In Situ Neutron Spectroscopy on the Martian Surface : Modeling the Hydra Instrument for Different Mission Scenarios

1. Introduction: Planetary neutron spectroscopy has proven to be highly successful in remotely detecting and measuring the abundance of water on planetary surfaces such as Mars and the Moon [1,2]. Because of the central role played by water on Mars and the need to make in situ measurements of water abundances for landed missions, neutron spectroscopy is also being investigated as a technique for quickly determining the near-surface water abundance for future Mars missions, such as the Mars Smart Lander (MSL) [3,4,5]. We are currently developing a water-and hydrate-sensing instrument called " HYDRA " that is being supported by the NASA Mars Instrument Development Program (MIDP). Previous work has been supported by the NASA Planetary Instrument Design and Development (PIDDP) Program. A detailed description of the science justifications for the HYDRA instrument are given as a companion paper in this conference [5]. Here we focus on summarizing results of modeling work that demonstrates surface based neutron spectros-copy is indeed feasible and can be successfully carried for a wide variety of mission scenarios. In particular, we have investigated 1) the effects of mounting a neutron spectrometer (NS) on the body of a rover and/or lander; and 2) the effects of making neutron measurements in the presence of a radioactive thermal generator (RTG) that produces copious amounts of neutrons. In both of these situations, we have determined that robust measurements of water content can be made using the technique of neutron spectroscopy. 2. Instrument Modeling The most straightforward way to make neutron measurements on the Martian surface is to use 3 He neutron detectors. These detectors are proportional counters filled with 3 He gas that has a high probability for absorbing neutrons. Advantages of 3 He detectors include a large efficiency for detecting neutrons, low mass (3 He sensors can be made for <100g), ruggedness (3 He sensors have survived shocks of up to 1500g's), and extensive space-flight heritage on NASA and Department of Energy based missions. Figure 1 shows a possible configuration of how a HYDRA instrument might be implemented. The configuration includes two 3 He tubes where one is covered in a layer of Sn and the other is covered in a layer of Cd. As with the Lunar Prospector NS [6], the Cd covered tube measures epithermal neutrons and the Sn covered tube measures both thermal and epithermal neutrons. Thermal neutron measure