The Effect of Obliquity on Abiotic Oxygen Production in Habitable Zone Planet Atmospheres

Oxygen is widely considered to be a reliable biosignature in the search for life elsewhere [1]. However, several mechanisms create abiotic oxygen, or oxygen from non-biogenic sources. These mechanisms must be thoroughly characterized to rule out false positive identifications of life in the future. Previous experiments from 1-D planetary evolution models have predicted significant abiotic oxygen accumulation resulting from a variety of initial volatile inventories, but the production of abiotic oxygen from atmospheric water loss after the magma-ocean phase did not consider the effect of planetary obliquity [2]. 3-D General Circulation Models (GCMs) have shown that stratospheric humidity increases on planets with higher degrees of obliquity through enhanced seasonality, which should result in significant levels of atmospheric water loss [3]. However, the minimum obliquity necessary to trigger significant stratospheric water vapor accumulation and subsequent water loss has not been studied.

We address this unknown obliquity parameter by using the ExoPlaSim GCM to quantify stratospheric humidity for a range of obliquity, eccentricity, insolation, and CO2 levels. We then develop a parameterization for the 3-D stratospheric humidity concentration that can be used in long-term 1-D planetary evolution models to study the effect of higher degrees of obliquity on atmospheric water loss. The ExoPlaSim GCM results support those seen from previous studies, with the stratospheric humidity increasing with increasing obliquity values [3]. Our results also reveal a marked regime change in the global humidity distribution after the planetary obliquity value passes the critical value of 54o. Finally, we will discuss the consequences of these results in the context of planetary habitability and exoplanet life detection.

[1] Meadows, V. S. (2017), Astrobiology, 1022–1052.

[2] Krissansen‑Totton, J., Fortney, J. J., Nimmo, F., & Wogan, N. (2021), AGU Advances, 2(2).

[3] Kang, W. (2019), The Astrophysical Journal, 877(1), L6.