Scientists from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, have created the first complete map of hydrogen abundances on the Moon’s surface using data collected over two decades ago. The map identifies two types of lunar materials containing enhanced hydrogen and confirms previous ideas about lunar hydrogen and water, such as the discovery that water likely played a role in the Moon’s original magma-ocean formation and solidification.
To create their map, APL’s David Lawrence, Patrick Peplowski, and Jack Wilson, along with Rick Elphic from NASA Ames Research Center, used orbital neutron data from the Lunar Prospector mission. The probe, which was deployed by NASA 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 emits cosmic rays, which are high-energy protons and neutrons that travel at nearly the speed of light through space. When cosmic rays collide with the surface of a planet or moon, they break apart the atoms present, sending protons and neutrons flying. By studying the motion of protons and neutrons, scientists can identify an element and determine where and how much of it exists.
We were able to combine data from Apollo missions’ lunar soil samples with what we’ve measured from space and finally put together a full picture of lunar hydrogen for the first time. When the Moon first formed, it’s widely assumed that it was molten debris from a massive impact with Earth.
David Lawrence
“Imagine you’re playing pool, and the cue ball represents neutrons, and the billiard balls represent hydrogen,” Lawrence explained. “When you hit a billiard ball with your cue ball, the cue ball stops moving and the billiard ball is pushed into motion, because both objects have the same mass. Similarly, when a neutron comes in contact with hydrogen, it dies and stops moving, and the hydrogen is sent into motion. So when we see a fewer number of neutrons moving about, it’s an indication hydrogen is present.”
The data was calibrated to quantify the amount of hydrogen by a decrease in neutrons measured by the Neutron Spectrometer, one of five instruments mounted on Lunar Prospector to complete gravitational and compositional maps of the Moon. The research was published in the journal Journal of Geophysical Research.
“We were able to combine data from Apollo missions’ lunar soil samples with what we’ve measured from space and finally put together a full picture of lunar hydrogen for the first time,” Lawrence continued.
The team’s map confirms enhanced hydrogen in two types of lunar materials. The first, at the Aristarchus Plateau, is home to the Moon’s largest pyroclastic deposit. These deposits are fragments of rock erupted from volcanoes, corroborating prior observations that hydrogen and/or water played a role in lunar magmatic events. The second is KREEP-type rocks. KREEP is an acronym for lunar lava rock that stands for potassium (K), rare earth elements (REE) and phosphorus (P).
“When the Moon first formed, it’s widely assumed that it was molten debris from a massive impact with Earth,” Lawrence explained. “As the melt cooled, minerals formed, 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 early data from the Lunar Prospector mission in 1998, said it took time to build on previous efforts to complete a full map of Earth’s nearest neighbor.
“Finalizing the analysis took a number of years,” said Lawrence. “As we were sorting through everything, we began making corrections to data that we discovered was not hydrogen. We went back and fine-tuned previous analyses, and in large part, we were able to do that because of discoveries from other missions. We are continuously building off of previous knowledge and stepping into new territory.”
This new map not only completes the inventory of hydrogen on the Moon, but it may also lead to quantification of how much hydrogen and water were present in the Moon when it was born. In 2013, APL researchers confirmed the presence of water ice at Mercury’s poles using data from the neutron spectrometer on the APL-built MESSENGER spacecraft. These discoveries are critical not only for understanding the solar system, but also for planning future human exploration of the solar system.
