Mercury Redux
FEATURE Mercury redux In January 2008, 33 years after Mariner 10 fl ew past the solar system’s innermost planet, MESSENGER crossed Mercury’s magnetosphere. Ancient volcanoes, contractional faults, and a rich soup of exospheric ions give clues to Mercury’s structure and dynamical evolution. Th e Mercury fl yby of the MESSENGER two have not been ruled out, but for those (Mercury surface, space environment, mechanisms shorter-wavelength magnetic geochemistry and ranging) probe was the features would be expected, which were not fi rst of three braking manoeuvres for the observed during the MESSENGER fl yby1. spacecraft , in preparation for its insertion Recent libration observations that require into a polar orbit in 2011. Th e probe a partially molten core11, and the limited achieved the closest approach (201 km) of contraction of Mercury, which implies a Mercury’s surface yet, and took a variety largely molten core, favour a convective of measurements in the magnetosphere, dynamo origin for Mercury’s magnetic fi eld. exosphere and on Mercury’s surface. Some Although Mercury’s magnetosphere of the fi rst results of the MESSENGER looks like a miniature version of Earth’s, mission1–6 reveal Mercury as a planet with Mercury’s relatively weak magnetic richly interconnected dynamics, from fi eld implies that its dynamo must work the dynamo in its molten outer core, a diff erently from that of the Earth. Th e crust and surface with great lobate faults geodynamo, which gives the Earth its and relatively young volcanoes, to a strong magnetic fi eld, is thought to operate magnetosphere that interacts with the core in a magnetostrophic regime in which the dynamo and the interplanetary solar wind.
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