Cliff collapse on Comet 67P/Churyumov-Gerasimenko -- I. Aswan

The Aswan cliff on Comet 67P/Churyumov-Gerasimenko collapsed on 2015 July 10. Thereby, relatively pristine comet material from a depth of ~12 m was exposed at the surface. Observations of the collapse site by the microwave instrument Rosetta/MIRO have been retrieved from 8 months prior to collapse, as well as from 5, 7, and 11 months post-collapse. The MIRO data are analysed with thermophysical and radiative transfer models. The pre-collapse observations are consistent with a 30 MKS thermal inertia dust mantle with a thickness of at least 3 cm. The post-collapse data are consistent with: 1) a dust/water-ice mass ratio of 0.9$\pm$0.5 and a molar $\mathrm{CO_2}$ abundance of ~30 per cent relative to water; 2) formation of a dust mantle after ~7 months, having a thickness of a few millimetres or a fraction thereof; 3) a $\mathrm{CO_2}$ ice sublimation front at 0.4 cm that withdrew to 2.0 cm and later to 20$\pm$6 cm; 4) a thermal inertia ranging 10-45 MKS; 5) a gas diffusivity that decreased from $0.1\,\mathrm{m^2\,s^{-1}}$ to $0.001\,\mathrm{m^2\,s^{-1}}$; 6) presence of a solid-state greenhouse effect parts of the time. The data and the analysis provide a first empirical glimpse of how ice-rich cometary material ages and evolves when exposed to solar heating.Comment: 22 pages, 24 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer revie

Collisional heating of icy planetesimals. I. Catastrophic collisions

Planetesimals in the primordial disc may have experienced a collisional cascade. If so, the comet nuclei later placed in the Kuiper belt, scattered disc, and Oort Cloud would primarily be fragments and collisional rubble piles from that cascade. However, the heating associated with the collisions cannot have been strong enough to remove the hypervolatiles that are trapped within more durable ices, because comet nuclei are rich in hypervolatiles. This places constraints on the diameter of the largest bodies allowed to participate in collisional cascades, and limits the primordial disc lifetime or population size. In this paper, the thermophysical code NIMBUS is used to study the thermal evolution of planetesimals before, during, and after catastrophic collisions. The loss of CO during segregation of $\mathrm{CO_2:CO}$ mixtures and during crystallisation of amorphous $\mathrm{H_2O}$ is calculated, as well as mobilisation and internal relocation of $\mathrm{CO_2}$. If an amorphous $\mathrm{H_2O}$ host existed, and was protected by a $\mathrm{CO_2:CO}$ heat sink, only diameter $D<20\,\mathrm{km}$ (inner disc) and $D<64\,\mathrm{km}$ (outer disc) bodies could have been involved in a collisional cascade. If $\mathrm{CO_2}$ was the only CO host, the critical diameters drop to $D<20$-$32\mathrm{km}$. Avoiding disruption of larger bodies requires a primordial disc lifetime of $<9\,\mathrm{Myr}$ at $15\,\mathrm{au}$ and $<50$-$70\,\mathrm{Myr}$ at $30\,\mathrm{au}$. Alternatively, if a $450\,\mathrm{Myr}$ disc lifetime is required to associate the primordial disc disruption with the Late Heavy Bombardment, the disc population size must have been 6-60 times below current estimates.Comment: 20 pages, 11 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer revie

Secondary gas in debris discs released following the decay of long-lived radioactive nuclides, catastrophic or resurfacing collisions

Kuiper-like belts of planetesimals orbiting stars other than the Sun are most commonly detected from the thermal emission of small dust produced in collisions. Emission from gas, most notably CO, highlights the cometary nature of these planetesimals. Here we present models for the release of gas from comet-like bodies in these belts, both due to their thermophysical evolution, most notably the decay of long-lived radioactive nuclides and collisional evolution, including catastrophic and gentler resurfacing collisions. We show that the rate of gas release is not proportional to the rate of dust release, if non-catastrophic collisions or thermal evolution dominate the release of CO gas. In this case, care must be taken when inferring the composition of comets. Non-catastrophic collisions dominate the gas production at earlier times than catastrophic collisions, depending on the properties of the planetesimal belt. We highlight the importance of the thermal evolution of comets, including crucially the decay of long-lived radioactive nuclides, as a source of CO gas around young (<50Myr) planetary systems, if large (10-100s kms) planetesimals are present.Comment: Submitted to MNRAS, 16 page

Airfall on Comet 67P/Churyumov–Gerasimenko

We here study the transfer process of material from one hemisphere to the other (deposition of airfall material) on an active comet nucleus, specifically 67P/Churyumov–Gerasimenko. Our goals are to: 1) quantify the thickness of the airfall debris layers and how it depends on the location of the target area, 2) determine the amount of H₂O and CO₂ ice that are lost from icy dust assemblages of different sizes during transfer through the coma, and 3) estimate the relative amount of vapor loss in airfall material after deposition in order to understand what locations are expected to be more active than others on the following perihelion approach. We use various numerical simulations, that include orbit dynamics, thermophysics of the nucleus and of individual coma aggregates, coma gas kinetics and hydrodynamics, as well as dust dynamics due to gas drag, to address these questions. We find that the thickness of accumulated airfall material varies substantially with location, and typically is of the order 0.1–1 m. The airfall material preserves substantial amounts of water ice even in relatively small (cm–sized) coma aggregates after a rather long (12 h) residence in the coma. However, CO₂ is lost within a couple of hours even in relatively large (dm–sized) aggregates, and is not expected to be an important component in airfall deposits. We introduce reachability and survivability indices to measure the relative capacity of different regions to simultaneously collect airfall and to preserve its water ice until the next perihelion passage, thereby grading their potential of contributing to comet activity during the next perihelion passage

Airfall on Comet 67P/Churyumov–Gerasimenko

We here study the transfer process of material from one hemisphere to the other (deposition of airfall material) on an active comet nucleus, specifically 67P/Churyumov–Gerasimenko. Our goals are to: 1) quantify the thickness of the airfall debris layers and how it depends on the location of the target area, 2) determine the amount of H₂O and CO₂ ice that are lost from icy dust assemblages of different sizes during transfer through the coma, and 3) estimate the relative amount of vapor loss in airfall material after deposition in order to understand what locations are expected to be more active than others on the following perihelion approach. We use various numerical simulations, that include orbit dynamics, thermophysics of the nucleus and of individual coma aggregates, coma gas kinetics and hydrodynamics, as well as dust dynamics due to gas drag, to address these questions. We find that the thickness of accumulated airfall material varies substantially with location, and typically is of the order 0.1–1 m. The airfall material preserves substantial amounts of water ice even in relatively small (cm–sized) coma aggregates after a rather long (12 h) residence in the coma. However, CO₂ is lost within a couple of hours even in relatively large (dm–sized) aggregates, and is not expected to be an important component in airfall deposits. We introduce reachability and survivability indices to measure the relative capacity of different regions to simultaneously collect airfall and to preserve its water ice until the next perihelion passage, thereby grading their potential of contributing to comet activity during the next perihelion passage

Division III: Commission 15: Physical Studies of Comets and Minor Planets

The business meeting of IAU Commission 15 (C15) took place in Beijing on 29 August 2012, from 14:00 to 18:00, in room 405 of the China National Convention Center. This report of the business meeting of Commission 15 at the 2012 IAU GA is based on the report provided by Alberto Cellino, past president, and on the minutes taken by Daniel Hestroffer, secretary of Commission 15 in the triennium 2009 to 2012, and current secretary. <P /

Outburst activity in comets: II. A multi-band photometric monitoring of comet 29p/Schwassmann-Wachmann 1

We have carried out a continuous multi-band photometric monitoring of the nuclear activity of comet 29P/Schwassmann-Wachmann 1 from 2008 to 2010. Our main aim has been to study the outburst mechanism on the basis of a follow-up of the photometric variations associated with the release of dust. We used a standardized method to obtain the 10 arc-sec nucleus photometry in the V, R, and I filters of the Johnson-Kron-Cousins system, being accurately calibrated with standard Landolt stars. Production of dust in the R and I bands during the 2010 Feb. 3 outburst has been also computed. We conclude that the massive ejection of large (optically-thin) particles from the surface at the time of the outburst is the triggering mechanism to produce the outburst. Ulterior sublimation of these ice-rich dust particles during the following days induces fragmentation, generating micrometer-sized grains that increase the dust spatial density to produce the outburst in the optical range due to scattering of sun light. The material leaving the nucleus adopts a fan-like dust feature, formed by micrometer-sized particles that are decaying in brightness as it evolved outwards. By analyzing the photometric signal measured in a standardized 10-arcsec aperture using the Phase Dispersion Minimization technique we have found a clear periodicity of 50 days. Remarkably, this value is also consistent with an outburst frequency of 7.4 outbursts/year deduced from the number of outbursts noticed during the effective observing time.Comment: 19 pages, 3 Tables, and 6 figure

The Comet Interceptor Mission

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum ΔV capability of 600 ms−1. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule

CO2-driven surface changes in the Hapi region on Comet 67P/Churyumov–Gerasimenko

Full list of authors: Davidsson, Bjorn J. R.; Schloerb, F. Peter; Fornasier, Sonia; Oklay, Nilda; Gutierrez, Pedro J.; Buratti, Bonnie J.; Chmielewski, Artur B.; Gulkis, Samuel; Hofstadter, Mark D.; Keller, H. Uwe; Sierks, Holger; Guettler, Carsten; Kueppers, Michael; Rickman, Hans; Choukroun, Mathieu; Lee, Seungwon; Lellouch, Emmanuel; Lethuillier, Anthony; Da Deppo, Vania; Groussin, Olivier; Kuehrt, Ekkehard; Thomas, Nicolas; Tubiana, Cecilia; El-Maarry, M. Ramy; La Forgia, Fiorangela; Mottola, Stefano; Pajola, Maurizio.Between 2014 December 31 and 2015 March 17, the OSIRIS cameras on Rosetta documented the growth of a 140-m wide and 0.5-m deep depression in the Hapi region on Comet 67P/Churyumov–Gerasimenko. This shallow pit is one of several that later formed elsewhere on the comet, all in smooth terrain that primarily is the result of airfall of coma particles. We have compiled observations of this region in Hapi by the microwave instrument MIRO on Rosetta, acquired during October and November 2014. We use thermophysical and radiative transfer models in order to reproduce the MIRO observations. This allows us to place constraints on the thermal inertia, diffusivity, chemical composition, stratification, extinction coefficients, and scattering properties of the surface material, and how they evolved during the months prior to pit formation. The results are placed in context through long-term comet nucleus evolution modelling. We propose that (1) MIRO observes signatures that are consistent with a solid-state greenhouse effect in airfall material; (2) CO2 ice is sufficiently close to the surface to have a measurable effect on MIRO antenna temperatures, and likely is responsible for the pit formation in Hapi observed by OSIRIS; (3) the pressure at the CO2 sublimation front is sufficiently strong to expel dust and water ice outwards, and to compress comet material inwards, thereby causing the near-surface compaction observed by CONSERT, SESAME, and groundbased radar, manifested as the ‘consolidated terrain’ texture observed by OSIRIS. © 2022 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.Parts of this research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. PJG acknowledges financial support from the State Agency for Research of the Spanish Ministerio de Ciencia, Innovacíon y Universidades through project PGC2018–099425–B–I00 and through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV–2017–0709). MRELM. is partly supported by the internal grant (8474000336–KU–SPSC). The MIRO instrument was developed by an international collaboration led by NASA and the Jet Propulsion Laboratory, California Institute of Technology, with contributions from France, Germany, and Taiwan. OSIRIS was built by a consortium led by the Max–Planck–Institut für Sonnensystemforschung, Göttingen, Germany, in collaboration with CISAS, University of Padova, Italy, the Laboratoire d’Astrophysique de Marseille, France, the Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain, the Scientific Support Office of the European Space Agency, Noordwijk, The Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain, the Universidad Politéchnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. We thank the Rosetta Science Ground Segment at ESAC, the Rosetta Mission Operations Centre at ESOC and the Rosetta Project at ESTEC for their outstanding work enabling the science return of the Rosetta MissionPeer reviewe

Modelling the water and carbon dioxide production rates of Comet 67P/Churyumov–Gerasimenko

The European Space Agency Rosetta/Philae mission to Comet 67P/Churyumov-Gerasimenko in 2014-2016 is the most complete and diverse investigation of a comet carried out thus far. Yet, many physical and chemical properties of the comet remain uncertain or unknown, and cometary activity is still not a well-understood phenomenon. We here attempt to place constraints on the nucleus abundances and sublimation front depths of H2O and CO2 ice, and to reconstruct how the nucleus evolved throughout the perihelion passage. We employ the thermophysical modelling code 'Numerical Icy Minor Body evolUtion Simulator', or nimbus, to search for conditions under which the observed H2O and CO2 production rates are simultaneously reproduced before and after perihelion. We find that the refractories to water-ice mass ratio of relatively pristine nucleus material is μ ≈ 1, that airfall material has μ ≈ 2, and that the molar abundance of CO2 relative H2O is near 30 per cent. The dust mantle thickness is typically ≥ 2 cm. The average CO2 sublimation front depths near aphelion were 3.8 m and 1.9 m on the northern and southern hemispheres, respectively, but varied substantially with time. We propose that airfall material is subjected to substantial fragmentation and pulverization due to thermal fatigue during the aphelion passage. Sub-surface compaction of material due to CO2 activity near perihelion seems to have reduced the diffusivity in a measurable way. © 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.Parts of the research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. BJRD and NHS acknowledge funding from National Aeronautics and Space Administration grant 80NSSC18K1272 awarded by the Rosetta Data Analysis Program. PJG acknowledges financial support from the State Agency for Research of the Spanish Ministerio de Ciencia, Innovación y Universidades through project PGC2018–099425–B–I00 and through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV–2017–0709).Peer reviewe