  Let $k$ be a field and $X \subset P^3_{k}$ a smooth cubic surface. Let
$\Delta \subset Pic(X)$ be the finite index subgroup spanned by norms of lines
on $X_{K}$ for $K$ running through the finite separable extensions of $k$. The
quotient $Pic(X)/\Delta$ is 3-primary. If $X$ contains a line defined over $k$,
then $\Delta=Pic(X)$.
