A History of the Solar System by Claudio Vita-Finzi

Author:Claudio Vita-Finzi
Language: eng
Format: epub, pdf
Publisher: Springer International Publishing, Cham

A common source of internal heating, the radioactive decay of the isotopes aluminium-26 and iron-60 (26Al and 60Fe, half-lives 7.3 × 105 yr and 2.62 × 106 yr respectively), imposes a limit on the autonomous activity of the accreted body which hinges largely on its content of the two isotopes and therefore its volume. For the Moon and Mercury this amounts to about 109 years [11] after which the magmatic activity that can be discerned on their surfaces came to an end. These dead planets, like their active companions, will of course continue to receive the energy of impacts , which may lead to localised melting from the impact to the point where major episodes of crustal displacement result [33].

Planetary crusts come in many varieties and dimensions, as do cores and the solid and liquid mantles that encircle them. Their definition may be thermal, chemical, dynamic or some combination of attributes. The Sun’s outer zones are thus generally defined by the mode of heat transfer (radiative, convective) and the Earth’s vary according to the application of the analysis (seismic, tectonic, petrological). The presence of subsurface oceans , of great interest in the search for extraterrestrial life, may be inferred from gravitational data derived from spacecraft behaviour, from the escape of geysers or from distinctive surface features.

The question remains how much a body’s differentiation is primarily or wholly owed to gravity (and thus density) acting benignly over time and how much to impact. Collision between embryos and planetesimals liberates high energies and may lead to the formation of a magma ocean [34]. If it is correct to assume that late stages 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 between silicates and metals leading, at least in the terrestrial planets and probably in several stages, to the formation of a metallic core, and radiometric dating shows that metal-silicate separation in the Solar System as a whole dates from the first 30 Myr of its history [45].

In contrast with the terrestrial planets, giant planets are thought to grow outwards by accreting nebular gas around a core composed of planetesimals, always providing that all the gas in the nebula has not been appropriated or has dissipated, a suggestion that appears valid for Jupiter-mass planets around solar-mass stars [24]. The process could be favoured in the outer nebula beyond the snowline because the planetesimal contribution to cores is hereabouts supplemented by ices and the parent star has too little gravitational influence to hamper growth.

The zone intermediate between crust and core —the mantle in Earth’s nomenclature—is depleted in metals to the benefit of the core and will generally be essentially rocky. This is the conclusion reached about Mars in the light of martian meteorites recovered on Earth coupled with modelling studies, [46] with every indication that the martian interior was dynamically active early in Mars’ history until heat transport fell below a critical level for active tectonics as well as dynamo activity.

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