The conclusion is supported by Messenger measurements of excess hydrogen at Mercury's north, the reflectance of Mercury's polar regions and the first detailed models of the surface and near-surface temperatures of Mercury's north polar regions, the space agency reported Thursday.
Because it is so close to the sun, Mercury would seem to be an unlikely place to find ice, but because the tilt of Mercury's rotational axis is almost zero there are pockets at the planet's poles that never see sunlight.
Scientists suggested decades ago there might be ice and other frozen volatiles trapped at Mercury's poles in permanently shadowed areas.
Images from Messenger confirmed radar-bright features at Mercury's north and south poles are within shadowed regions, a finding consistent with the water-ice hypothesis.
Messenger has used neutron spectroscopy to measure average hydrogen concentrations within Mercury's radar-bright regions, NASA scientists said.
"The neutron data indicate that Mercury's radar-bright polar deposits contain, on average, a hydrogen-rich layer more than tens of centimeters thick beneath a surficial layer 10 to 20 centimeters (4 to 8 inches) thick that is less rich in hydrogen," Messenger scientist David Lawrence wrote in a paper detailing the findings. "The buried layer has a hydrogen content consistent with nearly pure water ice."