Cyril Grima

Surface slope control on firn density at Thwaites Glacier, West Antarctica: Results from airborne radar sounding

We derive the surface density variations over Thwaites Glacier from a statistical analysis of airborne High-Capability Radar Sounder surface returns acquired in the 2004–2005 summer. We produce a 5 km gridded map with an estimated ±12.5 kg m−3 accuracy. The background pattern of densities decreases inland from ~ 480 kg m−3 to ~ 350 kg m−3. A remarkable ~ 30 km wide vein-shaped anomaly of up to 570 kg m−3 is located ~ 100 km from the coastline. Density anomalies correspond with surface slope breaks but are not necessarily coincident with smaller slopes. They could result from complex wind-driven snow redistribution and/or refreezing of former snowmelt. This inversion technique can significantly improve surface mass balance calculations to understanding of glacier dynamics at regional scales and is valuable to verify and improve Antarctic climate models. It is also a promising approach for future surface analysis of icy moons by planetary radars.

Estimating Subglacial Water Geometry Using Radar Bed Echo Specularity: Application to Thwaites Glacier, West Antarctica

Airborne radar sounding is an established tool for observing the bed conditions and subglacial hydrology of ice sheets and glaciers. The specularity content of radar bed echoes has also been used to detect the hydrologic transition of a subglacial water system from a network of distributed canals to a network of concentrated channels beneath the Thwaites Glacier. However, the physical dimensions of the distributed water bodies in these networks have not been constrained by observations. In this letter, we use a variety of simple radar scattering, attenuation, and cross-sectional models to provide a first estimate of the subglacial water body geometries capable of producing the observed anisotropic specularity of the Thwaites Glacier catchment. This approach leads to estimates of ice/water interface root mean square roughnesses less than about 15 cm, thicknesses of more than about 5 cm, lengths of more than about 15 m, and widths between about 0.5 and 5 m.

Discovery of a hypersaline subglacial lake complex beneath Devon Ice Cap, Canadian Arctic

A possible analog for saline lakes on planetary ice bodies lurks beneath a Canadian Arctic ice cap. Subglacial lakes are unique environments that, despite the extreme dark and cold conditions, have been shown to host microbial life. Many subglacial lakes have been discovered beneath the ice sheets of Antarctica and Greenland, but no spatially isolated water body has been documented as hypersaline. We use radio-echo sounding measurements to identify two subglacial lakes situated in bedrock troughs near the ice divide of Devon Ice Cap, Canadian Arctic. Modeled basal ice temperatures in the lake area are no higher than −10.5°C, suggesting that these lakes consist of hypersaline water. This implication of hypersalinity is in agreement with the surrounding geology, which indicates that the subglacial lakes are situated within an evaporite-rich sediment unit containing a bedded salt sequence, which likely act as the solute source for the brine. Our results reveal the first evidence for subglacial lakes in the Canadian Arctic and the first hypersaline subglacial lakes reported to date. We conclude that these previously unknown hypersaline subglacial lakes may represent significant and largely isolated microbial habitats, and are compelling analogs for potential ice-covered brine lakes and lenses on planetary bodies across the solar system.

Radar detection of the brine extent at McMurdo Ice Shelf, Antarctica, and its control by snow accumulation

We derive the surface density and brine infiltration depth/extent at McMurdo Ice Shelf, Antarctica, from combined analysis of radar profiles and radar statistical reconnaissance of the surface from 2011-2012 austral summer airborne observations. Most of the brine boundaries appear controlled, directly or indirectly, by the snow accumulation pattern. The infiltration is bounded westward by an ablation area and resides just above the pore close-off depth over most of its extent. The eastern brine limit matches a light-snow corridor, suggesting a reversed pressure gradient at depth that might sharply slow down the infiltration. Brine into ice is confirmed at the deepest locations north and east of Williams Field. The ice-ocean interface is undetected west of the infiltrated zone, except in localized patches. We hypothesize this echo-free zone to be due to high scattering below the surface, possibly from a network of accreted ice and/or ice platelets at the ice-ocean interface.

Clutter detection using two-channel radar sounder data

Surface clutter can corrupt both Earth and planetary Radar Sounder (RS) observations preventing definitive interpretation of subsurface features, which are often the primary interest of geologists and planetary scientists. Clutter is usually detected by manual or automatic techniques that require ancillary information about the topography of the surface. However, this topography information is not always available. In this paper, we propose a novel method for clutter detection that is independent from ancillary information. This method uses a two channel RS system to exploit the cross-channel interferometric phase difference and is made up three main steps: i) feature extraction and theoretical phase difference estimation, ii) RS interferogram formation and iii) comparison of theoretical and real phase difference distributions. The proposed method has been validated on RS data acquired in Greenland.

Comments on “An Inversion of Planetary Rough Surface Permittivity From Radar Sounder Observations”

Ye and Jin presented a method to derive the soil dielectric permittivity from radar surface echoes when a Digital Elevation Model of the observed region is known. However, this method has been already extensively applied on Mars, and the scientific protocol to demonstrate its efficiency only derives the numerical uncertainties inherent to the simulation tool used by the authors.

An Interferometric Approach to Cross-Track Clutter Detection in Two-Channel VHF Radar Sounders

Surface cross-track clutter can corrupt both earth and planetary radar sounder (RS) observations preventing definitive interpretation of subsurface features, which are often of primary interest to geologists and planetary scientists. This clutter is usually identified either by manual or automatic techniques that require ancillary information about the topography of the surface, or by using multichannel RS systems with arrays of antennas. However, topographic information is not always available and multichannel systems are generally too massive and costly to mount on satellites for the planetary exploration. In this paper, we propose a novel approach to clutter discrimination that is independent of ancillary information and limits the hardware complexity of the RS system. This approach uses a two-channel RS and exploits cross-channel interferometric phase differences to discriminate the clutter. Our approach includes three main steps: 1) manual feature extraction and theoretical phase-difference estimation; 2) RS interferogram formation; and 3) comparison of theoretical and real phase difference distributions. The proposed method was validated on RS data acquired in Greenland and provides a proof of concept for the surface clutter discrimination using RS data.

Characterizing near-surface firn using the scattered signal component of the glacier surface return from airborne radio-echo sounding

This work was supported by grants from UK NERC (NE/K004999), NASA (13-ICEE13-00018), NSERC (Discovery Grant/Northern Research Supplement), Alberta Innovates Technology Futures, the CRYSYS Program (Environment Canada), and a University of Alberta Northern Research Award.