Determination and localized analysis of intersatellite line of sight gravity difference: Results from the GRAIL primary mission

Abstract

[1] The line of sight (LOS) gravity difference between two coorbiting spacecrafts is determined in terms of intersatellite range-acceleration measurements available from the Gravity Recovery and Interior Laboratory (GRAIL). The precise orbit data are crucial for retrieving gravity difference from range acceleration and aligning the LOS data particularly in altitude. A relative orbit error of a few centimeters in position and a few tens μm/s in velocity is commensurate with the GRAIL-ranging instrument noise at a few μGal in LOS gravity difference. The power spectrum, as well as the topography correlation and admittance, is quantified by upward continuing the topographic potential, forward modeling the LOS gravity along the spacecraft trajectory (i.e., Bouguer correction) and comparing with the GRAIL LOS observations. Based on the data analysis from the primary GRAIL mission, I found that the LOS gravity difference observation produced near unity correlation with topography potential out to degree 550, higher than the global estimate, over the areas covered by the low-altitude orbit (~20 km). The crustal density was estimated to be 2500-2600 kg/m 3 with regional variations of about 10%, by minimizing the Bouguer coherence of the GRAIL data at the degree band 150-300. Systematic decrease in the density estimates by 3-4% or 100 kg/m 3 was observed at shorter wavelengths (degree band 300-500). It implies the inadequacy of a uniform density model across the entire lithosphere and suggests radial stratification of the bulk density (or porosity). Due to spatially localized characteristic, the LOS gravity difference data are well suited to regional analysis at the highest-possible resolution.

Global characteristics of porosity and density stratification within the lunar crust from GRAIL gravity and Lunar Orbiter Laser Altimeter topography data

“…We also examined a week (21–27 November 2012) of Level‐1B (L1B) data from the GRAIL XM, where the higher spatial resolution of gravity information is expected due to the lowered altitude of spacecrafts. We used the L1B intersatellite ranging data along with the orbit data to process the line of sight gravity difference and computed the Bouguer coherence spectrum following the method of Han []. The free‐air coherence spectrum shows the unity value extended to 20 km or so, confirming the higher sensitivity of the XM data.…”

“…Preliminary results for variation in bulk crustal densities from GRAIL by Wieczorek et al [] are on the order of 2550 ± 250 kg/m 3 , considerably less than the 2800–2900 kg/m 3 expected for typical anorthositic materials. Furthermore, Han [] estimated even lower density from the higher‐frequency gravity data. Wieczorek et al [] determined regionally varying densities that are consistent with 4–21% porosity within the entire lunar crust.…”

High‐resolution local gravity model of the south pole of the Moon from GRAIL extended mission data

“…We used Lunar Orbiter Laser Altimeter (LOLA) topography [ Smith et al , ] archived in the Planetary Data System (LOLA/PDS Data Node, http://imbrium.mit.edu). We used a density of 2400 kg/m 3 to account for the observed decrease in crustal density with higher degrees [ Wieczorek et al , ; Han , ]. In order to compare two solutions of the same size so we can subtract the same topography‐induced gravity potential, we generated global spherical harmonics for our local solution through a spherical harmonic transformation using Gauss‐Legendre (GL) quadrature.…”

GRAIL gravity constraints on the vertical and lateral density structure of the lunar crust

“…Since the acquisition of the last lunar seismic data, almost 40 years ago, the main way of investigating lunar crustal structure has been through gravity analysis via spacecraft tracking data from missions such as Clementine [ Zuber et al , ], Lunar Prospector [ Konopliv et al , ], SELENE [ Goossens et al , ], and Lunar Reconnaissance Orbiter [ Mazarico et al , ]. More recently, the Gravity Recovery and Interior Laboratory (GRAIL) mission has acquired highly accurate observations through the use of a pair of coorbiting spacecraft [ Zuber and Russell , ], and in this manner, the lunar gravity field has been measured to a much higher accuracy and resolution than previous missions both over the nearside and the farside of our satellite [ Zuber et al , ; Han , ; Konopliv et al , ; Lemoine et al , ; Konopliv et al , ; Lemoine et al , ]. The lunar anorthositic crust has been found to be much less dense than previously thought, and GRAIL data has revealed a significant bulk crustal porosity of ∼12% [ Wieczorek et al , ].…”