Essam Heggy

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar

The Philae lander provides a unique opportunity to investigate the internal structure of a comet nucleus, providing information about its formation and evolution in the early solar system. We present Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) measurements of the interior of Comet 67P/Churyumov-Gerasimenko. From the propagation time and form of the signals, the upper part of the “head” of 67P is fairly homogeneous on a spatial scale of tens of meters. CONSERT also reduced the size of the uncertainty of Philae’s final landing site down to approximately 21 by 34 square meters. The average permittivity is about 1.27, suggesting that this region has a volumetric dust/ice ratio of 0.4 to 2.6 and a porosity of 75 to 85%. The dust component may be comparable to that of carbonaceous chondrites.

Subsurface imaging in south-central Egypt using low-frequency radar: Bir Safsaf revisited

We present the capabilities of low-frequency radar systems to sound the subsurface for a site located in south-central Egypt, the Bir Safsaf region. This site was already intensively studied since the SIR-A and SIR-B orbital radars revealed buried paleodrainage channels. Our approach is based on the coupling between two complementary radar techniques: the orbital synthetic aperture radar (SAR) in C and L bands (5.3 and 1.25 GHz) for imaging large-scale subsurface structures, and the ground-penetrating radar (GPR) at 500 and 900 MHz for sounding the soil at a local scale. We show that the total backscattered power computed from L-band SAR and 900-MHz GPR profiles can be correlated, and we combined both data to derive the geological structure of the subsurface. GPR data provide information on the geometry of the buried scatterers and layers, while the analysis of polarimetric SAR data provides information on the distribution of rocks in the sedimentary layers and at the interface between these layers. The analysis of 500-MHz GPR data revealed some deeper structures that should be detected by lower frequency SARs, such as a P-band system.

Ground‐penetrating radar sounding in mafic lava flows: Assessing attenuation and scattering losses in Mars‐analog volcanic terrains

We conducted low-frequency (16 to 100 MHz) ground-penetrating radar surveys on the eroded lava flows at Craters of the Moon (Idaho, USA) volcanic field to evaluate the potential of future radar-sounding investigations on Mars to map shallow subsurface features. Radar-sounding profiles were obtained from three locations: above a lava tube, across a volcanic rift, and over a scoria cone. Results were combined with laboratory permittivity and magnetic permeability measurements of field-collected samples to deconvolve the electromagnetic attenuation and scattering losses from the total losses and therefore separately quantify both effects on the radar penetration depth. Our results demonstrate a constrained performance for low-frequency sounding radars to characterize mafic, arid volcanic terrains that contain a significant amount of ferro-oxides (∼14%), mainly in the form of olivine and magnetite. Penetration depths of 35 m were achieved at a frequency of 100 MHz, and depths of 80 m were achieved at 16 MHz, with an effective dynamic range of 60 dB. Results indicate that for frequencies below 100 MHz, the electromagnetic attenuation dominated the signal losses while above this frequency threshold the volume scattering dominated the losses. Over our frequency range, the observed electromagnetic attenuation and penetration depths were strongly dependent on the magnetic losses, ground porosities, and degree of heterogeneity rather than the sounding frequency. In light of these results, we suggest average attenuation and scattering losses measured in terms of dB/m and discuss the expected penetration depth for the Mars orbital radar-sounding instruments SHARAD and MARSIS in mafic volcanic terrains.

Local geoelectrical models of the Martian subsurface for shallow groundwater detection using sounding radars

Received 15 February 2002; revised 20 July 2002; accepted 28 August 2002; published 6 March 2003. [1] Low-frequency sounding radars should be able to probe the Martian subsurface layers down to varying depths, depending on the geoelectrical properties of the sounded sites. We present in this work four frequency-dependent geoelectrical models of the Martian subsurface in the 1–20 MHz frequency band, based on laboratory electromagnetic characterization of Martian soil analogues. Those models correspond to local Martian sites that we considered to be of particular interest in the search for water using mainly the Ground-Penetrating Radar (GPR) instrument of the Netlander mission. Results and discussion are also valid for both sounding experiments MARSIS and SHARAD. The four models of the Martian subsurface are designed to represent terrains where recent fluviallike features suggest the presence of near-subsurface ground ice and probably liquid water. We performed measurements on volcanic and sedimentary materials that may be present on these sites under the appropriate geophysical conditions that may exist in those terrains. We then simulated the backscattered radar echo arising from each site in the 2 MHz frequency band, using the Finite Difference Time Domain (FDTD) algorithm, in order to evaluate the instrument performances to probe the subsurface stratigraphy of each site. Our results confirm that the near-subsurface rich iron oxide mineralogy controls the instrument performances in terms of penetration depth and signal-to-noise ratio in the 2 MHz frequency band. We finally discuss the geophysical and geoelectrical sounding conditions that could lead to an ambiguous detection of shallow subsurface water on Mars for the Netlander GPR. INDEX TERMS: 3210 Mathematical Geophysics: Modeling; 1794 History of Geophysics: Instruments and techniques; 5144 Physical Properties of Rocks: Wave attenuation; 5109 Physical Properties of Rocks: Magnetic and electrical properties; KEYWORDS: Mars, hydrology, GPR, sounding, simulation, FDTD

Performances of ground penetrating radars in arid volcanic regions: Consequences for Mars subsurface exploration

A GPR field experiment in the Republic of Djibouti provides evidence for very low radar penetration in arid volcanic materials, in the range 100–500 MHz. This phenomenon is attributed to the high iron oxide and evaporite concentration in soils, which significantly increases the conductivity, thus leading to poor subsurface imaging performances. The geologic context in Djibouti is shown to provide a good terrestrial analogue to Mars geology. Results of this study show that the future sounding radar missions to Mars may not reach the penetration depths previously anticipated.

A Passive Probe for Subsurface Oceans and Liquid Water in Jupiter's Icy Moons

Abstract We describe an interferometric reflectometer method for passive detection of subsurface oceans and liquid water in jovian icy moons using Jupiter’s decametric radio emission (DAM). The DAM flux density exceeds 3000 times the galactic background in the neighborhood of the jovian icy moons, providing a signal that could be used for passive radio sounding. An instrument located between the icy moon and Jupiter could sample the DAM emission along with its echoes reflected in the ice layer of the target moon. Cross-correlating the direct emission with the echoes would provide a measurement of the ice shell thickness along with its dielectric properties. The interferometric reflectometer provides a simple solution to sub-jovian radio sounding of ice shells that is complementary to ice penetrating radar measurements better suited to measurements in the anti-jovian hemisphere that shadows Jupiter’s strong decametric emission. The passive nature of this technique also serves as risk reduction in case of radar transmitter failure. The interferometric reflectometer could operate with electrically short antennas, thus extending ice depth measurements to lower frequencies, and potentially providing a deeper view into the ice shells of jovian moons.

Probing structural elements of small buried craters using ground‐penetrating radar in the southwestern Egyptian desert: Implications for Mars shallow sounding

We report results from a field survey performed on a recently discovered impact field in the southwestern Egyptian desert, using a 270 MHz Ground-Penetrating Radar (GPR). This hyperarid region has significant similarities to the Martian heavily eroded mid-latitude cratered terrains in terms of crater density, size, and geomorphology. Profiles across small-buried craters revealed a coherent sequence of tilted layers constituting the cratonic infill resulting from aeolian deposits. In the intercrater areas the radargram revealed a poorly-defined subsurface stratigraphy and the presence of shallow structural elements associated with potential evidences of the consequences of the shock effects, i.e., faulting, fractures, and chaotic bedrock. The radar-penetration depth varied from 2 to 15 m, depending mainly on the amplitude of the volume and multiple scattering in the subsurface, caused by fractures and debris created by the impacts. We conclude that mid-frequency GPR onboard future Martian rovers can successfully perform similar structural mapping.

Permittivity measurements of porous matter in support of investigations of the surface and interior of 67P/Churyumov-Gerasimenko

Permittivity measurements on porous samples of volcanic origin have been performed in the 0.05–190 GHz range under labo- ratory conditions in support of the Rosetta mission to comet 67P/Churyumov–Gerasimenko, specifically with the MIRO radiometric experiment and CONSERT radar experiment. Methods. The samples were split into several subsamples with different size ranges covering a few μm to 500 μm. Bulk densities of the subsamples were estimated to be in the 800 to 1500 kg/m3 range. The porosities were in the range of 48% to 65%. From 50 MHz to 6 GHz and at 190 GHz, permittivity has been determined with a coaxial cell and with a quasi-optical bench, respectively. Results. Without taking into account the volume-scattering effect at 190 GHz, the real part of the permittivity, normalized by the bulk density, is in the range of 2.1 to 2.6. The results suggest that the real part of the permittivity of an ice-free dust mantle covering the nucleus is in the 1.5−2.2 range at 190 GHz. From these values, a lower limit for the absorption length for the millimeter receiver of MIRO has been estimated to be between 0.6 and 2 cm, in agreement with results obtained from MIRO in September 2014. At frequencies of interest for CONSERT experiment, the real part of the permittivity of a suspected ice-free dust mantle should be below 2.2. It may be in the range of 1.2 to 1.7 for the nucleus, in agreement with first CONSERT results, taking into account a mean tem- perature of 110 K and different values for the dust-to-ice volumetric ratio. Estimations of contributions of the different parameters to the permittivity variation may indicate that the porosity is the main parameter.

Mapping exposed and buried lava flows using synthetic aperture and ground-penetrating radar in Craters of the Moon lava field

The Craters of the Moon COM lava field has a multiple eruptivehistory.Burialofolderflowshasresultedincomplex subsurface stratigraphy. For the older eruptive periods, the locations of source vents and the extension of lava flows are eitherspeculativeorunknown,becausetheyareburiedunder more recent pyroclastics. In this study, we used surface and subsurface backscatter characteristics of the P- and L-band polarimetric airborne synthetic aperture radar AIRSAR data and ground-penetrating radar GPR soundings to resolve different exposed and buried lava flows. Our primary objective is to define the most effective polarization and frequency for mapping, resolving, and characterizing different lavatypesinthevolcanicfield.PolarimetricanalysisofAIRSAR images from COM allows a clear recognition of the aa and pahoehoe lava types as a result of the variability in their roughness. Our results suggest that the HV cross-polarized, AIRSAR L-band is capable of producing a detailed map delineating surface lava with different surface backscattering properties. An accuracy assessment utilizing the geological map of the Inferno Cone area was performed to quantify the reliability of differentiating lava types and mapping the lava flows extension below loose pyroclastics using AIRSAR data. Results shows an ability of P-band SAR to map buried structures up to 3 meters deep under loose cinder and ash deposits,resolvingburiedfissures,outcrops,andlavaflowsthat were validated with ground-truth GPR surveys. The techniquesusedinthisstudyprovideatooltoassessvolcanichazardsinremoteandinaccessibleplaces.Alsoitcouldbeanaid in the study of other planets and planetary bodies in the solar system.