Large scale dust jets in the coma of 67P/Churyumov-Gerasimenko as seen by the OSIRIS instrument on board Rosetta

Abstract

Context. During the most recent perihelion passage in 2009 of comet 67P/Churyumov–Gerasimenko, ground–based observations showed an anisotropic dust coma where jet–like features were detected at ∼ 1.3 AU from the Sun. The current perihelion passage is exceptional as the Rosetta spacecraft is monitoring the nucleus activity since March 2014, when a clear dust coma was already surrounding the nucleus at 4.3 AU from the Sun. Subsequently, the OSIRIS camera also witnessed an outburst in activity between April 27 and 30, and since mid–July, the dust coma at r^_{h} ∼ 3.7 − 3.6 AU preperihelion is clearly non–isotropic, pointing to the existence of dust jet–like features. Aims. To ascertain on the nucleus surface the origin of the dust jet–like features detected as early as in mid–July 2014. This will help to establish how the localised comet nucleus activity compares with that seen in previous apparitions as well as its evolution as the comet approaches its perihelion when most of the jets were detected from ground–based observations. Also, the determination of these areas allows for framing them in regions with spectroscopic or geomorphological distinct characteristics. Methods. Three series of dust images of comet 67P/Churyumov-Gerasimenko obtained with the Wide Angle Camera (WAC) of the OSIRIS instrument on board the Rosetta spacecraft have been processed with different enhancement techniques. This has been made in order to clearly show the existence of jet–like features in the dust coma whose appearance towards the observer changed due to rotation of the comet nucleus and due to the changing observing geometry from the spacecraft. The position angles of these features in the coma together with information on the observing geometry, nucleus shape and rotation, have allowed us to determine the most likely locations on the nucleus surface where the jets originate from. Results. Geometrical tracing of jet sources indicates that the activity of the nucleus of 67P, during July and August 2014, gave rise to large scale jet–like features from the Hapi, Hathor, Anuket and Aten regions, confirming that active regions may be present on the nucleus localized at 60◦ northern latitude as deduced from previous comet apparitions. Also, there are hints that large–scale jets observed from ground are possibly composed, in their origin on the nucleus surface, by numerous small scale features.