Experimental Constraints on Solid Nitride Phases in Rocky Mantles of Reduced Planets and Implications for Observable Atmosphere Compositions

Abstract

Astronomical surveys have discovered thousands of transiting exoplanets, revealing that rocky planets are common in the galaxy. A planet's interior chemistry is frequently inferred by average density, described by mass-radius (M-R) relationships. However, M-R relationships give rise to non-unique interpretations of a planet's interior composition, an issue that limits our ability to characterize far-away worlds. We present experimental and density functional theoretical results addressing the influence of an ultra-reducing (oxygen-poor) interior chemistry on rocky mantle phases and discuss the possible implications for atmospheric observables. We show that silicon carbide (SiC) and molecular nitrogen (N2) react to form solid silicon nitride (γ-Si3N4) at high pressures and high temperatures in a laser-heated diamond-anvil cell, consistent with ab initio computations. Si3N4 remains stable under extreme conditions and when quenched to ambient conditions. As SiC is a common compound found under very reducing conditions, these results indicate that nitrogen may form solid phases in an oxygen-poor rocky planet. If, by sequestering nitrogen in a planet's mantle, the distribution of nitrogen between a planet's interior and atmosphere is altered (i.e., a nitrogen-rich mantle and nitrogen-poor atmosphere), these results indicate that there may be atmospheric observables connected to the mantle-redox state of a rocky planet besides the oxygen-containing phases ubiquitous in exoplanet literature.