Extensive catalogue of exposures of volatiles on 67P/Churyumov-Gerasimenko comet nucleus revealed from the OSIRIS cameras onboard the Rosetta mission

We present the most extensive catalog of exposures of volatiles on the nucleus of  comet 67P/Churyumov-Gerasimenko from observations acquired with the OSIRIS imaging system on board the Rosetta mission.  We have analyzed medium and high-resolution images acquired with the Narrow Angle Camera (NAC) of OSIRIS at different wavelength in the 250-1000 nm range, investigating images from 109 different color sequences taken between August 2014 and September 2016, and covering spatial resolution from a few m/px to 0.1 m/px.

Exposure of volatiles are usually brighter than the comet dark terrain, and characterized by a neutral to moderate spectral slope in the visible range, which has been proven to be associated with a local enrichment in water ice thanks to joint observations carried out with the OSIRIS cameras and the VIRTIS visible and near infrared imaging spectrometer [2]. We applied the following methodology to identified exposure of volatiles on the 67P nucleus: bright spots exposing volatiles should both be brighter (at least 50%) than the comet dark terrain, and should have a neutral to moderate spectral slope values in the visible range (535 -882 nm) range. The spectral slope values has been chosen lower than 12%/(100 nm), but usually bright spots have spectral slope much lower than 8%/(100 nm).

Our analysis considerably expands by a factor of 10 the catalogue of bright spots previously identified (about 70  [1, 2, 3, 4, 5, 6]) on the comet, and produces the most extensive catalogue with more than 700 entries of exposure of  volatiles on 67P nucleus. For each bright spot we compute its surface, its precise location of the nucleus, its spectral slope, and its lifetime, and for some the estimated water ice abundance using geographical mixtures of the comet dark terrain and water ice. This lifetime of bright spots was estimated as the time in which a bright spot remains visible in different observing sequences. This estimation is of course biased by the observing frequency-conditions, thus the real lifetime is usually longer than the one reported. Volatiles may survive exposed to the surface for a period varying from a few days, to several months. Longer durations are usually found for water ice exposed after cliff collapse or formation of new scarps, exposing the underlying water-rich material.

Bright spots could be found isolated on the nucleus surface or grouped in a cluster, usually at the feet of cliffs. Isolated bright spots are observed in different type of morphological terrains, including smooth surface, on the top of boulders or close to irregular structures. Several of them are clearly correlated with the cometary activity, being the sources of jets or appearing after an activity event [7]. Even if numerous bright spots are detected, the total surface of exposed water ice is about 84000 m², that is 0.16% of the total 67P nucleus surface. This confirms that the surface of comet 67P is dominated by refractory dark terrains, while ice only occupies a tiny fraction

We also noticed an evolution of the spectral slope values, with the presence of several bright spots having negative slope only in the post-perihelion images, while during and pre-perihelion slopes values were close to zero or moderately positive. There is also a clear difference in the areal distribution of the bright spots pre- and post-perihelion. While bright spots have a larger median surface of about 5 m² in the pre-perihelion images, most of them have surfaces lower than 1-2 m² post-perihelion, clearly indicating that high spatial resolution is mandatory to identify exposure of volatiles on cometary surfaces. Our results on the bright spots area support the findings of Ciarniello et al. [8] and Fulle et al. [9] who deduced that the bright spots on comets are exposure of the primordial water-ice-enriched blocks (WEB) forming, together with the refractory matrix, cometary nuclei, and whose dominant size is of the order of 0.5-1 m. WEBs should be formed of water ice rich pebbles mixed with drier material, and exposed to the nucleus surface when the cometary activity erodes the dust mantle.

The fact that the majority of the bright spots are sub-meter sized is thus in agreement with

these predictions and with the radar measurements the 67P comet provided by CONSERT, which indicate that the nucleus is homogeneous up to scales of a few meters [10].

 

References:

 [1] Pommerol et al., 2015, A&A 583:A25; [2] Barucci et al., 2016, A&A 595, id.A102; [3] Deshapriya et al., 2016, MNRAS 462, S274  [4] Fornasier et al., 2016, Science 354, 1566; [5] Deshapriya et al. 2018, A&A 613, id.A36; [6] Oklay et al., 2017, MNRAS 469, S582, [7] Fornasier et al., 2019, A&A 630, A13; [8]  Ciarniello et al., 2022, Nat. Astron. https://doi.org/10.1038/s41550-022-01625-y: [9] Fulle et al., 2020, MNRAS 493, 4039; [10] Ciarletti et al., MNRAS 469, S805