Gamma Rays from Mercury Made by Solar Energetic Particles

R.C. Reedy (Los Alamos Nat'l Lab.)

Most gamma rays used to map elemental concentrations in planetary surfaces are made by neutron inelastic-scattering or capture reactions. These neutrons are produced by galactic cosmic rays interacting with the planet's surface. Some gamma rays are emitted in the decay of radionuclides made by solar energetic particles (SEPs). Prompt gamma rays made by SEPs are not observable because SEPs saturate the output of a gamma-ray spectrometer. Only SEP-produced radionuclides with half-lives of a day or longer are observable after an SEP event. The fluxes of such SEP-produced gamma rays from the Moon are fairly low (Reedy et al., 1973, JGR, 78, 5847). However, the fluence of SEPs increases with distance R towards the Sun as the square to cube of R. Thus the fluence of SEPs at Mercury (0.4 AU) is about 10 times higher than at the Moon. Mercury's magnetic field is weak enough that SEPs interact in the surface, producing some radionuclides. These radionuclides are made in the top centimeter of the surface, a much shallower depth than from which neutron-produced gamma rays come (tens of cm). SEP-produced gamma rays include those from the decay of Co-56 and Mn-54 made from Fe, V-48 from Ti, and Na-22 and Al-26 from Mg, Al, and Si. Besides mapping elemental concentrations in the very surface of Mercury, these gamma rays can be used to study past fluences of SEPs irradiating Mercury.