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New
images acquired by NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft
show that the Moon’s crust is pulling apart—at least in some small
areas. The high-resolution images from the Lunar Reconnaissance Orbiter
Camera (LROC) provide evidence that the Moon has experienced relatively
recent geologic activity.
In
the new LROC images, a team of researchers discovered small, narrow
trenches typically only hundreds to a few thousand meters (yards) long
and tens to hundreds of meters wide, indicating the lunar crust is being
pulled apart at these locations. These linear troughs or valleys, known
as graben, are formed when crust is stretched, breaks and drops down
along two bounding faults. A handful of these graben systems have been
found across the lunar surface. The team proposes that the geologic
activity that created the graben occurred less than 50 million years ago
(very recent compared to the Moon’s current age of over 4.5 billion
years).
In August, 2010,
the team identified physical signs of contraction on the lunar surface,
in the form of lobe-shaped ridges or scarps (known as lobate scarps),
using LROC images. They suggest that these scarps indicate the Moon
shrank globally in the geologically recent past and might still be
shrinking today. The team saw these scarps widely distributed across the
Moon and concluded that it was shrinking as the interior slowly cooled.
The new images of graben therefore present a contradiction—regions of
the lunar crust that are being pulled apart as the Moon shrinks.
“We
think the Moon is in a general state of global contraction due to
cooling of a still hot interior. The graben tell us that forces acting
to shrink the Moon were overcome in places by forces acting to pull it
apart,” says Thomas Watters of the Center for Earth and Planetary
Studies at the Smithsonian’s National Air and Space Museum, lead author
of a paper on this research appearing in the March issue of the journal
Nature Geoscience. “This means the contractional forces shrinking the
Moon cannot be large, or the small graben might never form.”
The
small graben indicate that contractional forces in the lunar crust are
relatively weak. The weak contraction indicates that unlike the
terrestrial planets—Mercury, Venus, Earth, and Mars—the Moon did not
completely melt in the very early stages of its evolution. Instead, an
alternative scenario better fits the observations: only the Moon’s
exterior initially melted forming a magma ocean.
“It
was a big surprise when I spotted graben in the farside highlands,”
says Mark Robinson of the School of Earth and Space Exploration at
Arizona State University, coauthor and principal investigator of LROC.
“I immediately targeted the area for high resolution stereo images so we
could create a 3-dimensional view of the graben. It’s exciting when you
discover something total unexpected. Only about half the lunar surface
has been imaged in high resolution. There is much more of the Moon to be
explored.”
As
the LRO mission progresses and coverage increases, scientists will have
a better picture of how common these young graben are, and more
importantly, what other types of tectonic features are nearby. The
number of graben systems the team finds may help scientists refine the
state of stress in the lunar crust.
Lunar Reconnaissance Orbiter Camera (LROC)
