Here on Earth, we’re used to hearing stories in the news about earthquakes in Japan or California. Most of us learned in grade school that as we stand here in our homes, offices, and coffee shops, we are surfing along on giant tectonic plates that float atop our planet’s liquid mantle. What’s less commonly known is that quakes aren’t unique to Earth. They’ve been observed on Mercury, Mars, and most bizarrely, the moon. How do quakes work and what does a series of Apollo-era seismic readings tell us about our only natural satellite?
The Floor Is Lava
Bodies large enough to maintain their own tidal forces—planets, planetoids, and large round moons like our own— are made up of layers. Depending on the type and size of the celestial body, the layers may differ, but when looking at rocky planets like Mars, Earth, Venus, and Mercury, as well as large rocky moons like our own, the layers follow a pretty similar formula. Working from the outside inward, you have the crust, followed by the uppermost layer of the mantle, the lithosphere. Both of these layers are solid. Next, there’s the molten section of the mantle, the asthenosphere. Below the mantle lies the core. The Earth and the moon both have a solid inner core at the center of the planet that is surrounded by a molten outer core. Other bodies, like Mars, have liquid cores.
Most celestial bodies have relatively uniform crust thickness. On Earth, the crust’s depth varies significantly across the planet. Continental crust is several times thicker than oceanic crust. Along with the lithosphere, this solid outer shell floats along on top of the asthenosphere. On most bodies, this outer shell moves as a single unit, drifting atop the molten inner portion of the planet or moon. Earth’s crust has developed faults along areas of varying thickness, snapping the crust into 18 different pieces of myriad size. These pieces are called tectonic plates, and they are responsible for most of the moving and shaking we feel around the planet every day.
Shake, Rattle, And Roll
Earthquakes are caused by friction along these fault lines. The moving tectonic plates create three primary types of faults: Normal, thrust, and strike-slip (sometimes referred to as extension, compression, and transform faults). Normal or extension faults occur when two plates pull apart from one another, creating a low-lying area in between. Thrust or compression faults happen when two plates but up against one another, which tends to result in a subduction zone on one side as one plate sinks below the other and a rise on the other side as the other plate is lifted. Finally, strike-slip or transform faults like the San Andreas fault occur when two plates rub up against one another laterally, grinding their edges along one another.
The motion of these plates causes massive tremors, which can be measured using seismographs. These devices read the vibrations in Earth’s crust and mantle to predict and record earthquakes. When the Apollo 11 astronauts set foot on the moon, they brought with them a seismograph to measure quake activity on the moon — you know, just in case. As it turns out, they recorded a surprising amount of seismic activity on what everyone had presumed was just a hunk of dead rock orbiting the planet. Since then, quakes have also been recorded on Mars, and evidence of seismic activity has been found on Mercury, Venus, several of Jupiter’s moons, and Titan, one of Saturn’s moons. How do bodies with single tectonic plates have seismic activity?
Rumblies In Their Tumblies
Just because other worlds lack the fractured crust that causes earthquakes doesn’t mean that their interiors are docile and unwavering. Mars’ interior occasionally lets out burps of energy that shake the planet and disrupt the status quo in the mantle. Venus’ tremors are thought to have a similar cause, but due to its corrosive atmosphere, our observations are limited to orbital surveillance. Even the sun experiences quakes from time to time, though the results tend to be a whole lot more explosive as they send ionized particles and plasma flying out into space at hundreds of miles per second. The moon and Mercury, however, are a little more docile.
Moonquakes and Mercury quakes are caused by age-related shrinkage. As the small rocky worlds cool, their molten insides contract. The solid layers above collapse and condense as well, generating tremors in the process. Throughout the moon’s cooling process, scientists estimate that it has shrunk over 600 feet in diameter. In addition to the Apollo program measurements, astronomers have also noticed scarps on the lunar surface that correspond with those on Venus, Mars, and Earth that were generated by seismic activity. Further analysis of these scarps and the seismic data gathered by NASA in the 1960s and 70s determined that the moon does in fact quake, and the tremors originate much closer to the surface than they do on Earth. Additionally, many of the quakes seem to have occurred when the moon was at its farthest point from Earth, and the pull of the planet is at its strongest.
Armed with the newfound knowledge that the moon isn’t some dead hunk of rock whirling around the globe, scientists planning for future crewed missions to the moon are considering these quakes when designing potential habitats. Astronauts planning to stay a while on the lunar surface will need housing that can withstand the moonquakes, and the missions will need to select stable landing areas. Further studies on the moon’s seismic activity could help us better understand its developmental process and how the inner workings of small rocky bodies function, bringing us one step closer to fully understanding how our solar system formed.