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Remote detection of a lunar granitic batholith at Compton–Belkovich

An Author Correction to this article was published on 25 September 2023

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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 increases the concentration of radiogenic elements1. Abundant 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 unknown2. Here we report microwave-wavelength measurements of an anomalously hot geothermal source that is best explained by the presence of an approximately 50-kilometre-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-gigahertz antenna temperatures of the Chang’e-1 and Chang’e-2 microwave instruments allow us to measure a peak heat flux of about 180 milliwatts per square metre, which is about 20 times higher than that of the average lunar highlands3,4. 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 region5. 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.

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Fig. 1: Latitudinally corrected 3-GHz TA shown at midnight local time.
Fig. 2: Microwave expectations from a geothermal source.
Fig. 3: Data minus forward model TA compared with the best-fit pluton model.
Fig. 4: Geophysical models of the Compton–Belkovich batholith.

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Data availability

The data used to make the figures in this paper are available at https://doi.org/10.5281/zenodo.7786749. The original Chang’e‐1 and Chang’e-2 MRM data can be downloaded from http://moon.bao.ac.cn/index_en.jsp. Our group has also produced a readable global gridded data product of all available Chang’e-1 and Chang’e-2 data at https://zenodo.org/record/7790013Source data are provided with this paper.

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Acknowledgements

This work was funded through Lunar Data Analysis Grant 80NSSC20K1430 and work related with the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer.

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Contributions

M.A.S.: primary writing, central ideas and concepts, figures and funding. J.F.: primary data processing, microwave modelling, writing, central ideas and data interpretation. K.L.-F.: petrologic model synthesis, writing and figure creation. J.C.A.-H.: gravity modelling, synthesis and writing. R.C.E.: petrologic model synthesis, writing and advised K.L.-F. M.S.C.: lead data product production (of global maps), science discussions, detailed review and editing. C.M.: aided in data product production (of global maps), science discussions, detailed review and editing. J.W.H.: science discussions, detailed review and editing. T.D.G.: science discussion, review and editing. M.N.W.: copy-editing and figure editing.

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Correspondence to Matthew A. Siegler or Jianqing Feng.

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Extended data figures and tables

Extended Data Fig. 1 Loss Tangent Derivation from Chang’e 2 data.

(a) Integrated loss tangent for each of the four frequencies derived from Chang’e 2 microwave amplitudes and thermal model fits (as in19) (b) Integrated loss tangent as a function of frequency for (black) the areas within 1 degree of CBVC, (red) the entire maps in ED1, and (blue) the highlands terrain model from19. The blue line is used for our modeling of the CBVC heat flux.

Source Data

Extended Data Fig. 2 Chang’e instrument antenna pattern models.

Simulated antenna patterns for each of the four MRM frequencies. Here they are plotted in antenna angle, leading to different spatial footprints for the Chang’e 1 vs. 2 missions due to their 200 and 100 km altitudes.

Source Data

Extended Data Fig. 3 Chang’e 2 antenna temperature data processing.

(a) Antenna temperatures as a function of time of day and distance from CBVC, (b) Gridded noontime antenna temperature data from the Chang’e 2 MRM mission centered at Compton-Belkovich overlain on LROC WAC topography- units in K. Note the trend in temperatures with latitude. (c) Gridded ΔTA data from the Chang’e 2 MRM mission centered at Compton-Belkovich after latitude correction overlain on LROC WAC topography- units in K. The 3 GHz 3c figure is used in main text Fig. 1.

Source Data

Extended Data Fig. 4 Forward model based of brightness temperatures.

(upper) Modeled surface temperature at night without a CBVC source. (lower) Full resolution modeled brightness temperature of four frequencies (again without a CBVC source) at night before convolution.

Source Data

Extended Data Fig. 5 Forward model of antenna temperatures, TA.

The modeled antenna temperature (in the absence of a CBVC heat source) at four frequencies after convolution.

Source Data

Extended Data Fig. 6 Data/forward model differences.

(a-d) The “data minus model” residual antenna temperature (units K) at four frequencies subtracted from the Chang’e 2 data. Contours map the LP-GRS Th enhancement (after Wilson et al.7), (e) The pixon-reconstructed LP-GRS Th concentrations with contours of CE-2 3 GHz data- model values. (f) CE-2 data minus model residual antenna temperature at four frequencies as a function of surface measured Th.

Source Data

Extended Data Fig. 7 Possible evolution of the CBVC subsurface system.

The resolution of the gravity and heat flow models is insufficient to inform internal variations in a body that must have formed from a complex system of magma chambers. This figure is one example of a system that could be represented by the Compton Belkovich batholith model and how that system could develop over time.

Extended Data Fig. 8 Model differences as a function of pluton geometry.

(a) Absolute data-model differences in peak TA for the 5 km e-folding density crustal model for various pluton diameters. Stars show minima for different crustal density models. (b) Absolute data-model differences in peak TA over the area within 15 km of the center of CBVC for the 5 km e-folding density crustal model for various pluton diameters.

Source Data

Extended Data Fig. 9 Model differences as a function of pluton geometry examining sensitivity to heat production of upper pluton.

(a) same as 8b, but for both the upper and lower pluton at 69.7 ppm Th. (b) 8b and 9a, but with no heat production from upper pluton and lower Pluton at 69.7 ppm Th, showing that most of the heat observed heat is coming from the lower magmatic system.

Source Data

Extended Data Fig. 10 Gravity models and GRAIL data for the CBVC.

Observed Bouguer gravity data and model corrected Bouguer gravity. Bouguer gravity for an assumed crustal density of (a) 2500 kg/m3, with corrections for the modeled density assuming lower intrusion densities of (b) 60 kg/m3, (c) 90 kg/m3, (d) 120 kg/m3, and (e) 200 kg/m3 and (f) North-south profiles of the observed and model corrected Bouguer gravity for assumed density contrasts of 60–120 kg/m3.

Source Data

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Siegler, M.A., Feng, J., Lehman-Franco, K. et al. Remote detection of a lunar granitic batholith at Compton–Belkovich. Nature 620, 116–121 (2023). https://doi.org/10.1038/s41586-023-06183-5

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