火星轨道器次表层探测雷达的水冰探测

Orbiter-based subsurface sounding radar for searching water ice on Mars

在地球上,水是生命存在的基础之一。大量证据表明火星表面曾经存在液态水,而目前的火星表面环境不支持液态水的长期存在。因此,水可能以不同的状态赋存于火星的次表层。寻找火星次表层的水一直是火星探测的关键科学目标之一。次表层探测雷达,如探地雷达、探冰雷达,是了解地下物质结构的有效方法,近年来在地外天体上得到大量应用。在过去十余年,欧洲的火星快车(Mars Express)上搭载的火星次表层和电离层探测先进雷达(Mars Advanced Radar for Subsurface and Ionosphere Sounding, MARSIS)和美国火星勘测轨道飞行器(Mars Reconnaissance Orbiter, MRO)上搭载的浅表层雷达(Shallow Subsurface Radar, SHARAD)已在火星轨道上获取了大量数据,被广泛应用于研究火星的地下结构,尤其是地下水冰探测。我国的天问一号火星探测器也携带了高低频轨道探测雷达和高低频火星车探地雷达,有望在不同的顺轨向、交轨向和距离向分辨率上揭示火星次表层不同深部的结构。本文综述了轨道器次表层探测雷达的探测原理和优势,简要介绍了雷达数据的处理和解译方法,重点总结了MARSIS和SHARAD近年来对火星水冰探测的最新进展,最后对天问一号环绕器雷达及其水冰探测作简要展望。

Water is the basic element for life on Earth. Abundant evidence has shown that flowing liquid water carved the surface of Mars. However, the current dry, cold and low-pressure surface allows only transient water. The ancient water may exist in the subsurface Mars in various forms such as water ice. Searching for evidence of subsurface water or water ice has been one of the significant objectives of all Mars missions. Penetrating radar is an effective way to investigate the subsurface structures on Earth, and has been more and more applied in planetary sciences. In the past two decades, the Mars Advanced Radar for Subsurface and Ionosphere Sounding(MARSIS) onboard Mars Express and the Shallow radar(SHARAD) on board Mars Reconnaissance Orbiter(MRO) have been orbiting Mars, collecting a large volume of radargrams, which have been widely applied in investigating the subsurface stratigraphy and searching for water ice. One orbiter-based radar and one rover-based radar are included in the payload of the Chinese Tianwen-1 mission. These radars are working in different frequencies with different resolutions. It is expected that new findings about the Martian subsurface can be revealed by using these radars. This paper first introduces the principle and advantages of the subsurface sounding radar, and subsequently demonstrates different processing methods for the radar observations and products that can be derived. Finally, this paper summarizes some important studies about searching subsurface water ice using MARSIS and SHARAD, followed by a prospect for the applications of radar observations in the Tianwen-1 mission.