Rosetta’s ‘rubber duck’ comet was once two separate comets

In the year and change since the European Space Agency’s Rosetta mission became the first spacecraft to orbit a comet, the probe has made all sorts of discoveries about the object known as Comet 67P/Churyumov-Gerasimenko-for instance, that it’s bumpy, not smooth as expected, and is covered in dark, carbon-rich compounds with surprisingly little ice.

The other hypothesis is that comet 67P is actually two comets, long ago welded together after a chance meeting.

The surface of the comet is made up of parallel layers of matter built up over time, in some places viewable along 650 meters deep into the comet’s body.

Rosetta’s first images of the comet surprised scientists by showing it to have two lobes that appeared to be connected by a narrower “neck”, a shape some compared to a rubber duck.

A rubber duck-shaped comet was formed after a collision between two objects, scientists say. “Except in this case we are considering two separate onions of differing size that have grown independently before fusing together”.

ROSETTA-OSIRIS view of 67P/Churyumov-Gerasimenko comet showing the irregular, fractured and stratified morphology of the Seth region of the main body.

Between 6 August 2014 and 17 March 2015 Rosetta took a series of high-resolution pictures of the geology of 67P and the results are now in.

According to the ESA, layers of material should typically form at right angles to the gravity of an object. “The heating and partial melting at the impact location and the subsequent cooling and gluing of the two bodies explain the shape of the neck region”. “It must have been a low-speed collision in order to preserve such ordered strata to the depths our data imply”.

“Our analysis… clearly shows that the layers of the body and the head of the comet are not related”, Massironi said.

Bjorn Davidsson, a co-author from Uppsala University, Sweden, said they also found “striking structural similarities between the two lobes” that implies they formed through “similar accretion process” despite their “initially independent origins”.

But there were many similarities, which suggested the two mini-comets “formed completely independently” before bumping into one another, the scientists said. “This growth is believed to have occurred about 4.5 billion years ago, contemporaneous with the formation of the solar system”.