Global ozone depletion on Mars during major SEP events observed by TGO/NOMAD

Ozone is an important trace gas for understanding the dynamics and photochemistry of planetary atmospheres and protecting life from harmful ultraviolet radiation on Earth. It is well known that ozone is efficiently destroyed by odd nitrogen (NOx) and odd hydrogen (HOx) generated by solar energetic particles (SEPs) on Earth [1]. It has never been proven that such phenomena can occur universally on the terrestrial planets. For instance, Mars lacks a global intrinsic magnetic field and has a thin CO2-dominated atmosphere, making it vulnerable to SEP impacts. Such SEP impacts are global and much closer to the surface than on Earth, as revealed by the global SEP aurora [2]. Nakamura et al. (2023) [3] predicted that such a space-atmosphere interaction induced by SEP would cause significant impacts on Mars’ atmospheric composition. Despite model predictions, observational understanding of such effects on Mars remains limited. This study aims to detect SEP-induced ozone depletion on Mars through observations.

In this study, we analyzed the absorption spectra of ozone around 200-300 nm obtained from the ultraviolet to visible (UVIS) channel of the NOMAD (Nadir and Occultation for MArs Discovery) instrument onboard the ExoMars Trace Gas Orbiter (TGO) [4]. We focused on the major SEP event in February 2024. During this event, three distinct peaks of SEP flux were observed by SEP measurements onboard MAVEN [5] over one month. Especially, the third peak shows a significant enhancement of the flux at 5 MeV ions, lasting for 4 days. Our Monte Carlo precipitation model, driven by SEP spectra measured by MAVEN/SEP, predicted that the SEP energy up to 5 MeV was sufficient to penetrate the Martian atmosphere down to ~50 km.

The ozone column density observed by NOMAD showed multiple decreases of approximately 10-20% at which timings coincided with the peak flux increase of the SEP event. However, regardless of the SEP, Martian ozone levels can naturally fluctuate due to factors such as clouds, atmospheric circulation, and dust. The remarkable point is that this global decline in ozone was observed simultaneously in both the northern and southern hemispheres. Such global ozone fluctuations are consistent with the predicted impact of the SEP event, rather than localized meteorological factors. This decrease is compared to our model prediction of ~10% decrease. Our result supports the model hypothesis that suggests water cluster ions (H⁺(H₂O)ₙ), generated by SEP, produce HOx compounds which destroy ozone in the CO2-dominated atmosphere. NOMAD observations demonstrate a global-scale ozone depletion coinciding with SEP penetration on Mars. This study presents remarkable observational results indicating that ozone depletion can occur due to SEPs even on planets with CO₂-dominated atmospheres, such as Mars. Understanding the impact of SEPs on Mars is crucial for elucidating the evolution of the Martian atmosphere from its early stages - when solar activity was more intense than it is today - to its current state.

References:

[1] Jackman et al. (2005) J. Geophys. Res., 110 A09S27.

[2] Schneider et al. (2018) Geophysical Research Letters, 45, 7391–7398.

[3] Nakamura et al. (2023) Earth, Plan. and Space, 75, 140.

[4] Vandaele et al. (2018) Space Sci. Rev., 214, 80.

[5] Larson et al. (2015 Space Sci. Rev., 195, 153.