Pinnacles on the surface of the comet 67P/Churyumov-Gerasimenko: regional distribution and morphology

Pinnacles are local topographic promontories of different shapes considered to be formed due to uneven surface erosion. In the case of comets, areal changes in the degree of erosion could be related to inhomogeneities of the nucleus. However, the amount of the solar radiation and the thermal gradient is different across the orbit for geomorphological regions, which can result in different erosion and shape for a similar composition among two differently illuminated areas. Therefore, a study of the areal distribution of pinnacles on the nucleus surface and their morphology may help to understand the structure and properties of the nucleus material. We mapped 166 pinnacles on the comet nucleus surface of 67P/Churyumov-Gerasimenko. About a third of them have planimetrically rounded shape (rounded pinnacles) and the rest are planimetrically elongated (local ridges). In the Southern hemisphere, number of both round pinnacles and local ridges is larger than in the Northern hemisphere. This difference possibly indicates the higher effectiveness of the pinnacles' formation in the Southern hemisphere. At the same time the mean values of the measured parameters, including the height, show no statistically reliable difference between the north and south. We found that the maximum height of the pinnacles is about a hundred metres. Suggesting that they have been formed by sublimational erosion, this value allows estimating the minimum thickness of the eroded material and thus the degree of the evolutionary changes of the nucleus. In our future study, we will model pinnacles formation based on the here presented analysis of observations.