Scientists at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, have compiled the first complete map of the amount of hydrogen on the moon’s surface using data collected more than two decades ago. The map identifies two types of lunar materials containing enhanced hydrogen and confirms previous ideas about lunar hydrogen and water, including the finding that water likely played a role in the initial formation and solidification of the moon’s magma-ocean. David Lawrence, Patrick Peplowski, and Jack Wilson of APL, along with Rick Elphic of NASA Ames Research Center, used orbital neutron data from the Lunar Prospector mission to create their map. The probe, which NASA deployed in 1998, orbited the moon for a year and a half and sent back the first direct evidence of enhanced hydrogen at the lunar poles before impacting 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 these cosmic rays come in contact with the surface of a planet or moon, they break apart the atoms located on those bodies and send protons and neutrons flying. Scientists are able to identify an element and determine where and how much of it exists by studying the motion of these protons and neutrons.
“Imagine you’re playing pool and the cue ball represents neutrons and the pool balls represent hydrogen,” Lawrence explained. ” When you hit a billiard ball with a cue ball, the cue ball stops moving and the billiard ball starts moving 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 set in motion. So when we see a smaller number of neutrons moving around, that’s a sign that hydrogen is present.”
The team calibrated the data to quantify the amount of hydrogen corresponding to neutron decay measured by the Neutron Spectrometer, one of five instruments mounted on the Lunar Prospector to complete gravity and compositional maps of the Moon. 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 measured from space, and we finally put together for the first time a complete picture of lunar hydrogen,” Lawrence continued.
The team’s map confirms enhanced hydrogen in two types of lunar materials. The first, on the Aristarchus Plateau, is home to the Moon’s largest pyroclastic deposit. These deposits are fragments of rock erupted from volcanoes, confirming previous observations that hydrogen and/or water played a role in lunar magmatic events. The second is KREEP type rocks. KREEP is short for lunar lava rock, which stands for potassium (K), rare earth elements (REE) and phosphorus (P).
“When the moon originally formed, it’s generally accepted that it was molten debris from a huge impact on Earth,” Lawrence said. “As it cooled, minerals formed 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 the initial data from the Lunar Prospector mission in 1998, said it takes time to build on existing efforts to complete a full map of Earth’s nearest neighbor.”It took several years to complete the analysis,” Lawrence said. “As we sorted everything out, we started making corrections to the data that we found out was not hydrogen. We went back and fine-tuned the previous analysis, and in large part we were able to do that through discoveries from other missions. They are constantly building on previous knowledge and entering new areas.”
This new map not only adds to the lunar hydrogen inventory, but could also lead to a quantification of how much hydrogen and water was present on the moon when it was born. In 2013, APL researchers also confirmed the presence of water ice at the poles of the planet Mercury using data from the neutron spectrometer on the APL-built MESSENGER spacecraft. These discoveries are important not only for understanding the solar system, but also for planning future human exploration of the solar system.
Source Journal reference: David J. Lawrence, Patrick N. Peplowski, Jack T. Wilson, Richard C. Elphic. Global Hydrogen Abundances on the Lunar Surface. Journal of Geophysical Research: Planets, 2022; 127 (7) DOI: 10.1029/2022JE007197
