Two decades ago, radio telescope observations showed radar-bright features in Mercury's polar regions. The radar characteristics are similar to those of the icy satellites of Jupiter and at the south polar ice cap on Mars. The radarbright spots on Mercury were therefore believed to be water ice, and images from MESSENGER provide new support for this idea. MESSENGER, the first spacecraft to orbit Mercury, has provided a view of Mercury's south polar terrain over a full Mercury day that makes it possible to identify areas in permanent shadow. Chabot et al. compared the MESSENGER images of craters in the south pole region with the distribution of radarbright features from previous radio telescope studies.