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Mercury: Space Environment, Surface, and Interior (2001) 8065.pdf

MERCURY: AN END-MEMBER OR A COSMIC ACCIDENT?. G. J. Taylor and E. R. D. Scott, Hawai`i Institute of Geophysics and Planetology, School of Ocean and and Technology, University of Hawai`i at Manoa, Honolulu, HI 96822, USA; [email protected]

Introduction: Mercury is unique. It is the densest for Mercury are critically dependent on the assumed planet, undoubtedly because it has a large metallic initial conditions and the reason for its small size. We core. Spectral observations indicate that its surface is suggest that Mercury may be small and chemically low in FeO (the best guess is around 3 wt% [1], but it distinctive because it also formed from a single em- might have none). Unambiguous have low bryo. FeO, indicating that their source regons in the mercu- Clues from : The notion that chondrites rian also contain little FeO, 2-3 wt% [2]; we were the building blocks of the has a long his- infer that the entire mercurian mantle is low in FeO. In tory, but enthusiasm for the idea has waned. S. R. contrast, the bulk Earth [3] and [2] con- Taylor [6] argued instead that stochastic events were tain more FeO (about 8 wt%), and contains about largely responsible for chemical differences between 18 wt% [4]. Do these compositional features of Mer- terrestrial planets and that groups and planets cury reflect its location near the ? Lewis [5] sug- are unrelated because they formed in separate loca- gested that gradients in the solar nebula tions. We have suggested [11] that certain chondrites led to variations in chemical composition of the plan- and terrestrial planets are related because chondritic ets. The FeO variation from Mercury to Earth and Ve- ingredients were transported across the nebula. nus to Mars is consistent with this idea. Alternatively, Ca-Al-rich inclusions. CAIs and refractory grains could Mercury simply be the product of stochastic with exceptionally 16O-rich compositions are found in events [6]? In this view, the terrestrial planets accreted all chondrite groups including CI, CM, E, and O chon- from material from the whole inner [7], drites, and . Their chemical and oxy- and Mercury's high is due to a large impact that gen isotopic compositions indicate formation in a dust- removed much of its original mantle [8] or other proc- enriched location, most probably near the protosun esses that fractionated metal from silicate [9]. So, is [12]. Efficient distribution to cooler regions favors Mercury an end-member planet that contains a record transportation by bipolar outflows [13]. of its formation near the Sun? Or is it an accident of grains. contain forsterite grains, and giant impacts? which are absent in the [14]. They Width of planetary accretion zones: Wetherill [7] probably formed close to the protosun either by an- modeled the accretion of the terrestrial planets from a nealing of interstellar grains [15] or by condensation. disk of lunar to Mars-sized embryos. He found wide- Outward transport was provided by mixing in the neb- spread gravitational mixing of the embryos and their ula [16,17] or by bipolar outflows. fragments, which ensured that each Chondrules. The inferred depletion of refractory formed from material originally located throughout the elements in chondrules from O and E chondrites and 16 inner solar system (0.5 to 2.5 AU). Thus any initial the O-rich of Al-rich chondrules and forsteritic radial chemical variations in the compositions of the condensates suggest that chondrules and their associ- would be largely erased as the planets ated Fe,Ni grains partly formed from condensates by a accreted. Wetherill [7] found no obvious tendency for CAI-related process [18]. Since chondrules and metal Mercury-like planets to be derived from material ini- grains in metal-rich chondrites were hurled outwards tially located at the inner edge of the disk of embryos. after they condensed [19], chondrules, CAIs and for- However, these models appear incompatible with the sterite grains may have been transported by one proc- inferred mantle FeO concentrations of the planets, ess [13]. If chondrules had formed by melting of dust- which increase with heliocentric distance from 2-3 wt% balls at the accretion location, we should find vesicular at Mercury to 18 wt% at Mars. chondrules in CM chondrites [20], but none is present. Subsequent modeling by Chambers and Wetherill This evidence for outward transport of thermally [10] confirmed that Earth- and Venus-like planets ac- processed materials from near the protosun suggests crete from broad zones. However, their models sug- that terrestrial planets could have formed from similar gest that Mars may be a single embryo, i.e., a body that materials. One class of chondrites may be particularly formed during earlier runaway growth when suitable as examples of Mercury building blocks. The were nearly circular and coplanar. Thus, Mars may be discovery of three new chondrite groups (CR, CH and derived from a restricted part of the nebula that was Bencubbin-like) among Antarctic has poorly sampled by the other planets. Accretion models greatly expanded the compositional range of chondrites Mercury: Space Environment, Surface, and Interior (2001) 8065.pdf

MERCURY: END MEMBER OR ACCIDENT?: G. J. Taylor and E. R. D. Scott

[21-25]. The new chondrites have normal levels of Conclusions: Mercury does not appear to be the refractory elements but are richer in metallic Fe (some accidental product of stochastic processes. While we were classed as meteorites) and poorer in volatile cannot rule out its formation from planetesimals elements like Na, K, and S than other chondrites. Ben- throughout the inner solar system or that it was modi- cubbin-like chondrites have more metal (~80 wt%) fied by a giant impact, a strong case can be made that it than Mercury (~70 wt%) and comparable FeO concen- is a surviving planetary embryo that formed from mate- trations. rials near its current location. Metal-rich chondrites are Did Mercury's formation involve a giant im- reasonable analogs for the material that accreted to pact? Most authors have embraced the idea that Mer- form Mercury. In fact, they might dominate the popu- cury’s high concentration of metal is a result of a giant lation of planetesimals that formed Mercury, Venus, impact that preferentially removed most of the silicate and Earth, the metal-rich, FeO-poor planets. mantle [8]. However, the existence of metal-rich This idea can be tested by the MESSENGER mis- chondrites and the evidence for formation of their con- sion. If similar to metal-rich chondrites, the bulk com- stituents near the protosun diminishes the need for such position of Mercury should be low in FeO (as it ap- an event. In addition, successful impact models re- pears to be), have chondritic relative abundances of quired extreme conditions: a) targets containing 32 refractory elements (Th/U, Al/Th), no enrichment in wt% metallic iron; b) head on collisions at 20 km/s or refractory elements over chondrites, and be low in oblique impacts at implausibly high collision speeds of volatile elements (K). 35 km/s [26]; c) removal of all silicate ejecta as sub- References: [1] Blewett, D. T. et al. (1997) Icarus centimeter-sized particles by the Poynting-Robertson 129, 217-231. [2] Robinson, M. S. and Taylor, G. J. effect to prevent from reaccreting on to Mer- (2001) MAPS 36, 841-847. [3] Jagoutz, E. et al. (1979) cury [8]. Most important, the impact model does not PLPSC 10th, 2031-2050. [4] Longhi, J. et al. (1992) account for the unusually low bulk FeO concentration Mars, 184-208. [5] Lewis J. S. (1972) EPSL 15, 286- of Mercury. 290. [6] Taylor S. R. (1991) Meteoritics 26, 267-277. Compositional diversity among the terrestrial [7] Wetherill G.W. (1994) GCA 58, 4513-4520. [8] planets: Chondrites are quite diverse in composition. Benz W. et al. (1988) Icarus 74, 516-528. [9] Weiden- We propose that primary chemical differences among schilling, S. J. (1978) Icarus 35, 172-189. [10] Cham- planetesimals were more important than giant impacts bers J. E. and Wetherill G. W. (1998) Icarus 136, 304- in establishing planetary compositions. However, the 327. [11] Scott, E. R. D., et al. (2001) MAPS. (ab- nebular processes responsible for diverse chondritic stract), in press. [12] Scott E. R. D. and Krot A. N. compositions are not clear. The low volatile abun- (2001) MAPS submitted. [13] Shu F. H. et al. (2001) dances of metal-rich chondrites are consistent with Astrophys. J. 548, 1029-1050. [14] Hanner M. S. Lewis’ equilibrium nebula condensation theory [5] and (1999) Space Sci. Rev. 90, 99-108. [15] Nuth J. A. et some metal grains have compositions consistent with al. (2000) Nature 406, 275-276. [16] Stevenson D. J. condensation above 1200 K [27]. However, these (1990) Astrophys. J. 348, 730-737. [17] Meibom A. et chondrites could not have accreted above 1200 K, as al. (2000) Science 288, 839-841. [18] Scott E. R. D. Lewis inferred for Mercury, because their metal grains and Krot A. N. (2001) MAP., submitted. [19] Krot A. cooled in days or weeks to below 600 K [28]. Rapid N. et al. (2001) Science 291, 1776-1779. [20] Maharaj condensation of metal and silicate in localized nebular S. V. and Hewins R. H. (1998) MAPS 33, 881-887. processes may have triggered accretion of planetesi- [21] Weisberg M. K. (1999) LPS 30, #1416. [22] Scott mals. Thus Mercury may be small and metal-rich be- E. R. D. (1988) EPSL 91, 1-18. [23] Grossman J. N. et cause only a small fraction of the metal and an even al. (1988) EPSL 91, 33-54. [24] Weisberg M. K. et al. smaller fraction of the silicate condensed and accreted . (1993) GCA 57, 1567-1586. [25] Weisberg M. K. et al. The remainder of the metal and silicate probably ac- (1995) Proc. NIPR Symp. 8, 11-32. [26] Wasson J. T. companied the volatiles into the Sun. The feeding zone (1988) In Mercury ed. F. Vilas et al., pp.622-650. [27] for Mercury might have included planetesimals from Meibom A. et al. (1999) JGR 104, 22,053-22,059. [28] further out in the nebula, as argued by Wetherill [7], Meibom, A. et al. (2000) Science, 288, 839-841. but it seems to have been dominated by planetesimals formed near its current location. Venus and Earth are also quite distinct from Mars, shown strikingly by their low FeO (about 8 wt%, versus 18 wt% for Mars). This suggests they might also have formed mostly from metal-rich chondrites (including enstatite chondrites), and possibly from planetesimals formed well within the of Mars.