Just like a recreational vehicle, they dump the toilet before setting off for deep space. Just so happens that they crap aluminum.

I know this because my tin-foil hat fell off one night and I found myself abducted and forced to have dinner with them. They served up what I am sure was a lovely dish on thier planet, consisting of bauxite ore with a side of alumina fresh out of thier Hall-Heroult smelting oven.

I tried to feed mine to thier...ah...er....dog but even he wouldn't eat it.

Case closed.