Lunar Heat Flow: Global Predictions and Reduced Heat Flux

Abstract

Need For Geothermal Heat Flux Measurements: Geothermal heat flow is a fundamental measure of the internal composition of a planet. On the Moon (which has lost much of its heat of formation, is not strongly tidally heated, and is not likely to have strong mantle convection) surface heat flux results predominantly from the subsurface column abundance of radiogenic material (e.g. U, Th, K…). As U and Th are refractory, the concentration of these elements can be directly related to the refractory composition of the bulk planetary body. These elements are also generally incompatible with rock forming minerals, leading them to be strongly partitioned into the lunar crust during the cooling of an early magma ocean. Therefore, while some heat may originate form the core and mantle, a majority of the flux seen at the surface is expected to be a crustal signal. As on Earth, surface heat flux measurements in areas of differing thickness crust may allow constraints on a reduced heat flux, which will allow for a separation of the mantle heat production component.Lunar Heat Flux Issues: However, the spatial distribution of these elements across the lunar crust has been found to be highly asymmetric. Orbital gamma ray spectrometer data shows most surface rare earth elements are contained in a single region known as the Procellarum KREEP (Potassium-Rare Earth Element-Phosphorous) terrain [Joliff et al., 2000], which is roughly coincident with the near-side mare. Somewhat unfortunately, this anomalous region dominates the near-side landing sites of the Apollo era, including the two heat flux measurements of the Apollo 15 and 17 missions. In Figure 1, the two Apollo sites are marked as black asterisks. These crustal surface measurements are also underpinned by a large theoretical literature [e.g. Laneuville et al., 2018; Siegler et al., 2016] suggesting further asymmetries within the lower crust unseen in surface data.Global Predictions: A global heat flux network of areas both within and without the Procellarum region would provide a much stronger constraint of the radiogenic composition of the Moon as a whole. Additionally, areas of especially thin crust would help isolate the mantle component of geothermal heat production. Figure 1 gives an example of global expectations for geothermal heat production. This is based on GRAIL constrained crustal thickness values [Wieczorek et al., 2014] and surface measured thorium values from the Lunar Prospector Gamma Ray Spectrometer (LP-GRS) measurements [e.g. Lawrence et al., 2003 with an update from Warren, 2005. The crust is given nominal parameters as described in Siegler and Smrekar (2015) here with a uniform crustal density of 2800kg/m 3 assumed. We use a relative column abundance of radiogenic materials as mixture of material characterized by the LP-GRS surface Th values and that of feldspathic lunar crust [Wieczorek and Phillips, 2000]. This map shows the resulting heat flux for a 10% GRS and 90% feldspatic mixture. Some lows are due to low GRS Th values, some due to thin crust as ...

The Volcanic and Radial Expansion/Contraction History of the Moon Simulated by Numerical Models of Magmatism in the Convective Mantle

“…Also shown are the horizontally averaged temperature at 4.4 Gyr for Case Ra2.15e4 (Ra = 2.15 × 10 4 ) and ET3 ( 𝐴𝐴 𝐴𝐴 * 𝑇𝑇 = 3). In (a), the gray and light-gray areas are the temperature distributions in today's lunar mantle inferred byKhan et al (2006Khan et al ( , 2014, respectively; the yellow areas are those estimated from the heat flux at the surface(Siegler & Smrekar, 2014;Siegler et al, 2022); the gray lines are those estimated under the assumption that the mantle consists of dry olivine (the solid line) and wet olivine with 0.01 wt % H 2 O (the dashed line) byKarato (2013). Also shown are (b) the average temperature in the mantle (bulk) and the temperature of the core; (c) the horizontal average of heat flux on the surface and the core-mantle boundary; (d) the root-mean-square average of matrix-velocity in the mantle all plotted against time.…”

Remote detection of a lunar granitic batholith at Compton–Belkovich

“…Here, we present the discovery that a large granitic batholith, similar in volume to terrestrial batholiths such as the Andean Altiplano-Puna Magma Body [12] underlays Compton-Belkovich. We identify this C-B batholith through a broad, increased local geothermal heat flux, which peaks at ~150-200 mWm -2 -approximately 20 times that of the background lunar highlands [13] and over eight times that measured at the anomalously hot Apollo 15 site. Such high heat flux requires a large mid-crustal body with much higher radiogenic element concentrations than previously observed from orbit [4] .…”

Remote Detection of a Lunar Granitic Batholith at Compton-Belkovich

“…Here, we present the discovery that a large granitic batholith, similar in volume to terrestrial batholiths such as the Andean Altiplano-Puna Magma Body [12] underlays Compton-Belkovich. We identify this C-B batholith through a broad, increased local geothermal heat flux, which peaks at ~150-200 mWm -2 -approximately 20 times that of the background lunar highlands [13] and over eight times that measured at the anomalously hot Apollo 15 site. Such high heat flux requires a large mid-crustal body with much higher radiogenic element concentrations than previously observed from orbit [4] .…”

“…Longer wavelengths will be required to map the lower heat flux seen over most of the Moon and other bodies, highlighting a path forward in future spacecraft instrumentation. Such data should be ground-truthed on the Moon by a globally distributed heat flux network [13,36] . Techniques such as in-situ heat flux, seismic, electromagnetic, long wavelength radar exploration, and sample geochemical analysis could further characterize the presence, size, and origin of the Compton-Belkovich pluton system.…”