A Hunt for Ice on the Moon Ends in Surprise: What the Moon’s Really Made of

Lunar Reconnaissance Orbiter Moon

This image based on data from NASA’s Lunar Reconnaissance Orbiter spacecraft shows the face of the Moon we see from Earth. The more we learn about our nearest neighbor, the more we begin to understand the Moon as a dynamic place with useful resources that could one day even support human presence. Credit: NASA / GSFC / Arizona State University

Radar Points to Moon Being More Metallic Than Previously Thought

What started out as a hunt for ice lurking in polar lunar craters turned into an unexpected finding that could help clear some muddy history about the Moon’s formation.

Team members of the Miniature Radio Frequency (Mini-RF) instrument on NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft found new evidence that the Moon’s subsurface might be richer in metals, like iron and titanium, than researchers thought. That finding, published July 1 in Earth and Planetary Science Letters, could aid in drawing a clearer connection between Earth and the Moon.

“The LRO mission and its radar instrument continue to surprise us with new insights about the origins and complexity of our nearest neighbor,” said Wes Patterson, Mini-RF principal investigator from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, and a study coauthor.

Substantial evidence points to the Moon as the product of a collision between a Mars-sized protoplanet and young Earth, forming from the gravitational collapse of the remaining cloud of debris. Consequently, the Moon’s bulk chemical composition closely resembles that of Earth.

Look in detail at the Moon’s chemical composition, however, and that story turns murky. For example, in the bright plains of the Moon’s surface, called the lunar highlands, rocks contain smaller amounts of metal-bearing minerals relative to Earth. That finding might be explained if Earth had fully differentiated into a core, mantle and crust before the impact, leaving the Moon largely metal-poor. But turn to the Moon’s maria — the large, darker plains — and the metal abundance becomes richer than that of many rocks on Earth.

This discrepancy has puzzled scientists, leading to numerous questions and hypotheses regarding how much the impacting protoplanet may have contributed to the differences. The Mini-RF team found a curious pattern that could lead to an answer.

Using Mini-RF, the researchers sought to measure an electrical property within lunar soil piled on crater floors in the Moon’s northern hemisphere. This electrical property is known as the dielectric constant, a number that compares the relative abilities of a material and the vacuum of space to transmit electric fields, and could help locate ice lurking in the crater shadows. The team, however, noticed this property increasing with crater size.

For craters approximately 1 to 3 miles (2 to 5 kilometers) wide, the dielectric constant of the material steadily increased as the craters grew larger, but for craters 3 to 12 miles (5 to 20 kilometers) wide, the property remained constant.

“It was a surprising relationship that we had no reason to believe would exist,” said Essam Heggy, coinvestigator of the Mini-RF experiments from the University of Southern California in Los Angeles and lead author of the published paper.

Discovery of this pattern opened a door to a new possibility. Because meteors that form larger craters also dig deeper into the Moon’s subsurface, the team reasoned that the increasing dielectric constant of the dust in larger craters could be the result of meteors excavating iron and titanium oxides that lie below the surface. Dielectric properties are directly linked to the concentration of these metal minerals.

If their hypothesis were true, it would mean only the first few hundred meters of the Moon’s surface is scant in iron and titanium oxides, but below the surface, there’s a steady increase to a rich and unexpected bonanza.

Comparing crater floor radar images from Mini-RF with metal oxide maps from the LRO Wide-Angle Camera, Japan’s Kaguya mission and NASA’s Lunar Prospector spacecraft, the team found exactly what it had suspected. The larger craters, with their increased dielectric material, were also richer in metals, suggesting that more iron and titanium oxides had been excavated from the depths of 0.3 to 1 mile (0.5 to 2 kilometers) than from the upper 0.1 to 0.3 miles (0.2 to 0.5 kilometers) of the lunar subsurface.

“This exciting result from Mini-RF shows that even after 11 years in operation at the Moon, we are still making new discoveries about the ancient history of our nearest neighbor,” said Noah Petro, the LRO project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The MINI-RF data is incredibly valuable for telling us about the properties of the lunar surface, but we use that data to infer what was happening over 4.5 billion years ago!”

These results follow recent evidence from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission that suggests a significant mass of dense material exists just a few tens to hundreds of kilometers beneath the Moon’s enormous South Pole-Aitken basin, indicating that dense materials aren’t uniformly distributed in the Moon’s subsurface.

The team emphasizes that the new study can’t directly answer the outstanding questions about the Moon’s formation, but it does reduce the uncertainty in the distribution of iron and titanium oxides in the lunar subsurface and provide critical evidence needed to better understand the Moon’s formation and its connection to Earth.

“It really raises the question of what this means for our previous formation hypotheses,” Heggy said.

Anxious to uncover more, the researchers have already started examining crater floors in the Moon’s southern hemisphere to see if the same trends exist there.

Read Surprising Find at the Bottom of Moon’s Craters Provides New Insights to Its Origin for more on this research.


Reference: “Bulk composition of regolith fines on lunar crater floors: Initial investigation by LRO/Mini-RF” by E. Heggy, E. M. Palmer, T. W. Thompson, B. J. Thomson and G. W. Patterson, 12 May 2020, Earth and Planetary Science Letters.
DOI: 10.1016/j.epsl.2020.116274

LRO is managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland for the Science Mission Directorate at NASA Headquarters in Washington. Mini-RF was designed, built and tested by a team led by APL, Naval Air Warfare Center, Sandia National Laboratories, Raytheon and Northrop Grumman.


View Comments

  • Question: Data shows the core structure of all the planets, with that in mind how accurate are those if we are just now learning the core of the moon?

    • We don't know the inner structure of planets very well, especially for gas giants:

      "The surviving core of a gas giant has been discovered orbiting a distant star by University of Warwick astronomers, offering an unprecedented glimpse into the interior of a planet. ... We have the opportunity to look at the core of a planet in a way that we can't do in our own solar system. There are still big open questions about the nature of Jupiter's core, for example, so strange and unusual exoplanets like this give us a window into planet formation that we have no other way to explore."
      [ ]

      Seismology has allowed to learn more of Earth inside, as well as Moon [Apollo seismometers] and now lately Mars [InSight seismometer]. I see some work at times, there is a lot of data, but also lots of uncertainties in models yet.

    • The best answer you would get is if you find a read a recent review of work on the topic.

  • Can't we use seismic waves as a method for gaining further insight on the moon's interior? For instance, an elaborate setup of multiple detonaters and receivers scattered accross the lunar surface.

  • How long until we had a space war over the control of the mineral deposits on the Moon?

    • The moon should be off limits to any mining efforts. It is of greatest value in tact.

      • I believe Mankind will find a way to mine the bejesus out of the moon until it is gone or loses the gravitational symbiotic relationship it has with Earth. The predictable tides will be gone, life will perish as we know it and we will perish along with it. Man always finds a way to destroy everything they touch for their own greedy purpose.

  • The moon is hollow and made of ti, al and f. The aliens covered it with a thin crust of cyber dirt in our universe simulation.

  • Maybe you should leave the moon as is .Humans are too smart for their own good.Your answers are here on this planet .If it were anywhere else you wouldn't be here .

  • These guys have no idea what they're talking about. My Mother told me the moon is made of cheese ;-)

    • If we're talking about the Collision of two protoplanets that are mostly formed then The cores would formed. The smaller planet colliding with the larger planet would likely eject the core to become the new moon. The core being the densest part of the entire planet would retain the most inertia and rip its way out of the other planet being slowed down by the Collision. The remains of that core would set itself up in orbit around the debris field slowly coalescing some of the debris. While the newly forming Earth would absorb the majority. I would postulate that there is in fact a solid metal core in the Moon. Evidence has shown that the moon had a magnetic field in the past.

NASA’s Goddard Space Flight Center

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