OSIRIS catches the wake-up of a dust jet

Rosetta’s scientific imaging system OSIRIS witnesses a new jet of dust emerging from the surface of comet 67P/Churyumov-Gerasimenko.

Sometimes it is all a matter of being in the right place at the right time. Or, as in the case of ESA’s space probe Rosetta, of opening your eyes at the exactly right moment. In mid-March, Rosetta’s onboard imaging system OSIRIS was able to catch the elusive moment, when a new dust jet from comet 67P/Churyumov-Gerasimenko awakes to life. Scientists from the OSIRIS team are now analyzing these images.

Over the past weeks comet 67P/Churyumov-Gerasimenko’s activity has increased constantly. As the comet’s nucleus gets warmer and warmer, more frozen gas sublimates from its surface carrying dust particles with it. Only four months from perihelion, the comet’s nucleus is now enshrouded in a dense coma; on the nucleus’ day side pronounced dust jets can be seen at all times.

The two images released today show the onset of such a jet for the first time. They were taken on March 12 from a distance of 75 kilometers. In the first image obtained at 7.13 CET several rays of dust jets frame the upper, illuminated side of the comet. The dark underside shows no such features. Two minutes later, the picture has changed: a spectacular new jet has emerged on the dark side hurtling dust into space and displaying a clearly discernable fine structure. 

“This was a chance discovery”, says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. “No one has ever witnessed the wake-up of a dust jet before. It is impossible to plan such an image”

Tracking variations in brightness along the jets, the researchers estimated the velocity of the dust grains to be at least eight meters per second. Measurements with Rosetta’s instrument GIADA have in the past rendered comparable speeds for emitted dust particles.

Surprisingly, the new-born jet arises from a shadowed area on the underside of the comet close to the center of the Imhotep region. So far, cometary activity has only been observed on the comet’s day-side. “In these images we see Imhotep on the brink of dawn”, OSIRIS scientist Jean-Baptiste Vincent from the MPS explains. “It is possible, that the first rays of sunlight hit some cliffs or outcrops that remained hidden to Rosetta due to the orbital position at the time.”

The onset of activity could also be the result of a different type of activity, more explosive, triggered by the heat wave reaching ices trapped in a deeper layer under the surface. This observation gives scientists the possibility to test different models of activity.

OSIRIS could not continue to observe the new feature after 7.17 CET. Imhotep was soon fully illuminated, making it impossible to discern individual jets in the overexposed coma. It is therefore not clear, whether Rosetta witnessed the birth of a continuous jet or a short-lived outburst.

“Usually, 67P’s dust-jets are rather long-lived”, says Vincent, who has been monitoring the comet’s activity over the past months. “Most of them last for a full dayside rotation of approximately six hours and even reappear in the next rotation”, he adds. However, scientists believe that also eruptive outbursts can occur. One of these might have been the first activity coma observation on 30 April, 2014. At that time the coma expanded over 1800 kilometers – and disappeared again a few weeks later.

Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. Rosetta is the first mission in history to rendezvous with a comet, escort it as it orbits the Sun, and deploy a lander to its surface.

The scientific imaging system OSIRIS was built by a consortium led by the Max Planck Institute for Solar System Research (Germany) in collaboration with CISAS, University of Padova (Italy), the Laboratoire d'Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Spain), the Scientific Support Office of the European Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), the Universidad Politéchnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institute of Computer and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially supported by the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.