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. Perennial firn aquifers are subsurface meltwater reservoirsconsisting of a meters-thick water-saturated firn layer that can form onspatial scales as large as tens of kilometers. They have been observedwithin the percolation facies of glaciated regions experiencing intenseseasonal surface melting and high snow accumulation. Widespread perennialfirn aquifers have been identified within the Greenland Ice Sheet (GrIS) viafield expeditions, airborne ice-penetrating radar surveys, and satellitemicrowave sensors. In contrast, ice slabs are nearly continuous ice layersthat can also form on spatial scales as large as tens of kilometers as aresult of surface and subsurface water-saturated snow and firn layerssequentially refreezing following multiple melting seasons. They have beenobserved within the percolation facies of glaciated regions experiencingintense seasonal surface melting but in areas where snow accumulation is atleast 25 % lower as compared to perennial firn aquifer areas. Widespreadice slabs have recently been identified within the GrIS via fieldexpeditions and airborne ice-penetrating radar surveys, specifically inareas where perennial firn aquifers typically do not form.However, ice slabs have yet to be identified from space.Together, these two ice sheet featuresrepresent distinct, but related, sub-facies within the broader percolationfacies of the GrIS that can be defined primarily by differences in snowaccumulation, which influences the englacial hydrology and thermalcharacteristics of firn layers at depth. Here, for the first time, we useenhanced-resolution vertically polarized L-band brightnesstemperature (TVB)imagery (2015–2019) generated using observations collected over the GrIS byNASA's Soil Moisture Active Passive (SMAP) satellite to map perennial firnaquifer and ice slab areas together as a continuous englacial hydrologicalsystem. We use an empirical algorithm previously developedto map the extent of Greenland's perennial firn aquifers via fittingexponentially decreasing temporal L-band signatures to a set of sigmoidalcurves. This algorithm is recalibrated to also map the extent of ice slabareas using airborne ice-penetrating radar surveys collected by NASA'sOperation IceBridge (OIB) campaigns (2010–2017). Our SMAP-derived maps showthat between 2015 and 2019, perennial firn aquifer areas extended over64 000 km2, and ice slab areas extended over76 000 km2. Combined together, thesesub-facies are the equivalent of 24 % of the percolation facies of theGrIS. As Greenland's climate continues to warm, seasonal surface meltingwill increase in extent, intensity, and duration. Quantifying the possiblerapid expansion of these sub-facies using satellite L-band microwaveradiometry has significant implications for understanding ice-sheet-widevariability in englacial hydrology that may drive meltwater-inducedhydrofracturing and accelerated ice flow as well as high-elevation meltwaterrunoff that can impact the mass balance and stability of the GrIS.

An empirical algorithm to map perennial firn aquifers and ice slabs within the Greenland Ice Sheet using satellite L-band microwave radiometry