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摘要
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. Plain Language Summary The recent investigation of the planet Mercury by NASA's MESSENGER mission has resulted in a wealth of new discoveries. Now there are high-resolution pictures of the entire planet that permit characterization of the geology of the planet. Further, there are measurements of topography, of gravity variations across the surface that indicate the structure of the subsurface, of the magnetic field that reveal how the planet interacts with the solar wind and how its deep interior has evolved, of the chemical composition of the rocks of the surface, and of the extended atmosphere (exosphere). We now have a view of Mercury that is quite different from the one that prevailed prior to MESSENGER's arrival at the planet. Rather than a long-quiet relic from the era of planet formation, Mercury has been a dynamic planet for most, if not all of its lifetime. Indeed, the data from MESSENGER contain signs of an ocean of magma covering the surface in the planet's earliest history and a billion years of voluminous volcanism that followed, as well as the generation of a magnetic field in its core in that same era. Further, modern Mercury has a magnetic field unique among the planets, exhibits variations in the character of its exosphere, and shows signs of activity such as the existence of water ice at the poles and loss of rock to space in enigmatic surface features termed hollows. Each new discovery has revealed new questions about the planet that collectively argue for continued exploration of the innermost planet.
- 出版日期2016-11
