THE ENIGMATIC ORIGIN OF MERCURY: EVIDENCE FROM THE MESSENGER MISSION

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.

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