It’s a smaller moon we see
Newly discovered cliffs on the moon indicate that it has shrunk in the past as it cooled off, and that it might even still be shrinking, researchers say.
The shrinkage isn’t dramatic — perhaps no more than a 300-foot reduction in the moon’s 2,000-mile diameter — but it is enough to cause cracks to form just like they would in the rind of a dried-up orange.
Researchers had first noticed the cliffs, technically called lobate scarps because they are semicircular, like a lobe, in images from the Apollo 15, 16 and 17 missions. But those images were collected only near the equator, and geologists thought they probably indicated a local phenomenon.
But new images from the entire surface of the moon collected by the Lunar Reconnaissance Orbiter, reported in Thursday’s edition of the journal Science, show that the scarps appear all over the body.
“This is the first evidence that the moon has been shrinking, and may still be shrinking,” NASA’s chief lunar scientist, Michael Wargo, said in a news conference. But, he added, “the kind of radius change and shrinking we are describing here is so small that you would never notice it.”
The scarps can be observed only with high-resolution cameras of the type that are aboard the lunar orbiter. The biggest is about 300 feet high and several miles long.
That is almost microscopic compared with similar scarps on Mercury, which can be more than a mile high and hundreds of miles long, according to the lead investigator, Thomas Watters of the Smithsonian Institution’s National Air and Space Museum. Mercury was much hotter than the moon when it formed, and thus contracted much more strongly during cooling.
The moon is thought to have formed early in the history of the solar system when a massive object crashed into Earth, knocking out a chunk that began to orbit our planet. Heat from the radioactive lunar core, the impact and subsequent meteor impacts caused the moon to collapse into its more spherical shape before cooling.
Planetary geologists estimate that the lunar scarps formed no more than 1 billion years ago — the moon, like the Earth, is about 4.5 billion years old — and perhaps as recently as 100 million years ago. There is a small possibility, Wargo said, that the process is continuing now.
The best evidence for the relatively recent age of some scarps is that many of them cut across meteor craters, deforming their shape. Many of the craters are kinds that disappear fairly quickly from the moon’s surface, Watters said. Moreover, none of the scarps show any evidence of having been struck by such impacts after their formation, even though meteor strikes are ongoing on the moon.
thomas.maugh@latimes.com
