T141 From Stardust to Planets: A Geological Tour of the Career of Harry Y. McSween Jr.

Monday, 26 September 2016
1:30 PM - 5:30 PM
Room 201

THE ENIGMATIC ORIGIN OF MERCURY: EVIDENCE FROM THE MESSENGER MISSION

Paper number: T141-9
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

Abstract

The MESSENGER mission measured the anomalous density, reduced mantle, chondritic major element abundance, lack of volatile depletion, and low albedo of planet Mercury [1-4]. These anomalies result either from gradual, orderly processes in the innermost protoplanetary disk [5, 6], or from chaotic processes (e.g., a mantle-stripping impact [7]).

Surface abundances of Ca, Al, and Ti do not rule out a giant impact, as ejecta would not be preferentially derived from the crust. Substantial reaccretion of silicates is likely: ejected material remains in Mercury’s orbit. Impact simulations can predict a high core/mantle ratio, but not Mercury’s reduced chemistry or its low albedo.

There is no strong evidence that primitive meteorites matching the redox state and volatile abundance of Mercury’s mantle did not form by ‘orderly’ processes [8, 9]. Equilibrium condensation in a vapor enriched in C-rich interplanetary dust produces both FeO-free silicates, refractory behavior of K, Cl and S, and highly reduced minerals, as well as fractionation of solid Fe from gaseous Si at high temperatures [6, 10]. Mechanisms for non-stochastic dynamical metal-silicate fractionation in the inner disk have been proposed [e.g., 11].

A giant impact cannot be entirely ruled out, but cannot explain Mercury’s suite of chemical anomalies. Mercury may represent a planetary embryo, recording extreme but orderly chemical and dynamical processes in the innermost nebula, perhaps including a final stochastic event. Extrasolar systems may contain highly reduced planets with a wide variety of core-to-mantle mass ratios close to their central stars.

[1] Hauck S. A. et al. 2013. J. Geophys. Res. 118:1204-1220.

[2] Peplowski P. N. et al. 2015. Planet. Space Sci. 108:98-107.

[3] Weider S. Z. et al. 2015. Earth Planet. Sci. Lett. 416:109-120.

[4] Evans L. G. et al. 2015. Icarus 257: 417-427.

[5] Chapman C. R. 1988. In Mercury, eds. F. Vilas et al. U. Arizona Press, pp. 1-23.

[6] Ebel D. S. and Alexander C. M. O’D. (2011) Planet. Space Sci. 59:1888–1894.

[7] Stewart S. T. and Leinhardt Z. M. 2012. Astrophys. J. 751:32–49.

[8] Weisberg M. K. and Kimura M. 2012. Chemie der Erde 72:101-115.

[9] Burbine T. H. et al. 2002. Meteor. Planet. Sci. 37:1233-1244

[10] Ebel D. S. and Sack R. O. 2013. Contrib. Mineral. Petrol. 166:923-934.

[11] Hubbard A. 2014. Icarus 241:329-335.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-284235

© Copyright 2016 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.