In the ever-evolving field of glaciology, the study of ice sheet fabrics and flow is undergoing a quiet revolution. The retreat of glaciers and ice sheets, a stark reminder of our changing climate, has long been a concern for scientists and communities worldwide. The Thwaites Glacier in Antarctica, for instance, has captured the attention of researchers due to its potential to cause significant sea level rise. But what makes this topic particularly fascinating is the intricate interplay between the fabric of the ice and its flow, a relationship that is now being unraveled with the help of cutting-edge radar technology. As an expert in this field, I find myself captivated by the emerging insights and the potential implications for our understanding of Earth's ice sheets and beyond.
The Fabric of Ice: A Memory and Modulator
The fabric of ice, the orientation of its crystals, is a key player in the story of ice flow. As ice deforms and flows, its crystals reorient, creating a memory of past flow patterns. This fabric not only influences the flow but is also influenced by it, creating a dynamic and complex system. The potential importance of fabric on large-scale ice flow has long been recognized, but the lack of observations made it difficult to quantify its effect. Over the past 20 years, radar polarimetry has emerged as a game-changer, offering a quicker and easier way to infer fabric, enabling observations at the scale of entire glaciers.
Radar: Unveiling the Fabric's Secrets
Ice-penetrating radar instruments emit electromagnetic waves that reflect off interfaces within and beneath glacial ice, including transitions in ice chemistry and the contact surface between the ice sheet and the ground or water below. The properties of the reflected waves are then measured when they return to the radar. Fabric introduces directional dependence in these measured electrical properties, with waves traveling at slightly different speeds depending on their polarization. This difference, though small, can compound and cause measurable changes in returned radar signals.
Fabric's Impact on Flow: A Complex Relationship
The effect of fabric on radar signal travel times accumulates through an ice column, making it more prominent in thicker ice with stronger horizontal fabric. In such cases, differences in travel times between polarizations can be measured even by standard radars. When fabric is weaker or ice is thinner, the offset is smaller and detectable only by systems that can identify the phases of radar returns. Small differences in fabric can also change the strength of returned signals, offering an independent way to identify fabric orientation and its depth variation.
Radar Studies: Validation and New Insights
The growing number of radar studies conducted near sites where ice cores have been collected has provided validation and bolstered confidence in the accuracy of fabric inference from radar. Researchers are now inferring fabric from radar in more dynamic areas, such as Thwaites Glacier, Whillans Ice Stream, and the Northeast Greenland Ice Stream (NEGIS), where ice fabrics change over short spatial scales and drilling ice cores is logistically difficult. Airborne radar surveys are particularly effective in these settings, efficiently mapping fabric variations across large, fast-moving areas.
Fabric's Role in Ice Viscosity
Observations of strong fabrics in fast-flowing regions suggest that fabric is an important control on ice viscosity. For example, at Rutford Ice Stream in Antarctica, ApRES data indicate that fabric causes sharp changes in viscosity in different directions with depth, a complexity not captured by current ice flow models. The fabric of the NEGIS varies substantially across the ice stream, facilitating horizontal shear that allows faster and more cohesive flow in the middle of the ice stream while simultaneously stiffening this ice against along-flow stretching.
Beyond Fabric: Other Anisotropic Properties
Most polarimetric radar studies have focused on fabric, but other ice characteristics can also cause directional effects. Bubbles trapped in ice, for instance, can affect radar waves differently in different directions. Ice at its melting point can also contain liquid water along boundaries between crystals, affecting radar returns. These properties have important influences on ice flow, but their implications are yet to be fully explored.
Expanding Horizons: Large-Scale and Planetary Applications
As polarimetric techniques mature, their applications are expanding. Researchers are moving from studying isolated profiles of ice fabric to mapping it across whole basins, a key shift for validating bespoke models of fabric and its effects on flow. These models are also rapidly developing to include additional physical processes and key simplifications that allow them to interface more easily with large-scale models used for projecting sea level rise.
The Future of Ice Sheet Mapping
The techniques pioneered for measuring ice on Earth may also prove useful elsewhere in the solar system. Orbital radar sounders have already probed Mars' ice masses, and the icy shell of Jupiter's moon Europa will soon be surveyed by single-polarization radars aboard NASA's Europa Clipper and the European Space Agency's Jupiter Icy Moons Explorer (JUICE). These radars might be useful for polarimetry at some locations on Europa, revealing past and present motion of ice features and answering fundamental questions about the moon.
Conclusion: A Transformative Technique
As polarimetric radar systems become routine tools for glaciologists and similar instruments begin operating on spacecraft exploring icy worlds, a technique once limited to a few isolated core sites on Earth could be poised to transform our understanding of ice across the solar system. This is not just a technical advancement; it is a profound shift in our ability to understand and predict the behavior of ice sheets, with far-reaching implications for climate science and beyond. In my opinion, this is a truly exciting time for glaciology, and I am eager to see what new insights and discoveries await us.
