Discrimination Between Dry And Water Ices By Full Polarimetric Radar Onboard ’Zhurong’ Mars Rover

Mars is the most Earth-like planets in the solar system,which also is a hot spot in deep space exploration.Many Martian geomorphologic characteristics and surface material composition indicate the existence of dry/water ice in the Martian subsurface.Therefore,detecting potential ice is significant to understand climate evolution,revealing the geological structure characteristics,and even explaining the origin of cosmic life on Mars.On May 22,2021,China’s first Mars rover,named "Zhurong" began its exploration on the surface of the Utopia Plain in the northern hemisphere of Mars.The rover is equipped with the Mars rover penetrating radar(Ro Pe R)which is aim to detect the potential water ice and/or dry ice in the Martian regolith of the landing site.Because Martian regolith may have a relative permittivity similar to that of dry/water ice,it is difficult to identify dry/water ice by analyzing the reflection intensity in traditional two-dimensional radar images.Therefore,this paper carries out dry/water ice laboratory detection experiments by carrying a similar full-polarimetric radar system as a high-frequency channel of Ro Pe R.Then,a reverse time migration imaging algorithm is applied to focus on the reflected signal of dry/water ice in the radar profile,and the H-α Polarization decomposition algorithm is used to analyze the polarization scattering characteristics of dry/water ice to discriminate the dry/water ice in Martian subsurface.This paper mainly elaborates on the following three aspects:Initially,a radar system similar to the high-frequency channel of the Ro Pe R is built in the laboratory,and a phase center calibration method identical to velocity spectrum analysis is used to calculate the position of the antenna phase center and obtain the accurate propagation time of electromagnetic waves.Then,a dihedral reflector was used to perform polarization calibration on the radar system.After calibration,the results showed that the polarization scattering was very close to the ideal scattering matrix.Additionally,this paper introduced the standard data processing steps of full polarization radar.The laboratory detection experiments of dry/water ice are carried out using the built polarized radar system.Then,the experimental data were preprocessed,and the reverse time migration was used to focus on the reflection signals of dry/water ice in the radar profiles.Finally,the H-α polarization decomposition was applied to extract the ice samples’ scattering characteristics and preliminary dry/water ice discrimination was achieved.FDTD numerical simulations also verified the experimental results.The results showed that the bottom of dry ice and water ice have different polarization scattering characteristics due to the differences in ice crystal structures.The needle-like structures inside the dry ice caused dipole scattering in the Z8 region of the bottom reflection.In contrast,the angular edges produced by the columnar structures inside the water ice resulted in both dipole scattering(Z8)and dipole scattering(Z7).It is feasible to discriminate the buried dry/water ice in Martian soil by H-α polarization decomposition.On the same token,this study introduced the data format of the Ro Pe R high-frequency channel and processed the measured Ro Pe R data.The discrimination method proposed in this study was used to analyze two suspected reflection signals.The results showed that the analyzed reflection signals have similar polarization scattering characteristics to water ice,so they could be identified as suspected water ice targets,providing a reference for subsequent data interpretation and analysis.Finally,the work done in this paper is summarized and according to the deficiencies in the current research,future follow-up research has been prospected.