Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p41a1572a&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P41A-1572
Physics
[5417] Planetary Sciences: Solid Surface Planets / Gravitational Fields
Scientific paper
Analysis of radio Doppler data generated by the Deep Space Network with Mariner 10 during its first and third encounters with Mercury yielded a quadrupole gravitational field with J2 equal to 6.0 ± 2.0 and C22 equal to 1.0 ± 0.5 in units of 10-5 (Anderson et al., Icarus 71, 337, 1987) . However, this underlying global field leaves systematic Doppler residuals for the third encounter (Esposito et al., COSPAR: Space Res. 17, 639, 1978), residuals that are most likely caused by a large gravity anomaly in the region of closest approach to Mercury at latitude 67.96° and east longitude 53.09°. We report here a detailed characterization of the likely sources producing the putative gravity anomaly. The recovered Doppler residuals and ground track from Esposito et al. (1978) are fit by a model that includes the spacecraft's six trajectory initial conditions along with Mercury's mass GM = 22,032.09 ± 0.91 km3 s-2 (Anderson et al., 1987) and the two quadrupole coefficients. After convergence to the best-fit trajectory, any remaining residuals represent an unmodeled signal that is assumed to arise from anomalous mass concentrations on Mercury plus noise. In order to reduce the noise evident in the Doppler residuals, we smooth them with a variable-width Gaussian filter (Palguta et al., Icarus 180, 428, 2006). The filter width in the time domain increases with the spacecraft altitude, reducing the noise before and after closest approach. Accelerations along the line of sight (LOS) are calculated by sampling the differentiated Doppler smoothing curve at a 10-second time interval, the sample interval for the Doppler frequency data. Multiple spherical-cap disk models are then used to fit the LOS acceleration data. The spherical-cap disk models not only provide the locations and magnitudes of anomalous mass concentrations on Mercury, but also their vertical and horizontal dimensions. We find that a minimum of four mass anomalies on or near Mercury's surface is required to explain the LOS acceleration data. Although at least four masses are required to reproduce all the major acceleration features, we show that the fit to the data can be improved by including up to seven spherical-cap disks. The modeled mass anomalies are on the order of 1018 - 1019 kg. Additionally, at least two negative and two positive mass anomalies are present in each model. Although there is some variability in the placement of the mass anomalies among the different models, we find that positive mass anomalies are typically found near the beginning and end of the Mariner 10 trajectory path (20° N, 25° E) and (55° N, 177° E).
Anderson John D.
Palguta Jennifer
Schubert Gerald
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