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Thread: Earth's Magnetic Field. Does it persist today from one or more large impacts, or something else?

  1. #1 Earth's Magnetic Field. Does it persist today from one or more large impacts, or something else? 
    Forum Junior Double Helix's Avatar
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    The formation of the Moon is believed to have resulted from one or more impacts of large planetesimals into the proto-earth during the earliest period of planetary formation in our solar system. Again, this may have resulted from one or more large impactors.

    The "orbital debris" from such impact(s) is believed to have formed the Moon. It is also possible that such impactors were also differentiated - they had molten iron-nickel cores. If any such object hits the earth at the right angle, it is likely that this metallic core would have contributed to the Earth's core, obtained during its primary formation. But differentiation of smaller bodies might not have occurred so quickly, preventing such a direct enhancement of Earth's core.

    Venus does not have a moon, and its magnetic field collapsed ca. 1 billion years after its formation, leaving it barren and lifeless. The two planets are nearly the same size, and it seems likely that it never accumulated a large iron-nickel core to retain its magnetic field to the present day.

    The difference between the two may very well be due to such impact event(s), with none of them hitting Venus, or else it should have a moon. Or could the Earth have formed in an iron-nickel rich accretion zone, which allowed it to develop a larger iron-nickel core, prolonging its magnetic field for much longer. Stranger things have been known to happen!

    Is there any strong evidence that impactors which likely led to formation of the Moon is the only reason life exists here today, or could there be another reason for Earth's long-lived magnetic field? Some have suggested that tidal forces of the Moon in its orbit could be helping to keep Earth's core hot. It is certainly not clear to at least one of us that a "most likely" answer is accepted.


    Last edited by Double Helix; January 27th, 2021 at 05:02 PM.
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    Venus doesn;t have a magnetic field to speak of due to its slow spin speed. It's metallic core size is on a par with Earths


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    Quote Originally Posted by unbrakochaz View Post
    Venus doesn;t have a magnetic field to speak of due to its slow spin speed. It's metallic core size is on a par with Earths
    You do bring up a good point about Venus' slow rotation playing a major role, at least with regard to the longer-lived field on Earth. But since so many are asking about why Venus does not have a magnetic field (1,2,3), rotation alone seems not to be required with a hot, convective core, which will apparently create its own field absent a planet's significant rotation. Indeed, why Venus does not have a magnetic field is a long standing puzzle not explained by its slow rotation (3). And rotation is not always the primary answer given by those speculating on this matter (4), and is what gave rise to the question of this thread.

    So perhaps the rotation of earth was imparted by the collision(s) which formed the moon, and that alone allows for a longer-lived field, without formation of a larger core. The degree of insulation slowing heat loss from Earth's core may also be involved.

    Most theories on the formation of the inner planets predict molten cores, inducing magnetic fields for Mercury, Venus, Earth and Mars in their early years of formation.

    While rotation helps, it alone does not explain such a rapid loss of the field. Faster heat loss from a smaller core, and thereby loss of convection, may be a better reason for its early dissipation. Less effective insulation slowing heat loss may have also played a role.

    Mars had a field for the first 500 million years or so based on magnetic observations of its oldest terrain. It has been postulated that the late heavy bombardment (LHB) accelerated heat loss and thus the early loss of Mars' field (5). Craters older than about 4 bya on Mars are magnetized, earlier ones are not, giving evidence dating the collapse of its active field - right around the time of the LHB.

    Such data cannot be acquired on Venus since it was "re-surfaced" within the last 800 million years (or less) by massive vulcanism (6). One wonders if the LHB may have also caused a more rapid cooling of Venus' core. And of course long-lasting and extensive vulcanism would also have resulted in faster cooling.

    Even Mercury retains a magnetic field induced by a hot convective core, although only about 1% that of Earth. (7)

    Still, the issue of contributions to Earth's molten core from one or more major impactors remains unresolved. But the slow rotation of Venus is a good addition to why it lost its magnetic field more quickly (combined with a more rapid core cooling), and not simply due to a smaller core upon formation. Perhaps Earth never acquired a larger core, and other factors actually are involved in its prolonged field strength, one of which is a significant rate of rotation.

    Can you provide a reference about the relative core sizes of Earth and Venus - you did say their sizes are "on par"?


    1. https://astrobiology.nasa.gov/news/i...ield-on-venus/

    2. https://www.lpi.usra.edu/vexag/meeti...-Magnetism.pdf

    3. sappho.eps.mcgill.ca/~olivia/666/2007/Why_does_Venus_lack_a_magnetic_field.pdf ("copy and paste" this link to download the pdf file for this artilcle from McGill)

    4. https://phys.org/news/2017-12-doesnt-venus-magnetosphere.html

    5. https://www.sciencemag.org/news/2009...himper-or-bang

    6. https://www.smithsonianmag.com/scien...kin-180957436/

    7. https://science.nasa.gov/science-new...mercuryupdate/
    Last edited by Double Helix; March 6th, 2021 at 05:00 PM.
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    I suppose a slow spinning planet could have a magnentic field if the core were ferro-magentic
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    Quote Originally Posted by karlbifs View Post
    I suppose a slow spinning planet could have a magnentic field if the core were ferro-magentic

    In order for the core to be ferromagnetic, it would have to be formed by something which induces magnetism. That is how iron magnets are made, by placing them in an applied magnetizing field.

    Craters on Mars which are 4 billion years or older retain magnetism believed to result by this very process - magnetized by a core geodynamo which existed in Mars' early years.

    Younger craters are not magnetized, suggesting the core cannot "auto-ferromagnetize" itself, so to say.

    So if a planet's iron core is ferromagnetic, how could that have been formed?
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    Is there any gravitational energy in proxima centuri?
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