Lamont-Doherty Earth Observatory
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With the highest uncompressed density among the planets, Mercury has long been known to possess an iron-rich core that occupies a much larger mass fraction of the planet than the cores of Earth, Venus, or Mars. Mercury is also the only inner planet other than Earth to host a modern global magnetic field. Measurements of Mercury's gravity field by the MESSENGER spacecraft, in orbit around the innermost planet since March 2011, together with measurements of Mercury's solid-body motions by Earth-based radar, yield both the moment inertia of the planet and the moment of inertia of the solid outer shell that participates in Mercury's 88-day libration forced by variations in the gravitational torque from the Sun during Mercury's eccentric orbit. The radius of the fluid outer core of Mercury is 2020±30 km, so the solid outer shell of the planet is only 420±30 km thick. The high density of that shell (3380±200 km/m3), despite a crust and mantle low in Fe, Ti, and Al compared with other terrestrial bodies, points to the possibility of contributions from another dense material. The chemically reduced conditions of Mercury's crustal materials suggest that Mercury's iron core, if similarly reducing, contains Si as well as S. Under the range of pressures and temperatures in Mercury's outer core, Fe-Si-S forms two immiscible liquids, so Mercury's outer core may be stratified, a solid layer of FeS may have formed at the top of the outer core, and, if so, such a layer would now contribute to the average density of Mercury's outer solid shell. Mercury is the only inner planet with a clear record, in its preserved tectonic features, for an extended history of interior cooling and global contraction. Prior to the MESSENGER mission, however, the contraction predicted from thermal history models, dominated by the cooling and partial solidification of the core, far exceeded the contraction inferred from shortening across tectonic landforms resolved in Mariner 10 images of the surface. A new analysis of contractional structures imaged by MESSENGER has increased the implied decrease in planetary radius by as much as a factor of five over previous estimates, bringing the geological observations at last into line with model predictions. Mercury's internal magnetic field, like that of Earth, is dominantly dipolar, albeit with a dipole moment less than that of Earth by a factor of approximately 103. Measurements of the location of Mercury's magnetic equator, however, show that Mercury's dipole, although aligned with the spin axis, is offset from the center of the planet by 20% of the planet's radius. An axially symmetric but equatorially asymmetric field had not been predicted by any dynamo model prior to the MESSENGER observations. Beginning in April 2013, the periapsis altitude of the MESSENGER spacecraft began to decrease progressively with each orbit, and in July 2014 the closest approach distance dropped to less than 100 km. Magnetic field measurements obtained at still lower altitudes have resolved crustal magnetic anomalies for the first time. The anomalies are consistent with thermoremanent magnetization acquired at least 3.7-3.9 billion years ago and indicate that Mercury's core dynamo has operated over most of the planet's history.