Rosetta mission describes surface change of comet in transit around the sun

Comets – small frozen bodies proceeding from the external regions of the Solar System – acquire their characteristic look when they approach the sun: the ice sublimates, giving rise to a tail and a coma – a gas envelope surrounding the nucleus. The weeks prior to or following the perihelion are a culminating period of activity for comets, and one of them has just been observed up close for the very first time by the Rosetta (ESA) mission.  Today Science magazine will publish a study analyzing the changes that occurred on the surface of comet 67P Churyumov-Gerasimenko during such a phase, which point to a more active past.

"The changes undergone by a comet, in the widest sense of the word, from the external Solar System to the perihelion, do not consist only in the development of a gas coma and a tail of dust. We can now affirm that  those large scale changes start out on a small of tens of meters or less, on the surface of the comet”, says Luisa M. Lara, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) taking part in the Rosetta mission.  

The appearance of a cometary nucleus in the phases around the perihelion used to be unknown, because it was always hidden by the coma. The OSIRIS camera on board the Rosetta mission was able to photograph the entire activity phase from its inception in May 2014 its point of maximum proximity to the sun which took place in August 2015, and its progressive distancing since.  

Now, the comparison of the images obtained in the course of two years, covering surfaces of a few meters or less, has made it possible to analyze the changes which have taken place on the surface of the comet on its journey around the sun.  

Among those changes, it is worth pointing out the collapse of cliffs in the Seth and Ash regions, the extension by some thirty meters of the half a kilometer fracture which crosses the neck of the comet, and the formation of smaller fractures parallel to this one, or the displacement of boulders: in the Khonsu region, a rock of more than twenty meters long weighing the equivalent on Earth to two hundred and fifty kilograms moved approximately one hundred and forty meters, possibly due to explosive events in its vicinity. 

Observations were also made of non consolidated material moving on the surface of the planet, revealing territory that used to be hidden: for example, on the Imhotep region, circular structures have been revealed which were hidden in the 2014 images, and which are similar to other regions that appeared and disappeared in the Hapi region and seem to indicate the work of erosion– in fact, the return to initial conditions has been frequent in the various changes observed.

“The rate of production of images and their comparison does not enable us to duplicate the evolution of certain processes, such as those responsible for moving big blocks of rock, but it does enable us to conclude that cometary activity involves violent processes, such as bursts of activity. Indeed, in a separate article published in Nature Astronomy we documented for the first time an unequivocal relation between one of those bursts and the massive collapse of a cliff", says Luisa M. Lara (IAA-CSIC).

"Moreover, more delicate processes may also be at work, but spread out in time, which cause the dust to rise and deposit again on the surface, and the walls to collapse and expose areas of subsurface ice”, the researcher added.   

Nevertheless, all these changes are local and have not affected the major orographic features of 67P, which indicates that the comet’s orography was set at an earlier historical phase.  It is known that the gravitational interaction with Jupiter has at least twice modified the orbit of 67P, in 1940 and 1959, when the minimal distance to the Sun went from 600 million kilometers (not enough to activate the comet) to 410 and 186 million kilometers, respectively.  

Researchers believe that the prominent orographic features on 67P may have formed during earlier orbits in the same orbital configuration, or in even earlier stages, but there is no doubt that the comet, in past times, went through a much more intense period of activity than that documented by the Rosetta mission.