Evolution of Rosetta's comet over 4.5 billion years
This diagram illustrates the evolution of Rosetta's dual-lobed comet, 67P/Churyumov–Gerasimenko, over the past 4.5 billion years.
The comet is thought to have formed this long ago in the primordial disc of the Solar System, perhaps as two small objects slowly collided and stuck together. Comets form in the icy outer Solar System and are stored there in vast clouds before beginning their journey inwards; comet 67P/C-G is thought to have entered the giant planet region hundreds of thousands to millions of years of ago. By this point a form of geological erosion named mechanical shear stress had taken hold, and was the dominant process sculpting and shaping the comet's surface and interior. A new study using data from Rosetta found this stress to peak in the region connecting the two lobes of the comet: the 'neck'. This neck bore the brunt of mechanical erosion, fracturing and thinning over time – as shown in the diagram by the cross-hatched lines.
The final steps cover the time period from tens of thousands of years to present day, a period during which sublimation erosion was dominant in shaping the comet's surface and interior. This kind of erosion takes place as the Sun warms ices within the comet, causing them to turn to gas and escape to space, carrying cometary material along with them. This weakened the comet's neck further, and the force grew stronger as it travelled inwards from Jupiter's orbit towards Mars.
It is important to note that the red arrows do not imply cometary rotation; instead, they represent shear deformation, and illustrate the torque generated at the neck.