New Research Reveals Relatively Recent (100 Million Years Ago)
Tectonic Activity
Newly discovered cliffs in the lunar crust indicate the moon
shrank globally in the geologically recent past and might still be
shrinking today, according to a team analyzing new images from
NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft. The results
provide important clues to the moon's recent geologic and tectonic
evolution.
The moon formed in a chaotic environment of intense bombardment
by asteroids and meteors. These collisions, along with the decay of
radioactive elements, made the moon hot. The moon cooled off as it
aged, and scientists have long thought the moon shrank over time as
it cooled, especially in its early history. The new research
reveals relatively recent tectonic activity connected to the
long-lived cooling and associated contraction of the lunar
interior.
"We estimate these cliffs, called lobate scarps, formed less
than a billion years ago, and they could be as young as a hundred
million years," said Dr. Thomas Watters of the Center for Earth and
Planetary Studies at the Smithsonian's National Air and Space
Museum, Washington. While ancient in human terms, it is less than
25 percent of the moon's current age of more than four billion
years. "Based on the size of the scarps, we estimate the distance
between the moon's center and its surface shrank by about 300
feet," said Watters, lead author of a paper on this research
appearing in Science August 20.
"These exciting results highlight the importance of global
observations for understanding global processes," said Dr. John
Keller, Deputy Project Scientist for LRO at NASA's Goddard Space
Flight Center, Greenbelt, Md. "As the LRO mission continues in to a
new phase, with emphasis on science measurements, our ability to
create inventories of lunar geologic features will be a powerful
tool for understanding the history of the moon and the solar
system."
Lunar Fault Line NASA Image
The scarps are relatively small; the largest is about 300 feet
high and extends for several miles or so, but typical lengths are
shorter and heights are more in the tens of yards (meters) range.
The team believes they are among the freshest features on the moon,
in part because they cut across small craters. Since the moon is
constantly bombarded by meteors, features like small craters (those
less than about 1,200 feet across) are likely to be young because
they are quickly destroyed by other impacts and don't last long.
So, if a small crater has been disrupted by a scarp, the scarp
formed after the crater and is even younger. Even more compelling
evidence is that large craters, which are likely to be old, don't
appear on top any of the scarps, and the scarps look crisp and
relatively undegraded.
Lobate scarps on the moon were discovered during the Apollo
missions with analysis of pictures from the high-resolution
Panoramic Camera installed on Apollo 15, 16, and 17. However, these
missions orbited over regions near the lunar equator, and were only
able to photograph some 20 percent of the lunar surface, so
researchers couldn't be sure the scarps were not just the result of
local activity around the equator. The team found 14 previously
undetected scarps in the LRO images, seven of which are at high
latitudes (more than 60 degrees). This confirms that the scarps are
a global phenomenon, making a shrinking moon the most likely
explanation for their wide distribution, according to the team.
Fault Scarp (left) At Apollo 17 Landing Site (arrow) NASA
Image
As the moon contracted, the mantle and surface crust were forced
to respond, forming thrust faults where a section of the crust
cracks and juts out over another. Many of the resulting cliffs, or
scarps, have a semi-circular or lobe-shaped appearance, giving rise
to the term "lobate scarps". Scientists aren't sure why they look
this way; perhaps it's the way the lunar soil (regolith) expresses
thrust faults, according to Watters.
Lobate scarps are found on other worlds in our solar system,
including Mercury, where they are much larger. "Lobate scarps on
Mercury can be over a mile high and run for hundreds of miles,"
said Watters. Massive scarps like these lead scientists to believe
that Mercury was completely molten as it formed. If so, Mercury
would be expected to shrink more as it cooled, and thus form larger
scarps, than a world that may have been only partially molten with
a relatively small core. Our moon has more than a third of the
volume of Mercury, but since the moon's scarps are typically much
smaller, the team believes the moon shrank less.
Because the scarps are so young, the moon could have been
cooling and shrinking very recently, according to the team.
Seismometers emplaced by the Apollo missions have recorded
moonquakes. While most can be attributed to things like meteorite
strikes, the Earth's gravitational tides, and day/night temperature
changes, it's remotely possible that some moonquakes might be
associated with ongoing scarp formation, according to Watters. The
team plans to compare photographs of scarps by the Apollo Panoramic
Cameras to new images from LRO to see if any have changed over the
decades, possibly indicating recent activity.
While Earth's tides are most likely not strong enough to create
the scarps, they could contribute to their appearance, perhaps
influencing their orientation, according to Watters. During the
next few years, the team hopes to use LRO's high-resolution Narrow
Angle Cameras (NACs) to build up a global, highly detailed map of
the moon. This could identify additional scarps and allow the team
to see if some have a preferred orientation or other features that
might be associated with Earth's gravitational pull.
"The ultrahigh resolution images from the NACs are changing our
view of the moon," said Dr. Mark Robinson of the School of Earth
and Space Exploration at Arizona State University, Tempe, Ariz., a
coauthor and Principal Investigator of the Lunar Reconnaissance
Orbiter Camera. "We've not only detected many previously unknown
lunar scarps; we're also seeing much greater detail on the scarps
identified in the Apollo photographs."
LRO NASA Image
The research was funded by NASA's Exploration Systems Mission
Directorate at NASA Headquarters, Washington. The team includes
researchers from the Smithsonian, Arizona State, the SETI
Institute, Mountain View, Calif., NASA Ames Research Center,
Moffett Field, Calif., Cornell University, Ithaca, NY, Institut fur
Planetologie, Westfalische Wilhelms-Universitat, Munster, Germany,
Brown University, Providence, RI, and the Johns Hopkins University
Applied Physics Laboratory, Laurel, MD.