On the right timescale, the Moon is a pretty tumultuous place. Since being chipped off the Earth some four and a half billion years ago, our natural satellite hasn’t simply been a shiny piece of eye-candy each month. It’s moving around an inch and a half away from us each year, and may eventually break out of its orbit. While it’s still in the neighborhood, we’ve confirmed that not only is it shrinking, but it’s also being yanked apart by the Earth at the same time.
When we sent astronauts to visit the Moon, they experienced a series of seismic events on the surface. Multiple types of quakes were detected, but the shaking alone didn’t always explain the origin of the activity. Looking at the rocky satellite from a different perspective, the Lunar Reconnaissance Orbiter (LRO) has shed new light on what’s been moving the Moon.
Crinkles from cooling
Fault scarps are like cliffs that form when pieces of terrain shift against each other. They’ve been seen in multiple places in the solar system, most dramatically on Mercury. The tiny planet closest to the Sun has huge scarps a mile high and hundreds of miles long. Their extreme size suggests that Mercury has seen some significant rearrangement of its surface due to cooling and shrinking. It was likely once completely molten with a larger volume. As the planet solidified, the cooling matter condensed into a smaller ball, leaving the surface to buckle and crinkle into the large scarps.
The fact that the LRO has found scarps across our Moon suggests a similar but gentler process at work. The Moon was likely partially molten at its formation, and has been cooling and shrinking ever since. By looking at scarps intersecting with other terrain features, like impact craters, scientists believe that the scarps are still being created today. The strange part is that they’re not popping up where you might expect if shrinkage was the Moon’s only problem.
Pull of a planet
Rather than a randomized distribution, the density of scarps on the Moon seems to be influenced by the Earth’s gravitational pull. Tidal forces were expected to be influencing seismic activity on the the Moon, but scarps buildup along a the sides closest and farthest from the Earth show just how much pull the Earth has. The data obtained from the LRO is showing a strong pattern in where these scarps are appearing, but to really confirm all this activity we’ll need some more seismic sensors on the Moon’s surface.
Source: NASA's LRO Discovers Earth's Pull is 'Massaging' our Moon by Nancy Neal-Jones / William Steigerwald, NASA.gov