The gravitational forces the moon and sun
exert are responsible for Earth's rising and falling tides. Earth's
gravity also exerts forces on the Moon in the form of solid body tides
that distort its shape. The Moon is slowly receding away from Earth and
forces build as the moon's tidal distortion diminishes with distance and
its rotation period slows with time. These tidal forces combined with
the shrinking of the moon from cooling of its interior have influenced
the pattern of orientations in the network of young fault scarps.
Credit: NASA/LRO
Earth’s gravity has influenced the orientation of thousands
of faults that form in the lunar surface as moon shrinks, according to new
results from NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft.
In August, 2010, researchers using images from LRO’s narrow
angle camera (NAC) reported the discovery of 14 cliffs known as “lobate scarpeds”
on the moon’s surface, in addition to about 70 previously known from limited
high resolution Apollo panoramic camera photographs. Due largely to their
random distribution across the surface, the science team concluded that the
moon is shrinking.
These small faults are typically less than 6.2 miles (10KM)
long and only tens or meters high. They are most likely formed by global
contraction resulting from cooling of the moon’s still hot interior. As the
interior cools and portions of the liquid outer core solidify, the volume
decreases, thus the moon shrinks and the crust solid crust buckles.
Now, after more than six years in the orbit, the Lunar
Reconnaissance Orbiter camera has imaged nearly three-fourth of the lunar
surface at high resolution, allowing the discovery of over 3000 more of these
features.
These globally distributed faults have emerged as the most
common tectonic land-form on the moon. An analysis of the orientation of these
small scarps yielded a surprising result: the faults created as the moon
shrinks are being influenced by an Unexpected source—gravitational tidal forces
from Earth.
Global contraction alone should generate an array of thrust
faults with no particular patter in the orientation of the faults, because the
contracting forces have equal magnitude in all directions.” This is not what we
found,” says Smithsonian senior scientist Thomas waters of the National Air and
Space Museum in Washington.
“There is a pattern in the orientation of the thousands of
faults and it suggests something else is influencing their formation, something
that is also acting on a global scale—“messaging’ and realigning them.
The other forces acting on the moon come not from its
interior, but from Earth. When the tidal forces are superimposed on the global
contraction, the combined stresses should cause predictable orientation of the
fault scarp from region to region. Watters says, “The agreement between the
mapped fault orientations and the fault orientations predicated by the modeled
tidal and constructional forces is pretty striking.”
The discovery of so many previously undetected tectonic
features as the LROC high-resolution image coverage continues to grow truly remarkable.
“Early on this mission we suspected that the tidal forces played a role in the
formation of tectonic feature, but we did not have enough coverage to make any
conclusive statements. Now that we have NAC images with appropriate lighting for
more than half the moon, structural patterns are starting to come into focus,”
said Robinson of Arizona State University.
The fault scarps are very young, so young that they are
likely still forming today. The teams modeling shows that the peak stresses are
reached when the moon is farthest from Earth. If the faults are still active,
the occurrence of shallow moonquakes related to slip events on the faults may
be most frequent when the moon is at apogee. This hypothesis can be tested with
a long-lived lunar seismic network.
“With LRO we’ve been able to study the moon globally in detail
not yet possible with any other body in the solar system beyond Earth,” Said
John Keller, LRO project Science at NASA’s Goddard Space Flight Center,
Greenbelt, Maryland.
Launched on June 18, 2009, LRO has collected a treasure
trove of data with its seven powerful instruments, making an invaluable
contribution to our knowledge about the moon. LRO is managed by NASA’s Goddard
Space Flight Center in greenbelt, Maryland, under Discovery program, managed by
NASA’s Marshall Space Flight Center in Huntsville for the Science Mission
Directorate at NASA headquarters in Washington, DC.
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