Abstract
Mercury, the Solar System’s innermost planet, has an unusually massive core prompting speculation that the planet lost silicate after it formed. Using the unusually high sulfur and low iron composition of its surface and space geodetic constraints on its core composition, we show Mercury’s chemistry to be compatible with formation in a larger planet at minimum 1.4–2.5 times Mercury’s present mass and possibly 2–4 times its mass by similarity with other rocky Solar System bodies. To do this, we apply an experimentally determined metal-silicate partitioning model for sulfur to Mercury’s silicate. The model is validated by applying it to Vesta, which, when evaluated at the conditions of Vestan self-differentiation, yields sulfur contents in its silicate in the range of HED meteorites. Mercury could have lost a substantial fraction of its rocky material through impacts or by being itself a remnant impactor. Independent of any stripping, because a significant amount of silicon resides in Mercury’s core, silicate meteoritic debris from Mercury would likely be characterized by 30Si isotopic enrichment >+ 0.10‰ relative to parent sources that could aid identification of a new meteorite class.
Highlights
Mercury is essentially a metal core draped with a thin silicate mantle
We use a model for sulfur, silicon, oxygen, carbon, and iron partitioning between metal and silicate to evaluate whether there are feasible conditions for Mercury that simultaneously satisfy the composition of the silicate mantle while yielding core compositions that satisfy space geodetic constraints
Mercury’s silicate composition (Table 3) may be used to find the P–T conditions where metal-silicate equilibrium is achieved if the metal composition is known
Summary
Mercury is essentially a metal core draped with a thin silicate mantle. Detailed models of its interior structure (Rivoldini et al 2009; Rivoldini and Van Hoolst 2013; Hauck II et al 2013; Dumberry and Rivoldini 2015) indicate core radii of 2000–2060 km and a core proportionately 58–70% of the planetary mass (M , 3.302 ×1023 kg (Rivoldini et al 2009)). Earth’s core is 32% of the planetary mass (Stacey 1992), Venus 24–32% (Fegley 2014), Mars’ and Vesta’s cores are 21 and 18%, respectively, of the planetary mass (Rivoldini et al 2011; Russell et al 2012), and the fractional mass of metallic elements in chondritic meteorites is ∼ 30% (McDonough and Sun 1995) These facts suggest that Mercury might have lost part of its original mantle at some point in the Solar System’s history through impacts with other bodies, stripping it of its silicate and leaving metal (Benz et al 1988; Benz et al 2007; Asphaug and Reufer 2014). Our objective is to show that Mercury’s surface chemistry is compatible with the formation in an originally larger body from which the mass was lost by impact processing
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