Greenland's bed topography is a primary control on ice flow, grounding line migration, calving dynamics, and subglacial drainage. Moreover, fjord bathymetry regulates the penetration of warm Atlantic water (AW) that rapidly melts and undercuts Greenland's marine‐terminating glaciers. Here we present a new compilation of Greenland bed topography that assimilates seafloor bathymetry and ice thickness data through a mass conservation approach. A new 150 m horizontal resolution bed topography/bathymetric map of Greenland is constructed with seamless transitions at the ice/ocean interface, yielding major improvements over previous data sets, particularly in the marine‐terminating sectors of northwest and southeast Greenland. Our map reveals that the total sea level potential of the Greenland ice sheet is 7.42 ± 0.05 m, which is 7 cm greater than previous estimates. Furthermore, it explains recent calving front response of numerous outlet glaciers and reveals new pathways by which AW can access glaciers with marine‐based basins, thereby highlighting sectors of Greenland that are most vulnerable to future oceanic forcing.

Dielectric multilayer reflectors that are non-polarizing are an important class of optical device and have numerous applications within optical fibres¹, dielectric waveguides² and light-emitting diodes³. Here, we report analyses of a biological non-polarizing optical mechanism found in the broadband guanine-cytoplasm ‘silver’ multilayer reflectors of three species of fish. Present in the fish stratum argenteum are two populations of birefringent guanine crystal, with their optical axes either parallel to the long axis of the crystal or perpendicular to the plane of the crystal, respectively. This arrangement neutralizes the polarization of reflection as a result of the different interfacial Brewster's angles of each population. The fish reflective mechanism is distinct from existing non-polarizing mirror designs^4,5,6,7 in that, importantly, there is no refractive index contrast between the low-index layers in the reflector and the external environment. This mechanism could be readily manufactured and exploited in synthetic optical devices.

Curie depths beneath Greenland are revealed by spectral analysis of data from the World Digital Magnetic Anomaly Map 2. A thermal model of the lithosphere then provides a corresponding geothermal heat flux map. This new map exhibits significantly higher frequency but lower amplitude variation than earlier heat flux maps and provides an important boundary condition for numerical ice‐sheet models and interpretation of borehole temperature profiles. In addition, it reveals new geologically significant features. Notably, we identify a prominent quasi‐linear elevated geothermal heat flux anomaly running northwest–southeast across Greenland. We interpret this feature to be the relic of the passage of the Iceland hotspot from 80 to 50 Ma. The expected partial melting of the lithosphere and magmatic underplating or intrusion into the lower crust is compatible with models of observed satellite gravity data and recent seismic observations. Our geological interpretation has potentially significant implications for the geodynamic evolution of Greenland.

Subglacial roughness can be determined at a variety of length scales from radio-echo sounding (RES) data either via statistical analysis of topography or inferred from basal radar scattering. Past studies have demonstrated that subglacial terrain exhibits self-affine (power law) roughness scaling behaviour, but existing radar scattering models do not take this into account. Here, using RES data from northern Greenland, we introduce a self-affine statistical framework that enables a consistent integration of topographic-scale roughness with the electromagnetic theory of radar scattering. We demonstrate that the degree of radar scattering, quantified using the waveform abruptness (pulse peakiness), is topographically controlled by the Hurst (roughness power law) exponent. Notably, specular bed reflections are associated with a lower Hurst exponent, with diffuse scattering associated with a higher Hurst exponent. Abrupt waveforms (specular reflections) have previously been used as a RES diagnostic for basal water, and to test this assumption we compare our radar scattering map with a recent prediction for the basal thermal state. We demonstrate that the majority of thawed regions (above pressure melting point) exhibit a diffuse scattering signature, which is in contradiction to the prior approach. Self-affine statistics provide a generalised model for subglacial terrain and can improve our understanding of the relationship between basal properties and ice-sheet dynamics.

We describe the bi-directed eyes of a mesopelagic teleost fish, Rhynchohyalus natalensis , that possesses an extensive lateral diverticulum to each tubular eye. Each diverticulum contains a mirror that focuses light from the ventro-lateral visual field. This species can thereby visualize both downwelling sunlight and bioluminescence over a wide field of view. Modelling shows that the mirror is very likely to be capable of producing a bright, well focused image. After Dolichopteryx longipes , this is only the second description of an eye in a vertebrate having both reflective and refractive optics. Although superficially similar, the optics of the diverticular eyes of these two species of fish differ in some important respects. Firstly, the reflective crystals in the D. longipes mirror are derived from a tapetum within the retinal pigment epithelium, whereas in R. natalensis they develop from the choroidal argentea. Secondly, in D. longipes the angle of the reflective crystals varies depending on their position within the mirror, forming a Fresnel-type reflector, but in R. natalensis the crystals are orientated almost parallel to the mirror's surface and image formation is dependent on the gross morphology of the diverticular mirror. Two remarkably different developmental solutions have thus evolved in these two closely related species of opisthoproctid teleosts to extend the restricted visual field of a tubular eye and provide a well-focused image with reflective optics.

We use polarimetric radar sounding to investigate ice crystal orientation fabric and its impact on ice viscosity within the near surface of Rutford Ice Stream, West Antarctica. The technique retrieves lateral and depth variations in the horizontal components of ice fabric but no direct information on the vertical fabric component. In the shallowest ice (depths 40–100 m), the fabric is consistent with flow‐induced development and correlates with the surface compression direction. Notably, toward the ice‐stream margin, the horizontal compression angle and azimuthal fabric orientation tend toward 45° relative to ice flow which is consistent with the early stages of flow‐induced fabric under simple shear. The fabric orientation in deeper ice (depths 100–300 m) is, in places, significantly misaligned with shallower ice and the surface compression direction due to sharp depth transitions in orientation. We then use a rheological model to bound effective anisotropic viscosities (directional hardness) of ice that are consistent with the radar measurements. Toward the shear margin, we show that the shallow‐ice fabric does not appreciably soften the ice to lateral shear although this may happen in deeper ice. In the center of the ice stream, we show that lateral and depth variations in the fabric alignment relative to ice flow result in corresponding changes in uniaxial ice viscosities relative to ice flow. Our results indicate that spatial variability in the fabric translates to variability in viscosity that widely used isotropic ice‐flow models are unable to consider.