Opaques in Mercury’s Crust: Additional Evidence for a Low-feo Magma Ocean

Introduction: Analysis of Mariner 10 and MESSENGER datasets reveal the importance of opaque components on Mercury's surface. A global darkening agent, suggested to be ilmenite or other Fe,Ti-bearing opaque mineral, has been invoked to explain the lower albedo of Mercury relative to the lunar highlands. Separately, a low-reflectance material (LRM) has been recognized as one of three dominant color terrains. We present laboratory reflectance spectra of ilmenite size separates and other candidate FeTi-bearing oxide minerals. These oxides cannot sufficiently darken Mercury without violating neutron spectrometer constraints on surface iron content. The spectra of all samples exhibit negative spectral slopes shortward of 500nm, consistent with the LRM. We review crystallization models of FeO-poor magma oceans and show that lack of a plagioclase flotation crust could lead to a thin quench crust with near surface layers of incompatibleand Ti-rich late stage cumulates, consistent with Mercury’s albedo and LRM. Mercury's surface composition is poorly constrained. Energy dependent neutron measurements by the MESSENGER Neutron Spectrometer (NS) and Earth-based microwave emission observations have Fe ≤6wt.% on Mercury's surface [1,2]. Earth-based telescopic and spacecraft spectra of Mercury lack any unambiguous features near 1μm that would indicate the presence of ferrous iron bearing silicates, limiting silicate FeO content to ≤2-3 wt.% [3-5]. Mercury's surface probably contains one or more opaque components, inferred from two lines of evidence. [5-8]. A global darkening agent has been suggested to explain Mercury's low overall albedo relative to the lunar highlands. Immature Mercury surface material is 30% darker than immature lunar highlands at 490nm [8]. Denevi and Robinson [8] suggest a mercurian crust with composition similar to the lunar highlands, but containing a global darkening agent, possibly ilmenite. The other expression of an opaque component is within a low-albedo, relatively blue terrain, termed “low-reflectance material" (LRM) [5]. MESSENGER multispectral images suggest a highopaque material associated with some craters and ejecta, indicative of a high-opaque layer underlying a moderate-opaque layer [5]. LRM spectra have negative slopes shortward of minima at 400nm (MASCS) or 600nm (MDIS) in ratioed spectra [4, 5]. The identity of the proposed opaque component may provide constraints on the FeO content and surface conditions of Mercury. To aid in the identification of the opaque components on Mercury's surface, we present new spectral reflectance data for plausible candidate opaque minerals. While ilmenite is suggested by analogy with the Moon, many other opaque minerals occur in lunar rocks [9]. We also present spectra of ilmenite size fractions to determine the effect of particle size on its albedo and spectral properties. Results: The reflectance spectra of powdered, synthetic Fe-Ti-rich members of major oxide solid solution series present on the Moon (Table 1, [9]), all show a negative slope shortward of 500nm (Fig. 1). In ilmenite this has been attributed to the long-wavelength shoulder of a reflectance peak at 330nm due to Fe-Ti charge transfer [10]. The negative slope shortward of 500nm is characteristic, perhaps diagnostic of Fe-,Tibearing oxides.