Valérie Ciarletti

Characterisation of the Soil Structure and Microwave Backscattering Based on Numerical Three-Dimensional Surface Representation: Analysis with a Fractional Brownian Model

Abstract In this paper, local structure of bare soil is analyzed from the fractal point of view. Soil surface profiles were created from three dimensional (3-D) stereoscopic images of soil surface leading to 3-D numerical reconstruction of the soil topography with very fine resolution. Investigations are done with a database (four soils) that includes three main soil classes according to the way of tillage: smoothed field from rainfalls, ploughed, and sowed fields. The fractional Brownian model developed by Mandelbrot is used to describe local structure of soil roughness. For the database soils, fractal nature of the profiles is demonstrated over a finite range of scales showing a good stability of fractal dimension for each one. This model also provides an excellent analytic fit to the experimental correlation function of soil. Therefore, a new method to calculate its shape at the origin and a more stable correlation length is presented. To study the influence of the band-limited fractal nature of the soil on radar signal, the Moment Method is used to evaluate the backscattered field and to obtain the radar cross-section by statistical averaging. Surfaces used for this electromagnetic simulation are cylindrical and perfectly conducting. A method is developed to generate soil surface profiles that have the same statistical properties and the same roughness parameters values (rms height, correlation length, and fractal dimension) as what has been found on our database soils. The generation method is based on an initially Gaussian correlated random profile, modified by the random midpoint displacement method to introduce the short-range disorder that depends on fractal dimension. The radar signal level computed on these surfaces by the Moment Method shows the dependence of backscattering on fractal dimension and new aspects in electromagnetic scattering behavior over soils.

Validation of a Rough Surface Model Based on Fractional Brownian Geometry with SIRC and ERASME Radar Data over Orgeval

During the 1994 SIRC/XSAR mission, part of the scientific activities were devoted to hydrology applications. To complement the shuttle radar data, the helicopter scatterometer ERASME was used on the Orgeval site. As a result, incidence angles ranging from 25° to 57° are available in the C band. Terrain measurements of soil moisture and roughness were also taken. The objective of this study was to take advantage of this data set to improve the geometrical characterization of local soil structure using a fractional Brownian model in order to simulate radar backscattering over agricultural fields. The experimental soil profiles are proved to be locally fractal over a spatial range of a few centimeters (clod structure). The relation between the shape of the correlation function and the fractal dimension is brought to light. A new empirical correlation function is used to adjust the experimental one. It leads to an improvement in the analytic backscattering model Integral Equation Model. Soil profiles, generated with combination between fractional Brownian local structure and global structure with respect to the surface root mean square (rms height) and the correlation length, are used in Moment Method backscattering simulations, which provide results in excellent agreement with radar data.

WISDOM GPR Designed for Shallow and High-Resolution Sounding of the Martian Subsurface

The Water Ice Subsurface Deposit Observation on Mars (WISDOM) Ground Penetrating Radar (GPR) is one of the instruments that have been selected as part of the Pasteur payload of the European Space Agencys (ESAs) 2018 ExoMars Rover mission. The main scientific objectives of the mission are to search for evidence of past and present life and to characterize the nature of the shallow subsurface. The Rover is equipped with a drill that can sample the subsurface down to a depth of approximately 2 m. The WISDOM GPR is the only instrumentation capable of obtaining information about the nature of the subsurface along the Rover path before drilling. WISDOM has been designed to explore the first ~3 m of the subsurface with a vertical resolution of a few centimeters. The paper presents a description of the WISDOM instrument with a particular emphasis on the electronic architecture and antenna design that have been chosen to meet the challenging technical objectives. Some preliminary measurements obtained with the prototype are given to illustrate the instruments potential performance.

Habitability on Early Mars and the Search for Biosignatures with the ExoMars Rover

Abstract The second ExoMars mission will be launched in 2020 to target an ancient location interpreted to have strong potential for past habitability and for preserving physical and chemical biosignatures (as well as abiotic/prebiotic organics). The mission will deliver a lander with instruments for atmospheric and geophysical investigations and a rover tasked with searching for signs of extinct life. The ExoMars rover will be equipped with a drill to collect material from outcrops and at depth down to 2 m. This subsurface sampling capability will provide the best chance yet to gain access to chemical biosignatures. Using the powerful Pasteur payload instruments, the ExoMars science team will conduct a holistic search for traces of life and seek corroborating geological context information. Key Words: Biosignatures—ExoMars—Landing sites—Mars rover—Search for life. Astrobiology 17, 471–510.

Electrical properties and porosity of the first meter of the nucleus of 67P/Churyumov-Gerasimenko - As constrained by the Permittivity Probe SESAME-PP/Philae/Rosetta

Comets are primitive objects, remnants of the volatile-rich planetesimals from which the solar system condensed. Knowing their structure and composition is thus crucial for the understanding of our origins. After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko in November 2014, for the first time, the Rosetta mission provided the opportunity to measure the low frequency electrical properties of a cometary mantle with the permittivity probe SESAME-PP (Surface Electric Sounding and Acoustic Monitoring Experiment−Permittivity Probe). Aims. In this paper, we conduct an in-depth analysis of the data from active measurements collected by SESAME-PP at Abydos, which is the final landing site of Philae, to constrain the porosity and, to a lesser extent, the composition of the surface material down to a depth of about 1 m. Methods. SESAME-PP observations on the surface are then analyzed by comparison with data acquired during the descent toward the nucleus and with numerical simulations that explore different possible attitudes and environments of Philae at Abydos using a method called the Capacity-Influence Matrix Method. Results. Reasonably assuming that the two receiving electrode channels have not drifted with respect to each other during the ten-year journey of the Rosetta probe to the comet, we constrain the dielectric constant of the first meter below the surface at Abydos to be >2.45 ± 0.20, which is consistent with a porosity <50% if the dust phase is analogous to carbonaceous chondrites and <75% in the case of less primitive ordinary chondrites. This indicates that the near surface of the nucleus of 67P/Churyumov-Gerasimenko is more compacted than its interior and suggests that it could consist of a sintered dust-ice layer.

Full polarimetric GPR antenna system aboard the ExoMars rover

A full polarimetric antenna system on board the ExoMars rover is part of the Experiment “Water Ice and Subsurface Deposit Observations on Mars” (WISDOM). The WISDOM-Experiment is a Ground Penetrating Radar (GPR) selected to be part of the Pasteur payload aboard the rover of the ExoMars mission. The Pasteur Panoramic Instruments (wide angle camera PANCAM, infrared spectrometer MIMA and WISDOM) will perform large-scale scientific investigations at the sites the Rover will visit. Among these instruments, WISDOM is the only that can provide a view of the subsurface structure prior to drilling. WISDOM has been designed to characterize the shallow subsurface structure of Mars. WISDOM will for the first time give access to the geological structure, electromagnetic nature, and, possibly, hydrological state of the shallow subsurface by retrieving the layering and properties of the buried reflectors. It will address important scientific questions regarding the planets present state and past evolution. The measured data will also be used to determine the most promising locations to obtain underground samples with the drilling system mounted on board the rover. The instruments objective is to get high-resolution measurements down to 2 m depth in the Martian crust. The radar is a gated step frequency system covering a frequency range from 500 MHz to 3 GHz. The radar is fully polarimetric and makes use of an ultra wideband antenna system based on Vivaldi antenna elements. The paper describes requirements, design and realization of the WISDOM antenna system accommodated on the ExoMars rover. Simulated and measured antenna performance is compared in this paper. Test measurements performed in permafrost regions on earth will be shown in the presentation.

A prototype for the WISDOM GPR on the ExoMars mission

This paper will describe a GPR prototype for the WISDOM - (Water Ice and Subsurface Deposit Observations on Mars) experiment on the ExoMars mission. The operation principle as well as the instrument design is explained. Simulations of the radar response to realistic geologic models of the Martian sub-surface are presented. Measurements with a commercial GPR system in Mars analog geology on earth are shown.

Modeling the Configuration of HF Electrical Antennas for Deep Bistatic Subsurface Sounding

In the frame of the European Space Agencys 2016 ExoMars mission, the Electromagnetic Investigation of the SubSurface (EISS) ground-penetrating radar has been designed and developed to perform deep soundings of the Martian subsurface from the surface. The EISS is designed to take advantage of the potential for bistatic radar investigations of the Martian subsurface between the fixed station (Lander) and the mobile platform (rover) and to characterize the 3-D structure and stratigraphy of the subsurface at depths ranging from 100 m to a few kilometers out to a 1-km radius around the lander. The EISS makes use of an electric dipole antenna made of two identical 35-m resistively loaded monopoles to transmit (and also receive in a monostatic mode) the high-frequency signal. However, the EISSs most innovative capability is its potential for bistatic operation, made possible by the accommodation of a small magnetic sensor on the rover (as initially planned for the ExoMars mission) which can measure the magnetic field (all three components) of the received waves whatever the direction and orientation of the rover. The aim of this paper is to show that the two monopoles of the antenna must be deployed on the surface in nearly opposite directions but not aligned to ensure good volume coverage around the transmitter. This paper is based on Finite Difference in Time Domain (FDTD) electromagnetic simulations. The simulated data have been used to study the impact of the angle between these two monopoles on the instrument performance.

Ultra light-weight antenna system for full polarimetric GPR applications

The motivation to develop an ultra light-weight antenna system was driven by a space borne radar application. The Experiment “Water Ice and Subsurface Deposit Observations on Mars” (WISDOM) is a Ground Penetrating Radar (GPR) selected to be part of the Pasteur payload on board the rover of the ExoMars mission. Among the Pasteur Panoramic Instruments on the ExoMars rover, only WISDOM can provide a view of the subsurface structure. WISDOM is the first GPR on a planetary rover. It has been designed to characterize the shallow subsurface structure of Mars. WISDOM will for the first time give access to the geological structure, electromagnetic nature, and, possibly, hydrological state of the shallow subsurface by retrieving the layering and properties of the buried reflectors. It will address important scientific questions regarding the planets present state and past evolution. The measured data will also be used to determine the most promising locations to obtain underground samples with the drilling system mounted on board the rover. The instruments objective is to get high-resolution measurements down to 2 m depth in the Martian crust. The radar is a gated step frequency system covering a frequency range from 500 MHz to 3 GHz. The radar is fully polarimetric and makes use of an ultra wideband antenna system based on Vivaldi antenna elements. The paper describes antenna requirements to fulfil for this very specific GPR application and it gives an overview about the light-weight design and its realization. Simulated and measured antenna performance is compared in this paper. Test measurements were performed in permafrost regions on earth.

Modelisation of roughness and microwave scattering of bare soil surfaces based on fractal Brownian geometry

Soil profiles are analysed from the fractal point of view. They are issued from a data base of three dimensional stereoscopic images of surfaces of different soil practices which are considered as isotropic. The fractal Brownian model is used for this description. Fractality is demonstrated for all soils. An excellent analytic fit to the experimental correlation functions is developed with a Brownian model formulation. A new method is introduced to calculate a more stable correlation length. For backscattering calculations, the moment method is used over generated profiles which respect to three parameters: the fractal dimension, the correlation length and the rms height. This numerical model proves the dependence of backscattering on fractal dimension.