Comets are impressive phenomena in the night sky. As their orbit brings them into the inner Solar System, their icy cores heat up, setting gas and dust free. The escaping gases, primarily derived from water ice, can also carry dust particles that form the coma and cometary tail. In September 2014, the European Rosetta spacecraft investigated the early activity of Comet 67P/Churyumov-Gerasimenko. At that time, local gas and dust jets were accompanied by daily recurring water ice patterns on the 'neck' of the comet. This was revealed by the observations performed by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS), which have now been evaluated.
"How and where exactly the sources of cometary activity arise has been a largely unsolved mystery in comet research," says Gabriele Arnold from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), responsible for the German scientific contributions to the VIRTIS instrument. The discovery of the ice patterns proves that, at certain times of the cometary day, water vapour flows from the comet interior to the surface, freezes in the shadowed regions, sublimates when exposed to sunlight and finally escapes into space. The international VIRTIS research team reports on this discovery in the current issue of the scientific journal Nature.
Water vapour migrates through porous comet material
"When we look at Comet Churyumov-Gerasimenko, we see an exceptionally dark body with a mostly ice-free surface," explains Arnold. "Nevertheless, the comet is very active, liberating water and other volatile components from its extensive internal reservoir to the exterior," explains the scientist from the To the Institute's website in Berlin. The researchers soon noticed that the ice patterns followed a day-night rhythm; ice formed as soon as the location of the escaping water vapour was shadowed during the comet's rotation. The researchers concluded that water vapour must arise in the ice-rich subsurface layers, which remain warm due to the sunlight received in the previous hours. Due to this, the subsurface water ice continues to sublimate and makes its way through the comet's porous interior to the surface, where it is deposited. The condensation of water vapour from the surrounding gas envelope – the coma – is not sufficient to explain the ice seen on the surface; this would only be possible closer to the Sun. "Thus, VIRTIS observations have, for the first time, uncovered one of the possible mechanisms driving the comet's local activity," says Arnold.
The comets 9P/Tempel 1 and 103P/Hartley 2 have also exhibited local water ice patterns, which could be explained by a similar day-night cycle. With their discovery, the scientists assume that this process is also found on other comets.
About the mission
The Rosetta spacecraft reached Comet 67P/Churyumov-Gerasimenko in August 2014 and, since then, has been observing the comet's increasing activity. On 13 August 2015, 67P reached perihelion, and is now moving slowly towards the outer Solar System on an orbit that lasts six-and-a-half years.
Rosetta is an ESA mission with contributions from its Member States and NASA. Rosetta's Philae lander touched down on the comet on 12 November 2014. The Philae lander is contributed by a consortium led by DLR, MPS, CNES and ASI.
VIRTIS is the Visible, InfraRed and Thermal Imaging Spectrometer on ESA's Rosetta spacecraft. It provides information on the composition of the solid materials on the nucleus as well as mapping their distribution on the surface, and of the gases and molecules in the coma. VIRTIS was built by a consortium under the scientific responsibility of the Institute for Space Astrophysics and Planetology (Istituto di Astrofisica e Planetologia Spaziali; IAPS) of the Italian National Institute for Astrophysics (Istituto Nazionale di Astrofisica; INAF) in Rome, which also guides the scientific operations. The consortium includes the Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique (LESIA) of the Observatoire de Paris (France) and the Institute of Planetary Research (Institut für Planetenforschung) of DLR (Germany). Instrument development was funded and managed by three national space agencies: Agenzia Spaziale Italiana (ASI, Italy), Centre National d'Études Spatiales (CNES, France) and the Deutsches Zentrum für Luft- und Raumfahrt (DLR, Germany). Full support from the Rosetta Science Operations Centre and the Rosetta Mission Operations Centre is gratefully acknowledged.
VIRTIS calibrated data will be available through the ESA Planetary Science Archive website.