Alain Herique

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

The landing(s) of Philae and inferences about comet surface mechanical properties

The Philae lander, part of the Rosetta mission to investigate comet 67P/Churyumov-Gerasimenko, was delivered to the cometary surface in November 2014. Here we report the precise circumstances of the multiple landings of Philae, including the bouncing trajectory and rebound parameters, based on engineering data in conjunction with operational instrument data. These data also provide information on the mechanical properties (strength and layering) of the comet surface. The first touchdown site, Agilkia, appears to have a granular soft surface (with a compressive strength of 1 kilopascal) at least ~20 cm thick, possibly on top of a more rigid layer. The final landing site, Abydos, has a hard surface.

Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar

The Philae lander provides a unique opportunity to investigate the internal structure of a comet nucleus, providing information about its formation and evolution in the early solar system. We present Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) measurements of the interior of Comet 67P/Churyumov-Gerasimenko. From the propagation time and form of the signals, the upper part of the “head” of 67P is fairly homogeneous on a spatial scale of tens of meters. CONSERT also reduced the size of the uncertainty of Philae’s final landing site down to approximately 21 by 34 square meters. The average permittivity is about 1.27, suggesting that this region has a volumetric dust/ice ratio of 0.4 to 2.6 and a porosity of 75 to 85%. The dust component may be comparable to that of carbonaceous chondrites.

Comet nucleus sounding experiment by radiowave transmission

Abstract We describe the radio science experiment proposed for the Rosetta cometary mission. The experiment consists in the transmission of electromagnetic waves between the landers and the orbiter through the comet to study its internal structure. In the paper, the electromagnetic model of the comet is presented and used to evaluate the potentiality of the experiment. Various modellings of the radio wave propagation are discussed. Finally, a description of the experiment and the instrument is made.

A characterization of a comet nucleus interior: inversion of simulated radio frequency data

Abstract Radio transmission through the nucleus of the comet Wirtanen, as developed in the CONSERT experiment on the ROSETTA mission, is designed to provide information on the electrical properties of the cometary interior. In order to determine what information can most readily be obtained, we first introduce an analytical model of the nucleus permittivity. We then use the WKB approximation to describe the propagation of waves through the nucleus model assuming a radio frequency of 90 MHz, the frequency chosen for the experiment. With one end of the propagation path on the surface of the comet, and the other in a vehicle orbiting the nucleus, it is shown that the signal at the orbiting end of the path must be back-propagated to the surface of the nucleus of the comet in order to obtain a correct interpretation of the data. We show how the mean permittivity along each propagation path can be obtained and how this can be used to obtain information about the large scale structure of the nucleus. Finally we discuss how the permittivity distribution found can be used to constrain the nature of the material in the interior of the nucleus.

A two dimensional simulation of the CONSERT experiment (radio tomography of comet Wirtanen)

Abstract We simulate two-dimensional (2-D) maps of the internal permittivity of comet Wirtanen from CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) phase measurements. We study the influence of a priori covariance matrices on the mapping process through Tikhonov or linear regression inverse formulas. We show that this a priori information permits a smooth imaging of the internal permittivity, sometimes affected by ghosts (false anomalies). Accounting for indirect propagation paths through the comet material (rays totally reflected by the comet surface) can alleviate this problem.

CONSERT suggests a change in local properties of 67P/Churyumov-Gerasimenko’s nucleus at depth

After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko, the Rosetta mission provided the first opportunity of performing measurements with the CONSERT tomographic radar in November 2014. CONSERT data were acquired during this first science sequence. They unambiguously showed that propagation through the smaller lobe of the nucleus was achieved. Aims. While the ultimate objective of the CONSERT radar is to perform the tomography of the nucleus, this paper focuses on the local characterization of the shallow subsurface in the area of Philae’s final landing site, specifically determining the possible presence of a permittivity gradient below the nucleus surface. Methods. A number of electromagnetic simulations were made with a ray-tracing code to parametrically study how the gradient of the dielectric constant in the near-subsurface affects the ability of CONSERT to receive signals. Results. At the 90 MHz frequency of CONSERT, the dielectric constant is a function of porosity, composition, and temperature. The dielectric constant values considered for the study are based on observations made by the other instruments of the Rosetta mission, which indicate a possible near-surface gradient in physical properties and on laboratory measurements made on analog samples. Conclusions. The obtained simulated data clearly show that if the dielectric constant were increasing with depth, it would have prevented the reception of signal at the CONSERT location during the first science sequence. We conclude from our simulations that the dielectric constant most probably decreases with depth.

Dielectric properties of comet analog refractory materials

Abstract Ground penetrating radio waves or radar provide a promising technique for the study of sub-surface planetary or cometary material. The comet nucleus sounding experiment by radiowave transmission (CONSERT) will be the only experiment on board the ROSETTA mission to provide information on the interior of the comet Wirtanen. Other projects planned or under development are Marsis/Mars Express and GPR for Net Lander. The aim is to perform a radar tomography, and the necessary data inversion of the measured permittivity requires knowledge of the complex dielectric permittivities of probable sub-surface materials, so laboratory studies of the dielectric properties of analog geological materials have become necessary. In connection with the Rosetta mission, we have characterized the dielectric permittivity of dunite, montmorillonite and kaolinite. We used granular materials identical to those of the KOSI comet simulation experiment conducted at DLR in Cologne (1987–1992). These materials were chosen as analog materials for the refractory component of comets and the compositions and grain size distributions of the powders are known. The present measurements covered temperatures increasing from 77 to 360 K at frequencies 120 Hz , 1.2 and 12 kHz . The powders were tested as received (exposed to atmospheric humidity), dried and after water impregnation. We find that the dielectric properties are consistent with the hypotheses on which the CONSERT instrument was designed.

Detectability of subsurface interfaces in lunar maria by the LRS/SELENE sounding radar: Influence of mineralogical composition

[1] The Lunar Radar Sounder (LRS) of the SELENE mission has detected horizontal subsurface features at depths of a few hundreds of meters within all major lunar maria. We have mapped these features at global scale and found a heterogeneous geographical distribution, which correlates negatively with the maps of TiO 2 and FeO obtained from UV-VIS measurements by the Clementine probe. High concentration of ilmenite (FeTiO 3 ) in the basaltic mare lava can explain this correlation, this mineral being a strong absorber for low frequency electromagnetic waves. Thus, absence of detection of subsurface interfaces by LRS on a large portion of lunar maria does not necessarily imply that these interfaces are actually absent.

Three-dimensional reconstruction of a comet nucleus by optimal control of Maxwell's equations: A contribution to the experiment CONSERT onboard space craft Rosetta

COmet Nucleus Sounding Experiment by Radio Wave Transmission (CONSERT) is one of 20 experiments onboard the ESA mission Rosetta and aimed at the reconstruction of the unknown internal material parameter distribution of a comet nucleus. The details on the experiment setup can be found in [1], [2]. CONSERT consists of a lander module which attaches to the surface of the comet and an orbiter module which circulates the comet in space. An electromagnetic sounding of the comet nucleus will be achieved by a signal link between the lander and orbiter antenna system. We propose an optimal control approach to solve for the unknown 3D material parameter distribution of the comet nucleus. We optimize the computed electromagnetic field distribution at the receiver locations for the measured (observed) electromagnetic field distribution by controlling the material parameters. The target functional, the difference between computed and observed field for all receiver locations and time steps is minimized by means of a gradient-based quasi-Newton optimization algorithm. The optimal control problem is solved if the target functional yields its global minimum.