The rubber-duck shaped comet 67P/Churyumov–Gerasimenko (67P) made up of a larger lobe and a smaller lobe separated by a thin ‘neck’ region were once two fully formed, distinct objects that merged together, notes a study published recently in the journal Nature .

The origin of the comet’s double-lobed form has been a key question since Rosetta first revealed its surprising shape in July last year. Till date, it has been unclear if comet 67P had formed by the fusion of two objects or if concentrated localised erosion of a single object led to the formation of the thin ‘neck.’

By using high-resolution images from the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) taken between August 6, 2014 and March 17, 2015 to study the layers of material seen all over the nucleus, scientists have unequivocally shown that the peculiar shape arose from a low-speed collision between two fully, separately developed comets.

“Gentle, low-velocity collisions occurred between two fully formed kilometre-sized cometesimals in the early stages of the Solar System,” notes the paper.

Matteo Massironi, the first author of the study from the University of Padova, Italy, has found that the two lobes with an “onion-like” stratification (layering) have “notable structural similarities” and surface composition. The structural similarities, in turn, indicate that the two comets that formed separately must have experienced similar accretion processes.

The researchers were able to conclude that comet 67P was formed due to fusion of two separate bodies by using the images to identify over 100 terraces distributed all over the surface of the comet and parallel layers of material clearly seen in exposed cliff walls and pits. Next, the directions in which the parallel layers were sloping and the depth to which they were extending were determined using a 3D shape model. They found that the “strata of the two lobes are clearly independent of each other,” suggesting that the two lobes had formed separately. The fact that the layers are inclined in opposite directions at the neck only confirms that the two lobes had been fused at the neck. The possibility of localised erosion to form the thin “neck” can be ruled out as erosion cannot alter the direction of inclination of the layers.

The researchers next studied the relationship between gravity and the orientation of individual features seen on the comet’s surface. Generally, the layering of strata should be perpendicular to the gravity of the object.

The scientists considered two scenarios — comet was first considered to be a single body with the centre of mass close to the neck and in the second case as two different comets with their respective centres of mass.

It was found that close to the neck, the orientation of a given layer and the direction of the local gravity are closer to perpendicular only in the model where the comet is considered as two separate objects.

Since “stratification is a primary structure”, and the ordered sequence of strata is left undisturbed to a depth of 650 metres, the two lobes should have fused together only through a low-velocity impact, the researchers state.

Published in The Hindu on October 5, 2015