Radar For Icy Moon Exploration Research Articles

Subsurface signatures of bright terrain formation models on Ganymede by 3D radar sounder simulations

The Radar for Icy Moon Exploration (RIME) on-board the JUICE mission will look for dielectric and mechanical interfaces below the icy crust of the Galilean moon Ganymede. Previous missions suggest that the surface of Ganymede is covered by two types of terrains, namely the dark terrain and bright terrain. The bright terrain covers two-thirds of the surface, and presents two main morphologies, i.e. the grooved and smooth bright terrains. For understanding the geological history of Ganymede, a still-open question regards the sequence and styles of processes that formed these morphologies. While in the literature different formation hypotheses are highly debated, direct evidence of these can be proven by subsurface probing, for which RIME will offer a unique opportunity. In this work, we aim to understand whether RIME can actually detect and differentiate between the different hypotheses. To this purpose, we perform 3D simulations of RIME radargrams, following a progressive insertion approach which allows to simplify the data interpretability. We focus on the bright terrain morphologies and analyze in detail the RIME detectability of geological components that can support the formation hypotheses of the grooved and smooth bright terrains. The results suggest that RIME can possibly detect the subsurface interfaces associated with the geological components, and differentiate between their radar responses. We also provide a key to interpretation of the RIME data over these targets, and link the radar responses to the formation hypotheses debated in the literature.

Non-linear radar response to the radial structure of Europa plume fallout deposits

Observations of water vapor plumes emanating from Europa's surface provide direct evidence of on-going activity within the ice shell that may have important implications for the potential habitability of Europa's putative global ocean. However, the scarcity of repeated observations suggest that Europa plumes are sporadic and may prove elusive when attempting to sample them directly with the instruments on-board either NASA's Europa Clipper or ESA's JUpiter ICy moons Explorer (JUICE) spacecraft. An alternative approach is then to search for and characterize the remnant plume fallout deposits preserved on Europa's surface. To this end, in this study we investigate the radar response of surficial plume fallout deposits when illuminated by both the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument on-board Europa Clipper as well as the Radar for Icy Moons Exploration (RIME) instrument on-board JUICE. Our results reveal distinct patterns in radial coherent surface reflectivity associated with different families of plume heights. Uniform fallout depositional rates surrounding low (≤25 km) altitude plumes result in a radially-constant coherent surface reflectivity that oscillates as a function of plume duration. In contrast, fallout deposits from high (≥50 km) altitude plumes exhibit radially variable, ripple-like patterns in coherent surface reflectivity as the velocity at which a particle is ejected from the plume is size dependent. The amplitude of the reflectivity oscillations as a function of fallout deposit thickness is directly dependent on porosity, with greater oscillation amplitudes being associated with older fallout deposits that have undergone some degree of densification (as opposed to the young, porous deposits of very recently active plumes). Finally, the results highlight that in the presence of near-surface structure, there is no direct proportionality between observed radar surface reflectivity and near-surface bulk density.

An Approach to the Generation and Analysis of Databases of Simulated Radar Sounder Data for Performance Prediction and Target Interpretation

Radar sounders (RSs) are low-frequency instruments that profile the shallow subsurface of planetary targets to obtain valuable scientific information. The prediction of the RS performance and the interpretation of the target properties from the RS data are challenging due to the complex electromagnetic interaction among many acquisition variables. Simulation of RS data can address this issue by modeling the complex interaction and producing simulated radargrams representing the acquisition scenario. In this article, we present an approach to generate databases of geoelectrical models and simulated radargrams corresponding to combinations of: 1) target geoelectrical hypotheses; 2) RS parameters; and 3) acquisition geometry configurations. The proposed approach exploits this database for: 1) predicting the detection performance and sensitivity of the RS and 2) understanding the interpretability of the underlying hypotheses. In order to identify hypothesis combinations that can be unambiguously inverted from the radargrams, we analyze the similarity between pairs of geoelectrical models and between the simulated radargrams, and the statistical distance between radargram features. The approach is demonstrated for the case of Radar for Icy Moons Exploration (RIME), using three selected targets on the Jovian moon Ganymede, with three different simulation techniques. The results are very promising and reveal the effectiveness of the proposed approach in extracting valuable information regarding: 1) the target detection performance of RIME; 2) the sensitivity to the dielectric contrast; 3) the separability of radargram features; and 4) the identification of hypothesis combinations producing significantly different radar response, and thus invertible.

Martian roughness analogues of Europan terrains for radar sounder investigations

Understanding the implications of Europa's roughness on backscattered radar signals is central to ensure that the future radar sounding instruments heading for the Jovian icy moons, the Radar for Icy Moons Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON), will yield fruitful observations over their Europan targets. In this paper we analyse the roughness of Mars in terms of piecewise monofractal behaviour, and compare the derived Allan profiles to those of different types of Europan terrains. Using the MOLA dataset, we created roughness analogue maps for 12 different types of Europan terrains, and performed additional analysis with High Resolution Imaging Science Experiment (HiRISE) DTM data to complement our study. We found that Aeolis Planum, the flanks of Tharsis Montes, and the chaos terrain south of Olympus Mons are particularly rich in Europan roughness analogues. To put our findings into context, we present optical image comparisons between some Europan terrains and a given Martian roughness analogue, as well as SHARAD and MARSIS tracks passing over some of these areas of interest.

Analysis of Temporal and Structural Characteristics of Jovian Radio Emissions for Passive Radar Sounding of Jupiter’s Icy Moons

Recent studies have proposed that Jovian decametric radiation (DAM) can be effectively exploited for probing the subsurface of Jupiter’s icy moons by using passive radio sounding. However, these studies were based on the assumption that Jovian noise is white and stationary. Therefore, additional investigations into the temporal stability, predictability, and spectral properties of Jovian noise are required to fully assess the potential of passive radar sounding and improve the acquisitions planning. In this article, we investigate these properties of the Jovian DAM to understand their impact on radar sounding performance. This is done by analyzing the recently available radio spectra acquired by the JUNO Waves instrument. Results are also evaluated for the specific case of the Radar for Icy Moon Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) that have been selected for European Space Agency (ESA) and NASA missions to Ganymede and Europa. Our results show that the Jovian DAM is not perfectly white, but no severe distortions in the range response should be expected. The results on spatiotemporal occurrence show that Jupiter’s DAM has a variable probability of occurrence, which is rather sporadic for some frequency ranges. The results on RIME and REASON flybys show that the Jovian DAM occurrence probability is relatively low for selected sub-Jovian flybys at Europa. For the RIME Ganymede orbital phase, a large number of high occurrence passive acquisition opportunities are expected. The experimental results also show that a large bandwidth receiver would enhance the chance of recording Jovian noise.

An Approach Based on Multiobjective Genetic Algorithms to Schedule Observations in Planetary Remote Sensing Missions

Data acquisition in planetary remote sensing missions is influenced by complex environmental resource- and instrument-specific constraints. This impedes to perform observations at any given time during the mission and with any of the instruments composing the scientific payload. This article presents an approach to automatic scheduling of acquisition operations of a remote sensing instrument composing the scientific payload of a mission. The methodology first subdivides the long available observation time intervals into shorter segments and then performs a selection of them, producing an acquisition schedule, optimized with respect to scientific requirements, instrument characteristics, and mission constraints. The scheduling problem is modeled as a multiobjective optimization problem and solved by using Genetic Algorithms (GAs). GAs are able to efficiently explore the solution space by considering different competing objective functions, reaching high-quality solutions. These solutions represent different optimized tradeoffs among the considered instrument-specific quality metrics. The approach is demonstrated on the operations of Radar for Icy Moons Exploration (RIME), a radar sounder onboard Jupiter Icy Moons Explorer. The obtained results show a high potential of the proposed methodology.

An Approach to the Simulation of Radar Sounder Radargrams Based on Geological Analogs

Simulation of radar sounder (RS) data is important for understanding the radar response of subsurface features to facilitate the interpretation of the real data. Conventional electromagnetic simulators require the definition of complex geoelectrical models of the investigated targets. They also involve time-complex solutions of Maxwell’s equations for computing the received electric field, which leads to very high computation time. Furthermore, the simulated radargrams are often not realistic as it is very difficult to model all the variables involved in the data acquisition. In this paper, we propose a novel simulation approach that exploits the data available from existing RSs in geologically analogous terrains, to produce realistic simulations of the investigated RS target. This simulation strategy is based on minimizing the difference between the analog and the investigated acquisition scenarios. This is done by applying a series of corrections, which depend on the relation between the radargram characteristics and the physical variables describing the acquisition process. The aim is to produce radargrams that resemble the investigated scenario in terms of the echo magnitude, bandwidth, range resolution, and along-track resolution. Experimental results present three case studies for different possibilities of the analog and the investigated scenarios. The validation of the simulated radargrams with actual data demonstrates the effectiveness of the proposed approach. Finally, we also present a real application of this approach for the simulation of Radar for Icy Moon Exploration (RIME) radargrams for a combination of instrument and target parameters, using the SHAllow RADar (SHARAD) radargram acquired over the geological analog of a selected RIME target.

Assessing the potential for passive radio sounding of Europa and Ganymede with RIME and REASON

Recent work has raised the potential for Jupiter's decametric radiation to be used as a source for passive radio sounding of its icy moons. Two radar sounding instruments, the Radar for Icy Moon Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) have been selected for ESA and NASA missions to Ganymede and Europa. Here, we revisit the projected performance of the passive sounding concept and assess the potential for its implementation as an additional mode for RIME and REASON. We find that the Signal to Noise Ratio (SNR) of passive sounding can approach or exceed that of active sounding in a noisy sub-Jovian environment, but that active sounding achieves a greater SNR in the presence of quiescent noise and outperforms passive sounding in terms of clutter. We also compare the performance of passive sounding at the 9MHz HF center frequency of RIME and REASON to other frequencies within the Jovian decametric band. We conclude that the addition of a passive sounding mode on RIME or REASON stands to enhance their science return by enabling sub-Jovian HF sounding in the presence of decametric noise, but that there is not a compelling case for implementation at a different frequency.

Radar signal propagation through the ionosphere of Europa

We review the current state of knowledge of the Europan plasma environment, its effects on radio wave propagation, and its impact on the performance and design of future radar sounders for the exploration of Europa׳s ice crust. The Europan ionosphere is produced in two independently-rotating hemispheres by photo-ionization of the neutral exosphere and interaction with the Io plasma torus, respectively. This combination is responsible for temporal and longitudinal ionospheric heterogeneities not well constrained by observations. When Europa׳s ionosphere is active, the maximum cut-off frequency is 1MHz at the surface. The main impacts on radar signal propagation are dispersive phase shift and Faraday rotation, both a function of the total electron content (up to 4×1015m−2) and the Jovian magnetic field strength at Europa (~420nT). The severity of these impacts decrease with increasing center frequency and increase with altitude, latitude, and bandwidth. The 9MHz channels on the Radar for Icy Moons Exploration (RIME) and proposed Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) will be sensitive to the Europan ionosphere. For these or similar radar sounders, the ionospheric signal distortion from dispersive phase shift can be corrected with existing techniques, which would also enable the estimation of the total electron content below the spacecraft. At 9MHz, the Faraday fading is not expected to exceed 6dB under the worst conditions. At lower frequencies, any active or passive radio probing of the ice shell exploration would be limited to frequencies above 1–8MHz (depending on survey configuration) below which Faraday rotation angle would lead to signal fading and detection ambiguity. Radar instruments could be sensitive to neutrals and electrons added in the exosphere from any plume activity if present.

Dielectric properties of Jovian satellite ice analogs for subsurface radar exploration: A review

AbstractThe first European mission dedicated to the exploration of Jupiter and its icy moons (JUpiter ICy moons Explorer—JUICE) will be launched in 2022 and will reach its final destination in 2030. The main goals of this mission are to understand the internal structure of the icy crusts of three Galilean satellites (Europa, Ganymede, and Callisto) and, ultimately, to detect Europa's subsurface ocean, which is believed to be the closest to the surface among those hypothesized to exist on these moons. JUICE will be equipped with the 9 MHz subsurface‐penetrating radar RIME (Radar for Icy Moon Exploration), which is designed to image the ice down to a depth of 9 km. Moreover, a parallel mission to Europa, which will host onboard REASON (Radar for Europa Assessment and Sounding: Ocean to Near‐surface) equipped with 9MHz and 60MHz antennas, has been recently approved by NASA. The success of these experiments strongly relies on the accurate prediction of the radar performance and on the optimal processing and interpretation of radar echoes that, in turn, depend on the dielectric properties of the materials composing the icy satellite crusts. In the present review we report a complete range of potential ice types that may occur on these icy satellites to understand how they may affect the results of the proposed missions. First, we discuss the experimental results on pure and doped water ice in the framework of the Jaccard theory, highlighting the critical aspects in terms of a lack of standard laboratory procedures and inconsistency in data interpretation. We then describe the dielectric behavior of extraterrestrial ice analogs like hydrates and icy mixtures, carbon dioxide ice and ammonia ice. Building on this review, we have selected the most suitable data to compute dielectric attenuation, velocity, vertical resolution, and reflection coefficients for such icy moon environments, with the final goal being to estimate the potential capabilities of the radar missions as a function of the frequency and temperature ranges of interest for the subsurface sounders. We present the different subsurface scenarios and associated radar signal attenuation models that have been proposed so far to simulate the structure of the crust of Europa and discuss the physical and geological nature of various dielectric targets potentially detectable with RIME. Finally, we briefly highlight several unresolved issues that should be addressed, in near future, to improve our capability to produce realistic electromagnetic models of icy moon crusts. The present review is of interest for the geophysical exploration of all solar system bodies, including the Earth, where ice can be present at the surface or at relatively shallow depths.