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: firstname.lastname@example.org.