Variegation of comet 67P/Churyumov-Gerasimenko in regions showing activity

Abstract

Aims. We carried out an investigation of the surface variegation of comet 67P/Churyumov-Gerasimenko, the detection of regions showing activity, the determination of active and inactive surface regions of the comet with spectral methods, and the detection of fallback material. Methods. We analyzed multispectral data generated with Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) narrow angle camera (NAC) observations via spectral techniques, reflectance ratios, and spectral slopes in order to study active regions. We applied clustering analysis to the results of the reflectance ratios, and introduced the new technique of activity thresholds to detect areas potentially enriched in volatiles.Results. Local color inhomogeneities are detected over the investigated surface regions. Active regions, such as Hapi, the active pits of Seth and Ma'at, the clustered and isolated bright features in Imhotep, the alcoves in Seth and Ma'at, and the large alcove in Anuket, have bluer spectra than the overall surface. The spectra generated with OSIRIS NAC observations are dominated by cometary emissions of around 700 nm to 750 nm as a result of the coma between the comet's surface and the camera. One of the two isolated bright features in the Imhotep region displays an absorption band of around 700 nm, which probably indicates the existence of hydrated silicates. An absorption band with a center between 800-900 nm is tentatively observed in some regions of the nucleus surface. This absorption band can be explained by the crystal field absorption of Fe 2+ , which is a common spectral feature seen in silicates.

Opposition effect on comet 67P/Churyumov-Gerasimenko using Rosetta-OSIRIS images

“…The details of image registration and photometric correction steps are described in Oklay et al (2016).…”

A mini outburst from the nightside of comet 67P/Churyumov-Gerasimenko observed by the OSIRIS camera on Rosetta

“…Although the diverse interactions of their different inhomogeneities are very complex it seems that a low conductivity layer blocking further penetration of the heat wave beneath an ice rich upper layer could promote small outbursts. Here, we may have a relation to the areas with low spectral slopes and classified as active by Oklay et al (2016) which are found in our source region because the low spectral slopes may be indicative of a larger ice content at or close to the surface. It is also remarkable that the suspected source region contains a number of circular features which may be dormant gas conduits and that circular features were also suggested as sources of the outbursts of 9P/Tempel-1 by Belton (2008).…”

“…Spectral slopes are presented in Fig. 8b) showing that these regions belong to the active regions group defined by Oklay et al (2016), due to their lower spectral slopes (10−12%/100 nm between 536 nm and 882 nm) than their vicinity (12−15%/100 nm), which is in the group of mixed regions. The latter correspond to either surfaces partially covered by material ejected from active spots or inactive areas peppered with small bright spots, which are currently interpreted as minor ice deposits.…”

“…The roundish features are ranging in diameter from 2−60 m and were interpreted by Auger et al (2015) as ancient degassing conduits. Oklay et al (2016) investigated large parts of the Imhotep region using spectral techniques and the potentially active regions are detected based on spectral slopes. The detected source region of the outburst (bottom panel of Fig.…”

“…7) and investigate the spectral properties of the region separately. Details of the spectral analysis methods and the definitions can be found in Oklay et al (2016).…”