JPL images of Mercury.
Scientists suggested decades ago there might be water ice and other frozen volatiles trapped at Mercury’s poles. The idea received a boost in 1991 when the Arecibo radio telescope in Puerto Rico detected radar-bright patches at Mercury’s poles.
Images taken from MESSENGER in 2011, and earlier this year, confirmed all radar-bright features at Mercury’s north and south poles lie within shadowed regions on the planet’s surface. These findings are consistent with the water ice hypothesis.