| Summary: | Coastal fast ice provides a protective barrier to continental land ice and an important biological habitat. Antarctic fast ice extent has been well quantified using satellite remote sensing but spaceborne measurement of sea ice thickness remains challenging, especially near the coast. Here, we present the first ICESat-2 laser altimeter fast ice freeboard measurements and thickness retrievals in Antarctica. In north Terra Nova Bay, ICESat-2 measured in situ validated fast ice freeboard with high accuracy, detected higher freeboard of thicker Ice Shelf Water-influenced fast ice with a sub-ice platelet layer beneath, and distinguished smooth-thermodynamic and rough-dynamic fast ice compositions. Estimates of sub-ice platelet layer thickness obtained by comparing ICESat-2 ice thicknesses with projected fast ice thicknesses and snow depths agreed with prior electromagnetic induction surveys. ICESat-2 has significant potential for large-scale assessment of fast ice thickness and for identifying regions of glacial meltwater outflow, provided snow depth is constrained.
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