As the title asks, what is the best name for the expression made with the mouth when sad?
To expand, frowning involves furrowing the brow, crying involves... crying. You can look sullen, you can pout or wince but these all convey specific emotions relating to the broader emotion of sadness.
Smiling on the other hand conveys happiness, and the multiple 'sub-emotions' (excitement, contentment, etc) to happiness.
So is there a word that conveys the upturned smile, without being specific to one type of sadness?
EDIT: k1eran suggested adding from my comments that Thesaurus.com offers the antonyms frown and glower, both relating to specific angry emotions. Other sites list, again, either expressions relating to specific versions of sadness, expressions which aren't the opposite of smiles (eg. pursed lips) or just the emotions themselves.