NASA Research Team Reveals Moon Has Earth-Like Core
WASHINGTON - State-of-the-art seismological techniques applied to
Apollo-era data suggest our moon has a core similar to Earth's.
Uncovering details about the lunar core is critical for developing
accurate models of the moon's formation. The data sheds light on the
evolution of a lunar dynamo -- a natural process by which our moon
may have generated and maintained its own strong magnetic field.
The team's findings suggest the moon possesses a solid, iron-rich
inner core with a radius of nearly 150 miles and a fluid, primarily
liquid-iron outer core with a radius of roughly 205 miles. Where it
differs from Earth is a partially molten boundary layer around the
core estimated to have a radius of nearly 300 miles. The research
indicates the core contains a small percentage of light elements such
as sulfur, echoing new seismology research on Earth that suggests the
presence of light elements -- such as sulfur and oxygen -- in a layer
around our own core.
The researchers used extensive data gathered during the Apollo-era
moon missions. The Apollo Passive Seismic Experiment consisted of
four seismometers deployed between 1969 and 1972, which recorded
continuous lunar seismic activity until late-1977.
"We applied tried and true methodologies from terrestrial seismology
to this legacy data set to present the first-ever direct detection of
the moon's core," said Renee Weber, lead researcher and space
scientist at NASA's Marshall Space Flight Center in Huntsville, Ala.
In addition to Weber, the team consisted of scientists from Marshall;
Arizona State University; the University of California at Santa Cruz;
and the Institut de Physique du Globe de Paris in France. Their
findings are published in the online edition of the journal Science.
The team also analyzed Apollo lunar seismograms using array
processing, techniques that identify and distinguish signal sources
of moonquakes and other seismic activity. The researchers identified
how and where seismic waves passed through or were reflected by
elements of the moon's interior, signifying the composition and state
of layer interfaces at varying depths.
Although sophisticated satellite imaging missions to the moon made
significant contributions to the study of its history and topography,
the deep interior of Earth's sole natural satellite remained a
subject of speculation and conjecture since the Apollo era.
Researchers previously had inferred the existence of a core, based on
indirect estimates of the moon's interior properties, but many
disagreed about its radius, state and composition.
A primary limitation to past lunar seismic studies was the wash of
"noise" caused by overlapping signals bouncing repeatedly off
structures in the moon's fractionated crust. To mitigate this
challenge, Weber and the team employed an approach called seismogram
stacking, or the digital partitioning of signals. Stacking improved
the signal-to-noise ratio and enabled the researchers to more clearly
track the path and behavior of each unique signal as it passed
through the lunar interior.
"We hope to continue working with the Apollo seismic data to further
refine our estimates of core properties and characterize lunar
signals as clearly as possible to aid in the interpretation of data
returned from future missions," Weber said.
Future NASA missions will help gather more detailed data. The Gravity
Recovery and Interior Laboratory, or GRAIL, is a NASA Discovery-class
mission set to launch this year. The mission consists of twin
spacecraft that will enter tandem orbits around the moon for several
months to measure the gravity field in unprecedented detail. The
mission also will answer longstanding questions about Earth's moon
and provide scientists a better understanding of the satellite from
crust to core, revealing subsurface structures and, indirectly, its
thermal history.
NASA and other space agencies have been studying concepts to establish
an International Lunar Network -- a robotic set of geophysical
monitoring stations on the moon -- as part of efforts to coordinate
international missions during the coming decade.
For more information about NASA science exploration missions, visit:
http://www.nasa.gov/topics/
Source: NASA
