A document by Alan R.A. Aitken. Click on the document to view its contents.

With radar sounders, coherent backscattering simulations from global planetary DEMs typically display a deficit in diffuse clutter, which is mainly due to the implicit assumption that roughness at scales below the resolution of the DEM is absent. Indeed, while polynomial approximations of the phase evolution across the facet allow for fast and mathematically rigorous simulators, the coarse resolution of these planetary DEMs leads to a potentially significant portion of the backscattering response being neglected. In this paper, we derive the analytical phase response of a rough rectangular facet characterised by Gaussian roughness and a Gaussian isotropic correlation function under the linear phase approximation. Formulae for the coherent and incoherent power scattered by such an object are obtained for arbitrary bistatic scattering angles. Validation is done both in isolation and after inclusion in different Stratton-Chu simulators. In order to illustrate the different uses of such a formulation, we reproduce two lunar radargrams acquired by the LRS instrument with a Stratton-Chu simulator incorporating the proposed rough facet phase integral, and we show that the original radargrams are significantly better-reproduced than with state-of-the-art methods, at a similar computational cost. We also show how the rough facet integral formulation can be used in isolation to better characterise subglacial water bodies on Earth.

Recent airborne radar-sounding surveys by NASAs Operation IceBridge revealed a large impact crater in the Hiawatha glacier region of northwest Greenland. The radar data used to identify the craters morphology included a 2016 survey carried out with a new ultrawideband radar system that demonstrated unprecedented levels of detail in the radar images. Notably, an unusual flat, specular reflector below the ice-bed interface was identified by visual inspection potentially as a groundwater table. However, this observation, and characterization of overlying material, has yet to be confirmed by a detailed radiometric analysis. This work analyzes four different flight segments with potential subglacial groundwater reflectors to constrain the bed geology and thermal regime, and probability that the sub-bed reflector is indeed a groundwater table. First, we exploit variation in the thickness of the subglacial layer between the ice-bed interface and the proposed ground water table to determine dielectric loss values. The bed material estimated is most likely a mixture of ice, dry sand, and air – with minuscule groundwater present in the layer between the ice-bed interface and the reflector. Lastly, we use the subglacial layer loss values to determine the radar reflectivity difference between the ice-bed interface and sub-bed reflector. The analyses are consistent with the presence of a groundwater table and are useful for providing additional geophysical constraints on the groundwater system beneath Hiawatha Crater. This detection is possible for subglacial settings that consist of a dry/frozen bed around 15 meters thick between the ice and water table in conjunction with a 150 to 520 MHz chirp radar system. Such groundwater sources are a commonly neglected but likely important component of glacier hydrology; they can drive water into till, elevate porewater pressures, reduce shear strength and significantly influence ice sheet dynamics and thus, sea level rise.

There has been possible evidence from ground based telescopes for plumes of water emanating from Europa’s ice shell. Because of their potential connection to the subsurface ocean these plumes are of significant interest as targets for the in-situ instruments on upcoming NASA and ESA missions to the Jovian system. Their potential similarity to tidally-modulated plumes observed at Enceladus have led to speculation that the occurrence of these plumes could be predicted accurately enough to plan in-situ sampling. However, the plumes could be the result of stochastic processes releasing near surface water from the ice shell and may not occur during the missions at all. Better observational constraints on plume distribution, predictability, and character would provide critical information for planning operations and measurements on the upcoming missions to Europa. The aim of this study is to demonstrate capability of the Gemini Planet Imager (GPI) for this task, modeling the detectability of Enceladus-like plumes on the surface of Europa using polarimetry. The polarized signal from an Encleadus sized plume is approximated by Mie scattering from micron-scale ice particulates at 1.5 - 1.8 µm. Ground-based observations with a nearly automated facility such as GPI could allow daily searches for plumes and enable dramatically more informed mission planning, increasing the potential for habitability characterization and even potentially life detection.

We use polarimetric radar sounding to investigate variation in ice crystal orientation fabric within the near-surface (top 40-300 m) of Rutford Ice Stream, West Antarctica. To assess the influence of the fabric on ice flow, we use an analytical model to derive anisotropic enhancements of the flow law from the fabric measurements. In the shallowest ice (40-100 m) the azimuthal fabric orientation is consistent with flow-induced development and correlates with the surface strain field. Notably, toward the ice-stream margins, both the horizontal compression angle and fabric orientation tend toward 45 degrees relative to ice flow. This result is consistent with theoretical predictions of flow-induced fabric under simple shear, but to our knowledge has never been observed. The fabric orientation in deeper ice (100-300 m) is significantly misaligned with shallower ice in some locations, and therefore inconsistent with the local surface strain field. This result represents a new challenge for ice flow models which typically infer basal properties from the surface conditions assuming simplified vertical variation of ice flow. Our technique retrieves azimuthal variations in fabric but is insensitive to vertical variation, and we therefore constrain the fabric and rheology within two end-members: a vertical girdle or a horizontal pole. Our hypotheses are that fabric near the center of the ice-stream tends to a vertical girdle that enhances horizontal compression, and near the ice-stream margins tends to a horizontal pole that enhances lateral shear.