We often think of the moon as a geologically dead fixture in our sky. That isn't a bad thing; scientists have long looked at the moon as a perfectly preserved slice of our solar system's history.
But new images returned from NASA's Lunar Reconnaissance Orbiter suggest that our natural satellite isn't dead at all. It's actually pretty active, having both shrank and grown fairly recently in its history.
The first evidence of an active moon came in 2010 when LRO’s camera returned high-resolution pictures of landforms called lobate scarps. Previously found only in the equatorial regions in images from Apollo missions 15, 16 and 17, these lobe-shaped cliffs have now been found scattered across the lunar surface.
Less than 10 meters high and several kilometers long, lobate scarps form along thrust faults — inclined fractures where blocks of a body's crust rise vertically. It's likely they formed when the moon's interior cooled and the rock contracted to force slices upward. These are common on the geologically active Earth, but finding them on the moon was a shock.
What's more, lunar scarps are fairly young; many have sliced ancient impact craters. As the moon's core has continued to cool in the relatively recent past, the crust has buckled under compression. It's sort of like an apple. As the fruit dehydrates, the skin buckles and forms around the shrinking core.
Based on the sizes of the scarps, scientists estimate the distance between the center of the moon and its surface shrank about 300 feet in the process. It's a very different moon than the dead one scientists have long imagined.
Now add features called graben to the picture. Graben are linear valleys typically much longer than they are wide, and finding these features on the moon was a total shock to mission scientists. They formed, like the lobate scarps, when the moon cooled. But instead of forming when the moon's crust buckled, they formed when the crust was stretched, broken and dropped down between two bounding faults. In short, it's growing.
Around the same time the moon was shrinking around its cooling core, forces in some places were acting to pull it apart. It's like a game of tectonic tug-of-war being played on the supposedly inert lunar surface. Taken together, these discoveries paint a very different picture of our moon, and it all relates to the moon's evolution, how it formed and how it lost its heat.
Most terrestrial planets in our solar system were so hot when they first formed that they completely melted. This put them in a general state of contraction — the inside stayed hot while the outer layer cooled.
The mix of scarps and graben found on the moon suggest that our satellite never completely melted in its earliest stages of formation. Instead, only the outer portion melted, covering the surface with an expansive layer of molten rock called a magma ocean. The balance of stresses acting on that early moon creates the right conditions for the somewhat contradictory surface features we see today.
These graben add to the evidence of recent geologic activity on the moon. Scientists think these features are only about 50 million years old, compared with the moon’s 4.5 billion-year life.
Our moon is shrinking in some places, growing in others, but worth further exploration everywhere.
Image: NASA's LRO in lunar orbit. Credit: NASA.