雷达对火星次表层的探测与研究现状

大量火星地貌及表面物质成分的研究表明火星上曾经存在地表水,而目前已探测到的火星上的水广泛存在于极区冰盖和次表层中。近十二年来,欧洲火星快车上搭载的火星次表层和电离层探测先进雷达和美国的火星勘测轨道器上搭载的浅表雷达已经开展了对火星次表层的探测和研究,并取得了一系列科学成果。火星次表层记录着火星形成与演化的重要历史信息。对火星次表层的探测和研究,可以为了解火星的物理特性和构造组成,探寻火星生命,研究火星的地质演化历史提供科学依据。综述目前利用雷达对火星次表层进行的探测和研究,介绍了雷达探测的原理与方法,总结了已取得的科学成果,并展望未来的火星探测雷达。     

雷达探测技术在探月中的应用

人类运用雷达探测技术对月球探测取得了丰硕的科学成果,开展了对月球地形地貌探测、月壤厚度反演、月球次表层结构探测、月球水冰探测等研究。以不同的探测方式和科学目的综述了国内外运用雷达探测技术对月球探测所取得的一系列科学成果,以及我国成功发射"嫦娥三号"卫星,其巡视器"玉兔号"上搭载的测月雷达(Lunar Penetration Radar)将用于探测月球次表层结构和月壤厚度与结构,同时也介绍了测月雷达的基本原理与一些基本指标参数。     

表层穿透雷达在月球和深空探测中的应用

对月球以及更远天体或者空间环境的探测是人类航天活动的重要方向。开展月球和深空探测有利于研究太阳系起源、演化与现状,以及生命起源与演变等重大科学问题,有利于催生基础性、前瞻性的学科与技术。相比光学探测方法,雷达具有强穿透性、极化特性以及不受光照限制等优势,是探测天体特性的有效手段之一,在月球和深空探测中发挥了重要作用。电磁波能够穿透几米到几千米的次表层,可用于探测月球和深空目标的表层介电常数、次表层结构、电离层及水冰等。按照探测方式的不同,表层穿透雷达探测主要包括地基雷达、环绕器雷达及巡视器雷达3种方式。针对不同的科学目标,不同的探测方式具有各自的优势和不足。回顾了表层穿透雷达在月球、火星以及小行星等探测中的科学应用,总结了已经投入使用以及计划中的各种雷达科学载荷的探测任务、参数设计、工作原理和探测结果,展望了在未来利用表层穿透雷达进行月球和深空探测的发展趋势。     

近地小行星地基雷达探测研究现状

地基雷达探测是研究太阳系中小行星的重要方法。雷达探测主要有两种方式:(1)连续波探测,可得到小行星表面的粗糙度等参数;(2)延迟多普勒探测,用于反演小行星的三维形状模型并确定自转轴状态。与其他探测方法相比,雷达观测具有分辨率高等优势。简要介绍了地基雷达探测的原理和方式,以及通过雷达成像建立小行星形状模型的方法。同时举例说明了雷达探测小行星的成果,并对包括雷达在内的多种探测方法进行了比较。     

基于测月雷达机制月壤相对介电常数反演方法模拟研究

嫦娥三号卫星于2013年12月2日发射,并成功着陆月球,其巡视器上搭载的测月雷达实现了首次雷达就位探测月球。基于测月雷达的探测机制,给出了两种计算月壤相对介电常数的方法,标定件标定法和双天线时延法,并通过仿真验证了两种计算方法的有效性,文中同时给出了两种计算方法存在的局限性,为后期基于嫦娥三号雷达数据进行月球次表层的介电特性反演提供了重要依据。     

面向宇宙再电离探测的基本数据处理方法

宇宙再电离时期(epoch of reionization, Eo R)的探测是SKA的重要科学目标之一,也是目前许多SKA探路者阵列的首要科学目标。由于宇宙再电离信号非常微弱,因此在数据处理的过程中存在许多难点,如高精度校准、大视场高动态成像等。对默奇森宽场阵列(Murchison Widefield Array, MWA)、低频阵列(Low Frequency Array, LOFAR)、21CMA阵列(21 Centimeter Array, 21CMA)等SKA低频先导干涉阵列的基本数据处理方法进行了综述,如干扰的识别与去除、数据校准、可视度研究以及成像研究等,并对数据处理时用到的一些常用技术与软件作了相应的介绍与总结。     

欧空局"火星登陆器"和"火星电离层和测地实验"计划

该文全面介绍了欧洲空间局的“火星登陆器(NetLander)”及其“火星电离层和测地实验(NEIGE)”项目。具体叙述了项目的科学目标与内容、实现途径、组织机构、工程技术方面的框架,以及目前最新的进展状况。希望借此能为我国开展相关工作提供参考与借鉴．     

火星为什么会有液态水

不久前,美国宇航局(NASA)的一次新闻发布会宣布,火星的近表面,甚至于表面就存在着液态水。事实上,火星研究者多年前就已经知道,火星上一定有水！因为在火星大气中早已探测到存在有水蒸气,在火星两极也发现有一定量的水冰。甚至于在火星的地下还有可能存在足够数量的冻结的水,它们融化之后,可以成为一个海洋。最近的发现,令人惊奇之处就在于:火星的近表面竟然就有液态水！     

嫦娥二号伽玛射线谱仪

嫦娥二号卫星是我国继嫦娥一号卫星成功发射后的第2颗探月卫星,其上搭载的伽玛射线谱仪(Gamma-Ray Spectrometer,GRS)的主要科学目标是探测月表O、Si、Fe、Ti、U、Th、K、Mg、Al、Ca等主量元素.相比嫦娥一号伽玛射线谱仪,嫦娥二号的能量分辨率和探测效率都大大提高.描述了嫦娥二号伽玛射线谱仪的设计和性能测试结果以及在轨飞行的初步探测成果.     

星载合成孔径雷达成像处理

合成孔经雷达(Synthetic Aperture Radar,SAR)是当今微波遥感领域的研究热点,其全天候、全天时、高分辨率以及穿透地表等特点使其具备其他传感器不可比拟的优势,因此得到了极其广泛的应用。成像处理是SAR技术的核心部分。分析了星载SAR点目标回波信号模型和几种常用的星载SAR数字成像处理算法。以距离-多普勒算法为例,阐述了距离压缩和方位压缩的实质,详细讨论了多普勒参数估计、距离徙动校正和斑点噪声的去除等几个关键的问题。采用ERS-1卫星数据做了成像实验,并对SAR成像技术加以总结。     

Top layers charaterization of the Martian surface: Permittivity estimation based on geomorphology analysis

Mars Express spacecraft inserted successfully Martian orbit at the end of 2003. On board this probe, a radar instrument called MARSIS (for Mars Advanced Radar for Surface and Ionosphere Sounding) is looking for water inside the first kilometers of Martian crust. To support MARSIS planning and data inversion, Laboratoire de Planétologie de Grenoble developed a MARSIS signal simulator.We show in this paper that MARSIS can also characterize some surface features, in addition to subsurface water and ionosphere sounding. We study a Martian surface region of special interest: Nilokeras Mensae, inside Acidalia Planitia. We discuss the previous geological studies of this region, and show the geomorphologies analyze of this surface area could lead to a simple terrain model. Then, we present a possible data inversion scheme and applying the MARSIS simulator, we test a first radar data inversion.Finally, we will show that complete dielectric characteristics of surface top layers can be retrieved, at least as often Mars Express flies over some layered terrain (at wavelength scale).     

Correction of the ionospheric distortion on the MARSIS surface sounding echoes

Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft has made numerous measurements of the Martian surface and subsurface. However, all of these measurements are distorted by the ionosphere and must be compensated before any analysis. We have developed a technique to compensate for the ionospheric distortions. This technique provides a powerful tool to derive the total electron content (TEC) and other higher-order terms of the limited expansion of the plasma dispersion function that are related to overall shape of the electron column profile. The derived parameters are fitted by using a Chapman model to derive ionospheric parameters like n₀, electron density primary peak (maximum for solar zenith angle (SZA) equal 0), and the neutral height scale H.

Our estimated ionospheric parameters are in good agreement with Mars Global Surveyor (MGS) radio-occultation data. However, since MARSIS does not have the observation geometry limitations of the radio occultation measurements, our derived parameters extend over a large range of SZA for each MEX orbit.

The first results from our technique have been discussed by Safaeinili et al. [2007, Estimation of the total electron content of the Martian ionosphere using radar sounder surface echoes. Geophys. Res. Lett. 34, L23204, doi:10.1029/2007GL032154].     

Integrating radar stratigraphy with high resolution visible stratigraphy of the north polar layered deposits,Mars

Shallow Radar (SHARAD) on board NASA’s Mars Reconnaissance Orbiter has successfully detected tens of reflectors in the subsurface of the north polar layered deposits (NPLD) of Mars. Radar reflections are hypothesized to originate from the same material interfaces that result in visible layering. As a first step towards verifying this assumption, this study uses signal analyses and geometric comparisons to quantitatively examine the relationship between reflectors and visible layers exposed in an NPLD outcrop. To understand subsurface structures and reflector geometry, reflector surfaces have been gridded in three dimensions, taking into account the influence of surface slopes to obtain accurate subsurface geometries. These geometries reveal reflector dips that are consistent with optical layer slopes. Distance–elevation profiling of subsurface reflectors and visible layer boundaries reveals that reflectors and layers demonstrate similar topography, verifying that reflectors represent paleosurfaces of the deposit. Statistical and frequency-domain analyses of the separation distances between successive layers and successive reflectors confirms the agreement of radar reflector spacing with characteristic spacing of certain visible layers. Direct elevation comparisons between individual reflectors and discrete optical layers, while necessary for a one-to-one correlation, are complicated by variations in subsurface structure that exist between the outcrop and the SHARAD observations, as inferred from subsurface mapping. Although these complications have prevented a unique correlation, a genetic link between radar reflectors and visible layers has been confirmed, validating the assumption that radar reflectors can be used as geometric proxies for visible stratigraphy. Furthermore, the techniques for conducting a stratigraphic integration have been generalized and improved so that the integration can be undertaken at additional locations.     

The influence of the ionosphere in subsurface Martian soil sounding experiments and a method of its correction

The problem of radar sounding of the Martian soil from an orbiting spacecraft is analyzed. The influence of the ionosphere on the results of remote sounding is revealed through the results of numerical modeling and actual subsurface soil sounding experiments carried out using the MARSIS radar sounder aboard the Mars-Express spacecraft. A method of correction of the dispersion distortion is proposed and tested through radiolocation of the polar cap on Mars.     

Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155° E and east MFF between 209 and 213° E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD data.     

Radar absorption due to a corotating interaction region encounter with Mars detected by MARSIS

Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is a subsurface and topside ionosphere radar sounder aboard the European Space Agency spacecraft Mars Express, in orbit at Mars since 25 December 2003, and in operation since 17 June 2005. The ionospheric sounding mode of MARSIS is capable of detecting the reflection of the sounding wave from the martian surface. This ability has been used in previous work to show that the surface reflection is absorbed and disappears during periods when high fluxes of energetic particles are incident on the ionosphere of Mars. These absorption events are believed to be the result of increased collisional damping of the sounding wave, caused by increased electron density below the spacecraft, in turn caused by impact ionization from the impinging particles. In this work we identify two absorption events that were isolated during periods when the surface reflection is consistently visible and when Mars is nearly at opposition. The visibility of the surface reflection is viewed in conjunction with particle and photon measurements taken at both Mars and Earth. Both absorption events are found to coincide with Earth passing through solar wind speed and ion flux signatures indicative of a corotating interaction region (CIR). The two events are separated by an interval of approximately 27 days, corresponding to one solar rotation. The first of the two events coincides with abruptly enhanced particle fluxes seen in situ at Mars. Simultaneous with the particle enhancement there are an abrupt decrease in the intensity of electron oscillations, typically seen by the Mars Express particle instrument ASPERA-3 between the magnetic pileup boundary and the martian bow shock, and a sharp drop in the solar wind pressure, seen in the proxy quantity based on MGS magnetometer observations. The decrease in oscillation intensity is therefore the probable effect of a relaxation of the martian bow shock. The second absorption event does not show a particle enhancement and complete ASPERA-3 data during that time are unavailable. Other absorption events are the apparent result of solar X-ray and XUV enhancements. We conclude that surface reflection absorption events are sometimes caused by enhanced ionospheric ionization from high energy particles accelerated by the shocks associated with a CIR. A full statistical analysis of CIRs in relation to observed absorption events in conjunction with a quantitative analysis of the deposition of ionization during space weather events is needed for a complete understanding of this phenomenon. If such analyses can be carried out, radar sensing of the martian ionosphere might be useful as a space weather probe.     

Multilayer simulations for accurate geological interpretations of SHARAD radargrams

SHARAD (SHAllow RADar) is a nadir-looking Synthetic Aperture Ground Penetrating Radar on board NASA's 2005 Mars Reconnaissance Orbiter. There are three main characteristics that define the performance of this instrument: ground penetration (due to the operational frequency, the observed echoes can be related to reflections from surface or subsurface), spaceborne operation (the first reflection does not necessarily correspond to the nadir reflection), and nadir looking SAR (there will always be left/right ambiguities). All this implies that there will be surface/subsurface range ambiguity and the geological interpretation of the radargrams cannot be straightforward. In order to avoid data misinterpretation, a simulator of SHARAD’s expected response for a given observation geometry and topography is needed. Simulations can take into account all surface/subsurface reflections in order to identify common families of ambiguities and facilitate the interpretation. In this work we present SHARSIM (SHARAD Radargram SIMulator), a software tool designed to simulate SHARAD radargrams taking as inputs Mars surface information and hypothetical subsurface structure. Its performance is analyzed by investigating typical artifacts and by a direct comparison with real radargrams. We show that SHARSIM simulations can help to discern between artifacts and real subsurface features in order to make accurate geological interpretations.     

MARSIS surface reflectivity of the south residual cap of Mars

The south residual cap of Mars is commonly described as a thin and bright layer of CO₂-ice. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is a low-frequency radar on board Mars Express operating at the wavelength between 55 and 230 m in vacuum. The reflection of the radar wave on a stratified medium like the residual cap can generate interferences, causing weaker surface reflections compared to reflections from a pure water ice surface. In order to understand this anomalous low reflectivity, we propose a stratified medium model, which allows us to estimate both the thickness and the dielectric constant of the optically thin slab. First, we consider the residual cap as single unit and show that the decrease in the reflected echo strength is well explained by a mean thickness of 11 m and a mean dielectric constant of 2.2. This value of dielectric constant is close to the experimental value 2.12 for pure CO₂-ice. Second, we study the spatial variability of the radar surface reflectivity. We observe that the reflectivity is not homogeneous over the residual cap. This heterogeneity can be modeled either by variable thickness or variable dielectric constant. The surface reflectivity shows that two different units comprise the residual cap, one central unit with high reflectivity and surrounding, less reflective units.     

A plasma flow velocity boundary at Mars from the disappearance of electron plasma oscillations

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft is capable of measuring ionospheric electron density by the use of two main methods: remote radar sounding and from the excitation of local plasma oscillations. The frequency of the locally excited electron plasma oscillations is used to measure the local electron density. However, plasma oscillations are not observed when the plasma flow velocity is higher than about 160 km/s, which occurs mainly in the solar wind and magnetosheath. As a consequence, in many passes, there is a sudden disappearance of the plasma oscillations as the spacecraft enters into the magnetosheath. This fact allows us to identify a flow velocity boundary on the dayside, between the ionosphere of Mars and the shocked solar wind. This paper summarizes the results of the measurement of 552 orbits mostly over a period from August 4, 2005 to August 17, 2007. The boundary points found using MARSIS have been verified by measurements from the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) Electron Spectrometer (ELS) instrument on Mars Express. The average position of the flow velocity boundary is compared to flow velocity simulations computed using hybrid model and other boundaries. The boundary altitude is slightly lower than the magnetic pile-up boundary determined using Phobos 2 and Mars Global Surveyor (MGS) crossings, but it is in good agreement with the induced magnetospheric boundary determined by ASPERA-3. Investigation of the effect of the crustal magnetic field revealed that the flow velocity boundary is raised at the locations with strong crustal magnetic fields.     

Schroeter's ratios for Martian craters: Radar results

Schroeter's ratios (ratios of the rim volume to the apparent volume) are determined for a sample of 29 large, degraded Martian craters selected from the Goldstone Mars radar altimetry data. On the average, the values of the calculated Schroeter's ratios are about two orders of magnitude smaller than the same ratios for fresh lunar craters. This indicates a severe rim volume deficit in degraded Martian craters and it provides an additional support to the notion of a widespread resurfacing of intercrater plains on Mars. Schroeter's ratios for degraded craters could provide a semi-quantitative measure of the effects of the modification processes that had been active on Mars and on the other planetary bodies.