• New explanation for Moon's half-century

    From ScienceDaily@1:317/3 to All on Thu Jan 13 21:30:34 2022
    New explanation for Moon's half-century magnetic mystery

    Date:
    January 13, 2022
    Source:
    Brown University
    Summary:
    A new study reveals how the diminutive Moon could have been an
    occasional magnetic powerhouse early in its history, a question
    that has confounded researchers since NASA's Apollo program began
    in the 1960s.



    FULL STORY ========================================================================== Rocks returned to Earth during NASA's Apollo program from 1968 to
    1972 have provided volumes of information about the Moon's history,
    but they've also been the source of an enduring mystery. Analysis of
    the rocks revealed that some seemed to have formed in the presence of
    a strong magnetic field -- one that rivaled Earth's in strength. But
    it wasn't clear how a Moon-sized body could have generated a magnetic
    field that strong.


    ==========================================================================
    Now, research led by a Brown University geoscientist proposes a new
    explanation for the Moon's magnetic mystery. The study, published in
    Nature Astronomy, shows that giant rock formations sinking through the
    Moon's mantle could have produced the kind of interior convection that generates strong magnetic fields.

    The processes could have produced intermittently strong magnetic fields
    for the first billion years of the Moon's history, the researchers say.

    "Everything that we've thought about how magnetic fields are generated
    by planetary cores tells us that a body of the Moon's size should
    not be able to generate a field that's as strong as Earth's," said
    Alexander Evans, an assistant professor of Earth, environmental and
    planetary sciences at Brown and co-author of the study with Sonia Tikoo
    from Stanford University. "But instead of thinking about how to power
    a strong magnetic field continuously over billions of years, maybe
    there's a way to get a high-intensity field intermittently. Our model
    shows how that can happen, and it's consistent with what we know about
    the Moon's interior." Planetary bodies produce magnetic fields through
    what's known as a core dynamo.

    Slowly dissipating heat causes convection of molten metals in a planet's
    core.

    The constant churning of electrically conductive material is what produces
    a magnetic field. That's how Earth's magnetic field -- which protects
    the surface from the sun's most dangerous radiation -- is formed.

    The Moon lacks a magnetic field today, and models of its core suggest
    that it was probably too small and lacked the convective force to have
    ever produced a continuously strong magnetic field. In order for a
    core to have a strong convective churn, it needs to dissipate a lot of
    heat. In the case of the early Moon, Evans says, the mantle surrounding
    the core wasn't much cooler than the core itself. Because the core's
    heat didn't have anywhere to go, there wasn't much convection in the
    core. But this new study shows how sinking rocks could have provided intermittent convective boosts.

    The story of these sinking stones starts a few million years after the
    Moon's formation. Very early in its history, the Moon is thought to have
    been covered by an ocean of molten rock. As the vast magma ocean began to
    cool and solidify, minerals like olivine and pyroxene that were denser
    than the liquid magma sank to the bottom, while less dense minerals
    like anorthosite floated to form the crust. The remaining liquid magma
    was rich in titanium as well as heat- producing elements like thorium,
    uranium and potassium, so it took a bit longer to solidify. When this
    titanium layer finally crystallized just beneath the crust, it was
    denser than the earlier-solidifying minerals below it. Over time, the
    titanium formations sank through the less-dense mantle rock underneath,
    a process known as gravitational overturn.



    ==========================================================================
    For this new study, Evans and Tikoo modeled the dynamics of how those
    titanium formations would have sunk, as well as the effect they might
    have when they eventually reached the Moon's core. The analysis, which
    was based on the Moon's current composition and the estimated mantle
    viscosity, showed that the formations would likely break into blobs as
    small as 60 kilometers and diameter, and sink intermittently over the
    course of about a billion years.

    When each of these blobs eventually hit bottom, they would have given
    a major jolt to the Moon's core dynamo, the researchers found. Having
    been perched just below the Moon's crust, the titanium formations would
    have been relatively cool in temperature -- far cooler than the core's estimated temperature of somewhere between 2,600 and 3,800 degrees
    Fahrenheit. When the cool blobs came in contact with the hot core after sinking, the temperature mismatch would have driven an increased core convection -- enough to drive a magnetic field at the Moon's surface as
    strong or even stronger than Earth's.

    "You can think of it a little bit like a drop of water hitting a hot
    skillet," Evans said. "You have something really cold that touches the
    core, and suddenly a lot of heat can flux out. That causes churning
    in the core to increase, which gives you these intermittently strong
    magnetic fields." There could have been as many as 100 of these
    downwelling events over the Moon's first billion years of existence,
    the researchers say, and each one could have produced a strong magnetic
    field lasting a century or so.

    Evans says the intermittent magnetic model not only accounts for the
    strength of the magnetic signature found in the Apollo rock samples,
    but also for the fact that magnetic signatures vary widely in the Apollo collection -- with some having strong magnetic signatures while others
    don't.

    "This model is able to explain both the intensity and the variability
    we see in the Apollo samples -- something that no other model has been
    able to do," Evans said. "It also gives us some time constraints on the foundering of this titanium material, which gives us a better picture
    of the Moon's early evolution." The idea is also quite testable, Evans
    says. It implies that there should have been a weak magnetic background
    on the Moon that was punctuated by these high- strength events. That
    should be evident in the Apollo collection. While the strong magnetic signatures in the Apollo samples stuck out like a sore thumb, no one
    has ever really looked for weaker signatures, Evans says.

    The presence of those weak signatures along with the strong ones would
    give this new idea a big boost, which could finally put the Moon's
    magnetic mystery to rest.

    ========================================================================== Story Source: Materials provided by Brown_University. Note: Content may
    be edited for style and length.


    ========================================================================== Journal Reference:
    1. Evans, A.J., Tikoo, S.M. An episodic high-intensity lunar core
    dynamo.

    Nat Astron, 2022 DOI: 10.1038/s41550-021-01574-y ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/01/220113111412.htm
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