Johns Hopkins APL assembles first global map of lunar hydrogen

Using data collected more than two decades ago, scientists from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, have compiled the first complete map of hydrogen abundance on the Moon’s surface. The map identifies two types of enhanced hydrogen-containing lunar material and corroborates previous ideas about lunar hydrogen and water, including the finding that water likely played a role in the formation and solidification of the Moon’s native ocean-magma.

David Lawrence, Patrick Peplowski and Jack Wilson of APL, along with Rick Elphic of NASA’s Ames Research Center, used orbital neutron data from the Lunar Prospector mission to create their map. The probe, deployed by NASA in 1998, orbited the Moon for a year and a half and sent back the first direct evidence of an increase in hydrogen at the lunar poles, before hitting the lunar surface.

When a star explodes, it releases cosmic rays, or high-energy protons and neutrons that travel through space at nearly the speed of light. When those cosmic rays come into contact with the surface of a planet, or moon, they break apart the atoms located in those objects, sending protons and neutrons flying. Scientists can identify an element and determine where and how much of it is present by studying the motion of its protons and neutrons.

“Imagine you’re playing billiards and the cue ball represents the neutrons and the billiard ball represents the hydrogen,” explains Lawrence. “When you hit a pool ball with your cue ball, the cue ball stops moving and the pool ball is pushed to move, because both objects have the same mass. Similarly, when a neutron comes into contact with hydrogen, it dies and stops moving, and the hydrogen is sent into motion. So when we see the number of neutrons moving less, that’s an indication of the presence of hydrogen.”

The team calibrated the data to measure the amount of hydrogen with a corresponding neutron drop as measured by the Neutron Spectrometer, one of five instruments installed on the Lunar Prospector to complete a map of the Moon’s gravity and composition. The findings were published in the Journal of Geophysical Research.

“We were able to combine data from lunar soil samples from the Apollo missions with what we had measured from space and finally piece together a complete picture of lunar hydrogen for the first time,” Lawrence continued.

The team’s map confirms an increase in hydrogen in two types of lunar material. The first, on the Aristarchus Plateau, is home to the largest pyroclastic deposits on the Moon. These deposits are rock fragments that erupted from volcanoes, corroborating previous observations that hydrogen and/or water played a role in lunar magmatic events. The second is KREEP type rock. KREEP is an acronym for lunar lava rock which stands for potassium (K), rare earth elements (REE) and phosphorus (P).

“When the Moon was originally forming, it was mostly accepted that it was liquid debris from a large impact with Earth,” Lawrence said. “As it cools, minerals form from the melt, and KREEP is thought to be the last type of material to crystallize and harden.”

Lawrence, who was part of the original team that studied preliminary data from the Lunar Prospector mission in 1998, said building on existing efforts to complete a complete map of Earth’s closest neighbours will take time.

“Completing the analysis took several years,” Lawrence said. “As we sorted everything out, we started to make corrections to the data that we found not hydrogen. We went back and refined the previous analysis, and for the most part, we were able to do so because of discoveries from other missions. We continue to build on previous knowledge and step into new territory.”

This new map not only completes the hydrogen inventory on the Moon, it could also lead to a quantification of how much hydrogen and water was on the Moon when it was born. In 2013, APL researchers also confirmed the presence of polar water ice on the planet Mercury using data from the neutron spectrometer on APL’s MESSENGER spacecraft. These discoveries are important not only for understanding the solar system but also in planning future human exploration of the solar system.

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