Seismic tomography and geochemical evidence for lunar mantle heterogeneity: Comparing with Earth

We present seismic tomography and geochemical evidence for the existence of significant lateral heterogeneities in the lunar mantle and make a comparison with the Earth's heterogeneity and seismicity. The Procellarum KREEP Terrane (PKT) is a unique province on the nearside of the Moon. It constitutes only about 15% or less of the lunar surface, but appears to owe a large portion of the Moon's radioactive heat-producing elements. We found a correlation between the Thorium (Th) abundance distribution and seismic tomography of the lunar nearside. The area with high Th abundance exhibits a distinct low shear-wave velocity, and the low-velocity anomaly extends down to 300-400. km depth below the PKT, suggesting that the thermal and compositional anomaly has a depth extent of 300-400. km in the lunar mantle. The distribution of deep moonquakes shows a correlation with the seismic-velocity variations in the deep lunar mantle, similar to the earthquakes which are affected or controlled by structural heterogeneities in the terrestrial crust and upper mantle. The presence of deep moonquakes and seismic-velocity heterogeneities in the mantle implies that the interior of the present Moon may be still thermally active.