An improved lunar gravity field model from SELENE and historical tracking data: Revealing the farside gravity features

Abstract

[1] A new spherical harmonic solution of the lunar gravity field to degree and order 100, called SGM100h, has been developed using historical tracking data and 14.2 months of SELENE tracking data (from 20 October 2007 to 26 December 2008 plus 30 January 2009). The latter includes all usable 4-way Doppler data collected which allowed direct observations of the farside gravity field for the first time. The new model successfully reveals farside features in free-air gravity anomalies which are characterized by ring-shaped structures for large impact basins and negative spots for large craters. SGM100h produces a correlation with SELENE-derived topography as high as about 0.9, through degree 70. Comparison between SGM100h and LP100K (one of the pre-SELENE models) shows that the large gravity errors which existed in LP100K are drastically reduced and the asymmetric error distribution between the nearside and the farside almost disappears. The gravity anomaly errors predicted from the error covariance, through degree and order 100, are 26 mGal and 35 mGal for the nearside and the farside, respectively. Owing to the 4-way Doppler measurements the gravity coefficients below degree and order 70 are now determined by real observations with contribution factors larger than 80 percent. With the SELENE farside data coverage, it is possible to estimate the gravity field to degree and order 70 without applying any a priori constraint or regularization. SGM100h can be used for global geophysical interpretation through degree and order 70.

Structure of the South Pole-Aitken Basin from KAGUYA Selenodesy Data

“…From one-year tracking data, lunar gravity field model SGM100h was obtained 2) and the model was refined into SGM100i taking into account VLBI data 3) . KAGUYA has a laser altimeter (LALT) which measures the distance between the satellite and the lunar surface with accuracy of 1 m 4) .…”

Ground Compatibility Tests for Gravity Measurement of SELENE: Accuracies of Two- and Four-Way Doppler and Range Measurements

“…At the same time, conventional Doppler and range measurements are carried out between Rstar and UDSC, which we refer to as two-way Doppler and range measurements. RSAT realized the first direct observation of the gravity field over the farside of the Moon, and enabled global gravity anomaly mapping of the Moon (Goossens and Matsumoto 2007;Matsumoto et al 2008Matsumoto et al , 2010.…”

“…Further, not only two-and four-way Doppler and range tracking data but also historical datasets from previous spacecraft missions are included in the gravity reduction processes of SELENE (Goossens and Matsumoto 2007;Matsumoto et al 2008Matsumoto et al , 2010. The relative weight of each dataset is evaluated by the expected variance in the Doppler and range measurements.…”

Science Summary of Kaguya Mission

“…An error of low-degree spherical harmonic coefficients which contribute to the polar moment of inertia and the k 2 Love number was also reduced in the Kaguya gravity model SGM100h comparing with the Lunar Prospector model LP100K. The differential VLBI technique by VRAD can be employed to refine further the coefficients by precise orbital determination 19) . The results obtained from the Kaguya mission cannot be used to determine accurately the elemental abundance of the entire Moon.…”