Multiple types of quakes move the Moon
The Moon is not as still and lifeless as it looks. Despite lacking Earth’s sometimes volatile tectonic plates, the Moon’s surface is more than an inert collection of dusty craters and human debris. On the exterior, bursts of flame have melted sands into glass beads. On the interior, the whole satellite regularly experiences multiple forms of seismic activity, some of which has yet to be explained.
A quartet of quakes
The first type of Moonquake is the most predictable. Every 27 days or so the Moon experiences a small seismic event, never more than 2 on the Richter scale. The regular schedule suggests that it’s due to the core of the Moon being pulled on by the Earth’s tides, thanks to shifting relationships in the to bodies’ centers of gravity.
The second type of seismic activity may be more like a the creaks and groans of your house as it cools down at night. The temperature range between day and night on the Moon is huge: 253º Fahrenheit in the sun vs. -243º Fahrenheit in the dark. As with other matter, the Moon’s two sides expand and contract from the changes in heat, causing warping and popping in the transitions. These quakes also aren’t especially huge, but they are frequent.
After that, meteoroid strikes can be large enough to cause some shaking to the local environment. The excess energy from the impact shakes the ground, like a small quake.
Finally, there are the more mysterious and shallow quakes. While they seem to only occur in the upper layers of the Moon’s geology, they can be larger than the previous types, and have been measured at a magnitude 5. They’re also frequent and last a long time, up to 10 minutes a quake. Some of this might be due to the Moon lacking bodies of water, which help absorb and dampen energy from quakes on Earth.
More measurements from Mars
It’s hard to use Earth as a model for these quakes though, thanks to the difference in composition and lack of tectonic movement. Astronauts visiting the Moon had set of seismometers, but those were shut down in 1977, a so no new data has really been gathered about this activity for decades. Our next chance to measure single-plated quakes is likely to be on the InSight mission to Mars, which will carry a seismometer with it in late 2016. It’s expected to detect a fair number of quakes, possibly demonstrating that Earth is the odd-man-out when it comes to our solar system’s seismic activity.
Source: Moonquakes and Marsquakes by Adrienne LaFrance, The Atlantic