The Rosetta team has cracked one of the longstanding questions regarding comet 67P: how its distinctive 'rubber duck' shape was formed. Credit: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0


The Rosetta spacecraft first revealed the ‘rubber duck’ shape of Comet 67P/Churyumov-Gerasimenko in July 2014, leading to speculation as to whether it was formed through a collision between two objects or through the erosion of a single comet to form its narrow ‘neck’ structure.

Using images captured by Rosetta between 6 August 2014 and 17 March 2015, the Rosetta team has confirmed the former to be true.

Matteo Massironi, associate scientist of the Rosetta OSIRIS team, led a study using images to identify over 100 terraces on the surface of the comet and parallel layers of material seen in exposed cliff walls and pits.

A 3D model was then created to determine the directions in which they were sloping and how far they extend below the surface.

“It is clear from the images that both lobes have an outer envelope of material organised in distinct layers, and we think these extend for several hundred metres below the surface,” says Massironi, from the University of Padova in Italy.

"You can imagine the layering a bit like an onion, except in this case we are considering two separate onions of differing size that have grown independently before fusing together."

The study revealed that the features on the comet are oriented around its lobes and in some places extend as deep as 650 metres.

“This was the first clue that the two lobes are independent, reinforced by the observation that the layers are inclined in opposite directions close to the comet’s neck,” says Massirioni.

“To be sure, we also looked at the relationship between the local gravity and the orientations of the individual features all around the reconstructed comet surface.”

Layers of material should form at right angles to an object’s gravity, so the study looked at comet 67P both as a single body with a centre of mass close to its ‘neck’ and also as two separate comets, each with its own centre of mass.


The calculations showed that the orientation of the layers and direction of gravity are closer to right angles in the model that posited the comet as two separate objects, rather than a single body.