The moon is a permanent feature in our skies, but is it as unchanging as it seems?
Scientists consider the Earth’s only natural satellite to be a pristine environment, an “open book” where the history of the solar system is written. But according to new observations by NASA’s Lunar Reconnaissance Orbiter (LRO), there’s more than just impact craters — born from the violent early days of our developing star system — written in the lunar landscape.
Reported in a new paper set for publication in the Aug. 20 issue of the journal Science, previously undetected landforms have been spotted by the LRO’s high resolution camera. These landforms are known as “lobate scarps” and were first identified in photographs taken by the Apollo 15, 16 and 17 missions.
However, the lobate scarps seen in these early missions appeared to be clustered around the equatorial regions. Now, very high resolution observations made by the LRO have revealed 14 more previously unknown scarps, signifying that they can be found all over the moon’s surface.
Lobate scarps are raised features approximately 9 meters high by several kilometers long that form along thrust faults. On Earth, thrust faults are obvious as older rock is forced to lift and fold over younger rock — this is caused by compression. For a geologically active body like the Earth, this is common, but the moon is not geologically active. What is creating this pressure?
“Relatively young, globally distributed thrust faults show recent contraction of the whole moon, likely due to cooling of the lunar interior. The amount of contraction [from the center of the moon to lunar surface] is estimated to be about 100 meters in the recent past,” said Thomas Watters of the Center for Earth and Planetary Studies at the National Air and Space Museum and lead author of the paper.
David Morrison, senior scientist at NASA’s Lunar Science Institute and NASA’s “Ask an Astrobiologist” expert who wasn’t involved with this research, likened this process to the wrinkles that form on an old apple as it dehydrates and shrinks. “You’re trying to fit the crust [of the moon] around a smaller interior, and the only way that can happen is if these scarps form,” Morrison told Discovery News.
“In the Earth system, where you have plate tectonics, you have as many new voids created as mountains thrust up,” he added, pointing out that there is no evidence on the lunar surface for these “voids” — or regions where the lunar crust has been pulled apart to balance out the regions of compression causing the scarps. The moon just has compression regions, wrinkling the surface. The logical conclusion is therefore that the core of the moon has shrunk and the crust buckled under the resulting compression.
Another interesting fact about these newly discovered scarps is their apparent young age. Along the thrust faults, some small impact craters have been overridden by the scarps (pictured above), giving an idea that the scarps are must have formed after some ancient lunar impacts.
Although this is fascinating, the moon isn’t the only celestial body to show signs of shrinkage.
“Ever since the first Mariner 10 flyby of the planet Mercury, one of the things that characterized the surface is the same lobate thrust faults also attributed to shrinkage,” Morrison said.
“Until now, Mercury was the only place that we’ve seen this with crustal shrinkage of several kilometers. Now it looks like it’s happened in our own back yard as well.”
Images (from top): A wrinkled apple (iStockPhoto), the LRO scarp observation plus overridden craters and an oblique view of a lobate thrust from the LRO (NASA/GSFC/Arizona State University).