RoI - Compton-Belkovich Th Anomaly

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Compton/Belkovich Th Anomaly

(Tier 2 Region of Interest for Constellation Program)

Official NASA Overview

ROI_-_Compton-Belkovich_Th_Anomaly.JPG
source: NASA Cx-LROC Tier 2 Targets (PDF); see expanded details below


Scientific Rationale

Farside KREEP Th (~ 10 ppm) anomaly

Lunar Thorium (Th) abundances on the lunar surface can aid in the various geological processes that formed during the moon's early history, and answer questions about its subsequent evolution over time. Signatures of the radioactive componant in Th show up in KREEP-like deposits more on the Moon's nearside rather than on its farside, and it's this relationship between the two components which signify one of the last crystalisation processes of a magma ocean that may have once globally covered the Moon. Recent, high resolution measurements by both the Lunar Prospector (USA ~ 1998-99) and Kaguya (Japan ~ 2007-08) space probes, however, do show that low concentrations of Th-KREEP deposits do exist on the farside; most especially found in zones around the South Pole-Aitken basin, and odd anomalous regions such as exists at the Compton-Belkovich borders. Both Thorium and the KREEP-rich (K - Potassium, REE - rare earth elements, P - Phosphorus) deposits are believed to have formed between the lunar crust and mantle when less dense cumulates floated upwards during the moon's evolution.

Compton-Belkovich Anomaly
The Th deposits at Compton-Belkovich lie in a rugged feldspathetic highlands crust region between the two inner rings (250 and 350 km in diameter respectively) of the Humboltianum Basin lying some ~ 300 km away to its south-west. Moreover, they are found to sit on the eastern side of a saddle-like outcrop whose topography is higher in a north-south direction than east-west, and whose abundance of Th is higher than on any other measured location on the lunar surface. On closer analyses of both the LP/Kaguya data, the Th-anomaly at Compton-Belkovich was found to be that of an outcrop of alkali anorthosite similar to those samples taken on location from Apollos 12 and 14. Such anorthosites are a componant in rocks that have an evolved composition associated to the Mg-suite (old ferroan anorthosites > 4.4 Bn years old) of magmas that intruded throughout the moon's crust early in pre-4 billion year lunar history. The problem, however, with the Th-anomaly at Compton-Belkovich is that no Mg-suite of rocks have been identified, and it is possible that KREEP lithologies may lay hidden below the outcrop of alkali anorthosites. More research and sampling of the area thus needs to be carried put, firstly, because the processes involved with late crystallisation aren't quite fully understood and, secondly, the anomaly may be due to a new type of rock that presently isn't available in current collections to date. - JohnMoore2


compton-belkovich-small.jpg

Left: Clementine image from Map-A-Planet.
Middle: Lunar Prospector Map of Compton-Belkovich Th-Anomaly - adapted from Thorium Abundances on the Lunar Surface paper (see Bibliography).
Right: Clementine image - adapted from The Compton-Belkovich Region of the Moon paper (see Bibliography).

Click image for larger view


compbel-thorium-small.jpg

Left: Lunar Prospector Map of Thorium of the moon's entire surface (1998-99).
Right: Kaguya Map of Thorium of the moon's entire surface (2007/08).

Click image for larger view


Additional Information


*Thorium, Potassium, Uranium, Iron, Oxygen, Silicon, Aluminium, Calcium, Magensium, Lithium.

LPOD Articles

Peaking Over the Limb, A New Mare to Study, Edge World.

General Bibliography

Mostly Thorium-related papers


Mostly KREEP-related papers