A Farside Geochemical Window into the Moon

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A Farside Geochemical Window into the Moon
Dewar area

• Thorium concentrations are highest on the
  nearside (Procellarium KREEP Terrane)
• Farside characterized by low Th and FeO
  abundances
• Anomalously high Th was found and studied near
  Dewar crater on the farside
• Th distribution vital for understanding the
  evolution of the lunar crust

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A Farside Geochemical Window into the Moon
Dewar anomaly has enhanced Th, FeO, Sm, and TiO2
values relative to surrounding highlands.
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A Farside Geochemical Window into the Moon

• Reflectance spectrum from the Dewar anomaly (red
  line) similar to near-side mare basalt (cyan
  line)
• - Mafic assemblage dominated by high-Ca
  pyroxene
• - Dewar anomaly likely caused by the presence of
  variable amounts of Th-rich mare basalt fragments
  in the regolith
• Surrounding highlands have typical anorthosite
  spectrum (white line)

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A Farside Geochemical Window into the Moon
How did the Dewar anomaly form? Body of evidence
does not support a deposit of mafic impact melt
breccias or surface deposits of mare basalts or
pyroclastics. The Dewar anomaly probably formed
from material excavated from a buried mare
basalt, or cryptomare, derived from a magma with
enhanced thorium concentrations.