NISAR Radar Reveals “Hummingbird” Fracture Pattern at Nunatak in East Antarctica

A recent radar image from the Earth-observation satellite NISAR reveals a striking fracture pattern around the Zaterjavshijsja nunatak in East Antarctica. The exposed rocky ridge protrudes from a broad ice stream and diverts its motion. In analysis, the web of lines resembles the outline of a hummingbird — internally the area is therefore referred to as "Hummingbird".

What the image shows

Tightly spaced cracks, crevasses and break-offs appear around the nunatak. The structures mark zones where the ice is bottlenecked, sheared or pulled apart as it flows northeast toward the ocean.

Context: What a nunatak does

Nunataks are rock outcrops that rise above glaciers or ice sheets. They disrupt an otherwise uniform flow, creating shear zones and local stresses. This generates areas of compressed ice beside zones of strong extension — preferred sites for wide crevasses and later detachments.

How L-Band radar works

Radar transmits microwaves and measures the backscattered echo. The longer wavelength of the L band penetrates dry snow better than C- or X-band signals and thus provides information about the upper ice layers and surface roughness. Independent of daylight and cloud cover, NISAR can image polar regions year-round and reveal fracture patterns that optical sensors often miss.

Why the pattern forms

When ice flows around a solid obstacle, the stress distribution changes: ice is compressed upstream of the nunatak, shear forces dominate to the sides, and tensile zones open in the wake. Cracks open along such weakness zones and can propagate into the ice stream.

Implications for calving and sea level

Fractures that reach the grounding line or the ice-shelf front increase the likelihood of larger calving events. Loss of ice shelves can accelerate the outflow of upstream glaciers — potentially contributing to sea-level rise. NISAR data help identify vulnerable areas and monitor their evolution.

Monitoring over time

A single image provides a snapshot; repeat observations are decisive. NISAR is designed for repeated measurements. This makes it possible to see how crack systems grow, where new crevasses appear and how quickly parts of the ice detach.

Limits and open questions

Radar images surface and near-surface structures well. Processes at depth — such as basal meltwater or the exact topography beneath the ice — remain only indirectly inferable. Reliable models therefore require additional data like boreholes, seismic surveys or gravity measurements.

Outlook

The NISAR image of the Zaterjavshijsja nunatak adds a building block to understanding Antarctic ice dynamics. Continuous radar observations can better constrain thresholds for fracture and calving and reduce uncertainties in sea-level rise projections.