"Bright patches" had been detected on Mercury in the past - namely by radar observations by the Arecibo radio telescope - but there was little supporting evidence to suggest these highly radar-reflective regions could be proof of water ice.
But with MESSENGER's help, a comprehensive global map is being constructed. By using the spacecraft's high-resolution imagery of Mercury's craters, the earlier Arecibo observations could be overlaid.
It turns out that these bright patches appear inside Mercury's shady craters, a potential indicator that water ice is present.
"All radar-bright features near Mercury's north pole are confined to shadowed areas in MDIS images to date, consistent with the water-ice hypothesis," said Johns Hopkins University planetary scientist Nancy Chabot in the conference proceedings (PDF).
But there's a catch. For water ice to remain stable inside these craters, an insulating layer of regolith - the thin, pulverized rock dust that forms on planetary bodies after eons of meteorite impacts - needs to blanket the surface, keeping the ice in a frozen state (preventing it from sublimating into space). For the ice to be preserved, around 20-30 centimeters (8-12 inches) of overlaying regolith needs to be present.