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Why study lunar water?
Decades of lunar exploration have proven that there are water ice enrichments in certain regions around the poles of our Moon. Some of these regions, called "Permanently Shadowed Regions" (or PSRs) at the lunar south pole may contain enough water to change our view of the formation and evolution of Moon, or they may contain enough water to support future human and robotic exploration of the solar system. Despite knowing these enrichments exist, we don't yet know exactly how much water ice lies within PSRs, and the origin and nature of the lunar polar volatile inventory is not well understood. That's in part because our current measurements of the bulk hydrated materials at the Moon's poles are not at high enough spatial resolution to resolve the PSRs and in some cases have provided inconclusive results. We also know that our Moon's polar volatile inventory appears to be much lower than on Mercury, where the PSRs are observed to have patchy, non-uniform distribution of hydrogen- and carbon-rich material. We need to place meaningful constraints on the bulk abundance and distribution of hydrated materials within permanently shadowed regions of the Moon to better understand how volatiles are delivered to the inner solar system, and to help inform models of lunar formation and evolution. We also need to improve our understanding of these small-scale (~tens of km) water-ice enrichments throughout the lunar poles in order to help plan future human and robotic lunar missions, as water is an extremely valuable in-space resource.
How will we characterize the lunar water abundance?
Neutron spectroscopy is a powerful tool used by many NASA missions for identifying hydrated materials on planetary surfaces. If you've ever seen global maps of water on planetary bodies (the Moon, Mars, Mercury, Vesta, Ceres) they were made using a neutron spectrometer. Lunar Prospector was the first planetary mission to carry a neutron spectrometer, but many have followed, including Mars Odyssey, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, Lunar Reconnaissance Orbiter, the Dawn mission to Vesta and Ceres and the Dynamic Albedo of Neutrons instrument on the Mars Curiosity Rover. Neutron detectors measure the energy distribution of neutrons that reach the detector, which is highly dependent upon the hydrogen content of the top meter of a planetary surface. The LunaH-Map Miniature Neutron Spectrometer (Mini-NS) is the first planetary science neutron detector to use CLYC (Cs2LiYCl6:Ce), an elpasolite class of inorganic scintillator for neutron detection. The large surface area of the Mini-NS, the high efficiency for epithermal neutrons (> 0.4 eV) of its CLYC sensors, and low spacecraft periapse at the Moon's South Pole will provide high-resolution maps of hydrogen within ~5 degrees of the pole. The primary science goal of the LunaH-Map mission is to evaluate uniformity of hydrogen across the lunar South Pole. Expected detector sensitivity will enable the Mini-NS to map hydrogen with good statistical confidence (~20% relative) at levels as low as 0.6% WEH (~600 ppm H) at spatial scales ~15 km2.
The primary science instrument on the LunaH-Map spacecraft is the Mini-NS, which is a neutron spectrometer using CLYC scintillator crystals. Mini-NS has 200 cm2 of detecting area covered in gadolinium foil making it sensitive to epithermal (>0.3eV) neutrons only. The Mini-NS consists of two detectors; each is independently operated with data and time synchronized to the spacecraft.
|Detector (2 per instrument)||2x2 Array of 4cm x 6.3cm x 2.0cm CLYC Crystal Modules|
|Sensitivities||Epithermal (E > 0.3 eV) neutrons|
|Dimensions||25cm x 10cm x 8cm|
|Power||3.6W (standby), 9.6W (data acquisition)|
|Data Acquisition Times||Counts binned every 1 second|
|Data Rate||14 Bytes/Sec (50 kBytes/Sec stored locally)|