Thermal evolution of Mercury as constrained by MESSENGER observations

Abstract

[1] Orbital observations of Mercury by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft provide new constraints on that planet's thermal and interior evolution. Specifically, MESSENGER observations have constrained the rate of radiogenic heat production via measurement of uranium, thorium, and potassium at the surface, and identified a range of surface compositions consistent with hightemperature, high-degree partial melts of the mantle. Additionally, MESSENGER data have placed new limits on the spatial and temporal variation in volcanic and tectonic activity and enabled determination that the planet's core is larger than previously estimated. Because Mercury's mantle layer is also thinner than previously thought, this result gives greater likelihood to the possibility that mantle convection is marginally supercritical or even that the mantle is not convecting. We simulate mantle convection and magma generation within Mercury's mantle under two-dimensional axisymmetry and a broad range of conditions to understand the implications of MESSENGER observations for the thermal evolution of the planet. These models demonstrate that mantle convection can persist in such a thin mantle for a substantial portion of Mercury's history, and often to the present, as long as the mantle is thicker than~300 km. We also find that magma generation in Mercury's convecting mantle is capable of producing widespread magmas by large-degree partial melting, consistent with MESSENGER observations of the planet's surface chemistry and geology.

Mercury’s global contraction much greater than earlier estimates

“…Models of an elastic-plastic lithosphere, however, allow for folding far below the threshold of elastic buckling stresses 42 . It is possible that these undulations are the result of mantle dynamic processes 43 , consistent with convective thermal models of Mercury's interior (including those in which mantle convection continues to the present 44 ). Three-dimensional models of convection within Mercury's thin mantle predict convection patterns characterized by long, sheet-like shapes and a polygonal pattern near the intersection of the sheet structures 43 ; to first-order, the linear undulations shown in Fig.…”

“…That we have documented changes in Mercury's radius of up to a factor of seven larger than previous results resolves a nearly four-decades-old paradox: the history of heat production and loss and the accumulated global contraction are now consistent [13][14][15] . Going forward, our findings are crucial to thermal history models that will address the bulk silicate abundances of heat-producing elements, the question of whether the mantle is convecting 44 at present and the history of cooling and present-day structure of the large metallic core, the source of Mercury's internal magnetic field 7,46 . Moreover, our synthesis of the surface manifestation of global contraction on Mercury provides fresh insight for spatial and temporal studies of volcanism and tectonics on the innermost planet.…”

“…Previous reports of 0.8-2 km of radial shortening 5,8,11 were derived from analyses of the 45% of Mercury's surface imaged by Mariner 10. A recent D max -L study 44 using MESSENGER data considered only the most prominent lobate scarps and high-relief ridges on Mercury and returned a value of ∼1.2 km for θ = 30…”

Mercury’s Interior

“…A variety of measurements provided by MESSENGER have yielded constraints for the thermal evolution of Mercury (Michel et al 2013). Surface Mg and S abundances are consistent with high interior temperatures and a high degree of partial melting in the mantle, while U, Th, and K abundances constrain the rate of radiogenic heat reduction.…”

“…Such a layer provides even greater constraint than required to explain a mantle that remains convecting over a longer period of time. Current models (Michel et al 2013) indicate that convection continued until less than 1 billion years ago (Fig. 2.7).…”