The moon has been shrinking, suggest scientists who spotted relatively young geological features that form when a planetary body cools and contracts.
Called lobate scarps, the features are made when land on one side of a geologic fault line is thrust upward, creating a slanting wall that can be several hundred feet high and several miles long.
"If you were walking up to one of these landforms, you would see basically what looks like a stair-step in the landscape," said study co-author Thomas Watters of the Center for Earth and Planetary Studies at the National Air and Space Museum in Washington, D.C.
"It would be steep in the front on the scarp face itself and then gently sloping on the backside."
The Apollo 15, 16, and 17 missions photographed about 70 lobate scarps near the moon's equator. (Read the original National Geographic magazine coverage of the Apollo 11 moon landing.)
But NASA's Lunar Reconnaissance Orbiter found 14 more scarps in several, widespread locations, suggesting the thrust faults are globally distributed across the moon's surface. (See some of the first moon pictures from the Lunar Reconnaissance Orbiter.)
Watters and colleagues think the scarps formed as the inside of the moon cooled from its original, molten state.
"As the interior cools, it contracts, and the crust of the moon has to adjust to the reduced volume," Watters said. "That causes this breaking of the crust into these small faults." Lobate scarps form when the contracting crust puts pressure along the fault planes.
What's more, all the scarps seen so far appear to have formed in the last billion years or so—relatively recent in the moon's roughly 4.6-billion-year history. (Take a moon mysteries and myths quiz.)
"They're very crisp, fresh looking features," Watters said. "There're constant micrometeorite bombardments on the moon, and features of this scale are probably not going to survive very long."
Based on the size of the scarps, Watters and his team estimate that the moon's width has shrunk by about 600 feet (182 meters) since the rocky body first formed.
(Related: Find out why Jupiter's Great Red Spot has been shrinking.)
Shrinking Moon a "Blockbuster" Discovery
Earth also has lobate scarps, although processes other than planetary cooling created the features. But scientists think scarps seen on Mars and Mercury—which are taller and longer than the moon's scarps—formed due to shrinkage.
The pristine state of the lunar scarps suggests the moon got cool enough to begin contracting only recently—which means it may still be shrinking, the study authors say.
This is surprising, given the moon's relatively small size, said Pat McGovern, a geophysicist at the Lunar and Planetary Institute in Houston, Texas, who called the new findings a "blockbuster result."
"Small planetary bodies tend to lose their internal heat very quickly," said McGovern, who was not involved in the new research.
"Heat is what drives the overall evolution of a planet, and the moon is thought to have lost most of [its heat] long ago. But the idea that you have very young faulting going on is very exciting, because it's somewhat unexpected."
While the moon might still be shrinking, its rate of contraction is probably slowing, and shrinkage will eventually halt completely, study co-author Watters said.
"The moon is going to contract less and less as time goes on, because its interior is cooling more and more," he said.
(Related: "Ancient Moon Had Earthlike Core?")
It's currently hard to say how much smaller the moon is likely to get, Watters added, but more images from NASA's lunar spacecraft could help answer that question.
"One of the exciting things about this is that we're continuing to image the moon with the Lunar Reconnaissance Orbiter," which is scheduled to operate for at least another two years, Watters said.
"Our hope is that with an extended mission we can produce a global map of the moon that's high enough resolution to see these lobate scarps everywhere."
The moon's shrinkage is described in this week's issue of the journal Science.