Let $(M,g)$ be a smooth Riemannian manifold, and let $S \subseteq M$ be a compact submanifold. 

Assume that for each $p \in M$, there exist a **unique** closest point on $S$, i.e a unique point $\tilde s(p) \in S$ such that $d(p,\tilde s (p))=d_S(p)$.

It is easy to see the map $\tilde s:M \to S$ is continuous. 

Is it differentiable? (at which points)?  If not, are there directional derivatives everywhere?

Does anything change if we assume every point has a unique minimizig geodesic to $S$?