Good points. I've always taken the spirit of that saying to be more like "thick enough to have to deal with, too thin to be enough". On the moon, you don't ha

Good points. I've always taken the spirit of that saying to be more like "thick enough to have to deal with, too thin to be enough". On the moon, you don't have to worry about reentry at all, so no need for a heat shield or any sort of aerodynamic protection, just some way to do a propulsive landing. On earth, you need the heat shield and some aero considerations, but you can land safely with parachutes alone. On mars, you need to deal with reentry but also some sort of propulsive landing system for anything of reasonable size.