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There's more metal on the moon than we thought

An image of the near side of the moon based on data from NASA's Lunar Reconnaissance Orbiter.
An image of the near side of the moon based on data from NASA's Lunar Reconnaissance Orbiter.
(Image: © NASA/GSFC/Arizona State University)

Earth's moon is more metal than scientists imagined.

NASA's prolific Lunar Reconnaissance Orbiter (LRO) found rich evidence of iron and titanium oxides under the surface of the moon, which may show a close connection with Earth's early history.

Scientists have been debating how the moon formed for decades. The leading theory suggests that a Mars-size world collided with Earth billions of years ago. The colliding world shattered upon impact and blasted part of the proto-Earth's surface into space. The debris surrounded Earth with a ring; the moon we see today is the result of that ring slowly collapsing under its own gravity.

Related: Amazing moon photos from NASA's Lunar Reconnaissance Orbiter

The moon's chemical composition, however, doesn't show clear evidence of that theory. The lunar highlands on the moon, visible from the Earth as bright regions, have rocks with smaller amounts of metal-bearing minerals relative to our planet.

That could make sense if Earth was already layered, with heavier metals sunk to the core — except that the moon's dark maria planes formed at the same time and have higher metal abundance even than Earth's rocks.

LRO's new findings could explain the discrepancy. The new research relies on a device called the Miniature Radio Frequency (Mini-RF) instrument, a radar probe designed to map lunar geology, look for water ice and test communications technologies.

The instrument scoured the terrain in the moon's northern hemisphere for an electrical property called the dielectric constant. This constant is a number comparing the ability of a material to transmit electric fields with that of the vacuum of space. 

Electric-field transmission is useful to find ice in the shadows of craters, where it is protected from the heat of the sun. But it can also identify areas where more metals, like iron and titanium oxides, are exposed to the surface.

And the scientists noticed that the dielectric constant increased with crater size, but only up to a certain point. Craters between 1 and 3 miles (2 and 5 kilometers) in diameter showed the dielectric constant increased steadily as the craters grew larger. For craters between 3 and 12 miles (5 to 20 km) wide, however, the constant held steady.

"It was a surprising relationship that we had no reason to believe would exist," Essam Heggy, co-investigator of the Mini-RF experiments from the University of Southern California in Los Angeles and lead author on the new research, said in a NASA statement

The team's theory was that the first few hundred feet (or meters) of the moon's surface has few of these oxides, but a richer source of metal lies further below. Then, as meteors collide with the lunar surface and scratch away upper layers, metals become exposed. That sort of pattern would also explain low metal levels in the lunar highlands and higher abundances in the darker and lower plains closer to the moon's subsurface.

To test their work, researchers compared Mini-RF's crater-floor radar images with metal oxide maps produced by a range of missions: LRO Wide-Angle Camera, Japan's Selenological and Engineering Explorer (SELENE) mission (also called Kayuga) and NASA's Lunar Prospector spacecraft. SELENE and Lunar Prospector are no longer operating, but their archival data remains.

Those observations showed that larger craters did indeed contain more metal, according to NASA, which the researchers believe support their hypothesis about buried metallic deposits that meteors excavate.

The results are even more intriguing in light of a puzzling phenomenon reported in 2019 by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, at the moon. Gravity measurements of the moon suggest there is a lot of dense material that is tens to hundreds of miles (or kilometers) underneath the moon's massive South Pole-Aitken basin. The GRAIL results, paired with LRO's new find, thus suggest metals may be more concentrated in certain regions of the moon.

The LRO results are one small step to better understanding how the moon formed, as the observations show how iron and titanium oxides are distributed beneath the moon's northern hemisphere. Next, the researchers will be looking at crater floors in the southern hemisphere to see how much metal is there.

A study based on the research was published Wednesday (July 1) in Earth and Planetary Science Letters.

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  • rod
    This is a very interesting find, especially as it relates to the origin of the Moon using the giant impact model. I note this from another report.

    "When comparing the metal content at the bottom of larger and deeper craters to that of the smaller and shallower ones, the team found higher metal concentrations in the deeper craters. What does a change in recorded metal presence in the subsurface have to do with our understanding of the Moon? The traditional hypothesis is that approximately 4.5 billion years ago there was a collision between Earth and a Mars-sized proto-planet (named Theia). Most scientists believe that that collision shot a large portion of Earth's metal-poor upper crust into orbit, eventually forming the Moon. One puzzling aspect of this theory of the Moon's formation, has been that the Moon has a higher concentration of iron oxides than the Earth—a fact well-known to scientists. This particular research contributes to the field in that it provides insights about a section of the moon that has not been frequently studied and posits that there may exist an even higher concentration of metal deeper below the surface. It is possible, say the researchers that the discrepancy between the amount of iron on the Earth's crust and the Moon could be even greater than scientists thought, which pulls into question the current understanding of how the Moon was formed. The fact that our Moon could be richer in metals than the Earth challenges the notion that it was portions of Earth's mantle and crust that were shot into orbit. A greater concentration of metal deposits may mean that other hypotheses about the Moon's formation must be explored.", ref - Higher concentration of metal in Moon's craters provides new insights to its origin
    Reply
  • Torbjorn Larsson
    rod said:
    This is a very interesting find, especially as it relates to the origin of the Moon using the giant impact model.

    There were already reports of finding Theia oxygen isotope remains the deeper the sample - perhaps connected to any iron oxide over-concentration.

    That said, the proxy of an extracted local difference in dielectric constant from radar measurement is a bit, well, remote. And the inflation of the find seems to be complete arm waving. Of course they should mention the possibility, but they need quantification to make it an hypothesis.
    Reply
  • rod
    FYI, periodically reports come out that cast a long shadow over the giant impact model for the origin of the Moon, this space.com article is another example. We have this metal report and recently carbon report too. Carbon emissions on the moon put theory of moon birth in doubt, ""The findings by the researchers suggest that the moon has a large amount of ancient carbon beneath its surface, and it has likely been there since the moon was formed. How it could have persisted on a very hot early moon remains a mystery. " Another example is, Reconsidering the Moon's Formation and Tidal Evolution, "The Giant Impact theory of lunar formation, in which Earth and Moon are product of a collision between two proto-planets, has been widely accepted for over three decades. It is the only scenario that simultaneously explains the Moon's low iron content and the angular momentum of the Earth-Moon system. In the course of the last few decades, theorists have recognized how important giant impacts are for terrestrial planet formation, making the Earth-Moon system less of a special case. However, in the 21st century, new and more precise analyses of isotope make-up of lunar samples collected during the Apollo landing have revealed that the Moon is remarkably Earth-like in its isotopic signature. This is in conflict with the "classical" Giant Impact theory, in which the Moon is mostly made from debris originating in a Mars-sized protoplanet that collided with proto-Earth."

    Good investigative reporting requires both sides to be heard concerning the giant impact model.
    Reply
  • JamesInSeattle
    "More metal on the moon than we thought"?

    Indeed. Which reminds me of:

    "Disaster Area - Their songs are on the whole very simple and mostly follow the familiar theme of boy-being meets girl-being beneath silvery moon, which then explodes for no adequately explored reason."

    -- The Hitchhiker's Guide to the Galaxy
    Reply
  • Wolfshadw
    If Theia formed in the same region of space as the Earth and other inner planetoids, why would it's composition be so different from Earth's?

    -Wolf sends
    Reply
  • rod
    Wolfshadw said:
    If Theia formed in the same region of space as the Earth and other inner planetoids, why would it's composition be so different from Earth's?

    -Wolf sends

    What defines the *same region of space as the Earth? Example, in the computer models for the giant impact, what is the astronomical unit distance from the Sun used in the simulations for the proto-Earth before Theia collision, and where did the proto-Theia form using the astronomical unit in relation to the proto-Earth? This is something I do not see reported on concerning the giant impact model. Remember, this is the proto-Earth and proto-Theia, not a fully formed Earth and Theia. There should be a simulation showing where both formed in the solar system and how they meet up :)
    Reply
  • Wolfshadw
    rod said:
    What defines the *same region of space as the Earth? Example, in the computer models for the giant impact, what is the astronomical unit distance from the Sun used in the simulations for the proto-Earth before Theia collision, and where did the proto-Theia form using the astronomical unit in relation to the proto-Earth? This is something I do not see reported on concerning the giant impact model. Remember, this is the proto-Earth and proto-Theia, not a fully formed Earth and Theia. There should be a simulation showing where both formed in the solar system and how they meet up :)

    It's my understanding that the majority of the proto-planets, if not all of them, were inside the orbit of Mars and that Mars is actually the last of the proto-planets; having escaped most of the collisions within it's orbit.

    No, I can't verify that Theia formed inside or near the orbit of Mars (a good point).

    -Wolf sends
    Reply
  • Roger M. Pearlman
    Nice, interesting.
    it appears to still be able to align with SPIRAL's ' Draw-Play' Lunar formation hypothesis.
    Reply
  • rod
    FYI. Theia impact for the origin of the Moon provides much modeling and fun to review. Here is an example from the NASA ADS Abstract about iron content of Theia and the Moon. https://ui.adsabs.harvard.edu/abs/2015AGUFM.P34C..03S/abstract
    "Several of these bodies were large enough to sterile the Earth by boiling the oceans. Alternatively, much of the terrestrial veneer came from the core of the moon-forming impactor Theia, rather than from later bodies. Recent lunar studies show that the Moon likely contains "excess" iron spewed from Theia's core. The mass of the lunar core is ~1.6×10^21 kg. There is also an excess FeO component in the lunar mantle of 1.3-3.5x10^21 kg as Fe. The total excess (core + mantle) lunar Fe is 3-5x10^21 kg or ~2% of Theia's core. This mass is comparable to the excess Fe of 2.3-10x10^21 kg in the Earth's mantle inferred from the Pt-group veneer and chondritic Fe/Pt. Most likely Fe metal from Theia's core entered the Moon-forming disk. Part of the Fe was oxidized by H2O and Fe2O3 in the disk, leaving the lunar mantle near the Fe-FeO buffer. The remaining iron metal condensed, gathered lunar Pt-group elements from the disk, and became the lunar core."

    I enjoy reports like this that show the oceans could be sterilized by boiling and the lunar core contains material from Theia in the giant impact model. Testing lunar samples and showing what is original lunar material and what iron came from Theia is interesting, especially in the lunar core :)
    Reply