The Refractory-to-Ice Mass Ratio in Comets

We review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the Refractory-to-Ice mass ratio inside the nucleus, which constrains the origin of comets. Such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. This makes the Refractory-to-Ice mass ratio inside the nucleus up to ten times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. We review the Refractory-to-Ice mass ratios available for the comet nuclei visited by space missions, and for the Kuiper Belt Objects with well defined bulk density, finding the 1-σ lower limit of 3. Therefore, comets and KBOs may have less water than CI-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc

Rebuilt by migration of undersempled 2D signals

Active geophysical prospecting techniques (seismics, ground penetrating radar) are based on the measurement of a reflecte d signal . This measurement is done for different points in the space and gives an image of reflected signal amplitude versus tim e (range) and versus one direction in space (x) . We use this data to rebuild an image of the subsurface . But the fact that directivit y of transmitter and receiver is low, induces that the image versus x and t is not equivalent to the subsurface image (x, z) . Migratio n techniques are developed to derive the subsurface image . In this paper, we use the f - k migration technique which gives a solution of the wave equation in the frequency-wavenumber domain and we study the effect of undersampling in the time domain on this method . We describe a method to characterize thi s effect and estimate the real wavelet . We calculate the analytic result of migration of undersampled signal for a simple reflector. We find that the correctly sampled part of the signal is focused by migration and the aliased part is spread out from this calculation . We deduce a method to rebuild the well-sampled 2D signal . The weakness of this technique is a decrease of the signal to nois e ratio and the generation of spurious images . We discuss all aspects of this new technique on synthetic data . The wavelet estimation and the frequency—wavenumber migration are applied to the ground penetrating data acquired i n Antarctica by the radar developed for Mars'98 mission.Active geophysical prospecting techniques (seismics, ground penetrating radar) are based on the measurement of a reflecte d signal . This measurement is done for different points in the space and gives an image of reflected signal amplitude versus tim e (range) and versus one direction in space (x) . We use this data to rebuild an image of the subsurface . But the fact that directivit y of transmitter and receiver is low, induces that the image versus x and t is not equivalent to the subsurface image (x, z) . Migratio n techniques are developed to derive the subsurface image . In this paper, we use the f - k migration technique which gives a solution of the wave equation in the frequency-wavenumber domain and we study the effect of undersampling in the time domain on this method . We describe a method to characterize thi s effect and estimate the real wavelet . We calculate the analytic result of migration of undersampled signal for a simple reflector. We find that the correctly sampled part of the signal is focused by migration and the aliased part is spread out from this calculation . We deduce a method to rebuild the well-sampled 2D signal . The weakness of this technique is a decrease of the signal to nois e ratio and the generation of spurious images . We discuss all aspects of this new technique on synthetic data . The wavelet estimation and the frequency—wavenumber migration are applied to the ground penetrating data acquired i n Antarctica by the radar developed for Mars'98 mission.Les techniques de prospection géophysique active (sismique, radar géophysique) reposent sur la mesure d'un signal réfléchi. Cette mesure est effectuée en différents points de l'espace, elle fournit une image de l'amplitude du signal réfléchi en fonction du temps et d'une dimension de l'espace (x). A partir de ces données, nous cherchons à obtenir une coupe du sous-sol. Mais, du fait de la faible directivité des capteurs et des sources utilisées, l'image en temps et en espace n'est pas immédiatement transposable en une image fonction de deux dimensions de l'espace (x, z), d'où l'utilisation des techniques de migration pour la reconstruire. Nous nous intéresserons ici à la migration f - k qui donne une solution de l'équation de propagation dans l'espace des fréquences et des nombres d'onde, et à son comportement face au sous-échantillonnage temporel. Pour cela, nous développerons une méthode de caractérisation du sous-échantillonnage et d'estimation de l'ondelette émise. Ensuite, nous calculerons analytiquement le résultat de la migration d'un signal sous-échantillonné dans le cas d'un réflecteur simple. Ce calcul montre que la partie correctement échantillonnée du signal est focalisée par la migration, tandis que la partie repliée est étalée. Nous en déduirons une méthode de compensation du repliement fondée sur la cohérence spatiale du signal. Cette compensation - partielle - se fait au détriment du rapport signal sur bruit et peut générer des fantômes. Nous discuterons les différents aspects de cette nouvelle méthode: des simulations permettront de caractériser les performances de la méthode. Nous appliquerons l'estimation de l'ondelette et la migration aux signaux réels acquis en Antarctique par le radar géophysique développé pour la mission spatiale Mars'98

Revealing the properties of Chuyurmov-Gerasimenko's shallow sub-surface through CONSERT's measurements at grazing angles

International audienceThe aim of the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) is the characterization of the inner structure and electrical properties of the Chuyurmov-Gerasimenko's nucleus. The instrument will sound the comet's nucleus between the lander Philae at the comet's surface and the Rosetta main spacecraft. A coarse three-dimensional model of the complex dielectric permittivity inside the nucleus will be reconstructed from the whole set of data obtained during the first science phase [3]. The work presented here show how a limited set of data acquired at grazing angles during a single low altitude fly-by can be used to characterize the shallow sub-surface of the nucleus. The study is based on simulated data obtained by two different electromagnetic models: the accurate pseudo spectral time-domain method and a much faster ray-based approximation taking into account material and path-loss

The <i>Castalia</i> mission to Main Belt Comet 133P/Elst-Pizarro

We describe Castalia, a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBC’s activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESA’s highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these

The WISDOM Radar: Unveiling the Subsurface Beneath the ExoMars Rover and Identifying the Best Locations for Drilling

The search for evidence of past or present life on Mars is the principal objective of the 2020 ESA-Roscosmos ExoMars Rover mission. If such evidence is to be found anywhere, it will most likely be in the subsurface, where organic molecules are shielded from the destructive effects of ionizing radiation and atmospheric oxidants. For this reason, the ExoMars Rover mission has been optimized to investigate the subsurface to identify, understand, and sample those locations where conditions for the preservation of evidence of past life are most likely to be found. The Water Ice Subsurface Deposit Observation on Mars (WISDOM) ground-penetrating radar has been designed to provide information about the nature of the shallow subsurface over depth ranging from 3 to 10 m (with a vertical resolution of up to 3 cm), depending on the dielectric properties of the regolith. This depth range is critical to understanding the geologic evolution stratigraphy and distribution and state of subsurface H2O, which provide important clues in the search for life and the identification of optimal drilling sites for investigation and sampling by the Rover's 2-m drill. WISDOM will help ensure the safety and success of drilling operations by identification of potential hazards that might interfere with retrieval of subsurface samples

Asteroids Inside Out: Radar Tomography

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After DART: Using the First Full-scale Test of a Kinetic Impactor to Inform a Future Planetary Defense Mission

NASA’s Double Asteroid Redirection Test (DART) is the first full-scale test of an asteroid deflection technology. Results from the hypervelocity kinetic impact and Earth-based observations, coupled with LICIACube and the later Hera mission, will result in measurement of the momentum transfer efficiency accurate to ∼10% and characterization of the Didymos binary system. But DART is a single experiment; how could these results be used in a future planetary defense necessity involving a different asteroid? We examine what aspects of Dimorphos’s response to kinetic impact will be constrained by DART results; how these constraints will help refine knowledge of the physical properties of asteroidal materials and predictive power of impact simulations; what information about a potential Earth impactor could be acquired before a deflection effort; and how design of a deflection mission should be informed by this understanding. We generalize the momentum enhancement factor β, showing that a particular direction-specific β will be directly determined by the DART results, and that a related direction-specific β is a figure of merit for a kinetic impact mission. The DART β determination constrains the ejecta momentum vector, which, with hydrodynamic simulations, constrains the physical properties of Dimorphos’s near-surface. In a hypothetical planetary defense exigency, extrapolating these constraints to a newly discovered asteroid will require Earth-based observations and benefit from in situ reconnaissance. We show representative predictions for momentum transfer based on different levels of reconnaissance and discuss strategic targeting to optimize the deflection and reduce the risk of a counterproductive deflection in the wrong direction

The ESA Hera Mission: Detailed Characterization of the DART Impact Outcome and of the Binary Asteroid (65803) Didymos

Hera is a planetary defense mission under development in the Space Safety and Security Program of the European Space Agency for launch in 2024 October. It will rendezvous in late 2026 December with the binary asteroid (65803) Didymos and in particular its moon, Dimorphos, which will be impacted by NASA’s DART spacecraft on 2022 September 26 as the first asteroid deflection test. The main goals of Hera are the detailed characterization of the physical properties of Didymos and Dimorphos and of the crater made by the DART mission, as well as measurement of the momentum transfer efficiency resulting from DART’s impact. The data from the Hera spacecraft and its two CubeSats will also provide significant insights into asteroid science and the evolutionary history of our solar system. Hera will perform the first rendezvous with a binary asteroid and provide new measurements, such as radar sounding of an asteroid interior, which will allow models in planetary science to be tested. Hera will thus provide a crucial element in the global effort to avert future asteroid impacts at the same time as providing world-leading science