Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse

Abstract

Pits have been observed on many cometary nuclei mapped by spacecraft. It has been argued that cometary pits are a signature of endogenic activity, rather than impact craters such as those on planetary and asteroid surfaces. Impact experiments and models cannot reproduce the shapes of most of the observed cometary pits, and the predicted collision rates imply that few of the pits are related to impacts. Alternative mechanisms like explosive activity have been suggested, but the driving process remains unknown. Here we report that pits on comet 67P/Churyumov-Gerasimenko are active, and probably created by a sinkhole process, possibly accompanied by outbursts. We argue that after formation, pits expand slowly in diameter, owing to sublimation-driven retreat of the walls. Therefore, pits characterize how eroded the surface is: a fresh cometary surface will have a ragged structure with many pits, while an evolved surface will look smoother. The size and spatial distribution of pits imply that large heterogeneities exist in the physical, structural or compositional properties of the first few hundred metres below the current nucleus surface.

Regional surface morphology of comet 67P/Churyumov-Gerasimenko from Rosetta/OSIRIS images

“…The Seth region also contains a number of circular pits. The largest is 210 m in diameter, ∼170 m deep, and shows evidence of dust emission from depth (Thomas et al 2015;Vincent et al 2015). Layering is also evident in Seth, in some cases, partially buried under a dusty cover (see Massironi et al 2015).…”

“…The acronyms in the header columns correspond to the following respectively: collapsing material (CM), cratered cones/Pancake-shaped mounds (CC/P-SM), smooth deposits (SD), and dust/airfall (D/A). Finally, the locations of active pits are from Vincent et al (2015).…”

Gas outflow and dust transport of comet 67P/Churyumov–Gerasimenko

“…It is therefore likely that a population of cm-sized grains would have been injected into the dust coma in this time period under normal condition. On the occasions of large outbursts as often seen between 2015 July and September, that might have been generated by thermal stress and/or cliff collapse (Vincent et al 2015(Vincent et al , 2016, particles of even metre-size could be ejected (Pajola et al 2015). The validity of this approximation depends on the radiative cooling process of the nucleus surface as examined by Sekanina (1988Sekanina ( , 1992.…”

“…The close-up observations of the coma activities and surface morphology by the remote-sensing instruments and a suite of in situ experiments as the comet moved around its perihelion point provided a wealth of information on the basic E-mail: ianlai@g.ncu.edu.tw properties of the sublimation-driven erosion of this short-period comet (Keller et al 2015;Sierks et al 2015;Vincent et al 2015). Of particular importance is the transport of dust material across the nucleus surface as it has significant effects on the short-term and long-term evolution of the outgassing behaviour.…”

The Agilkia boulders/pebbles size–frequency distributions: OSIRIS and ROLIS joint observations of 67P surface

“…Despite its small dimensions, ∼4 km in diameter, 67P shows a wide morphological diversity: enveloping strata (Massironi et al 2015), pits (Vincent et al 2015), boulders (Pajola et al 2015), high-reflectivity boulder clusters (Pommerol et al 2015), local fracturing (El-Maarry et al 2015), as well as dust covered terrains (Thomas et al 2015b) are only some notable features which can be found on 67P.…”