Space Sci Rev 2007 131 105 132 DOI 10 1007 s11214 007 9265 4 The Geophysics of Mercury Current Status and Anticipated Insights from the MESSENGER Mission Maria 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 Stanley Received 24 July 2006 Accepted 10 August 2007 Published online 18 October 2007 Springer Science Business Media B V 2007 M T Zuber G A Neumann S Stanley Department of Earth Atmospheric and Planetary Sciences Massachusetts Institute of Technology Cambridge MA 02139 4307 USA e mail Zuber mit edu O Aharonson Division of Geological and Planetary Sciences California Institute of Technology Pasadena CA 91125 USA J M Aurnou Department of Earth and Space Sciences University of California Los Angeles CA 90095 USA A F Cheng The Johns Hopkins University Applied Physics Laboratory Laurel MD 20723 6099 USA S A Hauck II Department of Geological Sciences Case Western Reserve University Cleveland OH 44106 USA M H Heimpel Department of Physics University of Alberta Edmonton AB T6G 2J1 Canada G A Neumann D E Smith Solar System Exploration Division NASA Goddard Space Flight Center Greenbelt MD 20771 USA S J Peale Department of Physics University of California Santa Barbara CA 93106 USA R J Phillips Department of Earth and Planetary Sciences Washington University St Louis MO 63130 USA S C Solomon Department of Terrestrial Magnetism Carnegie Institution of Washington Washington DC 20015 USA S Stanley Department of Physics University of Toronto Toronto ON M5S 1A7 Canada 106 M T Zuber et al Abstract Current geophysical knowledge of the planet Mercury is based upon observations from ground based astronomy and flybys of the Mariner 10 spacecraft along with theoretical and computational studies Mercury has the highest uncompressed density of the terrestrial planets and by implication has a metallic core with a radius approximately 75 of the planetary 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 dissipative process affecting the spin but the capture probability is enhanced if Mercury s core were molten at the time of capture The discovery of Mercury s magnetic field by Mariner 10 suggests the possibility that the core is partially molten to the present a result that is surprising given the planet s size and a surface crater density indicative of early cessation of significant volcanic activity A present day liquid outer core within Mercury would require either a core sulfur content of at least several weight percent or an unusual history of heat loss from the planet s core and silicate fraction A crustal remanent contribution to Mercury s observed magnetic field cannot be ruled out on the basis of current knowledge Measurements from the MESSENGER orbiter in combination with continued ground based observations hold the promise of setting on a firmer basis our understanding of the structure and evolution of Mercury s interior and the relationship of that evolution to the planet s geological history Keywords Mercury MESSENGER Core Rotational state Magnetic dynamos Thermal history 1 Introduction Mercury s internal structure and evolution collectively constitute one of the solar system s most intriguing geophysical enigmas In terms of its size and surface geology Mercury is often compared with Earth s Moon But in striking contrast to the Moon which is depleted in 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 of Earth Venus and Mars Wood et al 1981 In addition while the Moon is believed to have cooled rapidly subsequent to accretion Zuber et al 1994 Neumann et al 1996 Mercury appears to possess a liquid outer core Margot et al 2007 Such an internal structure is puzzling 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 and solidified by now A liquid core would survive if there is a sufficient amount of a light alloying 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 with the 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 that have been taken to imply global contraction Strom et al 1975 Watters et al 1998 associated with secular cooling Siegfried and Solomon 1974 Ancient intercrater plains and 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 mantle and crustal magmatism The evolution of Mercury s core state with time has implications for the planet s spin evolution Mercury currently displays a 3 2 spin orbit resonance and the presence of a fluid core would have enhanced considerably its probability of capture into this state Counselman 1969 It could be argued that the formation and dynamics of Mercury s core has had a greater influence on the geophysical evolution of the planet than for any other terrestrial planetary The Geophysics of Mercury Current Status and Anticipated Insights 107 body Consequently in this paper we treat the core as a point of focus as we review current understanding of Mercury s geophysics In the context of this review we emphasize recent advances in measuring and interpreting Mercury s rotational state in interpreting existing magnetic observations and in convective modeling of the planet s mantle and core We describe how anticipated future observations from NASA s MErcury Surface Space ENvironment Geochemistry and Ranging MESSENGER mission will provide a means of unraveling the unusual characteristics of Mercury s evolution 2 Physical and Chemical Characteristics 2 1 Geophysical Parameters The size shape and mass of Mercury have been measured from radio tracking of the Mariner 10 spacecraft and Earth based radar ranging Current knowledge of these parameters summarized in Table 1 is based on historical observations as well as more recent reanalysis of combined data sets Anderson et al 1987 1996 Mercury has the largest uncompressed density of the planets Ringwood 1979 and thus