Is there any closed expression for $\sum_{k=0}^\infty r^{k^2}$?

Question

Let $r$ be any real number with $0 < r < 1$. Then, of course, there is a closed expression for $$\sum_{k=0}^\infty r^k = \frac{1}{1-r}$$ But does there also exist a closed expression for $$\sum_{k=0}^\infty r^{k^2} \qquad ?$$ Decent approximations will be appreciated as well.