Mineralogy of the planets: a voyage in space and time

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Summary: Pertinentgeneral properties of the planetsare listed. Thecondensation of the solar nebula is set in the context of stellar evolution and meteoriteswith sections onastronomical observations, chemical composition of the solar nebula, physical properties of the solar nebula, andchemical and physical aspects of condensation, accretion, and planetary differentiation. A cool nebula is preferred to allow survival of pre-solar grains with isotopic anomalies. Equilibrium progressive condensation of the solar nebula is regarded as a useful theoretical boundary, but complex processes involving crystal-liquid differentiation in, and collisions between, planetesimals are used to interpret the properties of meteorites and terrestrial planets.Chemical differentiation in the nebulabegins with condensation and aggregation of dust, which can yield oxidized and reduced products depending whether C/O is less or greater than unity. Simple models for direct accretion of condensed materials into planets are reviewed but not adopted.Physical interactions involving small bodiesinclude collisional accretion of dust-covered bodies, and differentiation of silicate and metal from mechanical, magnetic, and electrostatic forces.Physical and chemical differentiation involving large bodiesinvolves head-on and glancing collision of planetesimals, orbital deflection, and disintegration within the Roche limit, and collision with debris rings and moons.Planetary accretion: dynamics, time scale, and heat sourcesinvolves more rapid growth of a larger body than a smaller one with ultimate development of one planet in each feeding zone, which flares out and ultimately overlaps with adjacent zones. Mars is small, and a planet did not develop in the asteroid belt, because of perturbations from Jupiter. The giant planets deflected material into the inner solar system. Melting of early planetesimals is invoked to explain differentiated meteorites.Chemical differentiation inside planet esimals and planetsdescribes the phase equilibria for metal, sulphide, and peridotite, either dry, wet, or containing CO2. A wet body could begin crystal-liquid differentiation near 1250 K with sinking of Fe,S-rich liquid and rising of basaltic melt. The peridotitic residuum might undergo a subsequent differentiation at higher temperature under volatile-free conditions. Mineralogical storage of H2O, CO2, S, Cl, F, and alkalies is discussed.Chemical differentiation in planetary atmospheresbriefly mentions escape of light species.For theEarth, theearly historyis constrained by Archaean rocks dating from −3.8 × 109yr whose properties indicate a non-reducing atmosphere, and a mantle that yielded volcanic rocks mostly similar to recent ones. Theupper mantle (above 200 km depth)contains peridotitic rocks attributable to crystal-liquid differentiation and metamorphism. Volatile elements exist in mica and other minerals, but are sparse. Abundances of siderophile and chalcophile elements are high enough to require late accretion of material rich in these elements, the presence of a barrier between upper mantle and core, and some extraction by sinking sulphide. Themantle (deeper than 200 km) and coreare inaccessible to direct study but interpretation of seismic data coupled with high-pressure laboratory studies requires inversion to dense phases in the mantle (especially perovskite?) and presence of light elements in the core (mainly S?). Thebulk compositionis modelled by cosmochemical analogy constrained by geophysical and geochemical parameters. The early condensate may be augmented by 1.5 ± 0.5? over (Mg+Si), and metal by 1.2±0.1?, while alkalies are probably depleted six-fold. Radial heterogeneity from a reduced interior to an oxidized exterior is suggested. For theMoon, sections coverobservations, petrologic interpretations, andbulk chemical composition. For theorigin of the Earth and Moon, age constraints, chemical constraints, anddynamical and accretional constraintsallowcomparison of suggested origins, with the conclusion that the Moon formed by either fission or disintegrative capture during early growth of the Earth, followed by simultaneous accretion coupled with disintegrative capture of planetesimals.Mercurymust be Fe-rich, but the silicate may not be just early condensate. For Venus, reviews are given of thesurface properties, atmosphere, speculations on bulk compositionandspeculations on surface mineralogy and atmospheric compositions. The CO2is in the atmosphere; most H may have been lost with concomitant oxidation of rocks; K/U ratios suggest basaltic and granitic rocks, and the high-surface temperature (c.740 K) implies granulitic metamorphism. For Mars, reviews are given ofsurface morphology, atmosphere and volatiles, mineralogy and petrology, andgeophysical and geochemical models. Prolonged emission of Fe-rich lavas is suggested. Volatiles were removed by mineralogical processes from the atmosphere to give ice caps and sediments affected by aeolian processes and oxidation from photochemically generated H2O2. Fe-rich layer silicates, maghemite, and Mg-sul-phate may dominate the sediments.The composition ofcomets and interplanetary dustmay be inferrable from micrometeorites whose complex properties are suggestive of carbonaceous meteorites.Asteroidsshould be supplying at least many of the meteorites to Earth. Ageneral descriptionand review ofremote-sensing studiesculminate in a review ofspatial descriptions and implications. The main belt is dominated by dark C-type asteroids assumed to be the primordial inhabitants produced by primary condensation and local accretion. The inner zone contains some brighter S-type asteroids interpreted as having undergone separation of metal from silicate as in stony-iron meteorites, as well as some E- and M-types perhaps matching enstatite-bearing meteorites and irons. Perhaps these differentiated(?) asteroids, as well as basaltic Vesta, are strays perturbed outwards from the inner solar system.A review of meteorites coverscarbonaceous meteorites, ordinary chondrites, enstatite chondrites and achondrites, reduced irons, forsterite-bearing meteorites and silicate inclusions in irons, irons, pallasites, chassignites and nakhlites, ureilites and lodranite, eucrites and shergottites, diogenites, howardites and mesosiderites, oxygen isotopes, andoriginal location of meteorites. Emphasis is placed on mineralogical properties demonstrating crystal-liquid differentiation, brecciation, agglomeration, and even aqueous alteration and vapour transfer, though some evidence remains of direct condensation of gas to solid. The meteorites demonstrate the existence of many parent bodies (probably over 60), and most macrometeorites are ascribed to collision debris from the main-belt asteroids that has been deflected past Mars. The range of oxygen isotopes is explained by mixing of supernova debris rich in16O with gas-solid differentiates of the nebula.The culmination of the review is asuggested synthesis, which emphasizes a new model of heterogeneous accretion from planetesimals whose diverse compositions range from reduced material near Mercury to oxidized material from the asteroid zone outwards. Growth of a terrestrial planet begins from near-by slow planetesimals, and ends with distant fast planetesimals. Only the Earth and Venus are big enough to retain debris from late volatile-rich planetesimals deflected to high speed by the giant planets. Mercury, Moon, and Mars are volatilepoor. The Earth is zoned from a reduced interior, composed of crystal-liquid segregates of planetesimals captured early, to an oxidized exterior containing some material captured from outside the orbit of Mars. A mantle barrier hindered chemical equilibration. Hydrogen loss augmented oxidation. The mantle may be chemically zoned inwards from olivine-rich composition to pyroxene-rich composition, and the core should be reduced and contain substantial S and perhaps C and P. Mercury and Venus should have accreted reduced material, and only Venus should contain volatiles obtained from late planetesimals from outside the Martian orbit. Mars should have accreted mainly S-type asteroids, and have captured little volatile-rich material.

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