47th Lunar and Planetary Science Conference (2016)
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EXPLORING THE DEPTH DISTRIBUTION OF LUNAR CRUSTAL MASS ANOMALIES USING GRAIL GRAVITY AND LOLA 1
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TOPOGRAPHY. Maria T. Zuber , David E. Smith , Sander J. Goossens Gregory A. Neumann , Frank G. Lemoine , 3
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Erwan Mazarico , Antonio Genova , David D. Rowlands . Department of Earth, Atmospheric and Planetary Sci-­‐
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ences, Massachusetts Institute of Technology, Cambridge, MA 02129-­‐4307, USA ([email protected]); CRESST, Uni-­‐
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versity of Maryland, Baltimore County, Baltimore, MD 21250, USA; NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; Introduction: Globally-­‐distributed, high-­‐
and 120< L <180 (30 – 45 km depth). Lower degrees (L resolution gravity [1] and topography [2] observations < 120) correspond approximately to mantle depths of the Moon from the Gravity Recovery and Interior [6]. Laboratory (GRAIL) mission [3] and Lunar Orbiter Laser We compare observations of Bouguer gravity de-­‐
Altimeter (LOLA) instrument [4] aboard the Lunar Re-­‐
constructed into various degree ranges to models to connaissance Orbiter (LRO) [5] provide the unique address what information might be obtained regard-­‐
opportunity to explore the shallow internal structure ing vertical density distribution. We extend depth sen-­‐
of the Moon, in greater detail than for any other solid sitivity when compared to earlier work using GRAIL planetary body beyond Earth. Here we develop mod-­‐
gravity [7]. els for mass anomalies within the crust and consider Illustrative Observations: We initially focus on their expression at the lunar surface, and compare highlands crust, where >98% of free air gravity is asso-­‐
results to observations from the GRAIL and LOLA data ciated with topography [1], so high degree and order sets. The objective is to understand the extent to mass anomalies that remain after the Bouguer correc-­‐
which source depths of mass anomalies can be con-­‐
tion constitute <2% of the full gravitational signal. We strained, given the inherent non-­‐uniqueness of gravity deconstruct the gravity field and plot the subsurface measurements. distribution of density anomalies for several repre-­‐
Approach: To study crust and upper mantle sentive regions of the lunar highlands, including sev-­‐
structure, we combine gravity and topography to pro-­‐
eral basins. duce Bouguer gravity that reveals the distribution of mass in the subsurface. Increasingly high degrees in the spherical harmonic expansion of the Bouguer gravity are sensitive to increasingly shallower struc-­‐
ture, with the sampling depth taken to correspond to the spatial block size or half wavelength of the degree and order of the spherical harmonic coefficients. ,
Figure 1. Model crust and associated depth zones de-­‐
fined by degree ranges in the power spectrum of the GRAIL-­‐LOLA Bouguer anomaly. We specify spherical harmonic degree (L) ranges to isolate contributions from different depths. We define a model crust illustrated in Figure 1. We consider a maximum of L= 540, corresponding to a spatial block size and depth range of 10 km, because the Bouguer spectrum is essentially error free to that point. We break the crust into degree ranges of 271 < L < 540 (10 – 20 km depth), 181 < L < 270 (20 – 30 km depth), Figure 2. Profile of Bouguer anomaly along the lunar equator from 190 E to 270 E. The full anomaly is at the top, with contributions below shown for degree rang-­‐
es 271 < L < 540 (10 –20 km depth), 181 < L < 270 (20 – 30 km depth), 120 < L < 180 (30 – 45 km depth). Figure 2 shows a profile of the Bouguer anomaly along the equator with contributions from depths shown in Figure 1, illustrating how the gravity field contains the most short wavelength content at shallowest depths, but many of the highest amplitude signals originate at approximately Moho depths. This point is further illus-­‐
trated in Figure 3, which shows a map view of decon-­‐
structed Bouguer gravity within a gravitationally smooth region of the equatorial farside highlands. 47th Lunar and Planetary Science Conference (2016)
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(a) Figure 3. Contributions to surface Bouguer gravity in the farside highland region 10 S – 10 N, 180 E – 190 E. Approximate depth ranges from left to right are 10 – 20 km, 20 – 30 km and 30 – 45 km, respectively. Depth Constraints: We next develop a forward model as an initial step to address the extent to which the depth of a mass source can be constrained. We 3
consider a block mass 10 x 10 x 10 km and calculate the surface gravity anomaly assuming the block at a range of depths, see Figure 4. The calculation shows that the surface signal is a maximum when the mass is taken to be at the correct depth. Further, the surface signal decreases rapidly at shallower depths, particu-­‐
larly when compared to the decrease at depths great-­‐
er than the depth of the mass. As a result, the ability to recover the corrected depth degrades for more deeply buried masses. These preliminary calculations demonstrate an approach for recovering the depth of crustal masses such as dikes, intrusions, major faults and other sub-­‐
surface structures within the lunar crust, taking max-­‐
imum advantage of the GRAIL and LOLA data sets. (b) Figure 4. (a) Surface Bouguer gravity anomaly associ-­‐
ated with a mass anomaly at depths of 15 (red), 25 (blue), 35 (green) and 45 (brown) km. The depth of the mass anomaly is circled in each case. (b) Same as (a) except Bouguer anomaly magnitude is plotted as percentage of maximum value. References: [1] Zuber M. T. et al. (2013) Science 339, 668-­‐671, doi:10.1126/science.1231507. [2] Smith D. E. et al. (2010) Geophys. Res. Lett. 37, doi:10.1029/2010GL043751. [3] Zuber M. T. et al. (2013) Space Sci. Rev. 178, 3-­‐24, doi:10.1007/s11214-­‐
012-­‐9952-­‐7. [4] Smith D. E. et al. (2010) Space Sci. Rev. 150, doi:10.1007/s11214-­‐009-­‐9512-­‐y. [5] Chin G. et al. (2007) Space Sci. Rev., 129, doi:10.1007/s11214-­‐
007-­‐9153-­‐y. [6] Wieczorek M. A. et al. (2013) Science 339, 671-­‐675, doi:10.1126/science.1231530. [7] Bes-­‐
serer et al. (2014) Geophys. Res. Lett. 41, doi: 10.1002/2014GL060240.