The landing of the Philae probe on the surface of Comet 67P/Churyumov-Gerasimenko is huge news in space exploration and understanding of the habits of comets. You’ve seen the pictures of its stark black landscape of ice, dust and rocky particles, and its impressive, spire-like structures that reach out into space. Thanks to those images, we’re all aware of how it looks.
But did you know how it sounds? Yes. The comet makes noise: it sings.
First, a quick background. The rocky structure where Philae landed is the comet’s nucleus. Comets are surrounded by gas, plasma and dust that give it, to terrestrial observers, their characteristic tails. Plasma is the fourth state of matter, and is an electrically conductive gas that can hold magnetic fields and electric currents. Comet 67P has a plasma “atmosphere,” known as a coma, surrounding it. While studying the interactions between the comet, its plasma and its interactions with the solar wind (which is also plasma, emitted from the Sun), scientists noticed something surprising.
Comet 67P is emitting a “song,” a repeated pattern of sound caused by oscillations in its magnetic field. These oscillations were picked up by Rosetta’s Plasma Consortium (RPC), a set of five instruments designed specifically to provide information on the comet’s plasma environment. RPC detected the sounds on November 11, a day before Philae descended to the surface. The “song” is being produced at 40 to 50 millihertz, which is far lower than the human ear can detect. To make it audible, the frequencies have been increased by a factor of 10,000.
Still, scientists are puzzled as to why this is happening. The general hypothesis is that the song is produced by the comet’s activity. They know that the song also releases neutral particles into space, where they become ionized, but other than that, the song of the comet remains a mystery, and one that the RPC team is eager to solve.