MESSENGER's Neutron Spectrometer detects low-energy neutrons colliding with hydrogen-rich matter in the top 40 cm (16 in) of Mercury's surface. Readings from the spectrometer point to an "excess" of hydrogen in some form at the planet's north pole.

"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 surface layer 10 to 20 centimeters thick that is less rich in hydrogen," writes MESSENGER scientist David Lawrence of Johns Hopkins University Applied Physics Laboratory. "The buried layer has a hydrogen content consistent with nearly pure water ice."