Scientists have constructed the first complete proton energy spectrum observed during an eruptive solar event in the Martian space, deepening our understanding of the radiation environment around Mars. This study was published in Geophysical Research Letters as a cover article.

This study was conducted by researchers from the University of Science and Technology of China, the Institute of Modern Physics of the Chinese Academy of Sciences (CAS), the Lanzhou Institute of Physics, and the University of Kiel in Germany.

Solar energetic particle (SEP) events, caused by solar eruptions, are among the most destructive space weather phenomena. During these events, high-energy charged particle flux in space may increase suddenly, posing significant threats to the safety of spacecraft and astronauts. Unlike Earth, Mars lacks a protective magnetic field and has a very thin atmosphere, making its surface particularly vulnerable to high-energy particles and secondary particles. Therefore, studying the impact of SEP events on Mars is crucial for radiation protection in future Mars exploration missions.

In November 2021, China's Tianwen-1 orbiter entered its science mission orbit around Mars, and its Mars Energy Particle Analyzer (MEPA) began measuring particle flux. With a wide energy measuring range of 2–100 MeV, MEPA significantly enhances the capability to monitor high-energy protons in Martian space, providing critical data for this study.

On February 15, 2022, MEPA recorded a SEP event with exceptionally high intensity and energy. The event was simultaneously detected by the European Space Agency's Trace Gas Orbiter (TGO), NASA's Mars Atmosphere and Volatile Evolution Orbiter (MAVEN), and the Curiosity rover on Mars' surface. This is the first SEP event observed by so many different radiation and particle detectors at Mars.

In this study, the researchers utilized data from multiple detectors to construct the proton energy spectra. The low- and medium-energy proton spectra were provided by Tianwen-1 and MAVEN, while the high-energy proton flux was derived by combining observations from the Curiosity rover on Mars with simulations of particle transport in the Martian atmosphere. By fitting the observed and derived fluxes at different energies, researchers successfully constructed the complete proton energy spectrum of the SEP event, spanning from 1 to 1000 MeV. 

The researchers then used this spectrum to calculate the radiation dose caused by the event in Martian orbit and on the Martian surface, which is consistent with actual dose measurements. This result validates the reliability of the Tianwen-1 MEPA data and the accuracy of the Martian radiation transport model.

This study not only serves as a reference for future research on similar space weather phenomena, but also highlights the necessity of continuous and coordinated radiation monitoring on Mars.

The first author of this study is ZHANG Jian, a PhD student at the University of Science and Technology of China, and corresponding authors are Prof. GUO Jingnan at the University of Science and Technology of China, Associate Prof. ZHANG Yongjie and Prof. SUN Zhiyu at IMP, and Prof. LI Cunhui at the Lanzhou Institute of Physics.

The research was supported by the Key Research Program and Strategic Key Project of CAS and the National Natural Science Foundation of China.

DOI：https://doi.org/10.1029/2024GL111775

Figure 1. The SEP particle event on February 15, 2022 was simultaneously detected by the Chinese Tianwen-1 orbiter, TGO, MAVEN, and the Curiosity rover on the surface of Mars. (Image from Geophysical Research Letters) 

Figure 2. Observed energy spectra and reconstructed 1-1000MeV proton spectra from SPE on February 15, 2022 (Image from Geophysical Research Letters)