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Funding Information: C. G. is supported by an ESA Research Fellowship. H. G. acknowledges support by the Swedish National Space Agency grant 108/18. B. S.-C. acknowledges support through UK-STFC grant ST/S000429/1. French co-authors acknowledge the support of CNES for the Rosetta and Comet Interceptor missions. I. M. acknowledges support through grants of Research Council of Norway (262941 and 275503). Publisher Copyright: © 2021, The Author(s), under exclusive licence to Springer Nature B.V. Comets hold the key to the understanding of our Solar System, its formation and its evolution, and to the fundamental plasma processes at work both in it and beyond it. A comet nucleus emits gas as it is heated by the sunlight. The gas forms the coma, where it is ionised, becomes a plasma, and eventually interacts with the solar wind. Besides these neutral and ionised gases, the coma also contains dust grains, released from the comet nucleus. As a cometary atmosphere develops when the comet travels through the Solar System, large-scale structures, such as the plasma boundaries, develop and disappear, while at planets such large-scale structures are only accessible in their fully grown, quasi-steady state. In situ measurements at comets enable us to learn both how such large-scale structures are formed or reformed and how small-scale processes in the plasma affect the formation and properties of these large scale structures. Furthermore, a comet goes through a wide range of parameter regimes during its life cycle, where either collisional processes, involving neutrals and charged particles, or collisionless processes are at play, and might even compete in complicated transitional regimes. Thus a comet presents a unique opportunity to study this parameter space, from an asteroid-like to a Mars- and Venus-like interaction. The Rosetta mission and previous fast flybys of comets have together made many new discoveries, but the most important breakthroughs in the understanding of cometary plasmas are yet to come. The Comet Interceptor mission will provide a sample of multi-point measurements at a comet, setting the stage for a multi-spacecraft mission to accompany a comet on its journey through the Solar System. This White Paper, submitted in response to the European Space Agency’s Voyage 2050 call, reviews the present-day knowledge of cometary plasmas, discusses the many questions that remain unanswered, and outlines a multi-spacecraft European Space Agency mission to accompany a comet that will answer these questionsby combining both multi-spacecraft observations and a rendezvous mission, and at the same time advance our understanding of fundamental plasma physics and its role in planetary systems.

The paper considers the free molecular flow of gas through the dusty porous surface layer of a comet nucleus. The study is based on computer models of generation of a porous medium and Knudsen gas diffusion. We consider various types of homogeneous and heterogeneous layers constructed from nonintersecting spheres, including layers that contain microcracks or inner cavities. Using the test-particle method, we quantitatively estimate the free path distribution function, layer permeability, and effective kinetic characteristics of sublimation products passing through a nonisothermal porous layer. In addition, in this approach, we consider the volumetric absorption of visible solar radiation in the near-surface absorbing layer. Simple approximation expressions are obtained for all the transport characteristics under study, which makes it possible to estimate the characteristics with sufficient accuracy for practical applications in the physics of comets. The results will be used to construct new consistent models of energy transfer in the near-surface layer of a comet nucleus and, first of all, to analyze the results of the observations of comet 67P/Churyumov‒Gerasimenko.

Ion acoustic waves were observed between 15 and 30 km from the centre of comet 67P/Churyumov–Gerasimenko by the Rosetta spacecraft during its close flyby on 28 March 2015. There are two electron populations: one cold at kBTe≈0.2 eV and one warm at kBTe≈2 eV. The ions are dominated by a cold (a few hundredths of electronvolt) distribution of water group ions with a bulk speed of (3–3.7) km s−1. A warm kBTe≈6 eV ion population, which also is present, has no influence on the ion acoustic waves due to its low density of only 0.25 % of the plasma density. Near closest approach the propagation direction was within 50∘ from the direction of the bulk velocity. The waves, which in the plasma frame appear below the ion plasma frequency fpi≈2 kHz, are Doppler-shifted to the spacecraft frame where they cover a frequency range up to approximately 4 kHz. The waves are detected in a region of space where the magnetic field is piled up and draped around the inner part of the ionised coma. Estimates of the current associated with the magnetic field gradient as observed by Rosetta are used as input to calculations of dispersion relations for current-driven ion acoustic waves, using kinetic theory. Agreement between theory and observations is obtained for electron and ion distributions with the properties described above. The wave power decreases over cometocentric distances from 24 to 30 km. The main difference between the plasma at closest approach and in the region where the waves are decaying is the absence of a significant current in the latter. Wave observations and theory combined supplement the particle measurements that are difficult at low energies and complicated by spacecraft charging.

The Hayabusa2 mission successfully collected samples from the asteroid Ryugu last year and will return these to Earth in December 2020. It is anticipated that the samples will enable the analysis of terrestrially uncontaminated organic matter and minerals. Such analyses are in turn expected to elucidate the evolution of organic matter through Solar System history, including the origination and processing of biogenically important molecules, which could have been utilized by the first organisms on Earth. In anticipation, studies have made predictions concerning the properties of Ryugu, including its composition. The spectral characteristics of Ryugu, such as albedo, have been employed to relate the asteroid to members of the carbonaceous chondrite group that have been identified on Earth. However, the recent Hayabusa2 touchdown highlights a disparity between the color of surfaces of displaced platy fragments, indicating a brightening trend for the surface exposed to space compared to that facing into the body. Here we present a mass balance calculation with reference to data from the literature, which indicates that Ryugu may contain a significantly higher abundance of organic matter (likely >50%) than the currently most accepted meteorite analogues. A high organic content may result in high levels of extractable organic matter for the second touchdown site, where the spacecraft sampled freshly exposed material. However, high abundances of insoluble aromatic/graphitic rich organic matter may be present in the first touchdown site, which sampled the surface of Ryugu that had been exposed to space. Moreover, we suggest that the potentially high organic abundance and the rubble-pile nature of Ryugu may originate from the capture of rocky debris by a comet nucleus and subsequent water-organic-mineral interactions and sublimation of water ice.

“The Great Comet of 1997” was a significant event in cometary physics. Numerous interesting scientific studies are devoted to this comet. Unfortunately, in the Russian scientific literature, there are few publications on investigations of Comet Hale–Bopp. This study is timed to the twentieth anniversary of finishing the main observations of this comet and reminds the readers about some of its unusual and even unique features without pretending to present an exhaustive review of the published materials. In this paper, an attempt is made to estimate the mass of the nucleus of Comet Hale–Bopp and it is supposed that it may be close to the initial mass of Halley’s comet which the latter had in the beginning of its evolution. Based on the published images of this comet, the approximate velocity of the gas-dust ejections and the spread of the velocities are calculated. A possible relation between the mass of the total losses and the dynamics of the comet nucleus is considered.

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 $\mathrm{H_2O}$ and $\mathrm{CO_2}$ 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 $0.1$-$1\,\mathrm{m}$. The airfall material preserves substantial amounts of water ice even in relatively small (cm-sized) coma aggregates after a rather long ($12\,\mathrm{h}$) residence in the coma. However, $\mathrm{CO_2}$ 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., Comment: 65 pages, 11 figures. Published manuscript

Periodic comets move around the Sun on elliptical orbits. As such comet 67P/Churyumov Gerasimenko (hereafter 67P) spends a portion of time in the inner solar system where it is exposed to increased solar insolation. Therefore given the change in heliocentric distance in case of 67P from aphelion at 5.68 AU to perihelion at similar to 1.24 AU the comet's activity the production of neutral gas and dust undergoes significant variations. As a consequence during the inbound portion the mass loading of the solar wind increases and extends to larger spatial scales. This paper investigates how this interaction changes the character of the plasma environment of the comet by means of multifluid MHD simulations. The multifluid MHD model is capable of separating the dynamics of the solar wind ions and the pick up ions created through photoionization and electron impact ionization in the coma of the comet. We show how two of the major boundaries the bow shock and the diamagnetic cavity form and develop as the comet moves through the inner solar system. Likewise for 67P although most likely shifted back in time with respect to perihelion passage this process is reversed on the outbound portion of the orbit. The presented model herein is able to reproduce some of the key features previously only accessible to particle based models that take full account of the ions' gyration. The results shown herein are in decent agreement to these hybrid type kinetic simulations.

The paper discusses four processes that may be responsible for shaping the relief of the comet landscape. The considerations take into account the following phenomena: intense sublimation of cometary ice, emission of dust from the surface comet and through jets, migration of dust on the cometary surface, and destruction of fragment the nucleus surface in the context of a change in cometary brightness. These processes appear to be of key importance for the formation of an irregular comet nucleus structure. It should be remembered that the processes discussed in this paper have an impact on the relief of the surface, but on a longer time scale, i.e., more than a few orbital periods.

We describe the AMBITION project, a mission to return the first-ever cryogenically-stored sample of a cometary nucleus, that has been proposed for the ESA Science Programme Voyage 2050. Comets are the leftover building blocks of giant planet cores and other planetary bodies, and fingerprints of Solar System’s formation processes. We summarise some of the most important questions still open in cometary science and Solar System formation after the successful Rosetta mission. We show that many of these scientific questions require sample analysis using techniques that are only possible in laboratories on Earth. We summarize measurements, instrumentation and mission scenarios that can address these questions. We emphasize the need for returning a sample collected at depth or, still more challenging, at cryogenic temperatures while preserving the stratigraphy of the comet nucleus surface layers. We provide requirements for the next generation of landers, for cryogenic sample acquisition and storage during the return to Earth. Rendezvous missions to the main belt comets and Centaurs, expanding our knowledge by exploring new classes of comets, are also discussed. The AMBITION project is discussed in the international context of comet and asteroid space exploration.

Anisotropic outgassing from comets exerts a torque sufficient to rapidly change the angular momentum of the nucleus, potentially leading to rotational instability. Here, we use empirical measures of spin changes in a sample of comets to characterize the torques and to compare them with expectations from a simple model. Both the data and the model show that the characteristic spin-up timescale, $\tau_s$, is a strong function of nucleus radius, $r_n$. Empirically, we find that the timescale for comets (most with perihelion 1 to 2 AU and eccentricity $\sim$0.5) varies as $\tau_s \sim 100 r_n^{2}$, where $r_n$ is expressed in kilometers and $\tau_s$ is in years. The fraction of the nucleus surface that is active varies as $f_A \sim 0.1 r_n^{-2}$. We find that the median value of the dimensionless moment arm of the torque is $k_T$ = 0.007 (i.e. $\sim$0.7\% of the escaping momentum torques the nucleus), with weak ($, Comment: 36 Pages, 6 Figures, AJ in press

We present a method for automatized detection and analysis of quasi-periodic lineament structures from images at pixel precision. The method exploits properties of the images’ frequency domain found by using the Fourier transform. We developed this method with the goal of detecting lineament structures in an image of the Hathor cliff of comet 67P/Churyumov–Gerasimenko, which are caused by layerings and furrows in the nucleus material. Using our method, we determined the orientation and wavelength range of these structures. The detected layering edges have similar orientations and spatial separations of 9–20 m, and are ubiquitous throughout the image. We suggest that the layerings are a global feature of the comet nucleus that provide information about formation and evolution of comet 67P. The furrows are non-uniformly distributed throughout the image. Their orientation is broadly parallel to the direction of the local gravity vector at the Hathor cliff, with spacings similar to those of the layering structures. The furrows are interpreted as signatures of local down-slope movement of cliff material. We demonstrate that the developed method is broadly applicable to the detection and analysis of various kinds of quasi-periodic structures like geological layering, folding and faulting, and texture analysis in general. In order to facilitate the application of our method, this paper is accompanied by a demo program written in matlab.

Here, we report the detection of phosphorus and fluorine in solid particles collected from the inner coma of comet 67P/Churyumov-Gerasimenko measured with the COmetary Secondary Ion Mass Analyser (COSIMA) instrument on-board the Rosetta spacecraft, only a few kilometers away from the comet nucleus. We have detected phosphorus-containing minerals from the presented COSIMA mass spectra, and can rule out e.g. apatite minerals as the source of phosphorus. This result completes the detection of life-necessary CHNOPS-elements in solid cometary matter, indicating cometary delivery as a potential source of these elements to the young Earth. Fluorine was also detected with CF$^+$ secondary ions originating from the cometary dust., 5 pages, 1 figure, 1 table, supplementary 2 tables and 2 figures

Context.Cometary outgassing is induced by the sublimation of ices and the ejection of dust originating from the nucleus. Therefore measuring the composition and dynamics of the cometary gas provides information concerning the interior composition of the body. Nevertheless, the bulk composition differs from the coma composition, and numerical models are required to simulate the main physical processes induced by the illumination of the icy body.Aims.The objectives of this study are to bring new constraints on the interior composition of the nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P) by comparing the results of a thermophysical model applied to the nucleus of 67P and the coma measurements made by the Reflectron-type Time-Of-Flight (RTOF) mass spectrometer. This last is one of the three instruments of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), used during the Rosetta mission.Methods.Using a thermophysical model of the comet nucleus, we studied the evolution of the stratigraphy (position of the sublimation and crystallisation fronts), the temperature of the surface and subsurface, and the dynamics and spatial distribution of the volatiles (H2O, CO2and CO). We compared them with the in situ measurements from ROSINA/RTOF and an inverse coma model.Results.We observed the evolution of the surface and near surface temperature, and the deepening of sublimation fronts. The thickness of the dust layer covering the surface strongly influences the H2O outgassing but not the more volatiles species. The CO outgassing is highly sensitive to the initial CO/H2O ratio, as well as to the presence of trapped CO in the amorphous ice.Conclusions.The study of the influence of the initial parameters on the computed volatile fluxes and the comparison with ROSINA/RTOF measurements provide a range of values for an initial dust mantle thickness and a range of values for the volatile ratio. These imply the presence of trapped CO. Nevertheless, further studies are required to reproduce the strong change of behaviour observed in RTOF measurements between September 2014 and February 2015.

We present an analysis of the layered structure on 67P/Churyumov–Gerasimenko’ s Hathor cliff and propose a mechanism (self-sustaining, dual-mode propagation of amorphous to crystalline ice phase-change fronts into the nucleus interior) for its origin. If this is a viable mechanism, the strata must be geologically young (∼106 y). The Hathor cliff exposes strata and orthogonal prominent linear structures deep into the small lobe over a radial distance of ∼650 m. We find an average radial thickness of a stratum (defined as the upper stratum boundary and the associated intra-strata material below) of ∼14 m and estimate its average horizontal dimension at 290 ± 110 m. Structures in intra-strata material are highly varied ranging from thin, often distorted, layers (∼1 - 3 m thick), to block-like and globular-shaped features (typically ∼1.5 m radius). Prominent lineaments are typically separated by ∼80 ± 30 m and are ∼5 m, or less, wide. Guided by the results of calculations by Tancredi, Rickman and Greenberg (1994, Astron. Astrophys. 286, 659 – 682) on the dual-mode propagation of such fronts in a simulated comet nucleus, we propose that the bi-modal rates of propagation of these fronts (an essentially stationary “quiescent” mode lasting ∼10 y (or more), alternating with an “active” rapid spurt propagating at ∼100 my−1 over ∼10 - 20 m (or more) in a few months) leads to the establishment of alternating strata boundaries (in the quiescent mode), and the intra-strata material (during the active mode). To achieve the apparent global coordination of the strata we hypothesize that the direction of propagation of the fronts are controlled by the radial outflow of CO coupled with the likely existence of a coarse layered structure in the primeval material below the front. Prominent lineaments were formed after the strata, and we propose that they are the result of tensional stress due to CO and other super-volatile gases released during the phase-change process. The array of different structural forms found in the intra-strata material of 67P are postulated to be the result of various modes of fluidization of the material during the active propagation mode. The independence of the strata systems in the two lobes of 67P (Massironi and 58 colleagues. 2015. Nature 526, 402 – 405) is explained by an assumed, low speed, break-up of the nucleus from a bi-lobate shape into a low energy close binary, soon after the time of onset of crystallization, as the surface is weakened. Eventually, the comet returns to a physically connected bi-lobate shape well after the crystallization of the interior of both lobes is complete (in about ∼2000 y). This “phase-change mechanism” during the Centaur phase may lead to internal layer geometries that vary from one Jupiter Family Comet (JFC) nucleus to another depending on their original shape and internal structure.

Imaging the interior structure of small planetary bodies facilitates a deep understanding of their origin and evolution, thus addressing fundamental questions about the formation of the Solar System. We show that high resolution 3D imaging of their interior structure is possible using radar waves that reflect from internal discontinuities of dielectric properties. A radar imaging mission at a comet nucleus would have the benefit of orbiting all around a finite and transparent body, collecting echoes that derive only from the target, and processing them collectively in phase. As is the case in the medical field, imaging a comet nucleus requires its illumination from multiple directions, which can be accomplished with a spacecraft in slow polar orbit around the studied object. Long acquisition time leads to a dense acquisition array resembling that of conventional synthetic aperture radar systems, but completely surrounding the nucleus ( 4 π steradians). Acquisition with an orbiter at large distance from the comet nucleus ( 5 × the mean diameter) results in relatively coarse data sampling relative to the radar wavelength, thus enabling 3D imaging with a short ( 90 days) mission. Radar migration is performed using techniques adapted from terrestrial exploration seismology. Wavefield migration identifies interior reflectors by time reversal of monostatic radar data redatumed to the known comet surface for computational speed-up. This technique is applicable to nuclei of arbitrary shape and interior complexity, and it is complementary to wavefield tomography tasked with constraining physical properties in-between the interior interfaces. Migration resolution is identical in all directions (range and cross-range) and it is equivalent to the range resolution of the radar system. Least-squares migration enhances resolution further by deconvolution of the radar wavelet from the reflectivity image.

Answering fundamental questions about the origin and evolution of small planetary bodies hinges on our ability to image their surface and interior structure in detail and at high resolution. The interior structure is not easily accessible without systematic imaging using, e.g., radar transmission and reflection data from multiple viewpoints, as in medical tomography. Radar tomography can be performed using methodology adapted from terrestrial exploration seismology. Our feasibility study primarily focuses on full wavefield methods that facilitate high quality imaging of small body interiors. We consider the case of a monostatic system (co-located transmitters and receivers) operated in various frequency bands between 5 and 15 MHz, from a spacecraft in slow polar orbit around a spinning comet nucleus. Using realistic numerical experiments, we demonstrate that wavefield techniques can generate high resolution tomograms of comets nuclei with arbitrary shape and complex interior properties.

We report the detection of emission from the OH 18 cm Λ-doublet transitions toward Comet C/2020 F3 NEOWISE using the Arecibo Telescope. The antenna temperatures are 113 ± 3 mK for the 1667 MHz line and 57 ± 3 mK for the 1665 MHz line. The beam-averaged OH column density (centered on the comet nucleus) derived from the 1667 transition is N OH = (1.11 ± 0.06) × 1013 cm−2. We implemented the Haser model to derive an OH production rate. The estimated OH production rate using the 1667 transition is Q OH = (3.6 ± 0.6) × 1028 s−1, a factor of 2.4 lower than optically derived values for the same observing day, the difference of which is likely explained by quenching.

Comets are icy remnants of the Solar System formation, and as such contain some of the most primitive volatiles and organic materials. Sampling the surface of a comet is a high priority for the New Frontiers program. Planetary simulants are crucial to the development of adequate in situ instruments and sample acquisition systems. A high-fidelity comet surface simulant has been developed to support hardware design and development for one Comet Surface Sample Return tool, the BiBlade Comet Sampler. Mechanical Porous Ambient Comet Simulants (MPACS) can be manufactured to cover a wide range of desired physical properties, such as density and cone penetration resistance, and exhibit a brittle fracture mode. The structure of the MPACS materials is an aggregated composite structure of weakly-bonded grains of very small size (diameter ≤ 40 μm) that are most relevant to the structure of the surface of a comet nucleus.

International audience; We suggest that the large organic molecules found in the dust of comet 67P/CG originated from the interstellar medium (ISM), and that this material is the source of absorption features in stellar spectra known as diffuse interstellar bands (DIBs). These large organic molecules were present in the ISM prior to the emergence of the pre-solar nebula and were conserved during comet formation in the solar nebula due to gentle, hierarchical accretion, a scenario based on many lines of evidences collected with Rosetta rendezvous mission (Davidsson et al. 2016). This is in contrast with the sublimation of H2O ice, according to diverse comet D/H values. While the organic to mineral mass ratios RC for comet Halley and 67P/CG were measured in the range ∼0.32–1.0, we estimate that the DIB carriers alone can provide RISM = 0.32, but that this ratio could be increased by other organic molecules in the ISM that do not show up in absorption. The decrease of DIB during line-of-sight crossings of the dense cores of interstellar clouds and the simultaneous steepening of the far-UV part of the reddening curve suggest that DIB carriers coagulate and are constituent of the very small grains eventually preserved in comets. This conclusion implies that a future sample-return mission of a comet nucleus would not only provide unique information on comets, but also on the exact nature of the interstellar species producing the hundreds of DIBs, an unanswered question since their discovery several decades ago.

We present results from our 47-night imaging campaign of Comet 41P/Tuttle-Giacobini-Kresak conducted from Lowell Observatory between 2017 February 16 and July 2. Coma morphology revealed gas jets, whose appearance and motion as a function of time yielded the rotation period and other properties. All narrowband CN images exhibited either one or two jets; one jet appeared as a partial face-on spiral with clockwise rotation while the second jet evolved from a side-on corkscrew, through face-on, and finally corkscrew again, with only a slow evolution throughout the apparition due to progressive viewing geometry changes. A total of 78 period determinations were made over a 7-week interval, yielding a smooth and accelerating rotation period starting at 24 hr (March 21&22) and passing 48 hr on April 28. While this is by far the fastest rate of change ever measured for a comet nucleus, the torque required is readily within what can exist given likely properties of the nucleus. If the torque remained constant, we estimate that the nucleus could have stopped rotating and/or began to tumble as soon as only two months following perihelion, and will certainly reach this stage by early in the next apparition. Working backwards in time, Tuttle-Giacobini-Kresak would have been rotating near its rotational break-up velocity 3-4 orbits earlier, suggesting that its extreme 7-magnitude outburst observed in 2001 might have been caused by a partial fragmentation at that time, as might the pair of 1973 8-magnitude outbursts if there had been an earlier spin-down and spin-up cycle., 27 pages text, 3 tables, 7 figures (in multiple parts)

Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., 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 pixel(-1), which allowed us to estimate an optical depth of similar to 0.65 and an ejected mass of similar to 2200 kg, assuming a grain bulk density of similar to 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(-1)) 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.© S. Fornasier et al. 2019, OSIRIS was built by a consortium led by the Max-Planck-Institut fur Sonnensystemforschung, Goettingen, Germany, in collaboration with CISAS, University of Padova, Italy, the Laboratoire d'Astrophysique de Marseille, France, the Instituto de Astrofisica de Andalucia, CSIC, Granada, Spain, the Scientific Support Office of the European Space Agency, Noordwijk, the Netherlands, the Instituto Nacional de Tecnica Aeroespacial, Madrid, Spain, the Universidad Politechnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and the Institut fur Datentechnik und Kommunikationsnetze der Technischen Universitat 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 Mission. S.F. acknowledges the financial support from the France Agence Nationale de la Recherche (programme Classy, ANR-17-CE31-0004). The authors thank H. Campins for his comments that helped us improve this article. The authors would like to thank Andrew Cooper and Marco Parigi for spotting the inconsistencies in regional boundaried of Fig. 1 in the online preprint version of the paper.

The aim of this paper is to contribute to the characterization of the general properties of the Long Period Comets (LPCs) family, and in particular to report on the dust environment of comet C/2009 P1 (Garradd).The comet was observed at two epochs pre-perihelion, at ~6 AU and at ~2.5 AU: broad-band images have been used to investigate its coma morphology and properties and to model the dust production rate.Comet C/2009 P1 (Garradd) is one of the most active and "dust producing" LPCs ever observed, even at the large heliocentric distance rh~6 AU. Its coma presents a complex morphology, with subtle structures underlying the classical fan-shaped tail, and, at rh~2.5 AU, also jet-like structures and spiralling outflows. In the reference aperture of radius ρ=5°×104 km, the R-Afρ is 3693±156 cm and 6368±412 cm, in August 2010 (rh~6 AU) and July 2011 (rh~2.5 AU), respectively. The application of a first order photometric model, under realistic assumptions on grain geometric albedo, power-law dust size distribution, phase darkening function and grain dust outflow velocity, yielded a measure of the dust production rate for the two epochs of observation of Qd=7.27×102 kg/s and Qd=1.37×103 kg/s, respectively, for a reference outflow dust velocity of vsmall=25 m/s for small (0.1-10 μm) grains and vlarge=1 m/s for large (10 μm-1 cm) grains.These results suggest that comet Garradd is one of the most active minor bodies observed in recent years, highly contributing to the continuous replenishment of the Interplanetary Dust Complex also in the outer Solar System, and pose important constraints on the mechanism(s) driving the cometary activity at large heliocentric distances. Fil: Mazzotta Epifani, C.. Istituto Nazionale Di Astrofisica; Italia. Osservatorio Astronomico di Roma; Italia Fil: Snodgrass, C.. The Open University; Reino Unido Fil: Perna, D.. Centre National de la Recherche Scientifique. Observatoire de Paris; Francia Fil: Dall’Ora, M.. Osservatorio Astronomico di Capodimonte; Italia Fil: Palumbo, P.. Università Parthenope; Italia Fil: Della Corte, V.. Istituto di Astrofisica e Planetologia Spaziale; Italia Fil: Alvarez Candal, A.. Ministério de Ciencia, Tecnologia e Innovacao. Observatorio Nacional; Brasil Fil: Melita, Mario Daniel. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Rotundi, A.. Università Parthenope; Italia

We present Rosetta observations from comet 67P/Churyumov-Gerasimenko during the impact of a coronal mass ejection (CME). The CME impacted on 5-6 Oct 2015, when Rosetta was about 800 km from the comet nucleus, \textcolor{black}{and 1.4 AU from the Sun}. Upon impact, the plasma environment is compressed to the level that solar wind ions, not seen a few days earlier when at 1500 km, now reach Rosetta. In response to the compression, the flux of suprathermal electrons increases by a factor of 5-10 and the background magnetic field strength increases by a factor of $\sim$2.5. The plasma density increases by a factor of 10 and reaches 600 cm$^{-3}$, due to increased particle impact ionisation, charge exchange and the adiabatic compression of the plasma environment. We also observe unprecedentedly large magnetic field spikes at 800 km, reaching above 200 nT, which are interpreted as magnetic flux ropes. We suggest that these could possibly be formed by magnetic reconnection processes in the coma as the magnetic field across the CME changes polarity, or as a consequence of strong shears causing Kelvin-Helmholtz instabilities in the plasma flow. Due to the \textcolor{black}{limited orbit of Rosetta}, we are not able to observe if a tail disconnection occurs during the CME impact, which could be expected based on previous remote observations of other CME-comet interactions., 14 pages, 12 figures

Consideration is given to the estimates for the strength of the consolidated material forming the bulk of the nucleus of comet 67P Churyumov–Gerasimenko and those for the strength of the surface material overlying the consolidated material at the sites of the first and last contact of the Philae lander with the nucleus. The strength of the consolidated material was estimated by analyzing the terrain characteristics of the steep cliffs, where the material is exposed on the surface. Based on these estimates, the tensile strength of the material is in the range from 1.5 to 100 Pa; the shear strength, from ∼13 to ⩾30 Pa; and the compressive strength, from 30 to 150 Pa, possibly up to 1.5 kPa. These are very low strength values. Given the dependence of the measurement results on the size of the measured object, they are similar to those of fresh dry snow at –10°C. The (compressive) strength of the surface material at the site of the first touchdown of Philae on the nucleus is estimated from the measurements of the dynamics of the surface impact by the spacecraft’s legs and the geometry of the impact pits as 1–3 kPa. For comparison with the measurement results for ice-containing materials in terrestrial laboratories, it needs to be taken into account that the rate of deformation by Philae’s legs is four orders of magnitude higher than that in typical terrestrial measurements, leading to a possible overestimation of the strength by roughly an order of magnitude. There was an attemp to put one of the MUPUS sensors into the surface material at the site of the last contact of Philae with the nucleus. Noticeable penetration of the tester probe was not achieved that led to estimation of the minimum compressive strength of the material to be ⩾4 MPa4 This fairly high strength appears to indicate the presence of highly porous ice with grains “frozen” at contacts.

Observations of Comet C/2010 X1 (Elenin) in August 2011 showed a pattern of evolution of coma morphology into an expanding, amorphous cloud of rapidly decreasing surface brightness shortly before perihelion passage, similar to C/1999 S4 (LINEAR) in 2000. However, the comet’s difficult viewing geometry at the time of disruption made it impossible to study the event using high-resolution imaging techniques and, in particular, to measure the ratio of dust to cometessimals which gives information on the internal structure of the nucleus before disruption. We report Herschel/PACS observations of the defunct Comet C/2010 X1 (Elenin), aimed at detecting its infrared ghost: the debris cloud and any remaining inert, or low-activity cometessimals that may have been released in the break-up of the nucleus, taking advantage of the comet’s close pass to Earth in mid-October 2011. These observations would have allowed us to detect, potentially, both individual surviving inert fragments of the nucleus of ≈260 m diameter (and significantly smaller, active cometessimals) and the expanding dust cloud, being particularly sensitive to coarse dust and boulders in the millimetre to metre size range. No discrete bodies were detected by Herschel to this limiting size.

The results of the testing analysis of the dynamic parameters of 114 comets undergoing nucleus splitting are presented in the article. Some aspects of the following idea as working hypothesis are studied: disintegration of the comet nucleus happens as results of their collision with meteoroid streams also. For the verification of this working hypothesis the position of splitted comet orbits have been analyzed relative to 125 meteor streams from Kronk's list. The number of comet orbit nodes separated by 0.001, 0.005, 0.01, 0.05 and 0.1 au from known meteor streams were computed and analyzed. The orbit of each meteorite stream was replaced by a set of similar pseudo-streams for comparison. Statistical methods and simulations showed that 31 out of the 125 streams have significant predominance over pseudo streams.

Context.An open question in the study of comets is the so-called cohesion bottleneck, that is, how dust particles detach from the nucleus.Aims.We test whether the CO pressure buildup inside the pebbles of which cometary nuclei consist can overcome this cohesion bottleneck.Methods.A recently developed pebble-diffusion model was applied here to comet C/2017K2 PANSTARRS, assuming a CO-driven activity.Results.(i) The CO-gas pressure inside the pebbles erodes the nucleus into the observed dust, which is composed of refractories, H2O ice and CO2ice. (ii) The CO-driven activity onset occurs up to heliocentric distances of 85 au, depending on the spin orientation of the comet nucleus. (iii) The activity onset observed at ≈26 au suggests a low obliquity of the nucleus spin axis with activity in a polar summer. (iv) At 14 au, the smallest size of the ejected dust is ≈0.1 mm, consistent with observations. (v) The observed dust-loss rate of ≈200 kg s−1implies a fallout ≥30%, a nucleus surface active area ≥10 km2, a CO-gas loss rate ≥10 kg s−1, and a dust-to-gas ratio ≤20. (vi) The CO-driven activity never stops if the average refractory-to-all-ices mass ratio in the nucleus is ≤4.5 for a nucleus all-ices-to-CO mass ratio ≈4, as observed in comets Hale–Bopp and Hyakutake. These results make comet C/2017K2 similar to the Rosetta target comet 67P/Churyumov–Gerasimenko. (vii) The erosion lifetime of cometary planetesimals is a factor 103shorter than the timescale of catastrophic collisions. This means that the comets we observe today cannot be products of catastrophic collisions.

From the operation of the MUPUS thermal probe Spohn et al. (2015) concluded that the material of the nucleus of 67P/Churyumov–Gerasimenko is likely to have a high strength, at least locally at the Philae landing site. In this work we consider the derived strength of the material in order to constrain its granulation. For this purpose we performed numerical simulations of the long-term sintering of ice–dust granular mixtures of different granulation, covered by a dust mantle. The dust mantle has a thickness of 0–16cm, and a (pore size and temperature-dependent) thermal conductivity. According to our simulations a hardened layer at least a meter thick forms beneath the dust only when the grains are tens of microns in radius, or smaller.

The results of the dynamic modeling of the process, that formed the dust tail of comet C/2012 S1 (ISON), are presented. The images of the comet were acquired with the 1-meter Zeiss-1000 telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS). To construct the model of the dust tail, the trajectories of 50 million dust particles were traced. The distribution of brightness in the dust tail of the comet was reproduced in the model. According to our model investigations, the observed tail could be formed by dust particles with the sizes ranging from 0.5 to 16.6 µm and the escape velocities from 17 to 130 m/s; the power exponent of the distribution by radius is–2.5, and the maximum age of the dust particles having formed the cometary tail is 25 days. In the paper, the brightness of the comet at the moment of observations is also estimated, and the morphology of the cometary coma is analyzed with the use of digital filters.

Previously proposed mechanisms have difficulty explaining the disruption of Comet C/2012 S1 (ISON) as it approached the Sun. We describe a novel cometary disruption mechanism whereby comet nuclei fragment and disperse through dynamic sublimation pressure, which induces differential stresses within the interior of the nucleus. When these differential stresses exceed its material strength, the nucleus breaks into fragments. We model the sublimation process thermodynamically and propose that it is responsible for the disruption of Comet ISON. We estimate the bulk unconfined crushing strength of Comet ISON's nucleus and the resulting fragments to be 0.5 Pa and 1-9 Pa respectively, assuming typical Jupiter Family Comet (JFC) albedos. However, if Comet ISON has an albedo similar to Pluto, this strength estimate drops to 0.2 Pa for the intact nucleus and 0.6-4 Pa for its fragments. Regardless of assumed albedo, these are similar to previous strength estimates of JFCs. This suggests that, if Comet ISON is representative of dynamically new comets, then low bulk strength is a primordial property of some comet nuclei, and not due to thermal processing during migration into the Jupiter Family., 37 pages, 4 figures

International audience; This review presents some of the recent advancements in our understanding of comets facilitated by laboratory studies, need for new laboratory simulations, and predictions for future explorations. With the spacecraft Rosetta at the comet 67P/Churyumov–Gerasimenko, a large volume of science data is expected to follow early results that have been published recently. The most surprising of them being hard ice shell that bounced thelander Philae a couple of times before settling on the comet. Long evaded molecular nitrogen has now been detected in the comet 67P/CG. The observed density of 470 kgm−3 is in line with other comet observations, whereas the nature and composition of hydrocarbons detected on the surface are still a puzzle. Observation of D/H ratio that deviates significantly from Earth’s water D/H ratio brings back to the table the long-standing question whether or not water on Earth was delivered by comet impacts. Our review summarizes some of the critical laboratory work that helps improve our understanding of cometary interior (whether amorphous or crystalline or containing clathrates), cometary surface (rich of complex organics), cometary coma and tail (D/H ratio, negative ions, and photoluminescence). Outstanding questions are also discussed.

In recent decades, several missions have detected signs of water and other volatiles in cold, permanently shadowed craters near the lunar poles. Observations suggest that some of these volatiles could have been delivered by comet impacts and therefore, understanding the impact delivery mechanism becomes key to explaining the origin and distribution of lunar water. During impact, the constituent ices of a comet nucleus vaporize; a significant part of this vapor remains gravitationally bound to the Moon, transforming the tenuous, collisionless lunar exosphere into a collisionally thick, transient atmosphere. Here, we use numerical simulations to investigate the physical processes governing volatile transport in the transient atmosphere generated after a comet impact, with a focus on how these processes influence the accumulation of water in polar cold traps. It is observed that the transient atmosphere maintains a certain characteristic structure for at least several Earth days after impact, during which time volatile transport occurs primarily through low-altitude winds that sweep over the lunar day-side. Meanwhile, reconvergence of vapor antipodal to the point of impact results in preferential redistribution of water in the vicinity of the antipode. Due to the quantity of vapor that remains gravitationally bound, the atmosphere is sufficiently dense that lower layers are shielded from photodestruction, prolonging the lifetime of water molecules and allowing greater amounts of water to reach cold traps. Short-term ice deposition patterns are markedly non-uniform and the variations that arise in simulated volatile abundance between different cold traps could potentially explain variations that have been observed through remote sensing.

High-quality white-light images from the Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) HI-1 telescope on board STEREO-B reveal high-velocity evanescent clumps (HVECs) expelled from the coma of the C/2011 L4 (Pan-STARRS) comet. The observations were recorded around the comet's perihelion (i.e., ∼0.3 AU) during the period 9–16 March 2013. Animated images provide evidence of highly dynamic ejecta moving near radially in the antisunward direction. The bulk speed of the clumps at their initial detection in the HI1-B images range from 200 to 400 km s−1 followed by an appreciable acceleration up to speeds of 450–600 km s−1, which are typical of slow to intermediate solar wind speeds. The clump velocities do not exceed these limiting values and seem to reach a plateau. The images also show that the clumps do not expand as they propagate. The white-light images do not provide direct insight into the composition of the expelled clumps, which could potentially be composed of fine, submicron dust particles, neutral atoms and molecules, and/or ionized atomic/molecular cometary species. Although solar radiation pressure plays a role in accelerating and size sorting of small dust grains, it cannot accelerate them to velocities >200 km s−1 in the observed time interval of a few hours and distance of 50 could possibly fit the observations. Just as likely, we find that an interaction with the solar wind and the heliospheric magnetic field can cause the observed clump dynamical evolution, accelerating them quickly up to solar wind velocities. We thus speculate that the HVECs are composed of charged particles (dust particles) or neutral atoms accelerated by radiation pressure at β > 50 values. In addition, the data suggest that clump ejecta initially move along near-radial, bright structures, which then separate into HVECs and larger dust grains that steadily bend backward relative to the comet's orbital motion due to the effects of solar radiation and gravity. These structures gradually form new striae in the dust tail. The near-periodic spacing of the striae may be indicative of outgassing activity modulation due to the comet nucleus' rotation. It is, however, unclear whether all striae are formed as a result of this process.

We have designed and built a laboratory facility to investigate the spectro photometric and morphologic properties of different types of ice bearing planetary surface analogs and follow their evolution upon exposure to a low pressure and low temperature environment. The results obtained with this experiment are used to verify and improve our interpretations of current optical remote sensing datasets. They also provide valuable information for the development and operation of future optical instruments. The Simulation Chamber for Imaging the Temporal Evolution of Analogue Samples (SCITEAS) is a small thermal vacuum chamber equipped with a variety of ports and feedthroughs that permit both in situ and remote characterizations as well as interacting with the sample. A large quartz window located directly above the sample is used to observe its surface from outside with a set of visible and near infrared cameras. The sample holder can be easily and quickly inserted and removed from the chamber and is compatible with the other measurement facilities of the Laboratory for Outflow Studies of Sublimating Materials (LOSSy) at the University of Bern. We report here on the results of two of the first experiments performed in the SCITEAS chamber. In the first experiment fine grained water ice mixed with dark organic and mineral matter was left to sublime in vacuum and at low temperature simulating the evolution of the surface of a comet nucleus approaching the Sun. We observed and characterized the formation and evolution of a crust of refractory organic and mineral matter at the surface of the sample and linked the evolution of its structure and texture to its spectro photometric properties. In the second experiment a frozen soil was prepared by freezing a mixture of smectite mineral and water. The sample was then left to sublime for 6 h to simulate the loss of volatiles from icy soil at high latitudes on Mars. Colour images were produced using the definitions of the filters foreseen for the CaSSIS imager of the Exomars/TGO mission in order to prepare future science operations. (C) 2015 Elsevier Ltd. All rights reserved.

International audience; The Rosetta encounter with comet 67P/Churyumov-Gerasimenko provides a unique opportunity for an in situ, up-close investigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the Sun.Observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within the thin coma.Methods. Observations from late October through mid-December 2014 show the continuous presence of the solar wind 30 km from the comet nucleus. These and other observations indicate that there is no contact surface and the solar wind has direct access to the nucleus. On several occasions during this time period, the Rosetta/ROSINA/Double Focusing Mass Spectrometer measured the low-energy ion composition in the coma. Organic volatiles and water group ions and their breakup products (masses 14 through 19), CO2+ (masses 28 and 44) another mass peaks (at masses 26, 27 , and possibly 30) were observed. Secondary ions include H3O+ and HCO+ (masses 19 and 29). These secondary ions indicate ion-neutral chemistry in the thin coma of the comet. A relatively simple model is constructed to account for the low H3O+/H2O+ and HCO+/CO+ ratios observed in a water dominated coma. Results from this simple model are compared with results from models that include a more detailed chemical reaction network.At low outgassing rates, predictions from the simple model agree with observations and with results from more complex models that include much more chemistry. At higher outgassing rates, the ion-neutral chemistry is still limited and high HCO+/CO+ ratios are predicted and observed. However, at higher outgassing rates, the model predicts high H3O+/H2O+ ratios and the observed ratios are often low. These low ratios may be the result of the highly heterogeneous nature of the coma, where CO and CO2 number densities can exceed that of water.

International audience; In this work we provide measurements of the bidirectional reflectance in the VIS-NIR spectral range for a selection of laboratory samples to deduce parameters such as their visible spectrum, phase curve, hemispheric albedo and phase reddening. The aim is to simulate the reflectance of the comet nucleus in order to aid the interpretation of data from the surface of comet 67P/Churyumov-Gerasimenko (67P) acquired by the Rosetta OSIRIS imaging system. We produced a broad variety of well characterised and reproducible samples containing water ice, carbonaceous compounds and complex organic molecules. We were able to reproduce the individual global spectrophotometric parameters (albedo, spectrum, phase reddening, phase curve) of 67P by using mixtures of activated charcoal, tholins and water ice. However, no mixture was able to fit all parameters simultaneously. The samples with highest porosities best match the phase curve of 67P. Spectral considerations from our experiments show that the particle size of the darkening agent on the cometary surface should have a lower limit of a few hundred nanometres to several micrometres and cannot be dominated by particles in the range of some tens of nanometres. Furthermore, our findings indicate that the bright ice patches observed by OSIRIS and VIRTIS should be relatively dust free at small scale.

We present 2-5 $\mu$m spectroscopic observations of the dust coma of 67P/Churyumov-Gerasimenko obtained with the VIRTIS-H instrument onboard Rosetta during two outbursts that occurred on 2015, 13 September 13.6 h UT and 14 September 18.8 h UT at 1.3 AU from the Sun. Scattering and thermal properties measured before the outburst are in the mean of values measured for moderately active comets. The colour temperature excess (or superheat factor) can be attributed to submicrometre-sized particles composed of absorbing material or to porous fractal-like aggregates such as those collected by the Rosetta in situ dust instruments. The power law index of the dust size distribution is in the range 2-3. The sudden increase of infrared emission associated to the outbursts is correlated with a large increase of the colour temperature (from 300 K to up to 630 K) and a change of the dust colour at 2-2.5 $\mu$m from red to blue colours, revealing the presence of very small grains ($\leq$ 100 nm) in the outburst material. In addition, the measured large bolometric albedos ($\sim$ 0.7) indicate bright grains in the ejecta, which could either be silicatic grains, implying the thermal degradation of the carbonaceous material, or icy grains. The 3-$\mu$m absorption band from water ice is not detected in the spectra acquired during the outbursts, whereas signatures of organic compounds near 3.4 $\mu$m are observed in emission. The H$_2$O 2.7-$\mu$m and CO$_2$ 4.3-$\mu$m vibrational bands do not show any enhancement during the outbursts., Comment: Accepted for publication in the Monthly Notices of the Royal Astronomical Society

Simultaneously to the ESA Rosetta mission, a world-wide ground-based campaign provided measurements of the large scale activity of comet 67P/Churyumov-Gerasimenko through measurement of optically active gas species and imaging of the overall dust coma. We present more than two years of observations performed with the FORS2 low resolution spectrograph at the VLT, TRAPPIST, and ACAM at the WHT. We focus on the evolution of the CN production, as a tracer of the comet activity. We find that it is asymmetric with respect to perihelion and different from that of the dust. The CN emission is detected for the first time at 1.34 au pre-perihelion and production rates then increase steeply to peak about two weeks after perihelion at (1.00±0.10) ×1025 molecules s−1, while the post-perihelion decrease is more shallow. The evolution of the comet activity is strongly influenced by seasonal effects, with enhanced CN production when the Southern hemisphere is illuminated.

The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae is equipped with two gas analyzers: The Cometary Sampling and Composition experiment (COSAC) and the gas chromatograph and mass spectrometer Ptolemy. On 12 to 14 November 2014 both instruments measured the organic composition of the comet nucleus material through seven measurements in sniffing mode during Philae's hopping and at its final landing site Abydos. We compare the temporal evolution of intensities of several ion species identified by both mass spectrometers. For COSAC this is the first analysis of the temporal behaviour of the measured ion species. All ion species showed the highest intensities in the first spectra measured by both instruments about 20 to 30 minutes after Philae's first touchdown at Agilkia, and a decay during the six consecutive measurements at Abydos. Both instruments measured a nearly identical decay of the water peak (m/z 18), and also CO (m/z 28) behaved similarly. In the COSAC measurements the peak at m/z 44 decays much slower than all the other ion species, including the water peak. In particular, the m/z 44 peak decays much slower in the COSAC measurements than in the Ptolemy data. This supports our earlier interpretation that COSAC for the first time analyzed a regolith sample from a cometary nucleus in situ, while Ptolemy measured cometary gas from the ambient coma. The m/z 44 peak measured by COSAC was likely dominated by organic species, whereas the peak measured by Ptolemy was interpreted to be mostly due to $CO_2$. Ion species heavier than m/z 30 tend to decay somewhat slower in the COSAC measurements than in the Ptolemy data, which may be related to differences in the exhaust designs between both instruments., Comment: 7 pages, 4 figures, Astronomy and Astrophysics in press

International audience; The ESA's comet chaser Rosetta has monitored the evolution of the ionized atmosphere of comet 67P/Churyumov–Gerasimenko (67P/CG) and its interaction with the solar wind, during more than 2 yr. Around perihelion, while the cometary outgassing rate was highest, Rosetta crossed hundreds of unmagnetized regions, but did not seem to have crossed a large-scale diamagnetic cavity as anticipated. Using in situ Rosetta observations, we characterize the structure of the unmagnetized plasma found around comet 67P/CG. Plasma density measurements from RPC-MIP are analysed in the unmagnetized regions identified with RPC-MAG. The plasma observations are discussed in the context of the cometary escaping neutral atmosphere , observed by ROSINA/COPS. The plasma density in the different diamagnetic regions crossed by Rosetta ranges from ∼100 to ∼1500 cm −3. They exhibit a remarkably systematic behaviour that essentially depends on the comet activity and the cometary ionosphere expansion. An effective total ionization frequency is obtained from in situ observations during the high outgassing activity phase of comet 67P/CG. Although several diamagnetic regions have been crossed over a large range of distances to the comet nucleus (from 50 to 400 km) and to the Sun (1.25–2.4 au), in situ observations give strong evidence for a single diamagnetic region, located close to the electron exobase. Moreover, the observations are consistent with an unstable contact surface that can locally extend up to about 10 times the electron exobase.

We present an extensive data set of ground-based observations and models of the dust environment of comet 67P/Churyumov–Gerasimenko covering a large portion of the orbital arc from about 4.5 au pre-perihelion through 3.0 au post-perihelion, acquired during the current orbit. In addition, we have also applied the model to a dust trail image acquired during this orbit, as well as to dust trail observations obtained during previous orbits, in both the visible and the infrared. The results of the Monte Carlo modelling of the dust tail and trail data are generally consistent with the in situ results reported so far by the Rosetta instruments Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) and Grain Impact Analyser and Dust\ud Accumulator (GIADA). We found the comet nucleus already active at 4.5 au pre-perihelion, with a dust production rate increasing up to ∼3000 kg s−1 some 20 d after perihelion passage. The dust size distribution at sizes smaller than r = 1 mm is linked to the nucleus seasons, being described by a power law of index −3.0 during the comet nucleus southern hemisphere winter but becoming considerably steeper, with values between −3.6 and −4.3, during the nucleus southern hemisphere summer, which includes perihelion passage (from about 1.7 au inbound to 2.4 au outbound). This agrees with the increase of the steepness of the dust size distribution found from GIADA measurements at perihelion showing a power index of −3.7. The size distribution at sizes larger than 1 mm for the current orbit is set to a power law of index −3.6, which is near the average value of in situ measurements by OSIRIS on large particles. However, in order to fit the trail data acquired during past orbits previous to the 2009 perihelion passage, a steeper power-law index of −4.1 has been set at those dates, in agreement with previous trail modelling. The particle sizes are set at a minimum of r = 10 μm, and a maximum size, which increases with decreasing heliocentric distance, in the 1–40 cm radius domain. The particle terminal velocities are found to be consistent with the in situ measurements as derived from the instrument GIADA on board Rosetta.

Among the comet 67P/Churyumov-Gerasimenko (67P/C-G) in situ measurements, the closest that have ever been performed at a comet nucleus, are also those of speed, mass, and cross-section of cometary grains performed by the Grain Impact Analyser and Dust Accumulator (GIADA) instrument. To interpret GIADA data, we performed dust dynamical numerical simulations with both spherical and non-spherical (spheroids) shapes. This allowed us to analyse how the grain non-sphericity affects the data interpretation. We find that some measured dust speeds are unlikely reproducible when a spherical shape is considered. We considered two GIADA observational periods, 2015 February 19-27 and 2015 March 13-28. Gas parameters calibrated with the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) measurements have been used to retrieve the gas conditions to set up the dust particle motion. The dust grains are assumed to be out of the near nucleus coma, i.e. where the gas velocity is radial and constant, therefore they are either aligned or have random but constant orientation with respect to the gas drag. We reproduced the GIADA dust speeds, using spheres and two different spheroidal shapes. We find that the particle shapes that reproduce best the GIADA dust speeds are consistent with the particle shape constrained by the GIADA data. We obtain different terminal velocities for spherical and non-spherical particles of the same mass. The shape, which reproduces the GIADA data, is oblate rather than prolate spheroid. We obtain rotational frequencies of the spheroidal particles that best fit the GIADA measurements in these periods.

We make the following predictions: (1) Comet 67P/Churyumov-Gerasimenko. The Secular Light Curve (SLC) of this comet exhibits a photometric anomaly in magnitude that is present in 1982, 1996, 2002 and 2009. Thus it must be real. We interpret this anomaly as a topographic feature on the surface of the nucleus that may be a field of debris, a region made only of dust or an area of solid stones but in any case it is depleted in volatiles. We predict that images taken by spacecraft Rosseta will show a region morphologically different to the rest of the nucleus, at the pole pointing to the Sun near perihelion. (2) Comet C/2012 K1 Panstarrs. This comet exhibits the same Slope Discontinuity Event (SDE)+magnitude dip after the event than 11 other comets listed in Table 1 all of which disintegrated. This group includes comet C/2012 S1 ISON. Thus it is reasonable to expect that this comet may disintegrate too. The probability of disintegration of this comet is 20%. (3) Comet C/2013 V5 Oukaimeden. This comet exhibits the same SDE+dip signature exhibited by 11 other comets in Table 1 all of which disintegrated. This group includes comet C/2012 S1 ISON. We predict that there is a 90% probability that this comet will disintegrate., 8 pages, 4 Figures, 1 Table, submitted for publication

The martian upper atmosphere could be strongly perturbed by the near collision with Comet C/2013 A1 (Siding Spring). Significant mass and energy will be deposited in the upper atmosphere of Mars if the comet coma is sufficiently dense. We predict that comet H 2 O production rates larger than 10 28 molecules/s would produce temperature increases exceeding 30 K and the H density in the upper atmosphere will approximately double. The temperature perturbation will persist for several hours and the increased H density for tens of hours. Drag on orbiting spacecraft may increase by substantial factors, depending upon comet activity, because of the thermal perturbation to the atmosphere. Observation of these perturbations may provide insight into the thermal and chemical balances of the atmosphere.

When comet nuclei approach the Sun, the increasing energy flux through the surface layers leads to sublimation of the underlying ices and subsequent outgassing that promotes the observed emission of gas and dust. While the release of gas can be straightforwardly understood by solving the heat-transport equation and taking into account the finite permeability of the ice-free dust layer close to the surface of the comet nucleus, the ejection of dust additionally requires that the forces binding the dust particles to the comet nucleus must be overcome by the forces caused by the sublimation process. This relates to the question of how large the tensile strength of the overlying dust layer is. Homogeneous layers of micrometer-sized dust particles reach tensile strengths of typically 10 3 to 10 4 Pa . This exceeds by far the maximum sublimation pressure of water ice in comets. It is therefore unclear how cometary dust activity is driven. To solve this paradox, we used the model by Skorov and Blum (Skorov, Y.V., Blum, J. 2012. Icarus 221, 361–11), who assumed that cometesimals formed by gravitational instability of a cloud of dust and ice aggregates and calculated for the corresponding structure of comet nuclei tensile strength of the dust-aggregate layers on the order of 1 Pa. Here we present evidence that the emitted cometary dust particles are indeed aggregates with the right properties to fit the model by Skorov and Blum. Then we experimentally measure the tensile strengths of layers of laboratory dust aggregates and confirm the values derived by the model. To explain the comet activity driven by the evaporation of water ice, we derive a minimum size for the dust aggregates of ∼1 mm, in agreement with meteoroid observations and dust-agglomeration models in the solar nebula. Finally we conclude that cometesimals must have formed by gravitational instability, because all alternative formation models lead to higher tensile strengths of the surface layers.

Long-period comet C/2013 A1 (Siding Spring) will experience a close encounter with Mars on 2014 Oct 19. A collision between the comet and the planet has been ruled out, but the comet's coma may envelop Mars and its man-made satellites. By the time of the close encounter, five operational spacecraft will be present near Mars. Characterizing the coma is crucial for assessing the risk posed to these satellites by meteoroid impacts. We present an analytic model of cometary comae that describes the spatial and size distributions of cometary dust and meteoroids. This model correctly reproduces, to within an order of magnitude, the number of impacts recorded by Giotto near 1P/Halley [1] and by Stardust near comet 81P/Wild 2 [2]. Applied to Siding Spring, our model predicts a total particle fluence near Mars of 0.02 particles per square meter. In order to determine the degree to which Siding Spring's coma deviates from a sphere, we perform numerical simulations which take into account both gravitational effects and radiative forces. We take the entire dust component of the coma and tail continuum into account by simulating the ejection and evolution of dust particles from comet Siding Spring. The total number of particles simulated is essentially a free parameter and does not provide a check on the total fluence. Instead, these simulations illustrate the degree to which the coma of Siding Spring deviates from the perfect sphere described by our analytic model (see Figure). We conclude that our analytic model sacrifices less than an order of magnitude in accuracy by neglecting particle dynamics and radiation pressure and is thus adequate for order-of-magnitude fluence estimates. Comet properties may change unpredictably and therefore an analytic coma model that enables quick recalculation of the meteoroid fluence is highly desirable. NASA's Meteoroid Environment Office is monitoring comet Siding Spring and taking measurements of cometary brightness and dust production. We will discuss our coma model and nominal fluence taking the latest observations into account.

We analyze the observed radial distribution of column densities of water-ice particulates embedded in the primary jet region (J 1 ) of 103P's inner coma at altitudes between 439 and 1967 m (Protopapa et al., 2014, Icarus 238, 191–204) and determine the speed and acceleration of particles and their mass-flow within the filaments of the jet. This is done by applying a CO 2 driven (Type IIa) jet model proposed by Belton (2010, Icarus 210, 881–897). The model utilizes water-ice particles dislodged in the source regions of the jet filaments and accelerated by CO 2 to explain the radial distribution of water-ice particulates. We provide an explanation for the remarkably different radial distribution of refractory dust particles by hypothesizing that the majority of the dust originates directly from the nucleus surface in inter-filament regions of the jet complex and is accelerated by H 2 O. Our model provides a mass-flow of water from the J 1 jet complex that is ∼40 times greater than the constant speed sublimation model discussed by Protopapa et al. but is still too small to explain the hyperactivity of the comet. Speeds in the flow are increased by a factor up to ∼20 over those found by Protopapa et al. To account for the hyperactivity, most of the mass dislodged in the filament source regions must be in weakly accelerated large chunks that achieve only low speeds en route to the region of observation. These chunks soon leave the filamentary jet structure due to the rotation of the nucleus and do not contribute to the column densities observed at higher altitudes in the jet filaments. Employing the results of Kelley et al. (2015 . Icarus 262, 187–189)) on total cross-section and mass-flow in the coma we find that large chunks with the same bulk properties as the nucleus can increase the active fraction of the comet by two or three times. With the exception that the chunks do not need to be “nearly pure water ice”, these results support the hypothesis that hyperactivity in comet 103P/Hartley 2 is the result of super-volatiles that “… drag out chunks of nearly pure water ice that then sublime to provide a large fraction of the total H 2 O gaseous output of the comet (A'Hearn et al., 2011. Science 332, 1396–1400). We find that the largest chunks (effective radius ∼4 m) are probably dislodged at an average rate less than 1 chunk/filament/1–3 rotation periods near perihelion and we estimate that the filamentary source regions, if 50–100 m in diameter, could be excavated to a depth of 44–171 m in a single perihelion passage. We note similarities with groups of active pits discovered on 67P (Vincent et al. , 2015. Nature 523, 63–66) and suggest that these features may have, in the past, supported Type IIa super-volatile jet outflows that are now essentially exhausted.

We have studied a small, very unusual stone, here named “Hypatia”, found in the area of southwest Egypt where an extreme surface heating event produced the Libyan Desert Glass 28.5 million years ago. It is angular, black, shiny, extremely hard and intensely fractured. We report on exploratory work including X-ray diffraction, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy with EDS analysis, deuteron nuclear reaction analysis, C-isotope and noble gas analyses. Carbon is the dominant element in Hypatia, with heterogeneous O/C and N/C ratios ranging from 0.3 to 0.5 and from 0.007 to 0.02, respectively. The major cations of silicates add up to less than 5%. The stone consists chiefly of apparently amorphous, but very hard carbonaceous matter, in which patches of sub- μ m diamonds occur. δ 13 C values (ca. 0‰) exclude an origin from shocked terrestrial coal or any variety of terrestrial diamond. They are also higher than the values for carbonaceous chondrites but fall within the wide range for interplanetary dust particles and comet 81P/Wild2 dust. In step heating, 40Ar/36Ar ratios vary from 40 to the air value (298), interpreted as a variable mixture of extraterrestrial and atmospheric Ar. Isotope data of Ne, Kr and Xe reveal the exotic noble gas components G and P3 that are normally hosted in presolar SiC and nanodiamonds, while the most common trapped noble gas component of chondritic meteorites, Q, appears to be absent. An origin remote from the asteroid belt can account for these features. We propose that the Hypatia stone is a remnant of a cometary nucleus fragment that impacted after incorporating gases from the atmosphere. Its co-occurrence with Libyan Desert Glass suggests that this fragment could have been part of a bolide that broke up and exploded in the airburst that formed the Glass. Its extraordinary preservation would be due to its shock-transformation into a weathering-resistant assemblage.