Martian atmospheric escape is a core scientific problem in Mars exploration. Studying the Escape of the Martian Atmosphere helps us gain a deeper understanding of the evolution of the Martian global climate environment. Studies have shown that the solar wind is the most effective driver of the escape of particles in the Martian atmosphere. This is because Mars does not have a global magnetic field, and the solar wind can interact directly with the Martian ionosphere or atmospheric ions, and through electromagnetic forces, they continue to erode and accelerate the escape of atmospheric ions into interplanetary space.
Early observations have shown that the interaction of the solar wind with the Martian ionosphere can drive or strip large clumps of Martian ionospheric ions (e.g., O+, O+), forming a "plasma cloud" that escapes explosively and integrally (Figure 1). However, due to the observation altitude of the spacecraft and the detection performance of the instrument, for many years there was little understanding of the formation and evolution mechanism of the "plasma cloud", especially the low-altitude "plasma cloud".
Figure 1 Schematic diagram of a Martian plasma cloud
To this end, using the data observation of a variety of high-performance scientific exploration instruments carried by the MARVEN Mars Probe in the United States, Zhang Chi, a doctoral student at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and his supervisors, Researcher Rong Zhaojin and Wei Yong, and many international scientific research institutions such as the Swedish Institute of Space Physics, the Max Planck Institute of solar system research in Germany, and the University of California, Berkeley, have jointly discovered and reported the periodic plasma cloud structure observed in the low altitude (600 km) range of Mars for the first time (as shown in Figure 2).
Figure 2 Periodic plasma clouds observed by MAVEN. From top to bottom, they are the timing changes of O+ energy spectrum, O+ energy spectrum, magnetic field, speed and spacecraft altitude
Unlike previous studies, this periodic low-altitude "plasma cloud" structure reveals a new set of observational features: the ion spectra exhibit dispersion characteristics (high-energy ions can be observed earlier), different Martian ion compositions have roughly the same velocity, and the appearance of plasma clouds is accompanied by an increase in the total magnetic field, and strong solar wind electron settlement characteristics. The results show that these plasma clouds originate in the low-altitude ionospheric region (~120 km height) and escape along the tail of the open magnetic field line. Estimates suggest that the "plasma cloud" could significantly increase the escape rate of ions in the Martian atmosphere during this event. This suggests that "plasma clouds" are a major way for ions to escape from the Martian atmosphere.
Based on these observational features, as shown in Figure 3, they propose that the possible formation mechanism of the "plasma cloud" should be: the periodic solar wind compresses the Martian magnetosphere, induces a periodic geomagnetic field reconnection between the solar wind magnetic field and the Martian crust magnetic field, and the magnetic field reconnection produces open magnetic field lines and a settled solar wind plasma, and the settled solar wind plasma will heat the low-altitude ionospheric plasma, so that these low-altitude plasmas periodically escape outward.
Figure 3 Schematic diagram of the formation mechanism of plasma clouds
The study reports the low-altitude plasma cloud structure of Mars for the first time and proposes the physical mechanism of its possible generation, which is of great scientific significance for our understanding of Mars ion escape and the interaction of the solar wind with Mars, and also provides an important guidance for the subsequent analysis of China's "Tianwen-1" Mars exploration data.
The research results were published in the international academic journals APJL (Zhang Chi, Rong Zhaojin*, Nilsson H, et al. MAVEN Observations of Periodic Low-altitude Plasma Clouds at Mars. The Astrophysical Journal Letters,2021, 922: L33. DOI: 10.3847/2041-8213/ac3a7d）。 This research has been funded by the National Natural Science Foundation of China (41922031, 41774188), the Class A Pilot Project of the Chinese Academy of Sciences (Honghu Special Project, XDA17010201), the Important Research Project of the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS-201904, IGGCAS-202102), and the National Scholarship Council (202104910297).
Proofreader: Wang Haibo
Editor: Chen Feifei