Apollo Data Retooled to Provide Precise Readings on Moon’s Core

Article written: 6 Jan , 2011
Updated: 24 Dec , 2015
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A new look at data from seismic experiments left on the Moon by Apollo astronauts has given researchers a better understanding of the lunar interior. The Moon’s core appears to be very similar to the Earth’s — with a solid inner core and molten liquid outer core — and its size is right in the middle of previous estimates.

“While the presence of a liquid core had previously been inferred from other geophysical measurements, we have made the first direct seismic observation of a liquid outer core,” said Dr. Renee Weber, a planetary scientist at NASA’s Marshall Space Flight Center, who led the team of researchers.

Apollo Passive Seismic Experiment from Apollo 15. Credit: NASA

The Apollo Passive Seismic Experiment measured seismic waves on the Moon and consisted of four seismometers deployed on the lunar near side during the Apollo missions between 1969 and 1972. The instruments continuously recorded ground motion until late-1977. But the data was thought to be rather weak because of the small number of stations, the lack of observation of far-side events, and interference from “moon quakes.” As this was the only direct measurements from the Moon available, various researchers differed on key characteristics such as the core’s radius, composition, and state (i.e., whether it was solid or molten.)

“The moon’s deepest interior, especially whether or not it has a core, has been a blind spot for seismologists,” said Ed Garnero, a professor at Arizona State University and a member of the research team. “The seismic data from the old Apollo missions were too noisy to image the moon with any confidence.”

Weber and her colleagues re-analyzed the Apollo data using a method usually used for processing seismic data on Earth. Called array processing, seismic recordings are added together or “stacked” in a special way and studied together. The multiple recordings processed together allow researchers to extract very faint signals. The depth of layers that reflect seismic energy can be identified, ultimately signifying the composition and state of matter at varying depths.

This method can enhance faint, hard-to-detect seismic signals by adding seismograms together.

“If seismic wave energy goes down and bounces off of some deep interface at a particular depth, like the Moon’s core-mantle boundary, then that signal “echo” should be present in all the recordings, even if below the background noise level,” said Patty Lin, a postdoctoral candidate at ASU and another member of the team. “But when we add the signals together, that core reflection amplitude becomes visible, which lets us map the deep Moon.”

Weber told Universe Today that the shear waves do not penetrate fluid regions. “So while we have observed compression reflections off the solid inner core, we have not (as expected) observed shear reflections off the inner core, since that energy is reflected at the outer core layer.”

Recent studies suggested the Moon had a relatively small iron-rich core, sized between about 250 and 430 km, or roughly 15 to 25% of its 1,737.1 km mean radius. The new measurements put the core slightly larger.
“We have placed the core-mantle boundary at a radius of 330 km, roughly 19% of the Moon’s mean radius,” Weber said in an email.

The iron-rich core has a solid inner ball nearly 240 km (150 miles) in radius, and an 90 km (55-mile) thick outer fluid shell.

The new research also points to volatile-depleted interior, with the lunar core containing a small percentage of light elements such as sulfur, similar to light elements in Earth’s core – sulfur, oxygen and others.

The retooled 30-year-old data also appears to confirm the the leading theory of how the Moon formed.

“The presence of a melt layer and a molten outer core supports the widely-accepted large-impact model of lunar formation, which predicts that the Moon could have formed in a completely molten state,” Weber said.

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15 Responses

  1. Member
    Aqua says

    – “The Moon’s core appears to be very similar to the Earth’s — with a solid inner core and molten liquid outer core…”
    – “…we have made the first direct seismic observation of a liquid outer core.”

    Its taken me a while to wrap my head around what this article is saying… that the moon, AT PRESENT, has a liquid outer core… right? So this would be magma similar to what is found on Earth? What’s keeping it hot? Orbital/tidal torsion? Degenerate radio nucleotides? Residual heat from formation? Residual heat from ancient impact? or some other perhaps exotic energy source?

  2. Torbjorn Larsson OM says

    Well, we know one thing: The Moon is not a product of EU fantasies.

    Actually, if the Moon is similar to Earth, its heat stems from the same sources as Earths. After coalescence it is gravity heating (gravity potential energy) that is the main mechanism while the material goes to hydrostatic equilibrium and sorts out. Thereafter radioactivity is the remaining heat source.

    And neither core are “kept hot” since the heat sources tapers off, but are subject to cooling.

  3. Jlazor says

    Isn’t the first image of Ganymede? Or are my eyes deceiving me =p

  4. capper says

    Torbjorn Larsson OM January 6, 2011 at 1:03 pm
    “Well, we know one thing: The Moon is not a product of EU fantasies.”

    -BOOOOOOOOO!!!!!!!!!!!

    Get em off the stage!

  5. GBendt says

    Every stuff contains radioactive nucleids, which decay following their respective radioactive half life. Tehe decay produces energy, which is dissipated at the stuffs surface. The larger the body, the more distant is its surface from its center. Heat that is produced close to the surface can be dissipated there, but heat that is produced closer to the center leads to a heat accumulation. If the body is large enough, the center may melt from that heat. If the body has enough weight to put sufficient load on the center of the melt, the resulting pressure may be so high that it causes the melt to solidify there, as the melting point of materials is raised by pressure. This is the reason why the Moon and the Earth both have a solid inner core and a liquid outer core.
    The moon´s energy output from radioactive decay is some 100 Gigawatts. The respective value for our Earth is some 9000 Gigawatts.

    • Torbjorn Larsson OM says

      True, but as I noted above, in models for large bodies the initial heat comes mainly from the coalescence and gravitational sorting. (I.e. it is all gravitationally potential driven.) At about 0.5 ~ 1 Gy radioactivity became the remaining heat source as regards Earth, IIRC. [Quoting from a half forgotten astrobiology text here, since it is currently hard to dig up. Literary.]

      Other bodies may of course have other relations between sources and sinks et cetera.

      • Torbjorn Larsson OM says

        Um, oh, yes. Warning for the period there. Maybe that was as short as the 30 My of believed max time for core formation, and I’m confusing it with the plate tectonics drivers of convection. [Bugger! Looks at stack of papers and books that should be archived; nay, still too large to dig into today.]

  6. Jim Krug says

    This sounds fishy to me. For decades and decades, the Moon was considered geologically “dead”, and now a liquid outer core is mentioned in passing to no fanfare whatsoever?

    One thing’s for sure: there’s something very fishy about the Moon’s interior…

  7. Paul Eaton-Jones says

    No it’s not at all fishy. Part of the reason the moon was considered geologically dead was that the seismic readings were too gross and the finer ones couldn’t be teased out of the background noise. No conspiracy – move along nothing to see here.

  8. tigermonkey says

    Does this data explain the gravity miscalculation made before the first lunar landing in1969? With previous suprises like that the new data does not seem to support
    the observations made by the astronauts who walked on the surface. I would like to
    see the data used calculate the true gravitational force on the surface. If the data and the known gravitation do not agree, prehaps more data needs gathered or a better hypohesis should be presented. I will wait on more results thank you, or an
    explanation which supports both the astronauts findings and the new gravitational computations which should complement one another.

  9. Paul Eaton-Jones says

    I’m sure I read years ago that the gravity anomolies thrown up around the Apollo landings were due to previoulsy undiscovered MASCONS which turned out to be either massive basalt extrusions or the remains of large iron meterorites/asteroids.

  10. manvantura says

    Dr. Weber told Universe Today that the shear waves do not penetrate fluid regions. “So while we have observed compression reflections off the solid inner core, we have not (as expected) observed shear reflections off the inner core, since that energy is reflected at the outer core layer. …”
    Are we to infer that they have observed shear reflections off the outer core? If not, then the claim that “we have made the first direct seismic observation of a liquid outer core … ” is not true.

    The implication is that the data was rather weak because of the small number of stations, the lack of observation of far-side events, and interference from “moon quakes.” It seems to me that the only source of seismic impulses for a passive system would be the moon quakes. Did the researchers use planned impacts of spacecraft or of asteroids as their source impulses?

    “The presence of a melt layer and a molten outer core supports the widely-accepted large-impact model of lunar formation, which predicts that the Moon could have formed in a completely molten state,” Weber said.
    I believe the ‘widely accepted’ theory of lunar creation referred to is a collision of a large body with the Earth with the ejection of a gouged-out chunk of the Earth and the body then forming the Moon. Models up to now have generally pictured a big gash out of the mantle, based on the idea that the Moon was just a rocky body. It seems that for the Moon to have a solid iron core the collision would have to have penetrated at least to the outer core of the Earth, and probably reformed it completely. Were the oceans present at that date? If so why is the Moon so dry?

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