The surface of the Moon is a bleak and forbidding place that could be hiding a lot of water ice within the dusty landscape. However, most possible ice deposits don't just stand out in an image. You need specialized mapping to find those deposits. So, scientists have devised another way to find the Moon's frosty hidden reserves. They use seismic waves from moonquakes as locator beacons.
Geologists at the University of Maryland, Lawrence Berkeley National Laboratory, and the University of Hawai'i recently published a paper (cited below) that examines the role that seismic waves play in locating ice deposits on and in the Moon. Finding water ice is important because the users of future lunar laboratories and habitats will need a steady supply of water. In addition, ice can be used to create oxygen supplies and rocket fuels for getting around the Moon. It's a far cheaper and more efficient source of water than bringing water and other supplies up from Earth.
A map of possible lunar water deposits around the south pole region of the Moon. Courtesy: Chandrayaan mission. Credit: ISRO.
How Did the Moon Get Water?
The origins of the lunar water caches remain something of a mystery, although the evidence points in a few directions. The most likely locations for substantial amounts of water are at the poles and other regions where some of the ice could have been delivered by comets or asteroids. There could be deposits in regions where volcanism transported water from inside the Moon to the surface, or via the solar wind or meteor showers. These bombard surface rocks, triggering chemical reactions that could release water molecules. The evidence for water is pretty substantial, based on studies of rocks brought back during the Apollo era, as well as studies made by such missions as the Chinese Chang'e 5 probe, the SOFIA project, and highly detailed images and impact modeling done by Chandrayaan scientists. It's more likely that much of the proposed lunar water accumulated over millions or billions of years, rather than just one major delivery by a comet or asteroid impact.
Lunar water is likely some kind of briny mixture of materials from the Moon mixed with materials delivered by delivery of chondritic materials via impacts. It's still possible that regions in the lunar interior could be warm enough to contain liquid water, although scientists need more data about the Moon's core to prove that. Some analyses also suggest that some lunar water could date back to the time of the Moon's formation during the impact between early Earth and a Mars-sized object. So, it's clear that a better understanding of the nature of lunar water (as well as its locations) is in order.
These images show a very young lunar crater on the side of the moon that faces away from Earth, as viewed by NASA's Moon Mineralogy Mapper on the Indian Space Research Organization's Chandrayaan-1 spacecraft. On the left is an image showing brightness at shorter infrared wavelengths. On the right, the distribution of water-rich minerals (light blue) is shown around a small crater. Both water- and hydroxyl-rich materials were found to be associated with material ejected from the crater. Credits ISRO/NASA/JPL-Caltech/USGS/Brown Univ.
Future Uses of Lunar Ice
According to Nicholas Schmerr of the University of Maryland, the Artemis missions and others will rely on lunar ice deposits, no matter where they come from. âItâs crucial to identify any materials on the moon that an astronaut can make use of while theyâre up there,â said Schmerr, who co-authored the team's study. âSince they will be limited by the few resources they brought from Earth, anything they find on the moon will help them basically live off the land, especially for longer-term missions or outposts.â
Lunar ice also provides another avenue of planetary science research. Ice in the shadowed craters may well contain undisturbed deposits of ice from comets and other events from early solar system history. âThe moon witnessed some of the most critical parts of the early solar system, including how water was delivered,â Schmerr said. âStudying the ice deposited there could reveal how water spread and ultimately how Earthâs oceans formed.â
Seismic Waves and Ice
Finding ice under the surface requires new thinking about how it can be located. The lunar surface is cold and dry. From studies here on Earth, scientists know that seismic waves behave differently depending on the types of material they pass through. When a quake occurs, it sends out these mechanical waves. They travel through all the materials beneath the surface, and their velocity changes depending on what they travel through at the time. The density and elasticity of materials are important factors. The velocity of a wave will tell us how deep the quake was, essentially where it occurred.
As an example, if a wave passes through an area that has a lot of ice, the waves travel two to three times faster than they would through dirt or sand. Areas with a lot of ice mixed in also see bounce back in the waves as they interact with the stiff, dense materials. If an area on the Moon has a lot of ice underground, scientists would expect to see changes in velocity of the earthquake waves as well as any possible bounce back. According to Schmerr, bounceback would also reveal other characteristics of the hidden ice. âWe can use seismic waves to not just see whether ice is present but also roughly how much of it there is,â he explained.
How to Test for Lunar Ice
It's one thing to study earthquake waves here on the home planet, but quite another to do it on the Moon. To figure out the best way to determine lunar ice deposits, the team, led by Harrison Lisabeth of Lawrence Berkeley National Laboratory, tried three approaches for detection. First, they took a volcanic rock from Arizona that is a close mimic of lunar rocks. If you crush that type of rock, it turns into an almost exact analog for lunar dust. They froze it and then X-rayed it to see how ice settled between dust grains. The second approach involved temperature models for the polar regions, where ancient ice deposits seem to exist undisturbed by sunlight. Finally, Schmerr ran computer simulations of minor moonquakes to see how they would interact with underground lunar ice. All three methods left very obvious signs of ice in the data.
Data from X-ray tomography of simulated lunar regolith showing a) the dry sample, b) a rendering of icy microstructures prepared with small amounts of water, c) grains with the ice removed, and d) ice with the grains removed. The ice acts like a cement in the pore spaces between the grains of dust. This test result showed how the ice could exist in the regolith of the Moon. Credit: Lisabeth, Schmerr, et al.
The results look very promising, and the research team was able to come up with some suggestions for further examinations of methods to find ice on the Moon. One approach is to study the microstructures of a wider array of lunar regolith analogs here on Earth. In addition, future computer models should take into account more accurate measurements of lunar topography. Finally, the team suggests that further seismic models and tests be made to determine more accurate locations and depths of ice. ? The hunt for lunar ice is a very real one and future explorers will need that resource. The team points out that recent spectroscopic data suggests that there is between 100 and 400 mg/g of water in the lunar soil. The job now is to identify deposits that are reasonably minable for industrial use by Artemis and other missions. Planned robotic and crewed missions to the moon in the next decade will provide opportunities to apply the tools of terrestrial near-surface geophysics to locating crucial lunar resources. The next viable mission is the Chinese Chang'e-7 is set to go to the Moon in .... and will carry a seismometer capable of looking for ice deposits in its landing site near Shackleton Crater. The 2028 Artemis mission should also be able to set a Lunar Environmental Monitoring Station that can also perform seismic studies.