Sen—I had the unnerving experience this week of being woken in the middle of the night by an earthquake. It was my first, since I live in a part of the world where they are infrequent and relatively mild. But it felt like something had suddenly struck the house. It also alarmed some very noisy and agitated gulls!
The quake, centred on the far south-east of England, only measured 4.2 on the Richter scale, a level that is a fairly common event for many countries in other parts of the world and which simply causes vigorous shaking. One can only imagine the horror of the experience of a major earthquake, such as those recently in Nepal, which were around 250,000 times more powerful.
But all these events remind us that we live on the thin and fragile crust of an active planet. Our solid ground is never more than 70 km (45 miles) thick and it sits on a layer of heated rock 2,890 km (1,795 miles) deep that is more flexible, like a viscous liquid.
Today we know that the Earth’s continents are moving about, or drifting, on a global network of tectonic plates that all run up against each other. At the edges of these plates, there is either new hot rock rising from within the Earth, or old rock being driven back underground, producing ocean trenches. Mountain ranges form when a continent wrinkles up at the edge of a colliding plate and fault lines occur where the plates rub against each other. They are often marked by ranges of volcanoes. The strains felt by colliding plates are suddenly released in waves of seismic energy, which produce earthquakes.
The UK lies well away from the boundary of any tectonic plate, with the nearest being along the Mid-Atlantic Ridge. But as I saw for myself, earthquakes still occur due to stresses building up and then being released in the moving crust.
The Moon used to be thought of as a cold, dead world, but NASA’s Apollo programme revealed that it too is seismically active. The astronauts on those missions left seismometers which clearly recorded moonquakes—you can see Buzz Aldrin placing a seismometer on the Apollo 11 mission in 1969 in our main picture.
NASA geologists studying the Apollo data concluded that there are four types of moonquake. One is caused by the Earth’s tidal pull, another by the extreme changes in temperature as the airless surface moves in and out of sunlight, and another by occasional meteoroid impacts. But there are also strange shallow quakes that occur less than 30 km (20 miles) underground.
It is not fully understood why these quakes occur, but they can register up to 5.5 on the Richter scale, which, on Earth, would be enough to cause minor structural damage. They also last longer than on Earth, continuing for up to ten minutes, against just two minutes for the biggest quakes on our own world.
An artist’s impression of NASA’s InSight lander exploring the interior of Mars to measure seismic activity. Image credit: NASA/JPL-Caltech
Apollo’s data made clear that the Moon wasn’t simply hard rock all the way through. It is now thought that it has a core, which is likely to be partly liquid, and then a softer, hot layer, about 150 km (nearly 100 miles) thick, between that and the base of a rocky layer called the mantle.
Apart from telling us more about the Moon’s structure, the Apollo findings will also be of practical use when humans eventually do build lunar bases. The building material will have to be flexible enough to withstand occasional but prolonged shaking.
Another potential future destination for human colonists is, of course, Mars. The Red Planet also experiences quakes, and so is seismically active, according to studies of its surface from orbit. NASA’s Mars Reconnaissance Orbiter picked out features with its High Resolution Imaging Science Experiment (HiRISE) camera including along a fault system called Cerberus Fossae, which cuts through a lava plain that is only a few million years old. The images revealed rocks that had rolled across the surface, leaving trails, and their patterns clearly pointed to marsquakes being responsible.
This exciting new aspect to Mars’ character will be studied by NASA’s next probe, InSight (it stands for Interior exploration using Seismic Investigations, Geodesy and Heat Transport), which is due to launch in March 2016, arriving at the planet six months later. Unlike the Curiosity rover, InSight will be a stationary lander, and its special task will be to dig deep into the martian surface to explore its interior.
One instrument, called the Seismic Experiment for Interior Structure (SEIS), is a seismometer that will precisely monitor marsquakes and other internal activity to add to our knowledge of the Red Planet. It should give us a clue as to nature of Mars’ own core, and tell us why the planet’s crust appears not to be made up of moving tectonic plates, such as are found on Earth.
Again, such information will be vital for those making long-term plans for human settlements or bases. But the data collected from studying quakes on Earth, the Moon and Mars are also invaluable in helping planetary scientists to understand how the rocky worlds of our Solar System formed and evolved.