Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
EARTH AND PLANETARY SCIENCE LETTERS 15 (1972) 286-290. NORTH-HOLLAND PUBLISHING COMPANY METAL/SILICATE FRACTIONATION IN THE SOLAR SYSTEM John S. LEWIS Planetary Astronomy Laboratory, * Department of Earth and Planetary Sciences, and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Mass. 02139, USA Received 2 February 1972 Fractionation between the metal and silicate components of objects in the inner solar system has long been recognized as a necessity in order to explain the observed density variations of the terrestrial planets and the H-group, L-group dichotomy of the ordinary chondrites. This paper discusses the densities of the terrestrial planets in light of current physical and chemical models of processes in the solar nebula. It is shown that the observed density trends in the inner solar system need not be the result of special fractionation processes, and that the densities of the planets may be direct results of simultaneous application of both physical and chemical restraints on the structure of the nebula, most notably the variation of temperature with heliocentric distance. The density of Mercury is easily attributed to accretion at temperatures so high that MgSiO3 is only partially retained but Fe metal is condensed. The densities of the other terrestrial planets are shown to be due to different degrees of retention of S, O and H as FeS, FeO and hydrous silicates produced in chemical equilibrium between condensates and solar-composition gases. It is proposed that Mercury and Venus Have cores of Fe °, Earth has a core of Fe ° containing substantial amounts of FeS, and Mars has a quite small core of FeS with more FeO in its mantle than in Earth's. Geophysical and geochemical consequences of these conclusions are discussed. The densities of the terrestrial planets can be most accurately determined by radar tracking o f Mercury, Venus, and Mars, and of spacecraft in interplanetary space. The most recent estimates o f the densities o f the planets, by Ash et al. [ 1] are : Mercury, 5.42 g/cm 3; Venus, 5.25; Earth, 5.51 ; and Mars, 3.96. Corrected to zero pressure, the densities become ~5.3, 4.40, 4.45, and ~3.85 respectively. Anderson and Kovach [2] have shown that the density may be converted into an equivalent parameter, the mean atomic weight (#). Their method applied to the data of Ash et al. yields mean atomic weights o f ~35.4, 26.6, 26.9, and 25.0, respectively, with errors of ~+-0.15 at worst. Two major theories have been proposed to account for the densities of the terrestrial planets. Urey [3] showed that, with the solar abundances o f the ele* Contribution no. 45 of the Planetary Astromony Laboratory. ments then accepted, profound fractionation of metalhc iron relative to silicates was required in order to effect the observed variations in density. Because of the inaccuracies in available estimates of the density o f Venus, Urey was unable to conclude that Earth was in fact more dense than Venus. A very different view was that of Ringwood [4] who hypothesized that the abundances o f the major rock-forming elements were the same in the various terrestrial planets, and that the density differences were caused by differences in oxidation state due to imcomplete reduction of primitive Fe 3+ to Fe ° by carbon. The density o f Mercury was attributed to volatilization of silicates by the Sun during its superluminous pre-main sequence phase. My present purpose is to reassess this problem in light of the recent upward revision in the iron abundance [ 5 - 1 1 ], Cameron's detailed physical models o f the solar nebula [12], and recent progress in understanding the chemistry of solar material [ 1 4 - 2 0 ] . J.S. Lewis, Metal/silicate fractionation in the solar system Cameron has presented calculations on the evolution of the primitive solar nebula in which the temperature, pressure, and density profiles are computed for the early nebula [ 12]. The structure of the inner portions of the nebula is governed by rapid turbulence, and the temperature-pressure profile is closely adiabatic. Within the central plane of the nebula the cooling process is essentially isobaric at any given point. I will present here a subset of my results from chemical equilibrium calculations on solar material. For the present it will suffice to simplify the chemical system under consideration to H, C, O, S, Si, Mg, Ca and Fe I I I I I I 1 28 7 in their solar proportions. Because there is still an uncer tainty of ~-+20% in the solar Fe: Si ratio, we will calculate the density of the condensate (as ~) vs. T for several different acceptable values of this solar abundance ratio. Fig. 1 presents the major features of the chemistry of solar material as a general function of both temperature and pressure, and is marked with an adiabat schematically indicating the temperaturepressure profile through the-nebula early in its history. Pressure can be directly related to heliocentric distance through Cameron's models, and the points of origin of the planets are indicated [ 12, 13]. Fig. 2 presents the density of the condensate along an isobaric section through fig. 1 at 10 .3 bar total pres- 2000 36 , T ' 1 ' I ' T ~ I ' I ' ' Mercury CaTiO 3 / 150C MgSi03 :54-- ? o W ~ • d Venus Earth Mars ::k g Fe o52 g g IOOC 500 'o~c "[ 28 I -6 condensation FeO / -7 MgSi03 50 FeS 0 K I I I ice I -5 -4 -3 -2 -I Ioglo PRESSURE ( b a r s } 1 g I 0 0 o~ +1 FeS format ion 26-- -0.86 Fig. l. Some major features o f the chemistry o f solar material, 0 to 2000°K and 10 7 to 101 bar total pressure. The condensation curves of CaTiO3 (a representative refractory mineral), Fe, MgSiO3 and ice; the appearance temperatures of FeS, trem o h t e (a h y d r o u s calcium silicate) and talc (a h y d r o u s magnesium silicate); and the line at which Fe metal is wholly oxidized to FeO (as FeSiO3 and Fe2SiO4) are indicated. An adiabat for the nebula is drawn in, and nebular pressures corresponding to the formation conditions o f the planets in Cameron's models are marked on the adiabat. The symbols, reading from the high-temperature end downward, are for Mercury, Venus, Earth, Mars, the asteroids, Jupiter, Saturn, Uranus, and Neptune. Temperatures of formation deduced from correlating the density data in fig. 2 with the observed densities of the terrestrial planets are in excellent agreement with this adiabat. 2400~16 ~- I ~ Fe/Si = 1,06 ~ _ ~ _ ~ . / " ~ k Fe/Si = 0 96 -'~/-~T /-~\/0t I ~ I ~ L ~ I ~ I 1400 1200 I 0 0 0 8 0 0 6 0 0 Ternperature, °K geO ond ./tremolile formation 'Orrn°tiOn i 400 Fig. 2. Density of condensed material inequilibrium with a solar-composition gas, 4 0 0 - 1 6 0 0 ° K at 10 - 3 bars. A simplified chemical system (the 20 m o s t a b u n d a n t elements) is employed for three different values of the Fe: Si ratio. The densities of the planets are excellently consistent with an Fe: Si ratio of 1.08, b u t the omission of rare elements and uncertainties in the abundances of major elements could displace the entire manifold of curves shghly. A true Fe: Si ratio below 1.0 is still possible. 288 .LS. Lewis, Metal/silicate fractionation in the solar system sure, ranging from the condensation temperature of Fe ° down to 400°K. Three different solar Fe: Si ratios are indicated. The most important features of these graphs are (1) condensation of MgSiO 3 near 1400°K, (2) removal of the H2S from the gas by reaction with solid Fe to make FeS, and (3) oxidation of the remaining elemental iron to FeO-bearing silicates. The density of the condensate drops rapidly from that of pure iron (for which B = 55.8) during MgSiO 3 condensation, rises briefly-by ~1% when FeS forms, and then drops by ~5% when Fe ° is oxidized to FeO. Thus the density of condensed material in chemical equilibrium with the nebula shows the same general features that are observed in the densities of the planets. The striking 1% increase in the density of Earth relative to Venus is attributed to the retention of the heavy volatile element sulfur by the Earth. Since the mean atomic weight of sulfur is 32, retention of sulfur in the Earth will cause a 1% increase in tt for each 5% of sulfur content. The correlation between the observed densities of the terrestrial planets and those predicted by the equilibrium theory is quite good. It is interesting that so good a fit can be obtained without invoking any mechanism for m e t a l - s i l i c a t e fractionation. The relative abundances of silicon, magnesium, iron, calcium, aluminum, etc. are exactly the same in Venus, Earth and Mars for this model. Mercury is seen to condense at such a high temperature that it is depleted in magnesium, silicon, and the alkali metals relative to iron, calcium, aluminum, titanium, nickel and other refractory materials. We conclude that Urey's [3] proposal of iron-silicate fractionation involving all the terrestrial planets is an artifact of the low cosmic abundance of iron then accepted, and that m e t a l - s i l i c a t e fractionation by physical means (magnetic forces, ease of fragmentation, etc.) need not be postulated in so far as the densities of the inner planets are concerned. Ringwood's [4] model for producing the terrestrial planets by in situ reduction of Fe304 to Fe ° by carbon involves several acl hoc hypotheses. In particular we note that Ringwood requires carbonaceous chondrite parent material, and faces the embarrassing task of removing a mass of CO from Earth roughly equal to the mass of Mars; that Ringwood postulates two separate processes to account for the densities of the terrestrial planets; that the density of Venus is explained as being less than that of Earth because of the "higher degree of oxidation of Venus", for which no cause is suggested. Ringwood's basic assumption that differences in oxidation state and in the degree of retention of volatiles can explain the densities of Venus, Earth and Mars is valid, but we find that, because of the high atomic weight of sulfur, density is not a monotonically decreasing function of oxidation state and volatile retention. In the present model, oxidation state and volatile retention do correlate monotonically with heliocentric distance, and the reason for this correlation is that the chemical composition of the condensate is a function of its temperature of formation. Because of the absence of any p r o o f that m e t a l silicate fractionation influences the density of the terrestrial planets, any hypothesis which requires that these planets have accumulated involatile chemical components incompletely [12], in a disequilibrium manner [4, 21 ], or with physical fractionation between components [3] must explain why the equilibrium theory with no fractionation does such a satisfactory job of explaining the data with far fewer arbitrary assumptions. The variation in Fe: Si ratio in the several families o f chondrites is attributed to their origin in parent bodies as small as 10 -12 times the mass of the Earth, in which statistical fluctuations in composition or local variations in conditions of accretion might be enormously larger than for objects of planetary mass. The models for the terrestrial planets which emerge from the present work are as follows: Mercury. A massive core of Fe-Ni alloy is surmounted by a small mantle o f Fe2+-free magnesium silicates. Ca, A1, Ti and other refractory oxides are present, but only traces of alkali metals, sulfur, FeO, etc. (due perhaps to "contamination" b y infalling debris) could be present. This agrees with Ringwood's picture of Mercury, but arises not as a result of a special additional postulate, but as an integral part of the model for formation of the planets. Venus. A massive core of Fe-Ni alloy is surmounted b y a massive mantle of Fe2+-free magnesium cilicates. Iron, magnesium, and silicon are present in cosmic proportions. A silica-rich crust similar in composition to Earth's is present [22]. Sulfur is probably virtually absent from the planet [23]. Earth. An inner core of Fe-Ni alloy and outer core of Fe-FeS melt are present. Certain chalcophile ele- J.S. Lewis, Metal/silicate fraetionation in the solar system ments are deficient in the mantle and crust but enriched in the outer core [ 2 4 - 2 6 ] . These elements, which are chalcophile above the Fe-FeS eutectic temperature at oxygen fugacities typical of ordinary chondrites, may include potassium, rubidium and cesium. Earth's mantlle contains ~10% FeO. The crust and upper mantle are oxidized relative to the rest of the Earth, due to preferential enrichment of Fe 3+ in upwardflowing differentiate melt relative to solid olivine, and by the effects of the Earth surface environment. Depletion of chalcophiles in the crest and the marked deficiencies of S, K, Rb and Cs in the crust and upper mantle are due to their extraction into an FeS-rich melt. The overall composition of the Earth is rather close to that of the H-group chondrites, but is not identical to nor derived from them or any other class o f meteorites. Mars. Mars is essentially devoid of free iron. It may contain a core of FeS with or without a small amount of Fe ° [27]. Its mantle is rich in FeO [FeO/(FeO + + MgO) ,-~ 0.5]. Hydrous minerals were retained in appreciable quantities during accretion. The present crust (assuming differentiation has taken place and been reasonably complete) should be similar to but more iron-rich than Earth's. Of the terrestrial planets, only Earth is likely to have accreted at a relatively low degree o f oxidation with both abundant Fe ° and FeS present. Only Earth is likely to have differentiated so as to extract the heavy alkali metals into the core. Thus the very large heat source of ~1020 erg/sec possible within the outer core of the Earth is impossible on Mercury or Venus and unlikely on Mars. The presence of a planetary magnetic dipole field on Earth and its absence on Mars is attributed to the difference in core composition. Heat flow through the crusts of Mars, Earth, and Venus should closely approximate that predicted by the chondritic model. On earth, deep convection of the mantle delivers heat from decay of deep-seated 40K to the base of the thin oceanic crust, and the continental crust is driven to positions over the subsiding parts of the upper mantle. The near-equality of the continental and oceanic heat flow and the close correspondence with the chondritic model are direct resuits of this model. In addition, the presence of the FeFeS eutectic melt very early in the thermal history of the Earth provides a simple explanation o f early differentiation of the Earth without the necessity of 289 postulating very rapid accretion. Because o f the postulated role of an Fe-FeS eutectic melt in permitting early differentiation of the Earth, the geochemical differentiation of Earth will differ in several respects from that of the other planets. The most important distinctions are in the potassium distribution and in the time of differentiation. Venus, Earth's near-twin, may take much longer to differentiate than Earth. In fact, Earth may differentiate before it is completely accreted, because adiabatic compression o f cold material is by itself adequate to heat most of the Earth's interior to above the Fe-FeS eutectic temperature. Heat flow through the crust of Mercury should be higher than that calculated for a potassium-free chondritic model because we expect complete retention of U and Th relative to Fe, but less dilution b y magnesium silicates. A heat flow value close to that for a chondritic model could result from this accidental compensation. The densities of the planets are not the only observable properties which may be strongly affected by the temperature of formation and accretion: a very interesting question is the different degree of retention of volatiles, such as H20, by the terrestrial planets. The data presented herein suggest that Earth accreted on the edge of the tremolite [Ca2-MgsSi8022(OH)2 ] stability field and Mars accreted well within it. Mars should thus contain roughly one water molecule for every two calcium atoms, or ~0.3% of the mass of Mars, and Earth should contain several times less water. A component of volatiles from accretion of a small proportion (<~ 1%) of cometary or carbonaceous chondrite material cannot be ruled out [29]. In this model the primitive material present throughout the asteroid belt is of carbonaceous chondrite composition. Further evidence for the existence of a significant radial temperature gradient and rather low temperatures in this region can be found in the densities of the satellites of Jupiter. The temperature gradient must not be substantially less than adiabatic or it becomes impossible to get accretion temperatures for the Galilean satellites low enough to explain their observed low densities [20]. The evidence from condensation models for the terrestrial planets combined with similar work on the satellites of the outer planets combine to suggest that the ordinary chondrites originated inside the orbit of Mars. The present orbits of 29(/ J.S. Lewis, Metal/silicate fraetionation in the solar system the o r d i n a r y c h o n d r i t e p a r e n t b o d i e s are speculative. A n d e r s a n d c o w o r k e r s [28, 29] have argued t h a t the E a r t h a n d t h e o r d i n a r y c h o n d r i t e s o r i g i n a t e d at very nearly the same t e m p e r a t u r e a n d pressure. I w o u l d claim t h a t this is n o t e v i d e n c e for a great u n i f o r m i t y o f c o n d i t i o n s over a wide range o f h e l i o c e n t r i c dist a n c e , b u t r a t h e r suggests an origin for o r d i n a r y c h o n dritic m a t e r i a l near 1.2 A U f r o m the S u n . A d e t a i l e d s t u d y o f the c h e m i s t r y o f solar m a t e r i a l is in p r e p a r a t i o n , and will b e p r e s e n t e d elsewhere. Acknowledgements I am g r a t e f u l to Prof. A.G.W. C a m e r o n for m a n y h e l p f u l discussions o f p h y s i c a l m o d e l s o f t h e p r i m i t i v e solar n e b u l a , t o Prof. H.C. U r e y for r e p e a t e d l y stimulating m y i n t e r e s t in this p r o b l e m over t h e past seven years, and t o Prof. E. A n d e r s for his h e l p f u l c o m m e n t s on t h e m a n u s c r i p t o f this p a p e r . This w o r k was supported by NASA under grant NGL-22 009-521. References [ 1 ] M.E. Ash, I.I. Shapiro and W.B. Smith, The system of planetary masses, Science 174 (1971) 551. [2] D.L. Anderson and R.L. Kovach, The composition of the terrestrial planets, Earth Planet. Sci. Letters 3 (1967) 19. [3] H.C. Urey, The origin and development of the Earth and other terrestrial planets, Geochim. Cosmochim. Acta 1 (1951) 209. [4] A.E. Ringwood, Chemical evolution of the terrestrial planets, Geochim. Cosmochim. Acta 30 (1966) 41. [5] T. Garz, H. Holweger, M. Kock and J. Richter, Revised solar iron abundance and its influence on the photospheric model, Astron. Astrophys. 2 (1969) 446. [6] T. Garz, M. Kock, J. Richter, B. Baschek, H. Holweger and A. Uns61d, Abundances of iron and some other elements in the Sun and in meteorites, Nature 223 (1969) 1254. [7] N. Grevesse and J.P. Swings, Forbidden lines of FelI in the solar photospheric spectrum, Astron. Astrophys. 2 (1969) 28. [8] B. Baschek, T. Garz, H. Holweger and J. Richter, Experimentelle Oszillatorenst~irken yon FeIl Linien und die solare Eisenh~ufigkeit, Astron. Astrophys. 4 (1969) 229. [9] J.M. Bridges and W.L. Wiese, The oscillator-strength scale for FeI, Astrophys. J. 16 (1970) L71. [10] T. Garz, J. Richter, H. Holweger and A. Uns61d, Abundance of iron in the solar photosphere, remarks concerning a paper by John E. Ross (1970), Astron. Astrophys. 7 (1970) 336. [ 11 ] S.J. Wolnik, R.O. Berthel and G.W. Wares, Shock-tube measurements of absolute gf-values for FeI, Astrophys. J. 162 (1970) 1037. [ 12] A.G.W. Cameron, Physical conditions in the primitive solar nebula, in: Meteorite Research, ed. P.M. Millman, (Reidel, Dordrecht, 1969). [13] A.G.W. Cameron, personal communications (1971). [ 14] H.C. Lord, Molecular equilibria and condensation in a solar nebula and cool stellar atmospheres, Icarus 4 (1965) 279. [ 15] J.W. Larimer, Chemical fractionations in meteorites I. Condensation of the elements, Geochim. Cosmochim. Acta 31 (1967) 1215. [16] J.W. Latimer and E. Anders, Chemical fractionation in meteorites - I1. Abundance patterns and their interpretation, Geochim. Cosmochim. Acta 31 (1967) 1239. [17] J.W. Larimer and E. Anders, Chemical fractionations in meteorites - III. Major element fractionations in chondrites, Geochem. Cosmochim. Acta 34 (1970) 367. [ 18] L. Grossman, Ph.D. Dissertation, Yale University (1971 ). [ 19] L. Grossman, Condensation in the primitive solar nebula, Geochim. Cosmochim. Acta (1972) in press. [20] J.S. Lewis, Low temperature condensation from the solar nebula, Icarus 16 (1972) in press. [21] K.K. Turekian and S.P. Clark, Jr., Inhomogeneous accretion of the Earth from the primitive solar nebula, Earth Planet. Sci. Letters 6 (1969) 346. [22] J.S. Lewis, An estimate of the surface conditions of Venus, Icarus 8 (1968) 434. [23] J.S. Lewis, Venus: Atmospheric and lithospheric composition, Earth Planet. Sci. Letters 10 (1970) 73. [24] V. Rama Murthy and H.T. Hall, The chemical composition of the Earth's core: Possibility of sulfur in the core, Phys. Earth Planet. Int. 2 (1970) 276. [25] J.S. Lewis, Consequences of the presence of sulfur in the core of the Earth, Earth Planet. Sci. Letters 11 (1971) 130. [26] H.T. Itall and V. Rama Murthy, The early chemical history of the Earth: Some critical elemental fractionations, Earth Planet. Sci. Letters 11 (1971) 239. [27] D.L. Anderson and T.H. Jordan, The composition and evolution of Earth and Mars, Trans. Am. Geophys. Union 52 (1971) 349 (Abstract). [28] J.S. Larimer, Composition of the Earth: Chond~itic or achondritic?, Geochim. Cosmochim. Acta 35 (1971) 769. [29] E. Anders, Chemical processes in the early solar system as inferred from meteorites, Accts. Chem. Res. 1 (1968) 289.