Snow? fog? Call in A-team for torturing some data? injudicious contrast enhancements to bring out the crack? buried by some recent snowfall?
I could envision blowing old snow, new snow, or the sun not oriented favorably for crack shadowing (eg, along-crack). While reflectance of incident radiation will vary greatly with wavelength, it is most unusual in Greenland for Sentinel radar to provide better resolution of ground features than cloud-free Landsat.
This is not snow-penetrating radar and a 15 m wide crack would not induce noteworthy polarization effects so it's not clear why Sentinel would see any better under the first two scenarios. However Sentinel sweeps a forward-looking oblique view whereas Landsat looks straight down (nadar view). Only the former sees terrain shadowing (crack depression by edge of crack).
I don't have the Sentinel orbital metadata handy but being in polar orbit, it presumably crosses the image at a slight angle to north-south. This would give it some terrain shadowing of the track because it has a goodly east-west component. It's always a good idea to draw a line on in the image showing the orbital track if there has been re-projection.
Meanwhile, metadata on the 18 Sep 15 Landsat above (LC82321132015261LGN00) shows fairly extreme sun elevation and azimuth. As indicated in the second image below, Landsats at EarthExplorer arrive re-projected to a local Mercator patch with north still up for Antarctica.
Sun Elevation 6.4º
Sun Azimuth 57.6º
Such a low sun angle is normally extremely favorable for showing slight variations in landscape elevation but here the odd azimuth may undo that benefit by more or less aligning with the crack, causing it not to be shadowed. Here the operative azimuthal convention has 180º due south. This makes it much harder to tweak contrast usefully while at the same time not introducing artifacts (first image).
The crack is barely visible in minimally enhanced band 8 of this Landsat (equilization). It can be drawn out somewhat by an adaptive contrast enhancement series, and perhaps more with a northeasterly shadowing convolution or embossing/bump map step. Note the azimuthal shadowing direction is quite apparent in the ridging.