The Role of Carbon in Core Formation Under Highly Reducing Conditions With Implications for the Planet Mercury

[摘要] Results from the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft have shown elevated abundances of carbon on the surface of Mercury. Furthermore, the X-Ray Spectrometer on board MESSENGER measured elevated abundances of sulfur and low abundances of iron, suggesting the planet's oxygen fugacity (fO2) is several log10 units below the Iron-Wstite (IW) buffer. Similar to the role of other volatiles (e.g. sulfur) on highly reducing planetary bodies, carbon is expected to behave differently than it would under higher fO2. As discussed by Nittler et al. and Hauck et al., under such highly reducing conditions, the majority of the iron partitions into the core. On Mercury, this resulted in a relatively large core and a thin mantle. Using a composition similar to the largest volcanic field on the planet (the northern volcanic plains), Vander Kaaden and McCubbin conducted sink-float experiments to determine the density of melts and minerals on Mercury. They showed that graphite would be the only buoyant mineral in a mercurian magma ocean. Therefore, Vander Kaaden and McCubbin proposed a possible primary flotation crust on the planet composed of graphite. Concurrently, Peplowski et al. used GRS data from MESSENGER to show an average northern hemisphere abundance of C on the planet of 1.4 +/- 0.9 wt%. However, as this result was only at the one-sigma detection limit, possible carbon abundances at the three-sigma detection limit for Mercury range from 0 to 4.1 wt% carbon. Additionally, Murchie et al. investigated the possible darkening agent on Mercury and concluded that coarse-grained graphite could darken high reflectance plains to the low reflectance material. To further test the possibility of elevated abundances of carbon in Mercury's crust, Peplowski et al. used the low-altitude MESSENGER data to show that carbon is the only material consistent with both the visible to near-infrared spectra and the neutron measurements of low reflectance material on Mercury, confirming that C is the primary darkening agent on Mercury. Confirmation of carbon on the planet prompts many questions regarding the role of carbon during the differentiation and evolution of Mercury. Given the elevated abundances of both S and C on Mercury's surface, it begs the question, what is the core composition of the planet? This study seeks to understand the impact of C as a light element on potential core compositions on Mercury.

[发布日期] 2017-03-20

[学科分类] 天文学(综合)