Lunar Ice Deposits are More Extensive than Previously Thought

Prior studies found signs of ice in the permanently shadowed regions near the south pole of the Moon, including areas within Cabeus, Haworth, Shoemaker and Faustini craters. A new analysis of data from NASA’s Lunar Reconnaissance Orbiter (LRO) shows there is widespread evidence of water ice within permanently shadowed regions outside the south pole, towards at least 77 degrees south latitude.

Ice could become implanted in lunar regolith through comet and meteor impacts, released as vapor (gas) from the lunar interior, or be formed by chemical reactions between hydrogen in the solar wind and oxygen in the regolith.

The permanently shadowed regions (PSRs) typically occur in topographic depressions near the lunar poles.

Because of the low Sun angle, these areas haven’t seen sunlight for up to billions of years, so are perpetually in extreme cold.

Ice molecules are thought to be repeatedly dislodged from the regolith by meteorites, space radiation, or sunlight and travel across the lunar surface until they land in a PSR where they are entrapped by extreme cold.

The PSR’s continuously cold surfaces can preserve ice molecules near the surface for perhaps billions of years, where they may accumulate into a deposit that is rich enough to mine.

“Our model and analysis show that greatest ice concentrations are expected to occur near the PSRs’ coldest locations below 75 Kelvin (minus 198 degrees Celsius, or minus 325 degrees Fahrenheit) and near the base of the poleward-facing slopes of PSRs,” said Dr. Timothy McClanahan, a researcher at NASA’s Goddard Space Flight Center.

“We can’t accurately determine the volume of the PSRs’ ice deposits or identify if they might be buried under a dry layer of regolith.”

“However, we expect that for each surface 1 m² residing over these deposits there should be at least about five more liters of ice within the surface top 1 m, as compared to their surrounding areas.”

Dr. McClanahan and colleagues used LRO’s Lunar Exploration Neutron Detector (LEND) instrument to detect signs of ice deposits by measuring moderate-energy, ‘epithermal’ neutrons.

Specifically, they used LEND’s Collimated Sensor for Epithermal Neutrons (CSETN) that has a fixed 30-km (18.6 mile) diameter field-of-view.

Neutrons are created by high-energy galactic cosmic rays that come from powerful deep-space events such as exploding stars, that impact the lunar surface, break up regolith atoms, and scatter subatomic particles called neutrons.

The neutrons, which can originate from up to about a 1-m (3.3-foot) depth, ping-pong their way through the regolith, running into other atoms. Some get directed into space, where they can be detected by LEND.

Since hydrogen is about the same mass as a neutron, a collision with hydrogen causes the neutron to lose relatively more energy than a collision with most common regolith elements.

So, where hydrogen is present in regolith, its concentration creates a corresponding reduction in the observed number of moderate-energy neutrons.

“We hypothesized that if all PSRs have the same hydrogen concentration, then CSETN should proportionally detect their hydrogen concentrations as a function of their areas,” Dr. McClanahan said.

“So, more hydrogen should be observed towards the larger-area PSRs.”

The findings were published this week in the Planetary Science Journal.

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T.P. McClanahan et al. 2024. Evidence for Widespread Hydrogen Sequestration within the Moon’s South Pole Cold Traps. Planet. Sci. J 5, 217; doi: 10.3847/PSJ/ad5b55

This article was adapted from an original release by NASA.