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    H2O from comets and wet asteroids
    mixed later? 
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    Lunar
    origin 
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    Bulk
    composition of moon like earths mantle 
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    Isotopic
    composition also very similar 
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    But is missing all the volatile elements (H in H2O,
    etc.) 
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    Dynamics
    of orbit also unusual compared to other 
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    moons 
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    Giant Impact Origin Theory 
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    Higher
    Mercury density also due to giant impact? 
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