The Mars Orbiter Subsurface Investigation Radar (MOSIR) on China’s Tianwen-1 Mission

China launched Tianwen-1 spacecraft successfully on July 23rd, 2020. The Mars Orbiter Subsurface Investigation Radar (MOSIR) is a subsurface radar sounder as a scientific instrument onboard Tianwen-1 orbiter. It is designed to study the compositions of Martian surface material, subsurface structure, and the ionosphere’s total electron content. It can also perform passive observations in a transfer orbit to Mars. The subsurface stratigraphic structure is critical to the study on Mars geological and climatic evolution history. Considering the optimal tradeoff between penetrating depth and range resolution, MOSIR operates at low-frequency and high-frequency channels, with the frequency bands of 10–15 MHz or 15–20 MHz and 30–50 MHz, respectively. MOSIR provides a penetration depth of more than 100 m with a vertical resolution of 7.5 m (20 MHz bandwidth) and 30 m (5 MHz bandwidth) in free space. The range and azimuth focusing techniques are applied in ground data processing to achieve the resolution of several hundred meters (along-track) and several thousand meters (cross-track). MOSIR is intended to search for water ice and liquid water that may be associated with signs of life in the polar layered deposits, Tianwen-1 landing site, and other selected areas.