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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.

Mini-NS: Miniature Neutron Spectrometer for Small Spacecraft

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. 

Mini-NS Specifications

Detector (2 per instrument)2x2 Array of 4cm x 6.3cm x 2.0cm CLYC Crystal Modules
SensitivitiesEpithermal (E > 0.3 eV) neutrons
Dimensions25cm x 10cm x 8cm
Mass 3.4 kg
Power3.6W (standby), 9.6W (data acquisition)
Data Acquisition Times Counts binned every 1 second
Data Rate 14 Bytes/Sec (50 kBytes/Sec stored locally)
Miniature Neutron Spectrometer
(Flight Unit)
Orbit Tracks

 Orbit ground track shown for entire 60 (Earth) day science phase: 141 passes over target area initially (and periodically) centered on Shackleton Crater (-89.9 degree latitude), with close-approach of 7 km at each perilune crossing. Yellow circle denotes LunaH-Map altitude of 8 km; green circle denotes LunaH-Map altitude of 12 km.

 

 

Fully Assembled Mini-NS Flight Unit

The Mini-NS is comprised of 8 neutron sensitive modules (above left) and 2 electronics board assemblies (above right). The detector is modular and each electronics board assembly can drive up to 4 modules.

LunaH-Map Mini-NS Calibration Team at Los Alamos National Laboratory Neutron Free In-Air Facility

(l to r): L.Heffern, E.Johnson, T.Prettyman, J.DuBois, R.Starr, B.Roebuck, K.Mesick, G.Stoddard, C.Hardgrove

Hydrogen Detection Using Neutron Spectroscopy

Every planet in our solar system is constantly bombarded by cosmic ray protons. The Moon doesn't have an atmosphere, so these protons constantly bombard the lunar surface, interacting with the regolith (e.g. rocks and soils) to producing neutrons. As these neutrons “leak” out from the surface, they lose energy and are slowed by collisions. Materials that are more enriched in hydrogen (i.e. lunar regolith within permanently shadowed regions) are more effective at moderating neutrons than unenriched regolith. A (over)simplified analogy for thinking about planetary neutron spectroscopy uses colliding balls. Think of a neutron as a ping pong ball. It will lose about half its energy when it hits another ping pong ball. Every time a neutron (ping pong ball) hits a hydrogen atom (another ping pong ball) it loses about half its energy. Collisions between neutrons and every other element in the periodic table are more like a ping pong ball hitting a bowling ball. This results in increased numbers of low energy (thermal) neutrons and decreased numbers of moderate energy (epithermal) neutrons when more hydrogen is present in the lunar regolith.

LunaH-Map will use a miniature neutron spectrometer (Mini-NS) which uses a neutron sensitive material called a scintillator. Scintillators produce a small light flash when neutrons or other particles interact within their structure. Photomultiplier tubes (or other photosensitive, light-catching and amplifying, devices) then amplify the signal and translate the light flashes into neutron counts. The Mini-NS is designed to detect the suppression of only epithermal neutrons. The Mini-NS uses a gadolinium shield to effectively "screen" (absorb) the thermal neutrons, making the detector sensitive to epithermal neutrons which are related primarily to the abundance of hydrogen. When making measurements over the permanently shadowed regions, where hydrogen is enriched, the Mini-NS will detect lower numbers of epithermal neutrons. These "neutron suppressed regions" correspond to hydrogen enrichments, most likely in the form of water (H2O) or hydroxide (OH).