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\begin{document}

\title{The Apollo Conundrum: The Moon Clearly Had a Magma Ocean. Did Earth?}



\author[1]{Jason Morgan}%
\affil[1]{SUSTech}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\begin{abstract}
When Apollo returned the first moonrocks, a major surprise was that the
lunar highlands are built of a single mineral - feldspar. Feldspar
crystals floating to the top of a moon-magma ocean can explain this
composition. If accepted as a key early stage in lunar evolution, it is
a logical leap to infer that Earth must have had a similar -- or larger
-- magma ocean during its early evolution. (The gravitational impact
energy per unit mass that is released during a planetesimal's accretion
scales as GM/R.) The nagging problem with the inference that Earth
passed through an early magma-ocean stage is that the oldest rocks on
Earth show no direct signs of a magma ocean. Instead the petrology of
the oldest preserved Earth rocks shows clear evidence that repeated
events of small to medium degrees of partial melting and melt
extraction, as opposed to pervasive fractional crystallization, has been
the modus operandi of terrestrial differentiation. The big difficulty is
how to effectively `remix' the products of an early terrestrial magma
ocean back into the quasi-uniform `primordial' pyrolite/peridotite
silicate lithology from which oceanic and continental crust are thought
to have evolved by partial melting events. Here I propose that a
partially molten silicate body is actually highly resistant to the
formation of a magma ocean. Jing and Karato (2012)'s experiments imply
that a silicate melt should absorb much more impact shock-energy than
either a silicate solid or an iron solid/melt. In this case, impact
energy will be heterogeneously added into the growing proto-Earth, with
silicate partial melts being shock-compression-heated to their
vaporization temperature before their surrounding silicate solids heat
to their melting point. The growing partially molten planetary surface
will tend to `explode' during impact events, with each
impact-induced-explosion using a relatively small mass of vaporized
silicate partial melt to fragment and rework much larger masses of cold,
shock-fractured overlying `lithosphere'. This explosive-armour-like mode
for silicate planetary accretion will strongly resist the magma
ocean-mode of planetary differentiation. A magma ocean would only tend
to form in the planetary body created from the accreting debris ring of
a giant impact event, a Moon.%
\end{abstract}%



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