A whole new Mercury: MESSENGER reveals a dynamic planet at the last frontier of the inner solar system

Abstract

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission yielded a wealth of information about the innermost planet. For the first time, visible images of the entire planet, absolute altimetry measurements and a global gravity field, measurements of Mercury's surface composition, magnetic field, exosphere, and magnetosphere taken over more than four Earth years are available. From these data, two overarching themes emerge. First, multiple data sets and modeling efforts point toward a dynamic ancient history. Signatures of graphite in the crust suggest solidification of an early magma ocean, image data show extensive volcanism and tectonic features indicative of subsequent global contraction, and low‐altitude measurements of magnetic fields reveal an ancient magnetic field. Second, the present‐day Mercury environment is far from quiescent. Convective motions in the outer core support a modern magnetic field whose strength and geometry are unique among planets with global magnetic fields. Furthermore, periodic and aperiodic variations in the magnetosphere and exosphere have been observed, some of which couple to the surface and the planet's deep interior. Finally, signatures of geologically recent volatile activity at the surface have been detected. Mercury's early history and its present‐day environment have common elements with the other inner solar system bodies. However, in each case there are also crucial differences and these likely hold the key to further understanding of Mercury and terrestrial planet evolution. MESSENGER's exploration of Mercury has enabled a new view of the innermost planet, and more importantly has set the stage for much‐needed future exploration.

Properties of Solar Wind Structures at Mercury's Orbit

“…In fact, Raines et al, 2015 found that the concurring contribution of the SW high dynamic pressure and the weak planetary magnetic field moves the dayside magnetospheric boundaries (viz., the bow-shock and magnetopause) approximately 8 times closer to Mercury's surface with respect to the Earth's case. So, to better understand the dynamics of the Hermean magnetosphere in response to SW plasma and IMF variations, a characterization of the solar inputs is needed (e.g., James et al, 2017;Johnson & Hauck, 2016). For this reason, studies aiming to the evaluation of changes in the shape and in the dynamics of the Hermean magnetosphere should take in proper account that SW variations are not random but depend on the occurrence of specific structures traveling in space.…”

The Influence of IMF Clock Angle on Dayside Flux Transfer Events at Mercury

“…Mercury's close proximity to the Sun exposes it to the extreme solar wind conditions present at an orbital distance of 0.31–0.47 AU, including an interplanetary magnetic field (IMF) strength of 20–40 nT (Blomberg et al, ), ∼5 times that measured at Earth, and solar wind number density of 30–70 cm −3 (Blomberg et al, ), an order of magnitude greater at Mercury (Baumjohann et al, ). Furthermore, the planetary dipole moment at Mercury is about 3 orders of magnitude lower than that at Earth (Johnson et al, ; Johnson & Hauck, ), with a value of 195 nT (Anderson et al, ) (where R M =2440 km is the radius of Mercury). The combination of this weak planetary field and the solar wind conditions means the Hermean magnetosphere is extremely small and strongly driven by variable conditions in the solar wind (Slavin et al, ).…”

A modelling approach to infer the solar wind dynamic pressure from magnetic field observations inside Mercury’s magnetosphere

“…Smith et al 2012;Hiremath 2012;Hauck et al 2013;Johnson & Hauck 2016). In our simulations we assume that Mercury has a uniform resistive interior with resistivity η = 10 7 Ω × m. Since the solar wind plasma and IMF orientation and strength remain constant at the inflow boundary of the simulations presented here, and since Mercury's mantle has very low conductivity, no electromagnetic induction is generated by the interior of Mercury over a constant solar wind and IMF.…”

“…Therefore, to understand the structure of Mercury's magnetosphere and its response to the solar wind plasma and IMF variations, we need to understand the interaction between the solar wind and Mercury's magnetosphere and distinguish between the contributions from external and internal magnetic sources (e.g. Raines et al 2015;Johnson & Hauck 2016;James et al 2017).…”

“…Anderson et al 2008Anderson et al , 2010, and nearly four years of magnetic field observations around Mercury from early 2011 until 2015 (e.g. Anderson et al 2011aAnderson et al , 2012Johnson & Hauck 2016). The global planetary field of Mercury has been estimated as a single dipole with a magnetic moment of 195 ± 10 nT × R 3 M displaced northward by 484 ± 11 km (∼0.2 R M ) from the centre of the planet, where R M = 2440 km is the radius of Mercury (Anderson et al 2011a.…”