A History of the Solar System1 by Claudio Vita-Finzi

Author:Claudio Vita-Finzi
Language: eng
Format: epub
Tags: gnv64
Published: 2016-07-11T16:00:00+00:00

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Differentiation

Fig. 5.3 Vesta, the largest asteroid in the Solar System (mean diameter 525 km) and the secondmost massive body in the asteroid belt after the dwarf planet Ceres. It was orbited by NASA’sspacecraft Dawn in 2011 and 2012. Image shows varied composition indicating differentiation con-sistent with its status as a protoplanet. Courtesy of NASA/JPL-CALTECH/UCLA/MPS/IDA/PSI

A common source of internal heating, the radioactive decay of the isotopesaluminium-26 and iron-60 ( 26 Al and 60 Fe, half-lives 7.3 × 10 5 yr and 2.62 × 10 6 yr respectively), imposes a limit on the autonomous activity of the accreted bodywhich hinges largely on its content of the two isotopes and therefore its volume.For the Moon and Mercury this amounts to about 10 9 years [ 11 ] after which themagmatic activity that can be discerned on their surfaces came to an end. Thesedead planets, like their active companions, will of course continue to receive theenergy of impacts, which may lead to localised melting from the impact to thepoint where major episodes of crustal displacement result [ 33 ].Planetary crusts come in many varieties and dimensions, as do cores and thesolid and liquid mantles that encircle them. Their definition may be thermal,chemical, dynamic or some combination of attributes. The Sun’s outer zones arethus generally defined by the mode of heat transfer (radiative, convective) and theEarth’s vary according to the application of the analysis (seismic, tectonic, pet-rological). The presence of subsurface oceans, of great interest in the search forextraterrestrial life, may be inferred from gravitational data derived from space-craft behaviour, from the escape of geysers or from distinctive surface features.The question remains how much a body’s differentiation is primarily or whollyowed to gravity (and thus density) acting benignly over time and how much toimpact. Collision between embryos and planetesimals liberates high energies andmay lead to the formation of a magma ocean [ 34 ]. If it is correct to assume that latestages of planetary accretion witness few, large collisions, these may well include hit-and-run-events, ‘perfect mergers’, and smashes [ 10 ] which retain two altered bodies.In magma oceans that form at the surface there takes place segregation betweensilicates and metals leading, at least in the terrestrial planets and probably in

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