Monitoring methods for Martian dust storms

Dust storms are the most prominent weather processes in Martian physical system, involving the surface, atmosphere, and space environments. Martian dust storms possess significant seasonality, but how dust storms occur, with different spatiotemporal scales and strengths, remains a mystery to the academic community. Martian dust storms can be classified according to either their spatial or temporal scales. For the spatial methods, dust storms can be categorized as local, regional, and global dust storms. Several local dust storms can combine to produce regional dust storms, several of which can develop into a planet-encircling global dust storm. In terms of seasonality and the temporal scale, dust storms can be classified into Types A, B and C. Type A dust storms are planet encircling Southern Hemisphere events in spring and summer. Type B dust storms are southern polar events that occur around the summer solstice. Type C dust storms start well after the end of Type B storms in the middle- and low-latitude regions. Dust storms are quantitatively evaluated using the parameter of the column integrated dust optical depth (DOD), which denotes how much of the radiation at a specific wavelength (from visible to infrared) would be absorbed or scattered by the airborne dust particles. The larger the DOD, the stronger the dust storm. The effective radii of the dust particles usually vary from similar to 0.5 to 2 mu m during the dust season and are proportional to the DOD. However, during large global dust storms, the dust particle effective radius can increase to 4 mu m or greater when the visible-light DOD exceeds 6. Dust storms greatly impact landers/rovers and sample return missions on the Martian surface by disturbing the atmospheric density and wind, threatening the safety of the entry-descent-landing system, degrading the efficiency of the solar cells due to both dust cover and obstructing the incident solar radiation, decreasing the environmental temperature and thermal control, blocking telecommunications, and polluting the instruments. For example, the Opportunity rover was absolutely lost during a 2018 global dust storm, and the Insight lander will end its mission in the late 2022 because of the accumulation of dust on its solar cells. Therefore, monitoring, investigating, and forecasting Martian dust storms is crucial for missions to the surface of Mars. Quantitative monitoring methods and DOD retrieval algorithms for Martian dust storms were systematically reviewed in this study. We propose three methods for China's future Martian dust storm monitoring system. First, smart multiwavelength visible cameras onboard the landers/rovers. These cameras can quantitatively monitor the dust activity near the landers/rovers in real time and provide key information for their operations. The second option utilizes visible-infrared multispectral imagers/spectrometers onboard the orbiters. The large-field multispectral imagers onboard the orbiters operating in nadir mode can capture the daily global picture of Mars at a moderate resolution and provide a qualitative evaluation of the global dust activity. A multispectral spectrometer working in limb mode can be used to retrieve the vertical profile of dust opacity, as well as some key parameters of the Martian atmosphere, such as surface brightness temperature, atmospheric temperature profiles, and water vapor volume mixing profiles. The third option is to use ground-based optical telescopes. There will be two telescopes in operation at the Lenghu Observatory for Planetary Science beginning in 2024 that an image the large-scale picture of Mars at several wavelengths in the visible to near infrared wavelengths during specific periods and provide high temporal resolution monitoring of the global dust activity on Mars. These multi-perspective Martian dust storm monitoring data cannot only provide a guide for China's Martian exploration mission, such as Tianwen-3 and other future missions, but also greatly promote our Martian system science research, improving our understanding of the evolution of the Martian environment and habitability.