Support of long‐wavelength topography on Mercury inferred from MESSENGER measurements of gravity and topography

Abstract

To explore the mechanisms of support of surface topography on Mercury, we have determined the admittances and correlations of topography and gravity in Mercury's northern hemisphere from measurements obtained by NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. These admittances and correlations can be interpreted in the context of a number of theoretical scenarios, including flexural loading and dynamic flow. We find that long-wavelength (spherical harmonic degree l < 15) surface topography on Mercury is primarily supported through a combination of crustal thickness variations and deep mass anomalies. The deep mass anomalies may be interpreted either as lateral variations in mantle density or as relief on compositional interfaces. Domical topographic swells are associated with high admittances and are compensated at 300-400 km depth in the lower reaches of Mercury's mantle. Quasi-linear topographic rises are primarily associated with shallow crustal compensation and are weakly correlated with positive mass anomalies in the mantle. The center of the Caloris basin features some of the thinnest crust on the planet, and the basin is underlain by a large negative mass anomaly. We also explore models of dynamic flow in the presence of compositional stratification above the liquid core. If there is substantial compositional stratification in Mercury's solid outer shell, relaxation of perturbed compositional interfaces may be capable of creating and sustaining long-wavelength topography.

Evolution of the interior of Mercury influenced by coupled magmatism-mantle convection system and heat flux from the core

“…The thermal and compositional structure of the mantle shown in Figure is in conflict with the mantle structure that James et al . [] suggest for Mercury in some of their models. In these models that are based on analyses of the gravity anomalies observed over the swells of the planet, they assume that there is a compositionally dense layer with laterally varying thickness on the CMB and suggest that this lateral variation dynamically supports the swells.…”

“…However, the thickness of this layer is laterally uniform, because the viscosity is only 10 19 –10 22 Pa after 2 Gyr in the deep mantle (see Figure g for 2.13 and 4.42 Gyr); lateral density variations are quickly relaxed at such a low viscosity, as Figure 11 of James et al . [] predicts. The low viscosity in the deep mantle is a consequence of the high temperature there maintained by the persistent compositional stratification of the mantle.…”

“…The third is the consistency with the observed topography and gravity anomalies of Mercury [ James et al ., ]. In Figures a and c the deep mantle is compositionally homogeneous after 3 Gyr, and the viscosity there is less than 10 20 Pa s throughout the calculated history of the mantle.…”

“…[] suggest that mantle convection ceases within 3–4 Gyr after the planetary formation. More detailed studies are now in progress to further impose and/or revise observational constraints on Mercury's interior [e.g., Weider et al ., , ; Byrne et al ., ; James et al ., ], and more refined numerical models are needed to understand outcome of these studies in the context of mantle evolution.…”

Global Distribution and Spectral Properties of Low‐Reflectance Material on Mercury

“…(a) Color composite of Mercury with the Mercury Dual Imaging System 1,000, 750, and 430 nm narrow-band filters in the red, green, and blue channels, respectively. (f) LRM map overlain on crustal thickness map fromJames et al (2015) The yellow boxes indicate the locations of craters A-Akutagawa, SA-Sholem-Aleichem, B-Basho, T-Tolstoj, R-Rachmaninoff, and C-Craters within Caloris, and three regional enhancements, R1-3.…”