Although the two moons of Mars, Phobos and Deimos, were discovered over a century ago, their origins, histories, and natures are largely a mystery. Even the most basic questions remain: Where did they come from? What are they made of? How do they relate to Mars and the rest of the solar system?
JAXA's Martian Moons eXploration (MMX) mission is on track to become the first dedicated mission to explore and sample the martian moons, with MEGANE as one of its scientific instruments. MEGANE will revolutionize our understanding of Phobos by characterizing the composition of its near-surface materials for the first time. These compositional data will be used to test the competing hypotheses for Phobos' origin, study surface processes on Phobos, support sample site selection for the MMX mission, and provide crucial global- and regional-scale geochemical context for the samples collected on Phobos and delivered to Earth for analysis.
MEGANE has three science goals that directly address core science objectives of the MMX mission:
MEGANE will determine the composition of Phobos' surface using gamma-ray and neutron measurements. MEGANE's measurements will take advantage of the fact that galactic cosmic rays are continually impacting the surface of Phobos and producing high-energy ("fast") neutrons. These neutrons interact with the surrounding surface materials, resulting in the emission of gamma rays whose energies identify the major elements in Phobos' surface. Naturally occurring radioactive elements, such as potassium (K), also emit gamma rays with energies that identify specific elements. In addition, the high-energy neutrons slow down by scattering off of the various atomic nuclei. Many of these gamma rays and neutrons escape Phobos' surface, and MEGANE, which will orbit above Phobos, will detect these escaping gamma rays and neutrons in order to determine the chemical composition of the moon's near-surface materials. Models of the hypothesized compositions of the martian moons indicate that MEGANE will determine the abundances of the following elements at Phobos: H, O, Mg, Si, K, Ca, Fe, and Th.
Gamma-ray and neutron spectroscopy have been used successfully on a number of other orbital missions to determine the chemical composition of planetary bodies, including MESSENGER at Mercury, Dawn at the asteroids Vesta and Ceres, Mars Odyssey at Mars, and Lunar Prospector at the Moon. MEGANE will build on the experiences gained from these previous investigations to produce the first compositional measurements of Phobos.