New electronic images, beamed back to Earth by NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft last week, show that the Moon is shrinking. Images of 14 recently discovered lunar cliffs reveal that our nearest neighbor in space has been slowly shrinking since it first started cooling off billions of years ago.
In Moon terms, the big cliffs aren't too old-about 100 million years young, but they reveal an odd tectonic phenomenon that happened in "recent" times. The distance between the lunar center and the surface shrank by more than 300 feet, based on the height of the largest cliff faces (that's the height of NASA's old Saturn-5 Moon rocket). The cliffs pictured in LRO images published last week are the result of global crustal contraction. They are akin to the ugly furrows of an apple left neglected on a kitchen counter for many weeks.
Called lobate scarps, Dr. Thomas Watters of the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum said the cliffs highlight the need for ongoing spacecraft-based global lunar observations. But with NASA's lunar exploration plans shrinking faster than the Moon's crust-all thanks to an anti-space White House administration-such desires may be but a sign of hypnagogia in the inauspicious Obama Space Age.
Since the Moon is over a third of the volume of the planet Mercury, its lobate scarps are smaller. As a result, according to NASA, the Moon shrank less than Mercury. And because the lunar cliffs appear to be very young, the Moon is still cooling and shrinking. Some moonquakes are probably signatures of ongoing shrinking.
Following the Apollo era, it was assumed there was nothing new in lunar geology. Since the U.S. Clementine and Lunar Prospector spacecraft of the 1990s, we now know that the Moon is truly a vast, unexplored world with lots of geological surprises just around the corner.
In addition to quakes, it is possible that the Moon's crustal contractions may also trigger small, transient volcanic eruptions. Such eruptions may be the source of so-called Lunar Transient Phenomena that have been observed by both astronomers and Apollo astronauts. For example, the lunar craters Aristarchus, Kepler and Tycho, among many others, are the scenes of occasional "mists" and flares that might be evidence of such short-lived eruptions. I explore this fascinating topic in selenology in my book "Inconstant Moon: Discovery and Controversy on the Way to the Moon" (Xlibris/Random House).
Back to lobate scarps. On the Moon they extend for many dozens of miles and some amateur astronomers are already searching for them with their large earth-based telescopes. In some cases, LRO images show small, recent craters breached by the cliffs, another sign that the "shrinkage" occurred in "recent" geological times. Most of the cliffs look fresh and recently formed.
According to NASA, lunar lobate scarps were first observed by Apollo astronauts orbiting the Moon. So NASA researchers are poring through the high-resolution Panoramic Camera images taken during the 1971-72 flights of Apollo 15, 16, and 17. This heritage of data now confirms that the cliffs span the entire Moon, thus making the idea of an entire shrinking world a reality.
According to a report by Space Daily, "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."
Beyond the Moon, the planet Mercury is another source for lobate scarps, but they are much grander in scale. Mercury's cliffs are over one mile high and run like faults for hundreds of miles.
What's in the Sky: The dwarf planet Pluto, at mag. 14, is in the northwestern portion of the constellation Sagittarius now. It is high in the south after sunset. Visit your public library or check online for Sky & Telescope magazine's Pluto finder charts in the July 2010 issue for details.
Lou Varricchio, M.Sc., was a science writer at the NASA Ames Research Center. He is also a member of the NASA-JPL Solar System Ambassador program. Readres may e-mail him at: email@example.com.