high pressure.

> **Comments:**
> **username_3**: This is consistent with wiki [Earth's internal heat budget](https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget#Radiogenic_heat), but then [Earth's magnetic field](https://en.wikipedia.org/wiki/Earth%27s_magnetic_field#Physical_origin) says: "The Earth's field originates in its core. This is a region of iron alloys extending to about 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core, with a radius of 1220 km, and a liquid outer core.[48]   ...  
> **username_3**: ... The motion of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary, which is about 3,800 K (3,530 °C; 6,380 °F).[49] The heat is generated by potential energy released by heavier materials sinking toward the core (planetary differentiation, the iron catastrophe) as well as decay of radioactive elements in the interior." It is implausible to me that that the relatively small inner core can stay so hot on remnant heat in the middle of the liquid outer core for billions of years. Should I Ask that?  
> **username_2**: @KeithMcClary - Note the lack of a citation on "as well as decay of radioactive elements in the interior" in that wikipedia article. That said, this is one of those areas where scientists agree to disagree. Geochemists say that there is very little, if any, of the four radioactive long-lived isotopes mentioned in my answer in the Earth's core due to the chemical nature of those three elements. Geophysicists argue that there has to be a good amount of at least one of those isotopes in the core to provide the heat flux they think is necessary to support the geodynamo.  
> **username_4**: So what in modern geothermal models allow the Earth as a whole (not the core) to be as warm as it is vs. the pre-20th century models which calculated an obviously false age of Earth to be less than a million years or so?  
> **username_5**: The lack of citation seems not entirely an accurate point, as any cursory google search on something like "why is the earth's core hot" will pull up dozens of pages from scientists and scientific organizations attributing most of the matter to radiogenic heat.  And the mantle, being way, way, way more voluminous than the core, is supposed to harbor most of those radioactive elements.  Their heat is then transported in part to the core.  The core doesn't need any radioactive elements; the mantle does, and it has them as a consequence of the lithophilia you mention.  
> **username_2**: @CarlWitthoft - Residual heat from the formation of the Earth, and heat generated by the formation of the solid core, with the latter coming in two forms. One is that forming solid iron/nickel from molten iron/nickel is an exothermic reaction. The other is that the solid inner core has fewer impurities (oxygen, silicon, sulfur, ...) than does the liquid outer core. In addition to being an exothermic reaction in and of itself, the formation of the core changes entropy.  
> **username_2**: @zibadawatimmy - Even at the core-mantle boundary, the mantle is cooler than is the core. The heat transfer is from the core to the mantle, not the other way around. That said, a warmer mantle means less heat transfer from the core to the mantle than does a cooler mantle. A warmer mantle however raises a new problem, which is that the Earth's magnetic field requires a certain amount (the exact amount is subject to debate) of heat transfer across the core-mantle boundary.  
> **username_1**: I am a bit confused by this notion. Neutrino observatories have established that a dominant part of Earth's internal heat budget comes from radioactive decay. Or are you simply saying "yes that's true, but not in the core"?  
> **username_2**: @AtmosphericPrisonEscape -- To the contrary. With one possible exception, neutrino observatories have established exactly the opposite: The dominant part of the heat budget of the Earth's core cannot come from radioactive decay. The one exception is potassium-40; the neutrinos emitted by the decay of potassium-40 are below the energy detection threshold of current neutrino detectors. You are perhaps conflating the Earth's internal energy budget, which includes the crust, mantle, and core, with the energy budget of the Earth's core.  (continued)  
> **username_2**: Uranium, thorium, and potassium are highly lithophilic elements, so very highly lithophilic that they tend to concentrate in the Earth's crust. Geochemists argue that the Earth's core has to be highly depleted in those elements. The only possible exception might be potassium, but even that remains controversial. There are some experiments that show that potassium might switch from lithophilic to siderophilic at sufficiently high temperatures and pressures. Then again there are other experiments that say this is nonsense.  
> **username_1**: @DavidHammen Yes, this was my question, thanks!  

---

### Answer by username_6 on 2019-12-10. Score: 3

There are two main factors that control if planets have magnetic field. There must be a fluid conducting medium (liquid iron for Earth, liquid metallic hydrogen for Jupiter), and the faster the core rotates the stronger the field.

Mercury rotates slowly,, not sure how much of it's large metallic core is liquid: weak field
Venus probably similar structure as earth because of similar density and mass so will have a liquid outer core like earth, but rotates very slowly : no field
Earth liquid iron outer core rotates fast
Mars: rotates about same period as earth, but core is probably solid. Smaller mass, any heat has leaked away, so core has frozen. The Insight mission is to sort out Mars's internal structure and if they can get the mole down the current heat flow.

There are other planets with magnetic fields.
Jupiter and Saturn, liquid metallic hydrogen as the conductor and both are fast rotators.

Uranus and Neptune have odd magnetic fields, but possibly salty ice(water methane)/mush or liquid and both rotate faster than Earth.

> **Comments:**
> **username_0**: Thanks for the detailed explanation. Is there a known reason why Venus rotates more slowly than Earth? Also, comments above stated Mars' core is partially liquid, based on observations. If that is the case, it stands to reason that its magnetic field should be stronger, right? And that it would've been stronger the further back in time you go. So maybe it lost its oceans and atmosphere later than previously expected?  
> **username_6**: Venus: there has been a recent study showing early oceans could have slowed the rotation before the oceans disappeared. Not sure if that method would result in retrograde rotation (axis tilted more than 90 deg). An old idea was to invoke a collision.  

---

