Linking surface morphology, composition, and activity on the nucleus of 67P/Churyumov-Gerasimenko

Abstract

Aims. The Rosetta space probe accompanied comet 67P/Churyumov-Gerasimenko for more than two years, obtaining an unprecedented amount of unique data of the comet nucleus and inner coma. This has enabled us to study its activity almost continuously from 4 au inbound to 3.6 au outbound, including the perihelion passage at 1.24 au. This work focuses identifying the source regions of faint jets and outbursts and on studying the spectrophotometric properties of some outbursts. We use observations acquired with the OSIRIS/NAC camera during July-October 2015, that is, close to perihelion. Methods. We analyzed more than 2000 images from NAC color sequences acquired with 7-11 filters covering the 250-1000 nm wavelength range. The OSIRIS images were processed with the OSIRIS standard pipeline up to level 3, that is, converted in radiance factor, then corrected for the illumination conditions. For each color sequence, color cubes were produced by stacking registered and illumination-corrected images. Results. More than 200 jets of different intensities were identified directly on the nucleus. Some of the more intense outbursts appear spectrally bluer than the comet dark terrain in the vivible-to-near-infrared region. We attribute this spectral behavior to icy grains mixed with the ejected dust. Some of the jets have an extremely short lifetime. They appear on the cometary surface during the color sequence observations, and vanish in less than some few minutes after reaching their peak. We also report a resolved dust plume observed in May 2016 at a resolution of 55 cm/pixel, which allowed us to estimate an optical depth of ∼0.65 and an ejected mass of ∼ 2200 kg, assuming a grain bulk density of ∼ 800 kg/m 3 . We present the results on the location, duration, and colors of active sources on the nucleus of 67P from the medium-resolution (i.e., 6-10 m/pixel) images acquired close to perihelion passage. The observed jets are mainly located close to boundaries between different morphological regions. Some of these active areas were observed and investigated at higher resolution (up to a few decimeter per pixel) during the last months of operations of the Rosetta mission. Conclusions. These observations allow us to investigate the link between morphology, composition, and activity of cometary nuclei. Jets depart not only from cliffs, but also from smooth and dust-covered areas, from fractures, pits, or cavities that cast shadows and favor the recondensation of volatiles. This study shows that faint jets or outbursts continuously contribute to the cometary activity close to perihelion passage, and that these events are triggered by illumination conditions. Faint jets or outbursts are not associated with a particular terrain type or morphology.Article number, page 22 of 27 S. Fornasier et al.: Jets and activity on comet 67P

Spectrophotometric characterization of the Philae landing site and surroundings with the ROSETTA/OSIRIS cameras

“…A1 (480-882 nm). The decrease of the spectral slope observed in Bastet, Wosret and Hatmehit was previously reported for the whole cometary nucleus by Fornasier et al 2016 and was attributed to the dust removal due to the intense cometary activity at close heliocentric distances (Vincent et al 2016;Fornasier et al 2019a).…”

“…The Wosret/Bastet side of the Abydos surroundings appeared to begin its activity about two weeks after perihelion, which coincided with the peak of activity of the whole comet nucleus (Fornasier et al 2019a). The earliest jet from this area is jet W-1 (see the top right corner of Fig.…”

“…All filters of a given sequence were first co-registered into a colour cube thanks to a python script based on the scikit-image library (Van Der Walt et al 2014), and the optical flow algorithm (Farnebäck 2003), in the same manner as described in Fornasier et al 2019a andHasselmann et al 2019. The orange filter (either NAC/F22 or occasionally NAC/F82, both centred at 649.2 nm) of the sequence served as the reference, and one of those transformations was subsequently applied to the remaining filters: affine (including translation, rotation and shearing), similarity (including 4), which will be discussed in Section 3. translation and rotation), projective and optical flow (which calculates the displacement field between two image frames).…”

“…The reflectance of selected ROIs were calculated by integrating the flux in a box of 3×3 pixels, and their relative reflectances were obtained by normalising the spectra to the green filter (NAC/F23 or occasionally NAC/F83, 535.7 nm) as commonly done in the literature for comet 67P in previous analyses (e.g. Fornasier et al 2015Fornasier et al , 2019aDeshapriya et al 2016;Feller et al 2016;Hasselmann et al 2017). If a sequence does not have a green filter, an artificial green filter was created through linear interpolation of the signals of the F24 and F22 filters.…”

“…The length and height of the long fracture were respectively 36.0±0.5 m and 4.1±0.1 m, and its width ranged between 2.0±0.5 at the "waist" near the middle to 4.0±0.5 m near both ends; while the depression had an irregular surface with depths below 0.8 m. If one applies a mean density of 537.8±0.7 kg/m 3 (Preusker et al 2017), the approximate mass losses from the three sites were 0.9-1.9×10 5 kg at site AA, 1.1-3.7×10 5 kg at site BB and 0.16-1.2×10 5 kg at site CC. These possible changes may have occurred during or close to the perihelion passage between June and October 2015 as the global cometary activity (Vincent et al 2016;Fornasier et al 2019a) caused dust removal on the whole surface of comet 67P, leading to an overall "bluer" spectral slope (Fornasier et al 2016, also discussed in Section 5), and their corresponding estimated mass losses are comparable with a few morphological changes reported in other regions on the surface of comet 67P: a cliff retreat with estimated mass loss of 0.8 × 10 5 kg in the Anhur region (Fornasier et al 2019b); cavities C2 and C3 of the Khonsu region with estimated mass losses of (4.1 ± 2) × 10 5 kg and (1.5 ± 0.5) × 10 5 kg, respectively (Hasselmann et al 2019). Other morphological changes reported by these two articles are linked to estimated mass losses ranging from a few to tens of million kilograms, which is 1-2 orders of magnitude higher than our estimations.…”