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CH 7 - Geophysical Processes in Planetary Differentiation

CH 7 - Geophysical Processes in Planetary Differentiation -...

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Heavier Elements Offshore Ocean Basin (West Side) Offshore Ocean Basin (East Side) Mountainous Regions Ocean Basin Dense Rock (Earth’s Mantle) Bulge Fluids Flow Out CHAPTER 7 GEOPHYSICAL PROCESSES IN PLANETARY DIFFERENTIATION The differentiation of the terrestrial planets and asteroids and also of the gas giants depends upon both geophysical and geochemical processes. Properly, geophysics refers to the physics of the Earth, i.e. “geo-” and geochemistry to the chemistry of the Earth. Applied to the planets and Solar System and the Universe as a whole, the science of chemistry is sometimes better called cosmochemistry. Presently we don’t use a similar term to describe a physics generalized to the description of processes on the planets. Some authors do use terms like, for example, selenophysics to describe the physics of the Moon. The geophysical processes involved in differentiation depend largely upon differential densities of materials, which might be either inherent or dependent upon their temperature. Buoyant materials rise and dense materials sink. .. The temperature caused buoyancy of mantle materials and fluid core is largely due to the slow freezing of the iron inner core and the release of the latent heat of fusion. 7.1 THE INTERNAL STRUCTURE OF EARTH, MOON & THE TERRESTRIAL PLANETS We know from the rotational dynamics of Earth, Moon and Mars that their internal density increases rapidly towards their centers. We infer the same for Venus and Mercury though we really haven’t obtained accurate measures of their moments of inertia. The typical structure of a typical terrestrial planet (Earth as model) or our Moon comprises: A thin outer crust of lighter silicates (largely granitic) where high standing and somewhat denser silicates (largely basaltic) where low standing. A very deep mantle of silicates , ever denser with depth. A core, largely composed of Fe, Ni and some alloying lighter elements such as O and S. The core may be frozen solid or, like Earth and Mercury, have an overlying melted shell. If the planet’s gravity is strong enough to hold volatiles against their evaporation into space, it may well have a substantial atmosphere. Earth has large oceans of liquid water.
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Picture 1 The crustal skin and mantle of the planet has little Fe and Ni. The crust is especially enriched in Ca, Na, K, Al, Si and O. The Fe and Ni has mostly sunk into the core. In the table below, one might note that overall, the Earth comprises about 15% by mass Si but the crustal abundance is about 28% Si; take care with the meaning of the normalization to Si = 1. The Sun’s photosphere roughly corresponds to what we see in meteorites that fall to Earth = thought to correspond to what the Earth looks like as a whole When we look at the crust we are looking at the scum that floated to the top of the Earth = doesn’t accord nearly as well o Earth as a whole doesn’t accord with the crustal rocks of the Earth = we have a lot of lighter elements in the Earth o
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CH 7 - Geophysical Processes in Planetary Differentiation -...

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