The Comet. It Sings!

Have you heard the comet singing? From the Rosetta Blog this press release:

Rosetta’s Plasma Consortium (RPC) has uncovered a mysterious ‘song’ that Comet 67P/Churyumov-Gerasimenko is singing into space. RPC principal investigator Karl-Heinz Glaßmeier, head of Space Physics and Space Sensorics at the Technische Universität Braunschweig, Germany, tells us more.

Sound_comet2
Artist's impression of the 'singing comet' 67P/Churyumov-Gerasimenko. Credit: ESA/Rosetta/NavCam
RPC consists of five instruments on the Rosetta orbiter that provide a wide variety of complementary information about the plasma environment surrounding Comet 67P/C-G. (Reminder: Plasma is the fourth state of matter, an electrically conductive gas that can carry magnetic fields and electrical currents.)

The instruments are designed to study a number of phenomena, including: the interaction of 67P/C-G with the solar wind, a continuous stream of plasma emitted by the Sun; changes of activity on the comet; the structure and dynamics of the comet’s tenuous plasma ‘atmosphere’, known as the coma; and the physical properties of the cometary nucleus and surface.

But one observation has taken the RPC scientists somewhat by surprise. The comet seems to be emitting a ‘song’ in the form of oscillations in the magnetic field in the comet’s environment. It is being sung at 40-50 millihertz, far below human hearing, which typically picks up sound between 20 Hz and 20 kHz. To make the music audible to the human ear, the frequencies have been increased by a factor of about 10,000.

The music was heard clearly by the magnetometer experiment (RPC-Mag) for the first time in August, when Rosetta drew to within 100 km of 67P/C-G. The scientists think it must be produced in some way by the activity of the comet, as it releases neutral particles into space where they become electrically charged due to a process called ionisation. But the precise physical mechanism behind the oscillations remains a mystery.

“This is exciting because it is completely new to us. We did not expect this and we are still working to understand the physics of what is happening,” says Karl-Heinz.

RPC may also be able to help in tracking Philae’s descent to the surface of 67P/C-G on 12 November, in tandem with the lander’s on-board magnetometer, ROMAP .

The contributing institutions to these instruments are:
RPC: Institutet för rymdfysik (IRF), Uppsala, Sweden; Southwest Research Institute (SwRI), USA; Institut für Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, Germany; Laboratoire de physique et chimie de l'environnement et de l'espace (LPC2E), Université d'Orléans, France, and Imperial College London, United Kingdom.
RPC-Mag: Institut für Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, Germany; Imperial College London, United Kingdom; Space Research Institute Graz, Austria

And here is the song:

https://soundcloud.com/esaops/a-singing-comet

And here is the alternate hypothesis for what is making this sound: