Radar Sounding 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.

An overview of GPR subsurface exploration of planets and moons

Geophysical techniques were first tested beyond Earth during the Apollo program. Of those examined, radio-wave propagation methods appeared to be the most suitable for the moon and other solar system bodies. This was due to the electromagnetic characteristics of planetary subsurfaces and the possibility to remotely perform measurements on board spacecrafts and rovers. After the first successful experiment on the moon, more than 20 years passed before ground-penetrating radar (GPR) was included in the payload of a planetary mission. Technological advancements in GPR design and successful results of radio echo sounding measurements for the detection of basal water below terrestrial ice sheets paved the way for the application of similar techniques to search for liquid water in the Martian subsurface. Since deployment of the first two subsurface radar sounders above Mars, the number of proposed planetary missions relying on GPR for surveying the subsurface of planets, moons, and other objects has grown progressively. Six orbiting radar sounders and five GPRs mounted on rovers/landers have been employed so far to explore the moon, Mars, and comet 67P/GC. Some of these are in full operation and some are just starting to operate. Planned missions to the icy moons of Jupiter will also depend heavily on radar sounders to detect evidence of an internal ocean on Europa and to understand the habitability conditions on Europa, Ganymede, and Callisto. Finally, planetary missions to Earth's twin, the planet Venus, could take advantage of GPR to understand the cause of its drastic change in climatic conditions and the geologic phenomena that contributed to changing a watery and hospitable surface into a hot and asphyxiating inhabitable planet.

Using MARSIS signal attenuation to assess the presence of South Polar Layered Deposit subglacial brines

Knowledge of the physical and thermal properties of the South Polar Layer Deposits (SPLD) is key to constrain the source of bright basal reflections at Ultimi Scopuli detected by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) radar sounder. Here we present a detailed analysis of attenuation, based on data acquired by MARSIS at 3, 4, and 5 MHz. We show that attenuation is frequency dependent, and that its behavior is consistent throughout the entire region. This suggests that the SPLD are compositionally homogeneous at Ultimi Scopuli, and our results are consistent with dust contents of 5 to 12%. Using these values as input, and plausible estimates of surface temperature and heat flux, we inferred basal temperatures around 200 K: these are consistent with perchlorate brines within liquid vein networks as the source of the reflections. Furthermore, extrapolation of the attenuation to higher frequencies explains why SHARAD (Shallow Radar) has thus far not detected basal reflections within the SPLD at Ultimi Scopuli.

The Lateral Continuity and Vertical Arrangement of Dust Layers in the Martian North Polar Cap From SHARAD Multiband Data.

Interpretation of radar sounder reflections to infer the structure and composition of the martian polar caps depends on whether bright returns correspond to single packed dust layers or a more finely layered structure. Reflections from multiple layers can create strong resonant scattering (interference) effects that impact analyses of radargram reflectors and inference of dielectric contrast. We identify resonant behavior for an areally extensive reflector in the north polar layered deposits from Shallow Radar data processed in two frequency bands. Echo strength varies by ∼2 dB between subband reflections across a region ∼400 km in extent, with the stronger echo shifting abruptly from the high‐ to low‐frequency band outside the central region of Gemina Lingula. This behavior can arise from resonant scattering between two layers of dust (0.3–0.6 m thick) separated by 0.5–3 m of ice. Such layering requires there be little postdepositional aeolian activity to preserve layer thickness and spacing.

Paths forward in radioglaciology

AbstractIce-penetrating radar sounding is a powerful geophysical tool for studying terrestrial and planetary ice with a rich glaciological heritage reaching back over half a century. Recent years have also seen rapid growth in both the radioglaciological community itself and in the scope and sophistication of its analysis of ice-penetrating radar data. This has been spurred by a combination of growing datasets and improvements in computational resources as well as advances in radar sounding instrumentation and platforms. Together, these developments are transforming the field and highlight exciting paths forward for future innovation and investigation.

Ionospheric effects from rocket launches against the background of geospace storms

Ionospheric effects accompanying launches and maneuvering system thruster firings of large rockets have been studied for about 60 years. Fairly complete and adequate models of generation and propagation of disturbances, which are caused by launches and maneuvering system thruster firings of large rockets in the Earth–atmosphere–ionosphere–magnetosphere (EAIM) system, are absent at present. It turns out a number of physical effects during ionospheric storms and rocket launches are similar. Therefore, the presence of ionospheric storm significantly complicates the search for the ionospheric response to the spacecraft launches. The aim of this study is to describe the results of observation of the ionospheric processes that accompanied rocket launches and maneuvering system thruster firings against the background of ionospheric storms. To analyze the measurements, observational data of the state of the ionosphere before, at the time, and after Soyuz and Proton rocket launches from the Baikonur cosmodrome (the Republic of Kazakhstan) were used. Observations were made at the Radiophysical Observatory of V. N. Karazin Kharkiv National University (near Kharkiv city, Ukraine). The Doppler vertical sounding radar was used for the measurements. Observations were made during solar cycle 24 (2009—2021). The number of the Soyuz rocket launches is 81, and 53 launches of the Proton rocket. Identification of the ionospheric response to the launch and maneuvering system thruster firings of a large rocket 2000 km away from the observation site against the background of a geospace storm by the Doppler method is usually possible at Kpmax   5, and at its larger values is very complicated or even impossible. In a number of cases, even though Kpmax = 4 the determination of the ionospheric response is complicated. To increase the detection reliability of the response to the launch and maneuvering system thruster firings of the rocket, the Doppler radar has to operate on a number of frequencies in the frequency range from 1.5...2 to 4…6 MHz. The existence of several groups of horizontal apparent speeds of disturbance propagation is confirmed: 1.7...3 km and more, 700...1000, 300...700, 150...260 m/s.

Cryolava Dome growth resulting from active eruptions on Jupiter's moon Europa

Europan domes are positive relief features that are typically circular to elliptical in planform shape, and have characteristic diameters <16 km. Although it cannot be ruled out that many of these domes may have been formed from the intrusion of diapirs into Europa's crust, a subset of domes have relatively smooth surfaces that do not mimic the surrounding terrain. These domes appear to obscure the preexisting terrain and have distinct margins which may be lobate or rounded. If all domes on Europa's surface represented structures where the icy crust had simply been “punched up” by diapiric intrusions, uplifts with these distinct morphologies would not be expected to exist. In this study, we revisit the hypothesis that a subset of europan domes formed in a manner similar to lava domes on Earth and Venus. Previously, we modeled dome formation as a consequence of the extrusion of viscous cryolava. However, that approach only allowed for the investigation of late-stage eruptive processes far from the vent and provided little insight into how cryovolcanic fluids may have arrived at the surface. Consideration of cryolava dome emplacement as fluids erupt onto Europa's surface is therefore pertinent. A volume flux approach, in which dome formation is modeled as fluid extrudes onto the surface at a constant rate, has been successfully applied to the formation of lava domes on Venus. That study showed that neglecting to consider changes in fluid rheology while a constant flux of lava is actively extruded onto the surface may result in overestimates, by several orders of magnitude, of initial lava viscosities at the time of eruption. Obtaining accurate viscosity estimates for Europa's cryovolcanic fluids is a critical step in understanding the properties of near-surface fluids that have participated in subsurface-surface exchange in the geologically recent past. To place improved constraints on the rheology and composition of europan cryolavas, and to better gauge the potential for dome formation on Europa via effusive eruptions, we apply this new volume flux approach to the formation of putative europan cryolava domes. We present a perturbation solution to the generalized form of the Boussinesq equation for fluid flow in a cylindrical geometry and explore dome formation while fluid is continuously extruding onto the surface. We find that at the time of eruption, dome-forming cryolavas may have had viscosities of 101–103 Pa s. These viscosity values suggest that cryolavas may be briny slurries composed of a mixture of water, salts, and ice crystals, rather than pure water (viscosity ~10−3 Pa s) or simple brines (viscosities between 10−3 and 10−1 Pa s). Nevertheless, the derived bulk viscosities indicate that dome-forming cryolavas have a rheology more similar to basalt than typical higher-viscosity andesite to rhyolitic dome-forming lavas on Earth. Several of the domes in our study may be connected to liquid reservoirs in Europa's crust, and subsurface-surface exchange may be ongoing today. As such, these domes represent compelling targets for multispectral imaging, radar sounding, and surface sampling by future missions to Europa.

Numerical simulations of radar echoes rule out basal CO2 ice deposits at Ultimi Scopuli, Mars

The principal objective of the radar sounder MARSIS experiment is to look for ice and water in the Martian subsurface. One particular focus of investigations, since 2005, has been the search for basal liquid water in the south polar layered deposits (SPLD). Anomalously strong basal echoes detected from four distinct areas at the base of the deposits at Ultimi Scopuli have been interpreted to indicate the presence of bodies of liquid water in this location, beneath a 1.5 km thick cover of ice and dust. Other explanations for the bright basal reflections have been proposed, however, including the possibility of constructive interference in layered media. Here, we test this mechanism through simulations of MARSIS radar signals propagating in models of CO2-H2O ice sequences. We then compare the results to real MARSIS data acquired over Ultimi Scopuli, finding that no CO2-H2O ice model sequence reproduces the set of real data. The results of our work have implications in relation to the global CO2 inventory of Mars.

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.

Extension of the unique interval of recovery of the initial phase of a signal of a three-channel two-frequency coherent radar

The article reveals the theoretical and practical provisions for ensuring the unambiguity of determining the initial phase of the radar signal, which, unlike the known ones, allows expanding the unambiguity interval of a multi-channel and multi-frequency coherent radar to tens of meters with radar sounding in the centimeter range with frequencies of the order of 10 GHz. Procedures have been developed to ensure the unambiguity of determining the initial phase of the radar signal by determining the phase difference behind different frequencies of the transmitting channels of a three-channel two-frequency station. A mathematical model is obtained for the formation of the phase difference behind different frequencies of the transmitting channels of a three-channel two-frequency station. The mathematical model makes it possible to find the coordinates of the projectile for the phase difference data for different frequencies of the station's transmitting channels. The procedures for restoring the total phase difference behind the known quadrature components of the signal of a three-channel two-frequency station are disclosed. Analytical dependencies for calculating precision values of projectile flight coordinates based on phase difference data for different frequencies of the transmitting channels of a three-channel two-frequency station have been developed and studies of the accuracy of their calculation have been carried out. To assess the accuracy of determining the coordinates of the flight of the projectile, a numerical simulation of the calculation of their absolute error for different values of the projectile flight speed and angle of throw was carried out. As reference values of projectile flight coordinates, the data obtained by solving the mathematical model of the spatial motion of the OF-540Zh projectile of the 152-mm self-propelled howitzer 2S3M were used. It is shown that the potential accuracy of calculating the projectile flight coordinates is based on the data of measuring the quadrature of the phase difference of radar signals of a coherent radar with continuous monochromatic radiation for different values of the flight speed and angle of the projectile is within (10-6/10-10) m.

New Insights Into Subsurface Stratigraphy Northwest of Ascraeus Mons, Mars, Using the SHARAD and MARSIS Radar Sounders

AbstractThe Tharsis Montes volcanoes on Mars are the source of laterally extensive lava flows and other volcanic deposits generating a complex stratigraphy throughout the Tharsis Volcanic Province. We use SHAllow RADar (SHARAD) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) observations in a region northwest of Ascraeus Mons to determine the composition, density, thickness, and spatial distribution of these emplaced volcanic materials. We identified subsurface reflectors along 43 SHARAD and five MARSIS observations. Reflectors in the volcanic plains are interpreted to be sequences of basaltic lava flows with interspersed pyroclastic material, dust, or regolith during a hiatus in activity. Others correspond to the base of three major flow fields. Several plain reflectors were detected by both MARSIS and SHARAD. Other notable reflectors were identified near Ascraeus' flank where lava buried glacially derived sediment. We derived thickness and other material properties for flows using their distinct topographic boundary visible in the radar images. Permittivity ranged from 7.0 to 11.2 corresponding to lava flow densities of 3.20–3.52 g/cm3. Flow thicknesses ranged from 19.8 to 60.2 m. Loss tangents were low for the flow fields ranging from 0.024 to 0.043. Loss tangents in the plains ranged from 0.010 to 0.076. Higher loss tangents correspond to lossier regions that may have higher concentrations of radar absorbing minerals like hematite. Surface roughness controls where reflectors are detected. SHARAD detects the base of three out of the four flow fields in this region with muted surface roughness from dust mantling and erosion.

Radar Characterization of Ice Crystal Orientation Fabric and Anisotropic Viscosity Within an Antarctic Ice Stream

AbstractWe use polarimetric radar sounding to investigate ice crystal orientation fabric and its impact on ice viscosity within the near surface of Rutford Ice Stream, West Antarctica. The technique retrieves lateral and depth variations in the horizontal components of ice fabric but no direct information on the vertical fabric component. In the shallowest ice (depths 40–100 m), the fabric is consistent with flow‐induced development and correlates with the surface compression direction. Notably, toward the ice‐stream margin, the horizontal compression angle and azimuthal fabric orientation tend toward 45° relative to ice flow which is consistent with the early stages of flow‐induced fabric under simple shear. The fabric orientation in deeper ice (depths 100–300 m) is, in places, significantly misaligned with shallower ice and the surface compression direction due to sharp depth transitions in orientation. We then use a rheological model to bound effective anisotropic viscosities (directional hardness) of ice that are consistent with the radar measurements. Toward the shear margin, we show that the shallow‐ice fabric does not appreciably soften the ice to lateral shear although this may happen in deeper ice. In the center of the ice stream, we show that lateral and depth variations in the fabric alignment relative to ice flow result in corresponding changes in uniaxial ice viscosities relative to ice flow. Our results indicate that spatial variability in the fabric translates to variability in viscosity that widely used isotropic ice‐flow models are unable to consider.

Double ridge formation over shallow water sills on Jupiter\u2019s moon Europa

Jupiter’s moon Europa is a prime candidate for extraterrestrial habitability in our solar system. The surface landforms of its ice shell express the subsurface structure, dynamics, and exchange governing this potential. Double ridges are the most common surface feature on Europa and occur across every sector of the moon, but their formation is poorly understood, with current hypotheses providing competing and incomplete mechanisms for the development of their distinct morphology. Here we present the discovery and analysis of a double ridge in Northwest Greenland with the same gravity-scaled geometry as those found on Europa. Using surface elevation and radar sounding data, we show that this double ridge was formed by successive refreezing, pressurization, and fracture of a shallow water sill within the ice sheet. If the same process is responsible for Europa’s double ridges, our results suggest that shallow liquid water is spatially and temporally ubiquitous across Europa’s ice shell.

Search for shallow subsurface structures in Chryse and Acidalia Planitiae on Mars

Chryse and Acidalia Planitiae (CAP) are known as one of the areas which have abundant sites of recurring slope lineae (RSL) on Mars. We present a radar survey of shallow subsurface structures across the CAP regions using the Mars SHAllow RADar sounder (SHARAD) onboard the Mars Reconnaissance Orbiter (MRO). A total of 25 subsurface reflectors were identified. The detected reflectors do not constitute apparent subsurface structures larger than 30 km. Because those have no counterparts in nearby tracks, we could not suggest that those are real subsurface structures. This study suggests that the CAP region does not have special wide-spread subsurface features which could be linked to the RSL sites.

Airborne ultra-wideband radar sounding over the shear margins and along flow lines at the onset region of the Northeast Greenland Ice Stream

Abstract. We present a high-resolution airborne radar data set (EGRIP-NOR-2018) for the onset region of the Northeast Greenland Ice Stream (NEGIS). The radar data were acquired in May 2018 with the Alfred Wegener Institute's multichannel ultra-wideband (UWB) radar mounted on the Polar 6 aircraft. Radar profiles cover an area of ∼24 000 km2 and extend over the well-defined shear margins of the NEGIS. The survey area is centered at the location of the drill site of the East Greenland Ice-Core Project (EastGRIP), and several radar lines intersect at this location. The survey layout was designed to (i) map the stratigraphic signature of the shear margins with radar profiles aligned perpendicular to ice flow, (ii) trace the radar stratigraphy along several flow lines, and (iii) provide spatial coverage of ice thickness and basal properties. While we are able to resolve radar reflections in the deep stratigraphy, we cannot fully resolve the steeply inclined reflections at the tightly folded shear margins in the lower part of the ice column. The NEGIS is causing the most significant discrepancies between numerically modeled and observed ice surface velocities. Given the high likelihood of future climate and ocean warming, this extensive data set of new high-resolution radar data in combination with the EastGRIP ice core will be a key contribution to understand the past and future dynamics of the NEGIS. The EGRIP-NOR-2018 radar data products can be obtained from the PANGAEA data publisher (https://doi.pangaea.de/10.1594/PANGAEA.928569; Franke et al., 2021a).

The Thickness of Talus Deposits in the Periglacial Area of SW Spitsbergen (Fugleberget Mountainside) in the Light of Slope Development Theories

Periglacial slopes are susceptible to recent climate change. The rate of morphogenetic processes depends on numerous factors. The most important of these is the warming of the air and ground, increased precipitation (extreme rainfall in particular), and the rate of snow cover decay. The dynamics of these processes may effectively modify contemporary slope development models. The paper shows the structure of selected talus slopes on a Fugleberget mountainside, based on field observations and radar (GPR) soundings. The results are then compared to classical slope models. The radar survey in April and May 2014 used a RAMAC CU II Malå GeoScience system equipped with a 30 MHz RTA antenna (Rough Terrain Antenna). Five GPR profiles of different lengths were obtained along the talus axes, transversally on Fugleberget and partly on the Hansbreen lateral moraine. According to the radar soundings, the maximum thickness of the debris deposits is 25–30 m. The thickness of the weathered material increases towards the talus cone’s terminal part, and debris deposits overlap marine sediments. The talus slopes’ morphometry shows that their current forms differ from standard slope models, which may be due to the significant acceleration of geomorphic processes resulting from climate change, including rapid warming in the last four decades.

Noise Removal and Feature Extraction in Airborne Radar Sounding Data of Ice Sheets

The airborne ice-penetrating radar (IPR) is an effective method used for ice sheet exploration and is widely applied for detecting the internal structures of ice sheets and for understanding the mechanism of ice flow and the characteristics of the bottom of ice sheets. However, because of the ambient influence and the limitations of the instruments, IPR data are frequently overlaid with noise and interference, which further impedes the extraction of layer features and the interpretation of the physical characteristics of the ice sheet. In this paper, we first applied conventional filtering methods to remove the feature noise and interference in IPR data. Furthermore, machine learning methods were introduced in IPR data processing for noise removal and feature extraction. Inspired by a comparison of the filtering methods and machine learning methods, we propose a fusion method combining both filtering methods and machine-learning-based methods to optimize the feature extraction in IPR data. Field data tests indicated that, under different conditions of IPR data, the application of different methods and strategies can improve the layer feature extraction.

On the Limitations of Using Polarimetric Radar Sounding to Infer the Crystal Orientation Fabric of Ice Masses

AbstractWe introduce a transfer matrix model for radio‐wave propagation through layered anisotropic ice that permits an arbitrary dielectric permittivity tensor in each layer. The model is used to investigate how crystal orientation fabrics without a vertical principal direction affect polarimetric radar returns over glaciers and ice sheets. By expanding the c‐axis orientation distribution in terms of a spherical harmonic series, we find that radar returns from synthetic fabric profiles are relatively insensitive to the harmonic mode responsible for a nonvertical principal direction; however, only for normally incident waves. Consequently, the strength of this mode might be relatively difficult to infer in glaciers and ice sheets, which in turn has implications for the ability to determine the full second‐order structure tensor, needed to infer the local flow regime, flow history, or to represent the directional viscosity structure of glacier ice for ice‐flow modeling.

A Range-Doppler Method for Focusing Radar Sounder Data Generated by Coherent Electromagnetic Simulators

Radar sounders (RS) are gaining importance in planetary missions thanks to their unique capability of providing direct measurements of subsurface structures. To support their design and data interpretation, several electromagnetic (e.m.) simulation techniques have been developed with enhanced capabilities of emulating the RS acquisition process. However, the raw simulated radargrams obtained from e.m. simulators are difficult to interpret and analyze without a focusing operation, which results in an underestimation of the RS detection performance. While frequency methods for range and azimuth compression of real RS data are well-established, their use on simulated data is not addressed in the literature and requires major modifications. This paper presents a novel method that implements azimuth compression using unfocused and focused processing on simulated raw data. The proposed method is based on an adaptation of the Range-Doppler algorithm to the case of raw data generated by a coherent RS simulator. The method is demonstrated in three case-studies to show the similarity between simulated and real data processing: 1) simple geometries; 2) a simulated SHARAD radargram compared with the real data product; and 3) a real application scenario for supporting the design of a new RS instrument. The results indicate higher fidelity of the focused simulated data with the real data product and the target structure, confirming the usefulness of the proposed approach in obtaining a realistic processing of simulated radargrams.

Clutter Detection and Surface/Subsurface Slope Determination by Combination of Repeat-Pass Sounder Orbits Applied to SHARAD Data

Nadir-Looking low frequency radar sounders cannot easily resolve off-nadir surface returns from the subsurface nadir echoes. Cross-track surface echoes (also named ‘clutter’) with time delays synchronized with subsurface returns are renowned for being a major challenge for scientists as they can affect the analysis of orbital radar sounders data. We present a method for clutter discrimination and surface/subsurface slope estimation using data acquired from radar sounders in closely spaced repeated orbits configuration. The method takes advantage of cross-track signal migration to discriminate off-nadir clutter from subsurface signal returns received at the nadir. The migration of the off-nadir signals is also used to determine the clutter Direction-of-Arrival (DOA) as well as the surface/subsurface cross-track slopes. The effectiveness of the method has been proven on the MRO’s Shallow Radar (SHARAD) dataset and provides a proof-of-concept demonstration for the surface clutter discrimination when radar sounders repeated-passes data are available.