Figure 8. The flux of (A) epithermal (0.4 eV–0.5 MeV) and (B) fast (>0.5 MeV) neutrons, as measured by the MESSENGER Neutron Spectrometer (orange data). There is a clear decrease in the flux of both epithermal and fast neutrons at high northern latitudes. For comparison, simulated model count rates are shown for the case where (i) no H is present (gray data) and (ii) where there is a thick layer of 100 wt% water ice at the surface of all radar-bright regions (blue data). Data are normalized to the 0 wt% H case in A.