Absence of atmospheric N2 as an indicator of an oxygen-poor mantle in rocky planets

Abstract:

Astronomical surveys have discovered thousands of transiting exoplanets, revealing that small rocky planets are common in our galaxy. A planet’s interior chemistry is frequently determined by average density, described by mass-radius (M-R) relationships. However, M-R relationships give rise to many non-unique interpretations of a planet’s interior composition. Here we show how planetary atmosphere compositions may hold important clues to their mantle chemistry. We present experimental results addressing the influence of an ultra-reducing (oxygen-poor) interior chemistry on atmospheric nitrogen levels. We show that silicon carbide (SiC) and molecular nitrogen (N₂) react to form solid silicon nitride (γ-Si₃N₄) at high pressures and high temperatures, which remains stable under both extreme and 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, rather than degas to the atmosphere as observed on Earth and on rocky bodies in our Solar System. We propose that the presence or absence of atmospheric N₂ may act as a proxy for the mantle-redox state of a rocky planet, a property that plays an important role in regulating atmospheric chemistry and geophysical processes. We present models for the presence of Si₃N₄-dust species in a super-Earth atmosphere as a method for identifying a reduced planetary body since the presence of N₂ may be difficult to rule out. Our results are an example of a ‘geomarker’, or an atmospheric signal giving an indication of planetary interior composition. Geomarkers provide a way to characterize rocky exoplanets in greater detail than through traditional M-R relationships.