GRAIL Constraints on Vertical and Lateral Density Structure of Lunar Crust
Abstract
Using the Gravity Recovery and Interior Laboratory (GRAIL) mission data [1], it has recently been shown that the lunar crust has a lower bulk density than previously thought: the near-surface porosity of ~12 % extends over at least the first few kilometers [2]. Here, we focus on short wavelength gravity and topography data (250<l<550) to infer the vertical and lateral density structure of the lunar crust. We applied a regional windowing to those data, using a single optimal space-limited taper based on [3]. The resulting local admittance - or effective density - spectra are then fitted to two kinds of theoretical spectra (assuming linear or exponential increase with depth). The resulting best-fit parameters enable us to estimate spatial variability of the vertical density structure. First, the mare regions exhibit a distinct decrease of density with depth, as expected from the high density of the mare basalts. Second, the farside is characterized by a general increase of density with depth, in agreement with [2]. A typical average value for the characteristic (e-folding) depth of the low density layer is 10 km, equivalent to a density gradient of ~30 kg.m-3/km. If density variation with depth is explained by porosity variation alone (i.e. pore closure), this would correspond to a typical surface porosity value of at least 20 %. Third, the Apollo 12 & 14 landing sites lie in regions that seem more similar to the farside (i.e. density increase with depth). The fourth and most significant result is that the South Pole-Aitken (SP-A) impact basin region appears different from the rest of the farside. Notably, it is characterized by a shallower low density region, with a characteristic depth of 5-10 km, instead of 15-20 km for the remaining parts of the farside. The singularity of the SP-A impact basin region probably reflects a shallower porous layer. This could be the result of either impact-induced removal of pre-existing fractured material, annealing of pre-existing fractures, thick (porous) SP-A ejecta deposits over the rest of the farside, or a combination of these factors. The apparent similarity between the Apollo 12 & 14 landing sites and the farside in term of shallow density structure points to the possibility of predicting the vertical seismic velocity structure in regions where no seismic data is available. [1] Zuber et al., 2013, Science 339, 668-671 [2] Wieczorek et al., 2013, Science 339, 671-675 [3] Wieczorek and Simons, 2005, GJI 162, 655-675
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.G31B..04B
- Keywords:
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- 6250 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS Moon;
- 1221 GEODESY AND GRAVITY Lunar and planetary geodesy and gravity;
- 3255 MATHEMATICAL GEOPHYSICS Spectral analysis