The phrase "surface layer" refers to the outermost part of something, typically the topmost or superficial layer. It is the visible or accessible part that exists above or on the surface of an object or organism. Full definition
Now let's consider the thin surface layer of the oceans. (judithcurry.com)
You need to dig deep and get past surface layer interests. (inc.com)
The distribution of the flow types varies with latitude — and the researchers think this means different surface layers of the asteroid contain different amounts of ice. (newsweek.com)