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

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 visible-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 pixel1, 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 m3. 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 pixel1) 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.