Planetary Composition

While planetary composition does non e'er track stellar limerick (e.g., the Earth does not accept the aforementioned bulk limerick as the Sun), it still is an intriguing arroyo to aid constrain the requirements for life and assist with target selection for life detection for upcoming ground and infinite-based observatories (Fujii et al., 2018).

From: Encyclopedia of Geology (2d Edition) , 2021

Comparative planetary evolution

Kent C. Condie , in Earth every bit an Evolving Planetary Organization (Quaternary Edition), 2022

Homogeneous accession

In homogeneous accession models condensation is essentially consummate earlier accretion begins (Taylor, 1999; Drake, 2000). Compositional zoning in the solar molecular deject is caused by decreasing temperature outward from the Sunday (Fig. 10.1 ) and this is reflected in planetary compositions. Refractory oxides, metals, and Mg-silicates are enriched in the inner function of the cloud where Mercury accretes; Mg-Iron silicates and metal in the region from Venus to the asteroids; and mixed silicates and ices in the outer part of the nebula where the behemothic planets accrete. Homogeneous accretion models produce an amazingly good match between predicted and observed planetary compositions. If, for instance, volatiles were diddled outward when Mercury was accreting refractory oxides, metal, and Mg-silicates, Venus and World should be enriched in volatile components compared to Mercury. The lower mean density of Venus and World is consistent with this prediction. Mars has a still lower density, and it is probably enriched in oxidized atomic number 26 compounds relative to Globe and Venus. Besides, the giant outer planets are composed of mixed silicates and ices, reducing their densities dramatically. If homogeneously accreted, how do the terrestrial planets become zoned? Information technology must be by melting, which results in segregation by such processes every bit fractional crystallization and sinking of molten fe to the core. Where does the rut come from to melt the planets and asteroids? Some of the major known heat sources are every bit follows:

(1)

Accretional energy. This energy is dependent upon impact velocities of accreting bodies and the amount of input energy retained by a growing planet. Accretional energy lone appears to have been sufficient, if entirely retained in the planet, to largely melt the terrestrial planets while they were accreting.

(2)

Gravitational collapse. Equally a planet grows, the interior is subjected to higher pressures and minerals undergo stage changes to phases with more than densely packed structures. Most of these changes are exothermic as discussed in Affiliate iii, and large amounts of free energy are liberated into planetary interiors.

(iii)

Radiogenic heat sources. Radioactive isotopes liberate significant amounts of heat during decay. Short-lived radioactive isotopes, such every bit 26Al and 244Pu, may take contributed meaning quantities of oestrus to planets during accretion. Long-lived isotopes, principally fortyK, 235U, 238U, and 232Thursday, are important heat producers throughout planetary history.

Because volatiles are retained in the terrestrial planets information technology is unlikely that they were ever completely molten. Hence, it is necessary to remove the early on oestrus very rapidly and convection seems the simply process capable of bringing rut from planetary interiors to the surface in a short time interval (≤   50 Myr) to escape complete melting. During the late stages of accession, collisions deposited enough free energy, if fully retained, to partly or fully melt World, perhaps multiple times with this magma bounding main extending nigh to the center of the planet (Drake, 2000). Metal volition sink through the magma ocean and accumulate at the center equally the core continues to abound (as discussed in Chapter five). This early core was disrupted and rapidly reformed during the Moon-forming collision issue. The temperature dependence of mantle viscosity appears to be the virtually important factor decision-making planetary thermal history (Carlson, 1994; Schubert et al., 2001). Initially, when a planet is hot and viscosity is low, chaotic mantle convection rapidly cools the planet and crystallizes magma oceans (Drake, 2000; Elkins-Tanton, 2012). As mantle viscosity increases, outset 50 Myr after accession, convection should absurd planets at reduced rates.

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Physics and Chemistry of the Solar System

In International Geophysics, 2004

Introduction

We have followed the development of matter from the hydrogen and helium ashes of the Big Bang through galaxy and star formation, heavy-chemical element nucleosynthesis and supernova explosions, the germination of the Solar Organization, and evolutionary processes in the realm of the outer planets. We then examined the chemical and physical differentiation of nearly homogeneous preplanetary material from the point of view of the Jovian and Uranian planets, ice-rich satellites, and comets. Next we surveyed the Centaur and TNO groups, and so moved closer to the Sun to study the Asteroid Belt, Trojan, and NEA populations and chronicle these asteroidal bodies to the laboratory report of meteorites, for the purpose of using them as guides to the materials and processes that gave ascension to the terrestrial planets. Before encountering the terrestrial planets Mars, Venus, and World in their full glory and complexity, we accept only to study the smaller, airless bodies in the inner Solar System. These bodies bridge the gap in size, degree of evolution, and complexity between meteoritic samples of asteroids and the 3 large terrestrial planets with atmospheres.

The general trend of planetary compositions and densities clearly favors the idea of germination of the Solar System out of a flattened, dusty gas disk within which the density, pressure, and temperature all dropped off steeply with increasing altitude from the middle. The bodies discussed in this chapter all allow us to test our ideas of planetary formation and development in specific but unlike ways. Phobos and Deimos appear to be relatively unaltered samples of the small bodies that passed through Mars' accretion zone late in the era of planetary formation. They therefore may incorporate much useful information on local environmental conditions during the formation of Mars.

Mercury orbits closer to the Lord's day than whatsoever other known Solar System torso and serves equally a probe of the high-temperature farthermost of conditions encountered during planetary formation. As recently as 1966 it waswidely believed that the Moon would prove to be a rather primitive object like to typical preplanetary solids, possibly wholly undifferentiated, similar chondritic meteorites. Only we now know that World 's Moon has a composition that deviates markedly from those of the terrestrial planets, in that the Moon seems to be strikingly deficient in native metals. Farther, the dynamical history of the Moon may be unique amongst the known bodies of the Solar Arrangement.

Io presents united states of america with our best example of large, rocky bodies in the outer Solar Organisation. Of all the vi lunar-sized satellites of the Jovian planets (Io, Europa, Ganymede, and Callisto about Jupiter; Titan at Saturn; Triton at Neptune) and the Pluto–Charon organization, it is the only one with rock-like density (3.52) and an water ice-costless surface. Even Europa, with its similar density of three.45, has a deep ice layer, possibly l km in thickness, that confined us from studying its rocky component. Io is also "Jupiter'south Mercury"; it is the high-temperature finish member of planetary formation in the Jovian subnebula, which spawned 4 bodies of planetary dimensions. The less massive, colder Saturnian subnebula produced no rocky satellites and no obvious systematic density tendency with Saturnocentric distance. The Uranian satellites likewise have depression density and no discernible trend, and the Neptunian system is unfortunately severely disturbed by the possible capture of Triton. Thus Io plays a unique role in our study of planetary formation: it represents a test of the generality of our ideas of planetary formation from what is effectively another nebular organisation.

The tale told about these minor rocky bodies by even the very simplest indicator of planetary limerick, bulk density, is already complex. Figure IX.ane shows the densities of the bodies in the inner Solar System, every bit corrected for gravitational self-compression. These corrected uncompressed (nothing-pressure) densities are to some caste model dependent, but they at to the lowest degree reflect an intrinsic belongings of the solid raw materials out of which the planets are made. The apparent similarity amid the observed densities of Mercury, Venus, and Earth hides a very big compositional difference: at nada force per unit area, Mercury (5.3 yard cm≠3) is much denser than Venus and Earth (both about 4.four, with Earth marginally denser by well-nigh 1 %). The simplest explanation for the density difference is that Mercury 's mass is about lxx % metal, compared with nigh 30 % for Venus and World. Mars has an observed density of iii.93, which converts to an uncompressed density of three.74, distinctly less than whatsoever of the other terrestrial planets. The Moon 's observed density of 3.34 yard cm≠3 is essentially unaffected by compression because of the Moon 's minor mass and depression interior pressures. The 2 modest satellites of Mars, Phobos and Deimos, accept densities of just almost 2 one thousand cm≠3, similar to the most volatile-rich carbonaceous chondrites (type CI). Thus the 7 bodies that orbit "permanently" in the inner Solar Arrangement correspond a minimum of five distinctly dissimilar compositions.

Effigy IX.one. Uncompressed densities of solid Solar Arrangement bodies. The solid line is the theoretical dependence of density on temperature for an equilibrium condensation scenario with a slightly elevated Atomic number 26:Si ratio. The dashed line is the dependence of density on accretion temperature for a rapid-accretion scenario, in which each condensate accretes into large bodies equally it condenses, without further reaction with nebular gas. Notation that, even with no allowance for radial mixing of nebular solids with unlike condensation temperatures, the predicted density of Mercury is as well low.

Nosotros should recall that many other modest bodies are presently in orbits in the same volume of space. The approximately 200 nearest asteroids include ane with a strikingly World-like orbit (1991 JW) and i that follows a Lagrange betoken on the orbit of Mars (1990 MB). The asteroids of the Aten family have orbital periods less than i year and patrol the space from 0.292 to i.513 AU from the Sun, and at least 25 % of the NEAs take orbital periods less than that of Mars. These bodies, withal, are merely temporary sojourners in the terrestrial planet region, exiles from the cold reaches of the asteroid chugalug or retired short-catamenia comets. After a few tens of millions of years they volition have suffered collision with ane of the terrestrial planets or experienced perturbation into Jupiter-crossing orbits and then ejection from the Solar Organization. Their presence should remind us that there is a constant supply of new interlopers of extremely diverse composition careering through the space occupied by the terrestrial planets. Indeed, Phobos and Deimos may be refugees from the Belt that have taken up long-term residence in orbits about Mars. Their compositions and histories, if known, might reveal the satellites to have either little or slap-up relevance to the history of Mars.

Because of their widely different locations, the bodies in this chapter experience a broad diverseness of collisional environments. The location of Phobos and Deimos in orbit nearly Mars causes their impact development to exist radically unlike from that of the other bodies. Mercury experiences impacts that are more energetic than those upon any other body in the inner Solar System, excepting only the Sun itself. The general idea of the dominant role of impacts in governing the surface development of small-scale rocky bodies seems challenged past Io, which exhibits not a single identifiable impact feature. Thus a comparison of the cratering histories of these bodies is of great importance.

Information technology is also interesting to note that every one of these bodies has a peculiar spin state: Phobos, Deimos, the Moon, and Io are all locked into a ane:1 spin–orbit resonance with their primaries. Mercury, with the most eccentric orbit of the group, is locked in a iii:2 resonance.

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Mineral Physics

T. Duffy , ... M.1000.Thou. Lee , in Treatise on Geophysics (Second Edition), 2015

two.07.v.one Composition of Super-Earths

The bulk composition of a planet should reflect that of its host star as modified by planet-forming processes within the protoplanetary disk. In general, a broad range of bulk compositions for terrestrial-type planets should be possible, in principle. The mineralogy of terrestrial super-Earths tin can be modeled assuming an Earth-like composition as a starting point. The composition of Earth is dominated by O, Fe, Mg, and Si with lesser amounts of Ca and Al (McDonough and Sun, 1995). The Globe's interior consists of a drape composed of Mg-rich silicates and oxides overlying a metallic Fe core, with other species in small-scale amounts (see Chapters two.03 , 2.06 , and 10.02).

Measured photospheric stellar abundances of the elements provide constraints on both internal and atmospheric compositions of exoplanets likewise as insights into what characteristics of protoplanetary clouds favor planetary germination (Delgado Mena et al., 2010). The Mg/Si, Iron/Si, and C/O ratios are important for determining the mineralogy and construction of terrestrial planets. The Mg/Si ratio will command the relative abundance of the major silicates such as pyroxenes (Mg/Si   =   ane) and olivine (Mg/Si   =   2). The solar and bulk Earth Mg/Si values are close to one (Asplund et al., 2009; Kargel and Lewis, 1993; McDonough and Sun, 1995). Photospheric measurements of planet-hosting stars show a range of Mg/Si values ranging from ~   0.vii to ~   ane.four (Delgado Mena et al., 2010). In simulations of planetary formation that included variable chemical composition in the protoplanetary disk, terrestrial planets with variable compositions, some very different from Earth, were found (Bond et al., 2010). Models of systems with low Mg/Si ratios yield planets that are Mg-depleted compared to World and are formed from silicates such equally pyroxenes and feldspars (Carter-Bond et al., 2012a ). However, studies of the human relationship between solar abundances, meteorites, and Earth majority compositions have shown that the human relationship between planetary compositions and stellar abundances is complex ( Drake and Righter, 2002).

The bulk Fe/Si ratio is an important parameter for determining the size of the metallic core in a terrestrial exoplanet. Values of core mass fraction used in typical models to engagement are based on analogy to our solar organisation and range from Earth-like (~   33%) to Mercury-like (~   66%), while end-member models ranging from pure core to a silicate-only coreless planet are useful for delineating the range of mass–radius space potentially occupied past rocky planets. The core mass fraction will depend not just on bulk Fe content but besides on cadre formation processes that sectionalization Fe between curtain and core (Elkins‐Tanton and Seager, 2008). It may also be affected by loss of a planet's mantle due to big impacts or evaporation due to stellar irradiation. For planets of our solar system, the Mg# (=   tooth Mg/Mg   +   Iron) of the curtain is known for just Earth and Mars and ranges between 0.seven and 0.9. The distribution of Fe betwixt drape and core has little overall result on the mass–radius human relationship (Elkins‐Tanton and Seager, 2008; Fortney et al., 2009).

The presence of water or other hydrous phases in a terrestrial-type planet is of particular interest as an essential component of habitability. The presence of water can also affect melting behavior and rheology and may touch on the likelihood of plate tectonics (Korenaga, 2010). Migration of behemothic planets may profoundly increase the efficiency of delivery of h2o and hydrous phases to growing terrestrial planets even promoting formation of ocean planets and wet Earths (Carter-Bond et al., 2012b). Close proximity to the host star, on the other hand, can atomic number 82 to intense heating, which would exist expected to strip volatiles from the planet.

Long-lived radioactive elements (40Grand, 232Th, 235U, and 238U) play an of import role as internal rut sources in the Earth, decision-making internal temperatures and thermal evolution. The amount of internal heating relative to basal heating in the drape will impact both internal and surface dynamics (van Heck and Tackley, 2011). Nearly models of extrasolar planets to engagement have causeless chondritic abundances of these elements (Tachinami et al., 2011), just the range of possible variability is poorly constrained. Ice-rich planets may be significantly libation in the interior compared to rocky planets of the same mass due to reduced affluence of radioactive elements (Sotin et al., 2007).

The C/O ratio is a chemical parameter that controls the distribution of Si amidst oxides and carbides (Bond et al., 2010). Under equilibrium conditions in the innermost region of the protoplanetary disk, a C/O value <   0.viii leads to interiors dominated by silicates ( Figure 12 ). When the C/O ratio is >   0.viii, Si exists as SiC with additional C also present. Further out in the disk, silicates boss simply the resulting planets can be carbon-rich. The types of rocky planets that may form under loftier C/O ratios have been classified as carbon planets (Kuchner and Seager, 2005). A wide range of C/O values take been observed in the photospheric abundances of exoplanet host stars with virtually ane-3rd of planet-hosting stars having C/O values >   0.viii (Delgado Mena et al., 2010; meet likewise Petigura and Marcy, 2011). On the other mitt, some recent studies accept suggested that previous determinations of loftier C/O in stars could have been overestimated due to systematic errors in estimating the C and O abundances and that carbon-rich main-sequence stars, and hence carbon planets, would exist rare (Fortney, 2012; Nissen, 2013).

Effigy 12. Chemic composition of refractory condensates expected in the protoplanetary disk of the Sun (C/O ratio   =   0.54) (top) and of the 55 Cancri system (C/O ratio   =   1.12) (bottom). The colored curves show molar mixing ratios of the major species, shown in the legend, in thermochemical equilibrium as a function of temperature in the protoplanetary disk midplane. A representative pressure of 10−   four  bar is assumed. The elemental abundances of the host star (Carter-Bail et al., 2012b; Delgado Mena et al., 2010) are assumed as initial weather condition, and the equilibrium computations were performed using thermochemical software (HSC Chemistry). The black filled circles at the acme of the figure show the temperatures of the deejay at dissimilar orbital separations and ages.

 Reproduced from Madhusudhan N, Lee KKM, Mousis O (2012) A possible carbon-rich interior in super-Earth 55 Cancri east. Astrophysical Journal Messages 759(2): L40. http://dx.doi.org/10.1088/2041-8205/759/2/L40, with permission.

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Mantle Dynamics

D. Bercovici , in Treatise on Geophysics (Second Edition), 2015

7.01.6 Major Unsolved Bug in Mantle Dynamics

7.01.6.1 Free energy Sources for Drape Convection

The discovery of radioactive elements at the turn of the nineteenth century was a key discovery in many regards, including providing show that the Earth has internal sources of heat. This was also the key argument to refute Lord Kelvin's estimate for the age of the Globe since he causeless that it was cooling freely (without oestrus sources) from an initially molten state (run into Section 7.01.2.4 ). Early estimates of the concentration of radioactive elements within the Earth are based on heat flow and geochemical measurements of crustal rocks, in conjunction with cooling models ( Chapter 7.05 ); these arguments tended to signal toward a high enough concentration of radioactive elements in the curtain to business relationship for as much every bit 70–fourscore% of the heat menses out of the Earth to be due to radiogenic heating (Schubert et al., 2001). This satisfied the status that the World has been cooling relatively slowly over its 4.v   Gy lifetime. However, recent estimates of radioactive element abundances from the study of chondrites (thought to exist representative of planetary building blocks) peradventure propose somewhat less radiogenic heating (see Chapter 7.06 ). If radiogenic heating is modest, then more of the Earth'southward heat catamenia is from loss of fossil heat (every bit with Lord Kelvin's assumptions; see as well England et al., 2007); this would demand rapid cooling from a recently excessively hot or even molten land, unless the physical procedure of convection was itself somehow very different in the past in order for the mantle to retain its heat (see Chapters 7.06 and 9.06 ). Alternatively, if chondritic estimates of these radiogenic sources are very incorrect, then it implies significant problems with the chondritic model for planetary composition and thus for our agreement of the World'southward formation. Either possibility leads to many intriguing directions for future inquiry.

7.01.6.2 Is the Curtain Well Mixed, Layered, Piled, or Plum Pudding?

Equally mentioned in the preceding text in Sections vii.01.3 and 125.5.2 , at that place are various lines of geochemical evidence – ranging from the disparity between trace element abundances in MORBs and those in ocean island basalts, sources and reservoirs of noble gases, source and origin of continental crust, etc. – that suggest that the drapery has isolated reservoirs, such as distinct layers. Although layering of the mantle at 660   km depth has probably been eliminated, the motivation notwithstanding persists to reconcile the geochemical inference of an unmixed drape with the geophysical bear witness of a well-stirred mantle. The problem remains unsolved, although various models and solutions take been proposed, in particular deep layering, or mechanisms for keeping the curtain poorly mixed (the 'plum-pudding' model). In recent years, deep-layering models have evolved into 'pile' models wherein the dense and long-lived abyssal layer is noncontiguous and gathered into at least two stable mounds or piles at the cadre–pall boundary; seismic prove for these piles has grown and the pall-pile model has become an active surface area of inquiry and argue (see Chapter 7.11 ). This issue comprises the major thrust of Affiliate 7.11 and is touched upon in Affiliate seven.x , also as in other volumes, notably Chapter ii.04 .

seven.01.six.iii Are In that location Mantle Plumes?

Convective plumes are relatively narrow cylindrical upwellings typical of convection and especially in fluids with strongly temperature-dependent viscosity, such every bit mantle rocks. The existence of plumes in the drape was proposed by Jason Morgan (see Affiliate 7.10 ) to explain dissonant intraplate volcanism such as at Hawaii. That hot spots appeared to be more or less immobile relative to plates suggested a deep origin, and it has often been supposed that plumes emanate from the most obvious heated boundary in the pall, the core–mantle boundary. Direct observation of plumes by seismic methods has achieved some success in recent years in identifying hot curtain anomalies below, for example, Hawaii (Wolfe et al., 2009), as well as by newer techniques that even so remain controversial (Montelli et al., 2004). Although evidence for the beingness of plumes has grown in the last decade, they remain the subject of ongoing fence. The subject of plumes and melting anomalies is discussed in Chapter seven.ten .

7.01.six.4 Origin and Crusade of Plate Tectonics

As noted already, the modern theory of drape convection was largely motivated to provide a driving mechanism for plate tectonics. Nosotros also argued in the preceding text ( Department 7.01.5.3 ) that plate tectonics is indeed convection. Withal, at that place notwithstanding remains no unified theory of mantle dynamics and plate tectonics, wherein plate tectonics arises naturally and cocky-consistently from mantle convection. There are some aspects of plate tectonics that are reasonably well explained by convective theory, in detail the beingness of cold planar downwellings akin to subducting slabs ( Chapter 7.09 ). Merely however, many first-lodge questions remain. Just with regard to subduction itself, at that place is however no widely accepted theory of how a subduction zone and sinking slab initiate from a thick cold and, by all appearances, immobile lithosphere and sink asymmetrically; that is, only 1 side of a convergent zone descends (see Capacity vii.02 , 7.07 , and vii.09 ).

Only many other issues remain in the problem of 'plate generation.' Although mid-ocean ridges are associated with upwelling from the mantle, all evidence points to the upwelling being shallow and the spreading being passive; that is, ridges are pulled apart by slabs at a altitude, rather than pried autonomously by a deep upwelling ( Chapter 7.08 ). How such passive upwelling occurs in convection is still not universally understood. Another long-standing enigma of the platelike form of curtain convection is the existence of toroidal motion, which involves strike-skid shear and spin of plates (Dumoulin et al., 1998; Hager and O'Connell, 1979; O'Connell et al., 1991). Toroidal motion is enigmatic considering it is not typical of convective menses; that is, it is non directly driven by thermal buoyancy forces and likewise does not transport heat and thereby serves no credible purpose (see Capacity 7.02 , 7.04 , and 7.07 ).

A generalizable theory of how plate tectonics is generated by drape convection is crucial to understanding not only how Globe works just also how planets in other solar systems work, that is, whether they are more probable to wait like Earth, Venus, Mars, or something unexpected. In particular, since plate tectonics is believed to stabilize the Globe's clement climate for hundreds of millions of years, it is considered a necessary condition for habitability and thus an active topic of argue in the report of extrasolar terrestrial planets. Much still remains to be learned in the fundamental problem of plate generation and is the main topic of Chapter vii.07 , also as beingness discussed throughout this treatise (encounter Affiliate nine.06 ).

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The bulk limerick of Mars

One thousand. Jeffrey Taylor , in Geochemistry, 2013

half-dozen.7 Comparing planet compositions

A central reason for determining planetary compositions is to compare them to deduce variations in compositions, atmospheric condition in the solar nebula and chemical uniformity of information technology, accretion processes (including the extent of mixing), and differentiation styles (east.g., floatation crust or non). Planetary scientists frequently emphasize differences among the terrestrial planets, but the similarities are hit ( Tabular array 6). The close similarities in the D/H ratios of Mars, Earth, carbonaceous chondrites, and Jupiter-family comets suggest a common source of h2o-bearing fabric in the inner solar organization (Alexander et al., 2012). K/Thursday (Table half dozen) varies amidst the terrestrial planets: Mercury, 5200   ±   1800 (Peplowski et al., 2011); Venus, ∼3000 (Surkov et al., 1987); Earth, 2900 (Jagoutz et al., 1979; McDonough and Sun, 1995; Taylor and McLennan, 2009); Mars, 5300   ±   220 (Taylor et al., 2006a). However, these variations seem less significant when compared to the K/Thursday ratios of the carbonaceous chondrites (19,000 – McDonough and Lord's day, 1995). The terrestrial planets are depleted in volatile elements compared to carbonaceous chondrites, but the depletions are non correlated with distance from the Sun. Oxygen isotopes are distinctive amid planets and meteorite groups (Mittlefehldt et al., 2008). The difference betwixt Earth (Δ17O of 0‰ by definition), and Mars (Δ17O of   +   0.25‰), is much smaller than the total range observed among chondrites Δ17O of −iv.3 to +2.v‰). Warren (2011) emphasizes the similarity among terrestral planets and differentiated meteorites compared to carbonaceous chondrites in the isotopic compositions of O, Cr, and Ti.

Tabular array 6. Comparisons for two of import chemical parameters amidst inner solar system objects.

1000/Th Refs FeO (wt%) Refs
Mercury 5200 a three f,one thousand
Venus ∼3000 b 8 f
Earth 2900 c 8 c
Moon 360 d 13 h
Mars 5300 due east 18 i

(a) Peplowski et al. (2011); (b) Surkov et al. (1987); (c) McDonough and Sun (1995); (d) Warren and Wasson (1979), Warren (1989); (e) Taylor et al. (2006a); (f) Robinson and Taylor (2001); (g) Nittler et al. (2011); (h) Taylor et al. (2006a); (i) Wänke and Dreibus (east.one thousand., 1988) and this paper.

Some chemical parameters do vary directly with heliocentric distance. Bulk silicate FeO (Tabular array vi) increases from ∼3   wt% in Mercury (Robinson and Taylor, 2001; Nittler et al., 2011), to viii   wt% in World (McDonough and Dominicus, 1995) and Venus (Robinson and Taylor, 2001) to 18   wt% in Mars. FeO and the size of the metallic cores are inversely correlated. These trends suggest a range in oxidation conditions correlated with heliocentric altitude, in contrast to the credible weak correlation with heliocentric distance for D/H, K/Th, and oxygen isotopes. Equally these differences can be produced by varying oxidation conditions, they do not advise the terrestrial planets were formed from fundamentally different materials. On the reverse, the broad chemic similarities among these planets betoken substantial mixing throughout the inner solar arrangement during planet germination, as suggested by dynamical models (O'Brien et al., 2006; Walsh et al., 2011).

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