Arizona State University | 2011 Jan 06
At Long Last, Moon's Core 'Seen'The Moon, Earth’s closest neighbor, has long been studied to help us better understand our own planet. Of particular interest is the lunar interior, which could hold clues to its ancient origins. In an attempt to extract information on the very deep interior of the Moon, a team of NASA-led researchers applied new technology to old data. Apollo seismic data was reanalyzed using modern methodologies and detected what many scientists have predicted: the Moon has a core.
According to the team’s findings, published Jan. 6 in the online edition of Science, the Moon possesses an iron-rich core with a solid inner ball nearly 150 miles in radius, and a 55-mile thick outer fluid shell.
“The Moon’s deepest interior, especially whether or not it has a core, has been a blind spot for seismologists,” says Ed Garnero, a professor at the School of Earth and Space Exploration in ASU’s College of Liberal Arts and Sciences. “The seismic data from the old Apollo missions were too noisy to image the Moon with any confidence. Other types of information have inferred the presence of a lunar core, but the details on its size and composition were not well constrained.”
Sensitive seismographs scattered across Earth make studying our planet’s interior possible. After earthquakes these instruments record waves that travel through the interior of the planet, which help to determine the structure and composition of Earth’s layers. Just as geoscientists study earthquakes to learn about the structure of Earth, seismic waves of “moonquakes” (seismic events on the Moon) can be analyzed to probe the lunar interior.
When Garnero and his graduate student Peiying (Patty) Lin heard about research being done to hunt for the core of the Moon by lead author Renee Weber at NASA’s Marshall Space Flight Center, they suggested that array processing might be an effective approach, a method where seismic recordings are added together in a special way and studied in concert. 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.
“Array processing methods 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. But when we add the signals together, that core reflection amplitude becomes visible, which lets us map the deep Moon,” explains Lin, who is also one of the paper’s authors.
The team found the deepest interior of the moon to have considerable structural similarities with the Earth. Their work suggests that the lunar core contains a small percentage of light elements such as sulfur, similar to light elements in Earth’s core – sulfur, oxygen and others.
“There are a lot of exciting things happening with the Moon, like Professor Mark Robinson’s LRO mission producing hi-res photos of amazing phenomena. However, just as with Earth, there is much we don’t know about the lunar interior, and that information is key to deciphering the origin and evolution of the Moon, including the very early Earth,” explains Garnero.
Science NOW | Richard A. Kerr | 2100 Jan 06
Seismic Detection of the Lunar Core - RC Weber et alApollo astronauts may be garnering another prize from their exploits of more than 3 decades ago. They left seismometers across the face of the moon to probe its interior, but no one had been able to paint a clear picture from the data the sensors collected. Now, two independent groups have reanalyzed the Apollo data using modern but very different techniques, and both teams say they have detected lunar seismologists' prime target: a core of iron that is still molten 4.5 billion years after the moon's formation.
The Apollo seismic experiment was challenging from the start. Moonquakes are sparse and feeble, the moon's impact-shattered crust garbles any seismic signals, and computers of that era couldn't handle the complete data set. Today, computers are faster, and terrestrial seismologists have developed far more powerful analytical techniques, so lunar researchers have taken another crack at the Apollo seismic data, which were recorded by the five sensors and radioed back until the mid-1970s.
Like an earthquake, a moonquake sets off ripples of motion called seismic waves that speed through surrounding rock. Both groups combed the data for signs of quakes' waves that may have reflected off the core, but each group took a very different approach. Planetary scientist Renee Weber of NASA's Marshall Space Flight Center in Huntsville, Alabama, and her colleagues analyzed four types of seismic waves—which differ in the direction of vibration—from deep quakes clustered in 38 spots. They combined seismic records from each cluster to bring out any reflected signals and filtered the combined records to remove some of the noise. Seismologist Raphaël Garcia of the University of Toulouse in France and his colleagues, on the other hand, analyzed two wave types from three moonquakes after calibrating the seismic stations.
In back-to-back talks at last month's meeting of the American Geophysical Union in San Francisco, California, the groups reported that, like Earth, the moon has a molten core. Garcia and colleagues found a liquid core with a radius of 365 kilometers. Weber and her colleagues reported a core radius of 330 kilometers, which they also report online today in Science. Given the uncertainties, the two estimates are indistinguishable. In addition, Weber found seismic reflections from a solid inner core with a radius of 240 kilometers—a feature Earth has as well—and reflections from a layer of mostly rock with a bit of magma 150 kilometers thick lying above the liquid iron outer core.
"I'm surprised they could get this much information from this data," says planetary physicist David Stevenson of the California Institute of Technology in Pasadena. If the seismic results hold up, he adds, they would be by far the strongest evidence yet for a liquid core. Then researchers could use the detailed seismic picture of the moon's interior to understand better the evolution of a planetary body assembled from the vaporous debris of a giant impact on the still-forming Earth. But they're not quite there yet. "The Apollo [seismic] data have all sorts of weirdness in them," says seismologist Jesse Lawrence of Stanford University in Palo Alto, California. "As is so often the case, more work needs to be done."
- Science (online 2011 Jan 06) DOI: 10.1126/science.1199375
Universe Today | Nancy Atkinson | 2011 Jan 06
Details of the Moon's Core Revealed by 30-year-old DataA 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.
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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.)
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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. ...
Space.com | Science & Astronomy | 2011 Jan 06