Leonardo Carrer

Intense volcanism has played a significant role in shaping Venus's surface and geology. The existence of lava tubes (i.e., pyroducts) on Venus has been largely hypothesized but never confirmed. Being a subsurface structure, the presence of a lava tube can be revealed by a localized collapse of the roof denoted as skylight. Between 1990 and 1992, the Synthetic Aperture Radar (SAR) instrument on board the Magellan spacecraft mapped the Venusian surface. By leveraging a SAR imaging technique developed for detecting and characterizing accessible subsurface conduits in the proximity of skylights, we analysed the Magellan radar images in locations where there is evidence of localized surface collapses. Our analyses reveal the existence of a large and open subsurface conduit in the Nyx Mons region. This feature is hypothesized to be a pyroduct, characterized by a diameter of about 1 km, a roof thickness of at least 150 m and an empty void height of no less than 375 m. The conduit extends in the subsurface for at least 300 meters from the skylight.

The comprehension of pit craters on Venus’ surface is still limited both in terms of genetic process and geometric characteristics. Pit craters are valuable features for planetary scientists and geologists as they offer a window into a planetary body’s history, geological processes, and environmental conditions. In this context, morphometry is a valuable tool for unraveling the geological history of these structures and for comparative investigations with analogous features occurring on other celestial bodies. In this study, we present a methodology utilizing orbital Synthetic Aperture Radar (SAR) data to estimate the characteristics of Venusian pits. Our findings demonstrate that it is feasible to estimate the depth of a given pit crater by analyzing reflections from the pit’s wall facing the radar illumination direction. This method offers an additional estimate of the pit’s depth, complementing literature techniques based on radar shadows, which may not always be available depending on the acquisition geometry. Depending on the configuration of the pit’s interior structure, the depth estimation derived from the pit wall’s reflections is potentially less prone to geometric distortions than the one obtained from the radar shadow. The morphometric analysis of Venusian pits, based on the radar-derived parameters from Magellan SAR data, reveals that these pits exhibit very large collapse volumes and remarkable morphological similarities with lunar pit craters. This discovery has implications for comparative planetology, shedding light on potential shared mechanisms and processes governing the formation and evolution of such features.

Pits are depressions in the ground that occur due to the collapse of the surface layer. The characterization from orbit of their internal structure using optical images is challenging due to uncontrolled illumination geometry. In this paper, we propose a methodology for the characterization of pits’ interiors by exploiting Synthetic Aperture Radar (SAR) images. The methodology analyzes the amplitude and range of the radar echoes originating from the pit’s interior for determining its geometric characteristics through data inversion. The experimental results demonstrate that a set of bright reflections in the radar image can be attributed to the response of pits’ vertical walls and floor. By applying the proposed methodology and interpreting the radar reflections, we are able to derive a geometric characterization (e.g., depth) of a given pit. The retrieved geometric parameters from SAR data of a terrestrial pit denoted as Well of Barhout align well with the ground truth. The findings of this study have implications for both Earth observation and planetary exploration.

Lava tubes are terrestrial tunnel-like natural subsurface caves. Mounting evidence suggests their presence on the Moon and Mars. Planetary radar sounders are nadir-looking instruments operating in the HF/VHF part of the spectrum with subsurface penetration capabilities. Recently, several studies either proposed future mission concepts for lava tubes detection or attempted to locate them on the Moon and Mars with the available radar sounding data. Lava tubes are typically modelled as quasi-cylindrical structures but their actual geometry and its influence on the radar backscattering in near-nadir regime has never been investigated in the literature. These are crucial information for understanding the feasibility of detecting lava tubes by current and future planetary radar sounding systems. Accordingly, in this paper (i) we assess whether lava tubes are self-affine fractal surfaces at horizontal scales relevant to radio and microwave scattering and (ii) we evaluate the effect of lava tube topography on the radar backscattering response in near-nadir regime. Our experimental results, which are inferred from 3D terrestrial laser scanning data of planetary lava tube analogs, show that lava tubes (i) are self-affine fractals at horizontal scales relevant to radar sounding and (ii) they are electromagnetically rough surfaces especially in VHF band. We provide quantitative values on the lava tube fractal parameters and radar roughness losses along with a discussion on both (a) the implication of our results on current radar sounding systems’ ability to detect lava tubes and (b) the planning of future missions devoted to lava tube detection and characterization.

Caves are one of the last frontiers of human exploration on Earth. They are very relevant scientific targets as they host significant biodiversity and unique geologic formations. The presence of underground passages accessible for human or robotic exploration are revealed by localized collapse of the near-surface ceiling of a cave system (skylight). Remote sensing systems are a valuable tool for skylights detection as these features are often located on very remote and often inaccessible regions of the Earth. However, with the available remote sensing techniques and data analysis methodologies it is very difficult to determine whether a skylight is providing access to a cave continuation or it represents only a closed depression with no extensions. In this paper we propose a methodology, based on very high resolution (VHR) orbital synthetic aperture radar (SAR) imaging systems, to estimate both caves geometric characteristics and accessibility information in the proximity of a skylight. To test our methodology, we acquired radar data over different Earth’s location by exploiting the Capella Space X-band microsatellite radar constellation. The experimental results show that our methodology effectively determines the caves geometric characteristics and accessibility under a variety of surface conditions. We also detected several unknown and unexplored large cave systems located near Volcan Wolf and Ecuador, Isla Isabela, Galapagos. The presented work has relevant implications for the field of geological studies, ecology and space exploration research since optical imaging shows the evidence of potential cave systems accessible from skylights on other planetary bodies such as Mars.

Spaceborne radar sounders are high frequency (HF)/very high frequency (VHF) nadir-looking sensors devoted to subsurface investigations. Their data interpretation can be severely hindered by off-nadir surface clutter. Recent literature showed that the clutter suppression capabilities of this class of systems can be greatly enhanced by deploying an array of orbiting sensors in formation flight synthesizing a narrow radar antenna beam. In this article, we assess the capability of distributed radar sounding to discriminate clutter from subsurface returns by exploiting direction of arrival (DOA) estimation techniques. This is achieved by first outlining an approach for designing and evaluating the distributed radar sounder DOA estimation performance as function of the radar system parameters (e.g., intersensor distance) and external noise factors such as ionospheric scintillations. Then, the theory is complemented by radar simulations of several acquisitions over Greenland assuming a variety of subsurface geometries. The simulations confirm that clutter discrimination through DOA estimation is a viable approach to further improve the array capability in disambiguation of subsurface echoes from surface ones.

Several studies are in progress for proposing an Earth-orbiting radar sounder (EORS) mission. Some of them consider as baseline system architecture a recently proposed distributed radar sounding array in formation flight with enhanced capabilities of clutter suppression. Besides clutter, the detectability of subsurface targets may also be affected by the propagation of the radar signal through Earth’s ionosphere. These effects include frequency-dependent phase dispersion and scintillations. In this letter, we present a subsurface detection performance assessment of an EORS with distributed architecture focusing on the ionospheric effects. The novel contributions of this work are: 1) simulation of the coherent radar response of a representative polar ice target (englacial layering) in the distributed radar sounding case; 2) inclusion of spatially dependent ionospheric scintillation effects on the distributed beam pattern; 3) inclusion of phase dispersion effects for different values of total electron content (TEC); and 4) analysis of the subsurface detection performance. Detectability analysis is performed after applying a state-of-the-art technique for compensating ionospheric phase-dispersion effects. The results show that the englacial layering is detectable by compensating for the dispersion effects in the range between 1 and 21 TECU in the ionosphere. The layering is also detectable at higher values of TEC by improving the accuracy of TEC estimation. Moreover, even without compensation, the worst case ionospheric phase scintillations of 25° produce a negligible effect on the detectability.

Research based on Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) data detected unusual radar bright basal reflections located at about 1.5 km depth in a Mars region denoted as Ultimi Scopuli. These reflections were interpreted as a signature of subglacial liquid water even though this interpretation is still being debated in the literature. In this article, we propose a novel approach to the detection and imaging of candidate subglacial liquid water from radar sounding data. The approach combines the radar echo power traces with a suitable digital elevation model to provide a bidimensional representation of the surface. Even if the imaging method reconstructs a representation of the surface, we prove that it can be used to identify subsurface bright reflections in icy regions. Imaging is feasible even if the basal interface is not directly included in the processed data for image generation. To support this experimental evidence, we show that a relationship exists between the value of the reflected echo power originating from the englacial layers and the basal-to-surface-echo-power ratio. The observed relationship holds on both Ultimi Scopuli radar sounding data acquired on Mars and Lake Vostok data acquired on Earth. Our results show that the 2-D imaging provides an alternative way for locating candidate subglacial liquid water bodies on Mars over large areas also where the basal interface is not directly measured. The proposed approach complements previous research for further evaluation of the actual presence of liquid water on Mars.

Passively sounding icy and rocky bodies in our solar system provides a way to observe the surface and subsurface of these objects without the need for costly transmitters. Jupiter's decametric radiation provides a suitable source of radio frequency signals for sounding on geological scales of interest, but its spectral structure can introduce undesired artifacts. Recent studies have shown that Jovian bursts could be effectively applied as a source of passive sounding. However, it has been noted that the spectral properties of Jovian bursts, as measured with JUNO data, are not perfectly modeled as white noise, which can result in degraded sensitivity and vertical resolution for passive sounding. In response to this finding, we present conditioning processes that improve the echo detectability and sounding resolution for Jovian burst-like signals. More than 18 h of Jovian burst recordings are used to simulate conditioning of the natural spectral variation, demonstrating that high-quality corrections are possible with these processes for noise conditions in both the Jovian and Earth/Moon systems.