The Geophysics of Mercury: Current Status and Anticipated Insights from the MESSENGER MissionAbstractIntroductionPhysical and Chemical CharacteristicsGeophysical ParametersBulk CompositionCrustal and Mantle StructureOrbital and Rotational ParametersSurface Constraints on Thermal EvolutionMajor Impact BasinsVolcanismTectonicsInformation from the Rotational StateSpin-Orbit ResonanceDetermination of Core StateCore-Mantle CouplingFree Motions?The Changing Cassini State PositionMagnetic Field: Observations and Possible ExplanationsObservations from Mariner 10Remanent MagnetizationCore Dynamo ModelsThin-Shell Dynamo ModelsThick-Shell Dynamo ModelsThermal Evolution ModelsMantle Structure and DynamicsParameterized Mantle ConvectionLooking AheadAcknowledgementsReferencesSpace Sci Rev (2007) 131: 105–132DOI 10.1007/s11214-007-9265-4The Geophysics of Mercury: Current Status andAnticipated Insights from the MESSENGER MissionMaria T. Zuber ·Oded Aharonson ·Jonathan M. Aurnou ·Andrew F. Cheng ·Steven A. Hauck II ·Moritz H. Heimpel ·Gregory A. Neumann ·Stanton J. Peale ·Roger J. Phillips ·David E. Smith ·Sean C. Solomon ·Sabine StanleyReceived: 24 July 2006 / Accepted: 10 August 2007 / Published online: 18 October 2007© Springer Science+Business Media B.V. 2007M.T. Zuber () · G.A. Neumann · S. StanleyDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology,Cambridge, MA 02139-4307, USAe-mail: [email protected]. AharonsonDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena,CA 91125, USAJ.M. AurnouDepartment of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USAA.F. ChengThe Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USAS.A. Hauck IIDepartment of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, USAM.H. HeimpelDepartment of Physics, University of Alberta, Edmonton, AB, T6G 2J1, CanadaG.A. Neumann · D.E. SmithSolar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USAS.J. PealeDepartment of Physics, University of California, Santa Barbara, CA 93106, USAR.J. PhillipsDepartment of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USAS.C. SolomonDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015,USAS. StanleyDepartment of Physics, University of Toronto, Toronto, ON, M5S 1A7, Canada106 M.T. Zuber et al.Abstract Current geophysical knowledge of the planet Mercury is based upon observationsfrom ground-based astronomy and flybys of the Mariner 10 spacecraft, along with theoreti-cal and computational studies. Mercury has the highest uncompressed density of the terres-trial planets and by implication has a metallic core with a radius approximately 75% of theplanetary radius. Mercury’s spin rate is stably locked at 1.5 times the orbital mean motion.Capture into this state is the natural result of tidal evolution if this is the only dissipativeprocess affecting the spin, but the capture probability is enhanced if Mercury’s core weremolten at the time of capture. The discovery of Mercury’s magnetic field by Mariner 10 sug-gests the possibility that the core is partially molten to the present, a result that is surprisinggiven the planet’s size and a surface crater density indicative of early cessation of significantvolcanic activity. A present-day liquid outer core within Mercury would require either a coresulfur content of at least several weight percent or an unusual history of heat loss from theplanet’s core and silicate fraction. A crustal remanent contribution to Mercury’s observedmagnetic field cannot be ruled out on the basis of current knowledge. Measurements fromthe MESSENGER orbiter, in combination with continued ground-based observations, holdthe promise of setting on a firmer basis our understanding of the structure and evolution ofMercury’s interior and the relationship of that evolution to the planet’s geological history.Keywords Mercury · MESSENGER · Core · Rotational state · Magnetic dynamos ·Thermal history1 IntroductionMercury’s internal structure and evolution collectively constitute one of the solar system’smost intriguing geophysical enigmas. In terms of its size and surface geology, Mercury isoften compared with Earth’s Moon. But in striking contrast to the Moon, which is depletedin iron and has a small (if any) metallic core, Mercury’s size and mass (Anderson et al. 1987,1996) indicate a high metal/silica ratio and a metallic mass fraction more than twice that ofEarth, Venus, and Mars (Wood et al. 1981). In addition, while the Moon is believed to havecooled rapidly subsequent to accretion (Zuber et al. 1994; Neumann et al. 1996), Mercuryappears to possess a liquid outer core (Margot et al. 2007). Such an internal structure ispuzzling, as simple thermal evolution models (Cassen et al. 1976; Solomon et al. 1981;Schubert et al. 1988) predict that a pure iron or iron-nickel core should have cooled andsolidified by now. A liquid core would survive if there is a sufficient amount of a lightalloying element such as sulfur to lower the melting point (Schubert et al. 1988).Mercury’s internal structure and its thermal evolution ultimately must be reconciled withthe planet’s surface geology. Mercury has a heavily cratered surface (Murray et al. 1974;Murray 1975; Trask and Guest 1975) with ancient compressional tectonic structures thathave been taken to imply global contraction (Strom et al. 1975; Watters et al. 1998)as-sociated with secular cooling (Siegfried and Solomon 1974). Ancient intercrater plainsand somewhat younger smooth plains of possible volcanic origin (Strom et al. 1975;Trask and Strom 1976; Robinson and Lucey 1997) constrain the early history of the mantleand crustal magmatism.The evolution of Mercury’s core state with time has implications for the planet’s spinevolution. Mercury currently displays a 3 : 2 spin–orbit resonance, and the presence of a fluidcore would have enhanced considerably its probability of capture into this state (Counselman1969).It could be argued that the formation and dynamics of Mercury’s core has had a greaterinfluence on the geophysical evolution of the planet than for any other terrestrial planetaryThe Geophysics of Mercury: Current Status and Anticipated Insights 107body. Consequently in this paper we treat the core as a point of focus as