Oscillations in the Composition and Oxidizing Capacity of the Martian Atmosphere Driven by Obliquity Variations

Owing to gravitational perturbations from the giant planets, the absence of a large stabilizing moon, and its non-spherical shape, Mars could have experienced large obliquity variations over its history. Numerical simulations suggest that over the past 10 Myr, Mars’s obliquity has spanned a range of ~30°, varying between ~15° and ~45°, with the long-term mean shifting from ~35° to ~25° around 5 Myr ago and superimposed rapid oscillations of up to ~20° on ~100-kyr timescales.

High obliquity increases polar insolation, accelerating the sublimation of surface ice and thereby raising atmospheric water vapor, whereas low obliquity favors cold trapping at the poles and a much drier atmosphere. Because the photolysis products of water vapor act as key catalysts in Martian photochemistry, variations in Mars’s obliquity can strongly influence atmospheric chemistry by modulating the atmospheric water content.

We use a fully coupled 3D photochemistry–radiation–dynamics model, the Mars Planetary Climate Model (PCM), to test this hypothesis and to quantify how Martian atmospheric composition and chemistry respond to obliquity variations over the recent past. A key strength of this class of models is its ability to self-consistently simulate the spatiotemporal distribution of atmospheric water vapor through polar sublimation–condensation and 3D atmospheric transport, as well as the atmospheric CO₂ abundance through the seasonal exchange of CO₂ with the polar caps.

We first evaluate the capability of the model to reproduce the present-day composition of the Martian atmosphere. One-dimensional photochemical models underestimated CO by up to ~85%, a discrepancy that has persisted for more than three decades. The Mars PCM reproduces a much more realistic CO abundance, yielding a global annual mean of ~750 ppmv, close to observed values of 800–960 ppmv. We find that tuning key reaction rates or including heterogeneous chemistry on airborne dust particles can further improve agreement with observations. However, the model simultaneously predicts H₂ abundances more than an order of magnitude higher than observed, transforming the long-standing CO deficit problem into an H₂ surplus problem.

We then simulate the Martian atmosphere across obliquities from 5° to 45°. The results confirm the expected obliquity control on atmospheric water vapor. Near the present-day obliquity, increasing obliquity—and hence atmospheric water vapor—enhances the production of OH, a photolytic product of water vapor and a key atmospheric oxidant, thereby increasing the oxidizing capacity of the atmosphere and reducing the abundance of reduced species such as CO.

At obliquities below ~15°, extremely low polar temperatures lead to the formation of a massive CO₂ polar ice cap, substantially reducing the atmospheric CO₂ column. The weakened UV shielding enhances H₂O photolysis, resulting in a further decline in CO as obliquity decreases.

At high obliquity, rapid H₂O photolysis increases odd-hydrogen radicals by orders of magnitude, but the abundance of H₂O₂, which is derived from odd-hydrogen radicals, remains relatively stable, only modestly higher than present-day levels. This limits the likelihood that extremely elevated H₂O₂ concentrations at high obliquity would have sterilized organic matter produced by ancient life at the surface or in the shallow subsurface.