Radar Evidence of Subglacial Liquid Water on Mars

Strong radar echoes from the bottom of the martian southern polar deposits are interpreted as being due to the presence of liquid water under 1.5 km of ice

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 charac- teristics 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 strongly depend on radar sounders to detect evi- dence of an internal ocean on Europa and to understand the habit- ability conditions on Europa, Ganymede, and Callisto. Finally, planetary missions to Earth’s twin, the planet Venus, can potentially take advantage of the use 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

Electromagnetic characterization of a crushed L-chondrite for subsurface radar investigations of solar system bodies

Radar sounders are becoming essential methods for the exploration of the Earth, Planets and the interior of small Solar System bodies. The performance of a radar is strongly related to the electromagnetic parameters of the materials composing the body surface and subsurface. Given the very limited access to planetary soil and rocks, meteorites represent important analogues of asteroids and Jovian icy moon crusts. Electromagnetic properties of meteorites are poorly known as they have seldom been investigated through extensive laboratory measurements as a function of frequency, temperature, and density. The present work builds upon a previous study on a solid chondrite sample, extending the measurements to the complex dielectric permittivity and magnetic permeability of a crushed L5 chondrite in the frequency range typical of the radars proposed for future space missions, such as AIDA, JUICE and EUROPA CLIPPER. The results show that for such dry granular materials the main parameter controlling the electromagnetic properties is the sample bulk density. Moreover, our results highlight the importance to account for magnetic properties in radar signal attenuation estimation for planetary exploration

Coaxial-Cage Line for Geo-materials Electromagnetic Characterization

We present the results obtained with a custom coaxial-cage line built to measure the complex dielectric permittivity and magnetic permeability of granular and/or liquid materials. The coaxial-cage line presents an open structure to facilitate the insertion and to control the compactness of granular materials. The measurements have been performed using a Vector Network Analyzer and the electromagnetic parameters of the samples have been retrieved through the Nicolson-Ross-Weir algorithm (in case of magnetic materials) or the Boughriet algorithm (in case of non-magnetic ones). The cell has been used to characterize the electromagnetic properties of some geo-materials (clay soils and magnetite samples). The electromagnetic parameters are shown as a function of frequency (1MHz-1GHz) and temperature (about 200-298K)

Loss tangent estimation from ground-penetrating radar data using Ricker wavelet centroid-frequency shift analysis

A novel method to estimate the loss tangent using ground-penetrating radar (GPR) data and assuming a Ricker wavelet as the transmitted radar signal is presented. This method proposes a new relation between centroid frequency and loss tangent, which allows retrieval of the loss tangent of a material through a probabilistic inversion approach. The reliability of the results is carefully evaluated by performing an uncertainty analysis that accounts for the white Gaussian noise affecting the radar data. The method is validated in low- and high-loss scenarios, using synthetic data generated by a well-established finite-difference time-domain model (gprMax). The simulations are performed assuming a dipping sharp reflector separating two materials with different electric properties. The loss tangent values retrieved using the centroid-frequency method are compared, for different levels of noise, with those estimated using the maximum frequency method. The results find that the centroid-frequency method is, in general, more precise when the noise level is high, and it is more accurate in a high-loss environment. Finally, the new approach is applied to real GPR data collected with 500 MHz and 1 GHz antennas on a volcanic ash deposit on Mount Etna (Italy), using the signal reflected from a sloping interface separating two distinct volcanic ash layers

A critical analysis on the uncertainty computation in ground-penetrating radar-retrieved dry snow parameters

The use of the ground-penetrating-radar (GPR) technique to estimate snow parameters such as thickness, density, and snow water equivalent (SWE) is particularly promising because it allows for surveying a large area in a relatively short amount of time. However, this application requires an accurate evaluation of the physical parameters retrieved from the radar measurements, which requires estimating each quantity involved in the computation along with its associated uncertainty. Conversely, the uncertainties are rarely reported in GPR snow studies, even if they represent essential information for data comparisons with other techniques such as the snow rod or snow pit methods. Snow parameters can be estimated from radar data as follows: The snow thickness can be computed from two-way traveltime if the snow average wave velocity is known; the snow density can be estimated from wave velocity using an appropriate mixing formula, and SWE can be computed once these two parameters have been calculated. Starting from published data, we have estimated the accuracy achievable by computing the overall uncertainty for each GPR-retrieved snow parameter and evaluated the influence of the different sources of uncertainties. The computation was made for three antenna frequencies (250, 500, and 1000 MHz) and various snow depths (0-5 m). We find that for snow thicknesses of less than 3 m, the main contribution to the uncertainties associated with snow parameters is given by the uncertainty on two-way traveltime estimation, especially for low antenna frequencies. However, for thicker snow depths, other factors such as the uncertainty on the antenna separation affect the overall accuracy and cannot be neglected. Our studies highlight the importance of the uncertaintiy assessment and suggest a rigorous way for their computation in the field of quantitative geophysics

Laboratory and field Ground Penetrating Radar measurements for buried weapons detection

In this work we present the results obtained in two different experiments aimed at evaluating the capability of Ground Penetrating Radar to detect buried metal weapons. The first experiment was conducted acquiring radar data on a layer of synthetic sand in which a metal gun was hidden at shallow depth. The second experiment was conducted in the field, collecting radar data in an area where four different objects were buried (at about 30cm) in sandy volcanic soil. The radar survey was performed for different soil water contents (dry and wet conditions). The results show how the capability to detect the targets and define their depth and shape is strongly dependent on the target depth and orientation with respect to the antenna electric field, as well as to the host material dielectric properties

Dielectric measurements and radar attenuation estimation of ice/basalt sand mixtures as martian Polar Caps analogues

The nature of the materials underlying the superficial deposits of Mars can be inferred, applying an inversion algorithm, from the data acquired by the orbiting HF radars MARSIS and SHARAD. This approach requires the knowledge of the electromagnetic properties of the shallow deposits and an accurate evaluation of the signal attenuation. The present work is focused on the determination of the dielectric parameters of several geo-materials. We performed the measurements of the complex permittivity, in a wide range of temperature (150–250 K) and frequency (20 Hz–1 MHz), on pure water ice, dry basalt sand and ice/basalt mixtures with different sand volume fractions. The data are presented in terms of attenuation as a function of basalt volume fraction, frequency and temperature, and discussed in terms of extrapolation to MARSIS and SHARAD frequency bands. The results show that, besides the expected dependence of the attenuation from temperature, the presence of the solid inclusions in the ice strongly affects the behaviour of the attenuation versus frequency

Thermal and electromagnetic models for radar sounding of the Galilean satellite icy crusts

The icy crusts of the Galilean satellites of Jupiter are supposed to be one of the potentially habitable zones of the solar system, and subsurface water oceans are believed to lie beneath them. The ice penetrating radar sounder RIME (Radar for Icy Moon Exploration) housed on board the JUICE (JUpiter ICy moons Explorer) mission is expected to probe the icy crust of Europa, Ganymede and Callisto up to a depth of about 9 km. The main objectives of RIME are: the characterization of the ice shell of Ganymede; the detection of melt materials inside the Europa's ice shell; the determination of the regolith thickness on Callisto. The penetration of the radar signal is strictly connected to the electromagnetic properties of the ice, that in turn depends on the presence of contaminants and temperature profile inside the satellite's crust. Laboratory measurements carried out in the temperature range of (100-273)K provided the dielectric properties of pure and doped ice (salts, acids and dusts), whereas temperature profiles are obtained taking into account the heat conduction and diapiric convection models for the Galilean satellites. The combination of electromagnetic and thermal properties of the icy crust, allowed us to generate simulated radar data at the operation frequency of RIME (9 MHz). Such simulations are important to determine the radar performance in the icy crusts, and in particular to estimate the signal penetration and the capability to resolve buried layers