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
PSRDpresents
www.psrd.hawaii.edu/April08/Dewar-Th.html
A Farside Geochemical Window
into the Moon
Dewar anomaly has enhanced Th, FeO, Sm, and TiO2 values
relative to surrounding highlands.
PSRDpresents
www.psrd.hawaii.edu/April08/Dewar-Th.html
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)
PSRDpresents
www.psrd.hawaii.edu/April08/Dewar-Th.html
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.
PSRDpresents
www.psrd.hawaii.edu/April08/Dewar-Th.html