Lunar Ice Prospecting: Neutron Spectrometer Mapping

Scan Controls

Orbital altitude 2.0 km

Integration time 4.0 s

Scan speed 1.0×

Survey Readouts

Area surveyed0%

Estimated ice mass0 t

Mean estimate error— pp

How it works

Fast (epithermal) neutrons are kicked loose from the top ~1 m of regolith by cosmic-ray spallation. Hydrogen nuclei are almost exactly the neutron's mass, so they are unusually good at slowing (moderating) those neutrons on the way back out — a real technique flown on Lunar Prospector and LRO's LEND instrument. Wherever hydrogen (bound in water ice) is more abundant, the epithermal count rate the orbiter sees is suppressed:

N(H) ≈ N0 · (1 − k·H)      k ≈ suppression coefficient
Detected signal:  S = 1 − N/N0  ∝  H  (ice abundance)

Neutron counting is a Poisson process, so the fractional noise on any single measurement falls as 1/√counts. Two controls trade directly against that noise floor:

Altitude — lower orbits collect more counts per pass (flux ∝ 1/altitude²) for a cleaner signal, but their footprint on the ground is also smaller, so a low pass covers less area and needs more passes for full coverage. Higher orbits blur several craters into one averaged reading.
Integration time — longer dwell per sample accumulates more counts (less noise) at the cost of scanning fewer locations per minute.
Scan speed — how fast the spacecraft sweeps the boustrophedon (lawnmower) ground track below.

Each pass blends its noisy estimate into every cell under the instrument footprint, weighted by how many counts that pass actually collected — exactly how real polar-hydrogen maps are built up orbit after orbit until the water-ice signal rises above the noise.

Estimated map

Dry regolith Water ice signal Unsurveyed

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