祝融号火星车次表层探测雷达有效探测深度

Effective depth of detection of Zhurong Mars rover subsurface penetrating radar

2021年5月,中国首次火星探测任务“天问一号”携带祝融号火星车,成功着陆在乌托邦平原南部.祝融号火星车上搭载的次表层探测雷达(RoPeR)对于研究火星近表层精细结构和物理性质,以及了解是否赋存水冰提供了重要依据.深入研究祝融号火星车次表层探测雷达的有效探测深度,将不仅帮助分析次表层探测雷达数据的有效性,而且还能表征近表层物质对电磁波的散射和吸收特征.本文首先基于祝融号火星车次表层探测雷达的探测参数,建立了有效探测深度与介电常数和损耗率的关系模型;其次,根据实测次表层探测雷达数据的相似性,确定有效信号深度;最后,通过祝融号火星车雷达的两个通道实际数据的能量衰减特性,计算出不同深度的损耗角正切值,与探测深度模型和有效信号深度进行匹配.祝融号火星车次表层探测雷达的有效探测深度研究,将帮助分析火星近表层物质成分,为我国深空探测助力.

On May 15,2021,Tianwen-1's lander,which was China's first mission to Mars,successfully landed in the southern region of the Utopia Planitia(109.925°E,25.066°N),located west of the Elysium Mons.Subsequently,the Mars rover named Zhurong was deployed and drove toward the south from the landing spot.The Mars Rover Penetrating Radar(Ro Pe R),aboard the Zhurong rover,facilitates on-site measurement of subsurface materials'fine-scale structure and dielectric features though two different frequency channels.Channel one(CH-1)operates within 15 to 95 MHz frequency range,whereas channel two(CH-2)operates at 0.45 to 2.15 GHz.CH-1 revealed two sets of weak-to-strong reflection variation patterns in the radar image,suggesting the occurrence of episodic hydraulic flooding sedimentation.The Tianwen-1 landing site presents a larger heterogeneity in both the underground structure and the permittivity distribution by CH-2.It suggests that the space weathering degree(defined to include the continuous impact of large and small meteoroids and the steady bombardment of the surface by charged atomic particles from the sun and the stars)of the lunar regolith is higher than the Martian counterpart,which may be caused by the protection offered by the Martian atmosphere and the consequential fewer surface impacts compared to the Moon.The Subsurface Penetrating Radar carried by Zhurong rover serves as a crucial resource for examining the fine structure and physical properties of the superficial layer of Mars.It also aids in comprehending whether the water ice exists on this area.An extensive examination of Ro Pe R's effective detection can assist in evaluating the effectiveness of its data,and in defining how near-surface materials on Mars scatter and absorb electromagnetic waves.In this context,this paper carries out a study of the effective depth of detection of Ro Pe R on Zhurong rover.Firstly,this paper establishes a relationship model between effective detection depth and dielectric constant and loss tangent based on the detection parameters of Ro Pe R.It introduces a theoretical model for the effective detection depth of Ro Pe R's two channels,which have a limited bandwidth.The bandwidth sets the upper and lower limits of the model,depicted by dotted lines.The solid line represents the relationship between penetration depth and loss tangent,determined using the center frequency.Next,the depth of effective signal is determined by analyzing the similarity of the measured Ro Pe R data.The correlation coefficient of the radar data from the first channel suggests an effective detection depth of approximately 88 m.For the second channel's actual radar data,the data above 5 m is reliable,while the data below this depth necessitates the application of more robust denoising techniques for further utilization.Finally,the loss tangent is calculated at different depths through the energy attenuation characteristics of the Ro Pe R data,which is matched with the detection depth model and the effective signal depth.The first channel detected a loss tangent of0.0379±0.0006(R4)for the near-surface material,implying that the material close to one hundred meters deep has relatively high loss.The second channel detected a loss angle tangent of 0.0087±0.0002(R3)for the surface material within 5 m,indicating that the surface Martian material has higher loss compared to lunar soil but lower loss compared to deep Martian material.This research on the effective detection depth of the Ro Pe R can aid in the analysis of Martian near-surface material composition and advance China's exploration of deep space.