A porosity gradient in 67P/C-G nucleus suggested from CONSERT and SESAME-PP results: an interpretation based on new laboratory permittivity measurements of porous icy analogues

The Rosetta spacecraft made a rendezvous with comet 67P/Churyumov-Gerasimenko (67P) in 2014 August, soon after the Philae module landed on the small lobe of the nucleus on 2014 November 12. The CONSERT instrument, onboard Rosetta and Philae, sounded the upper part of the interior of 67P with radiowaves at 90 MHz and determined an average of the real part of the permittivity (hereafter ) equal to about 1.27. The SESAME-PP instrument, onboard Philae, sounded the near-surface of the small lobe in the 400–800 Hz range and determined a lower limit of equal to 2.45. We use a semi-empirical formula obtained from measurements of performed in the laboratory at 243 K on water ice and ice-basaltic dust mixtures, with a controlled porosity in the 31–91 per cent range and a dust-to-ice volumetric ratio in the 0.1– 2.8 range, to interpret the results of the two instruments, taking into account the temperature and frequency dependences. A graphical method is proposed to derive ranges of porosity and dust-mass fraction from a value of derived from observations. The non-dispersive behaviour of below 175 K, allows us to compare the values of obtained by CONSERT and SESAME-PP. We show that the porosity of the small lobe of 67P increases with depth. Based on new measurements of analogues of complex extraterrestrial organic matter, the so-called tholins, we also suggest that, for the dust component in the cometary material, the presence of silicates has more effect on than organic materials

The Basal Detectability of an Ice-Covered Mars by MARSIS

International audienceThe detection of anomalously strong relative basal reflectivity beneath the Martian South Polar Layered Deposits (SPLD) from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) has led to hypotheses suggesting the presence of basal materials such as liquid water. Here, we propose a forward approach to assess whether such a high signal could be produced by a Martian terrain currently exposed at the surface without liquid water. We convert existing MARSIS surface reflectivity measurements into a basal reflectivity as if it were overlaid by an SPLD-like ice deposit. 0.3%-2% of the surface could produce basal reflections of magnitude similar to the SPLD measurements in the assumption of a 10% impure ice. An ice loss tangent >$>$0.01 is required to prevent any of the current Martian surface from producing a bright SPLD-like basal reflection. The detected bright terrains are gathered within volcanic constructs of diverse geologic epoch

Microwave imaging from experimental data within a Bayesian framework with realistic random noise

International audienceThis paper deals with the reconstruction of three-dimensional targets from experimental multiple-frequency data measured in the anechoic chamber of the Institut Fresnel (Marseille, France). An inverse iterative scheme is implemented with an adequate choice of the cost functional. Indeed, a Bayesian approach is considered in order to take into account the random noise which is present in the experiment. This leads to the definition of an adequate cost functional, where the weighting coefficients are changing with the frequency, the incidence angle and the receiving angle. The inversion scheme turns out to be more robust and accurate

Probing the interior of asteroid Apophis: a unique opportunity in 2029

International audienceThe near Earth asteroid (99942) Apophis, discovered in 2004, (with a diameter of about 270 meters) will come back very close to the Earth on April 13, 2029. The close approach of Apophis to the Earth in 2029 will present an unique opportunity for characterizing this object, serving both science and mitigation purposes. The object will be easily visible from the Earth and it can be expected that its shape and thermal properties will be well determined from ground based observations. However, the characterization of its interior will not be achievable from purely terrestrial observations. Such a characterization, beyond its high scientific value, is essential for planning any mitigation operation, should it be necessary in the future. Near Earth objects are a precious source of information as they represent a mixture of different populations of small bodies containing fundamental information on the origin and early evolution of the solar system. Monitoring the response of Apophis to the gravitational constraints induced by its close approach to the Earth may provide a way to access information on its internal structure. A study to identify some affordable mission scenarii for such a mission is presently underway at CNES (the French Space Agency). We will present the scientific and mitigation objectives of such a mission as well as the preliminary results of the mission analysis and the main system characteristics

The Asteroid Impact and Deflection Assessment (AIDA) mission: Science Proximity Operations

International audienceThe moon of the near-Earth binary asteroid 65803 Didymos is the target of the Asteroid Impact and Deflection Assessment (AIDA) mission. This mission is a joint concept between NASA and ESA to investigate the effectiveness of a kinetic im-pactor in deflecting an asteroid. The mission is composed of two components: the NASA-led Double Asteroid Redirect Test (DART) that will impact the Did-ymos moon (henceforth Didymoon), and the ESA-led Asteroid Impact Mission (AIM) that will survey the Didymos system. Using much of ESA's Rosetta experience , the AIM mission will undertake all the proximity operations both before and after the impact produced by DART. The physical and dynamical characterization of both Didymain (primary) and Didymoon is of maximum importance in the joint AIDA mission and the main purpose of the AIM spacecraft. The characterization includes measuring before and after the DART impact, the internal properties of the primary and secondary, the mass of Didymoon, the surface geology and regolith properties of both objects, and the dynamical state of Didymoon [1]. In this abstract, we summarize the proximity operations needed to achieve the scientific objectives of the AIM spacecraft using the broad suite of experiments it will carry to satisfy its mission objectives