Space Facts

#40

A Groaning Planet

Closer to home, our planet makes a deep groan every time its crust shifts, and sometimes those low-frequency sounds carry all the way into space. During an earthquake, the ground's shaking can produce vibrations in the atmosphere, usually with a frequency between one and five Hz. If the earthquake is strong enough, it can send infrasound waves up through the atmosphere to the edge of space.

Of course, there's no clear line where Earth's atmosphere stops and space begins. The air just gradually gets thinner until eventually there's none. From about 80 to about 550 kilometers above the surface, the mean free path of a molecule is about a kilometer. That means the air at this altitude is about 59 times too thin for audible sound waves to travel through, but it can carry the longer waves of infrasound.

When a magnitude 9.0 earthquake shook the northeastern coast of Japan in March 2011, seismographs around the world recorded how its waves passed through the Earth, and the earth's vibrations also set off low-frequency vibrations in the atmosphere. Those vibrations traveled all the way up to where the European Space Agency's Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite maps Earth's gravity from low orbit, 270 kilometers above the surface. And the satellite recorded those sound waves - sort of.

GOCE has very sensitive accelerometers on board, which control the ion engine that helps keep the satellite in a stable orbit. On March 11, 2011, GOCE's accelerometers detected vertical displacement in the very thin atmosphere around the satellite, along with wavelike shifts in air pressure, as the sound waves from the earthquake passed by. The satellite's thrusters corrected for the displacement and saved the data, which became an indirect recording of the earthquake's infrasound.

The indirect recording was buried in the satellite's thruster data until a team of researchers led by Raphael F. Garcia happened across it and published a paper on their findings.

Sorry these have been about sound lately. But I really didn't know that the planets and stars made sounds and it's really interesting. Thank you guys for pointing this out. And I apologize for giving incorrect information :(