Remote Detection of a Lunar Granitic Batholith at Compton-Belkovich

Abstract

Granites are nearly absent in the Solar System outside of Earth. Achieving granitic compositions in magmatic systems requires multi-stage melting and fractionation, which also increase radiogenic element concentrations. Water and plate tectonics facilitate these processes on Earth, aiding in remelting. Although these drivers are absent on the Moon, small granite samples have been found, but details of their origin and the scale of systems they represent are unknown. We report microwave-wavelength measurements of an anomalously hot geothermal source that is best explained by the presence of a ~50 km diameter granitic system below the thorium-rich, farside feature known as Compton-Belkovich. Passive microwave radiometry is sensitive to the integrated thermal gradient to several wavelengths depth. The 3-37 GHz antenna temperatures of the Chang’E 1 and 2 microwave instruments allow us to measure peak heat flux of ~184 mWm^-2; ~20 times higher than the average lunar highlands. The surprising magnitude and geographic extent of this feature imply an Earth-like, evolved granitic system larger than believed possible on the Moon, especially outside of the Procellarum region. Furthermore, these methods are generalizable: similar uses of passive radiometric data could vastly expand our knowledge of geothermal processes on the Moon and other planetary bodies.

Formation of silicic crust on early Earth and young planetary bodies in an Iceland-like setting

“…Surface water on early Mars and hydrous minerals (amphibole) in meteorites 26,51 provide the necessary clues that this may have been possible early in Mars' history, before losing much of its water. On Venus and the Moon, which are thought to be lacking water through much of their histories 26,28 transport of water to depth to generate silicic magma may be more problematic. Given the low geothermal gradient and lack of water, hydrous partial melting of basalt to produce silicic crust on Venus is unlikely.…”

“…An Iceland setting can also be used as an analogue for generating silica-rich rocks on planetary bodies, other than the early Earth, that do not have subduction processes linked to active plate tectonics. In particular, Venus, Mars, and the Moon [26][27][28] are examples of stagnant lid regimes, where the entire crust is a single plate. Although these planetary bodies are dominated by basaltic magmatism, there is evidence for the presence of silicic igneous material on all three [26][27][28] , possibly analogous to the Eoarchaean continental crust formed on Earth.…”

“…In particular, Venus, Mars, and the Moon [26][27][28] are examples of stagnant lid regimes, where the entire crust is a single plate. Although these planetary bodies are dominated by basaltic magmatism, there is evidence for the presence of silicic igneous material on all three [26][27][28] , possibly analogous to the Eoarchaean continental crust formed on Earth. Furthermore, on Venus, steep-sided volcanic domes also likely represent eruption of silica-rich magma within dominantly basaltic terrains 27 , again perhaps analogous to Iceland.…”

In Situ Measurements of Thermal Environment on the Moon's Surface Revealed by the Chang'E-4 And Chang'E-5 Missions

“…Direct in-situ measurements of the thermophysical and optical properties of lunar regolith and rocks, as well as lunar surface temperature, were conducted by Apollo 15 [12], [13], Apollo 17 [14], and the CE-4 mission [15]. Numerical analysis has been extensively employed for studies on the global lunar surface temperature [16], [17], the effects on lunar surface temperature from solar and Earth radiation and lunar heat flow [18], the thermal stability of water ice deposits in permanently shadowed regions [19], global heat flow [20], and geological interpretations [21]. However, there is currently a rare of reports on the influence of the lunar surface thermal environment on the temperatures of detectors and their payload instruments.…”