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[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?

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    $\begingroup$ 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... $\endgroup$ – AtmosphericPrisonEscape Dec 5 '19 at 20:52
  • $\begingroup$ 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 $\endgroup$ – RonS Dec 5 '19 at 21:00
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    $\begingroup$ 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, ...? $\endgroup$ – AtmosphericPrisonEscape Dec 5 '19 at 21:26
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    $\begingroup$ The internal structure of Venus isn't particularly well-constrained (it's difficult to put seismometers there, the surface conditions are somewhat hostile), but there's some evidence that there is a liquid outer core. The lack of magnetic field may be related to the slow rotation and the details of how heat flows through the planet. $\endgroup$ – antispinwards Dec 5 '19 at 22:35
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    $\begingroup$ 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. $\endgroup$ – David Hammen Dec 6 '19 at 10:25
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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.

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  • $\begingroup$ This is consistent with wiki Earth's internal heat budget, but then Earth's magnetic field 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] ... $\endgroup$ – Keith McClary Dec 6 '19 at 6:34
  • $\begingroup$ ... 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? $\endgroup$ – Keith McClary Dec 6 '19 at 6:40
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    $\begingroup$ 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? $\endgroup$ – Carl Witthoft Dec 6 '19 at 15:44
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    $\begingroup$ @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. $\endgroup$ – David Hammen Dec 7 '19 at 13:30
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    $\begingroup$ @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. $\endgroup$ – David Hammen Dec 7 '19 at 13:33
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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.

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  • $\begingroup$ 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? $\endgroup$ – RonS Dec 11 '19 at 7:18
  • $\begingroup$ 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. $\endgroup$ – TazAstroSpacial Dec 12 '19 at 23:41

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