Using data collected over two decades ago, scientists at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, have created the first complete map of hydrogen occurrence on the lunar surface. The map identifies two types of lunar materials that contain enhanced hydrogen and confirms previous ideas about lunar hydrogen and water, including findings that water likely played a role in the moon’s original magma-ocean formation and solidification.
APL’s David Lawrence, Patrick Peplowski and Jack Wilson, along with Rick Elphic of the NASA 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 returned 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 a moon, they break up atoms that are on those bodies, sending protons and neutrons into the air. Scientists are able to identify an element and determine where and how much of it is present by studying the movement of these protons and neutrons.
“Imagine you’re playing billiards 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 starts moving because both objects have the same mass. Likewise, when a neutron comes into contact with hydrogen, it dies and stops moving, and the hydrogen is set in motion. So when we see fewer numbers of neutrons moving, that’s an indication that hydrogen is present.
The team calibrated the data to quantify the amount of hydrogen by the corresponding 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 results were published in the Journal of Geophysical Research: Planets.
“We were able to combine data from lunar soil samples from the Apollo missions with our measurements from space and finally, for the first time, create a complete picture of lunar hydrogen,” Lawrence continued.
The team’s map confirms elevated hydrogen in two types of lunar materials. The first, on the Aristarchus Plateau, hosts the moon’s largest pyroclastic deposit. These deposits are rock fragments erupted from volcanoes, confirming previous observations that hydrogen and/or water played a role in lunar magmatic events. The second is KREEP-like rocks. KREEP is an acronym for Lunar Lava Rock, which stands for Potassium (K), Rare Earth Elements (REE), and Phosphorus (P).
“When the moon originally formed, it is widely believed to have been molten debris from a massive impact on Earth,” Lawrence said. “On cooling, minerals formed from the melt, and KREEP is believed to be the last type of material to crystallize and harden.”
Lawrence, who was part of the original team that examined the first data from the Lunar Prospector mission in 1998, said it would take time to build on existing efforts to produce a complete map of Earth’s nearest neighbor.
“It took several years to complete the analysis,” Lawrence said. “As we sorted it all out, we started correcting data that we found wasn’t hydrogen. We went back and refined previous analyzes and in large part we were able to do this because of discoveries from other missions. We continuously build on previous knowledge and break new ground.”
This new map not only completes the hydrogen inventory on the moon, but 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 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.
David J. Lawrence et al, Global Hydrogen Abundances on the Lunar Surface, Journal of Geophysical Research: Planets (2022). DOI: 10.1029/2022JE007197
Provided by the Johns Hopkins University Applied Physics Laboratory
Quote: Assembling the first global map of lunar hydrogen (2022, July 20), retrieved July 20, 2022 from https://phys.org/news/2022-07-global-lunar-hydrogen.html
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