* Astronomy

Title: What drives the dust activity of comet 67P/Churyumov-Gerasimenko?
Author: B. Gundlach, J. Blum, H. U. Keller, Y. V. Skorov

We use the gravitational instability formation scenario of cometesimals to derive the aggregate size that can be released by the gas pressure from the nucleus of comet 67P/Churyumov-Gerasimenko for different heliocentric distances and different volatile ices. To derive the ejected aggregate sizes, we developed a model based on the assumption that the entire heat absorbed by the surface is consumed by the sublimation process of one volatile species. The calculations were performed for the three most prominent volatile materials in comets, namely, H_20 ice, CO_2 ice, and CO ice. We find that the size range of the dust aggregates able to escape from the nucleus into space widens when the comet approaches the Sun and narrows with increasing heliocentric distance, because the tensile strength of the aggregates decreases with increasing aggregate size. The activity of CO ice in comparison to H_20 ice is capable to detach aggregates smaller by approximately one order of magnitude from the surface. As a result of the higher sublimation rate of CO ice, larger aggregates are additionally able to escape from the gravity field of the nucleus. Our model can explain the large grains (ranging from 2 cm to 1 m in radius) in the inner coma of comet 67P/Churyumov-Gerasimenko that have been observed by the OSIRIS camera at heliocentric distances between 3.4 AU and 3.7 AU. Furthermore, the model predicts the release of decimeter-sized aggregates (trail particles) close to the heliocentric distance at which the gas-driven dust activity vanishes. However, the gas-driven dust activity cannot explain the presence of particles smaller than ~1 mm in the coma because the high tensile strength required to detach these particles from the surface cannot be provided by evaporation of volatile ices. These smaller particles can be produced for instance by spin-up and centrifugal mass loss of ejected larger aggregates.

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Title: Measurements of the Near-Nucleus Coma of Comet 67P/Churyumov-Gerasimenko with the Alice Far-Ultraviolet Spectrograph on Rosetta
Author: Paul D. Feldman (1), Michael F. A'Hearn (2), Jean-Loup Bertaux (3), Lori M. Feaga (2), Joel Wm. Parker (4), Eric Schindhelm (4), Andrew J. Steffl (4), S. Alan Stern (4), Harold A. Weaver (5), Holger Sierks (6), Jean-Baptiste Vincent (6) ((1) JHU, (2) UMd, (3) LATMOS, (4) SwRI, (5) JHU/APL, (6) MPIS)

Aims. The Alice far-ultraviolet spectrograph onboard Rosetta is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/ Churyumov-Gerasimenko and to determine their spatial distribution and evolution with time and heliocentric distance.
Methods. Following orbit insertion in August 2014, Alice made observations of the inner coma above the limbs of the nucleus of the comet from cometocentric distances varying between 10 and 80 km. Depending on the position and orientation of the slit relative to the nucleus, emissions of atomic hydrogen and oxygen were initially detected. These emissions are spatially localised close to the nucleus and spatially variable with a strong enhancement above the comet's neck at northern latitudes. Weaker emission from atomic carbon and CO were subsequently detected.
Results. Analysis of the relative line intensities suggests photoelectron impact dissociation of H2O vapor as the source of the observed H I and O I emissions. The electrons are produced by photoionisation of H2O. The observed C I emissions are also attributed to electron impact dissociation, of CO2, and their relative brightness to H I reflects the variation of CO2 to H2O column abundance in the coma.

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Title: Comet 67P/Churyumov-Gerasimenko: Constraints on its origin from OSIRIS observations
Author: H. Rickman (1,2), S. Marchi (3), M. F. A'Hearn (4), C. Barbieri (5), M. R. El-Maarry (6), C. Güttler (7), W.-H. Ip (8), H. U. Keller (9), P. Lamy (10), F. Marzari (5), M. Massironi (11,12), G. Naletto (12,13,14), M. Pajola (12), H. Sierks (7), OSIRIS Team ((1) P.A.S. Space Research Center, Warszawa, (2) Uppsala University, (3) Southwest Research Institute, (4) University of Maryland, (5) University of Padova, (6) University of Bern, (7) Max-Planck Institut für Sonnensystemforschung, (8) National Central University, Taiwan, (9) TU Braunschweig, (10) Laboratoire d'Astrophysique de Marseille, (11) Dipartimento di Geoscienze, University of Padova, (12) C.I.S.A.S., University of Padova, (13) Department of Information Engineering, University of Padova, (14) CNR-IFN UOS, Padova LUXOR)

One of the main aims of the ESA Rosetta mission is to study the origin of the solar system by exploring comet 67P/Churyumov-Gerasimenko at close range. In this paper we discuss the origin and evolution of comet 67P/Churyumov-Gerasimenko in relation to that of comets in general and in the framework of current solar system formation models. We use data from the OSIRIS scientific cameras as basic constraints. In particular, we discuss the overall bi-lobate shape and the presence of key geological features, such as layers and fractures. We also treat the problem of collisional evolution of comet nuclei by a particle-in-a-box calculation for an estimate of the probability of survival for 67P/Churyumov-Gerasimenko during the early epochs of the solar system. We argue that the two lobes of the 67P/Churyumov-Gerasimenko nucleus are derived from two distinct objects that have formed a contact binary via a gentle merger. The lobes are separate bodies, though sufficiently similar to have formed in the same environment. An estimate of the collisional rate in the primordial, trans-planetary disk shows that most comets of similar size to 67P/Churyumov-Gerasimenko are likely collisional fragments, although survival of primordial planetesimals cannot be excluded. A collisional origin of the contact binary is suggested, and the low bulk density of the aggregate and abundance of volatile species show that a very gentle merger must have occurred. We thus consider two main scenarios: the primordial accretion of planetesimals, and the re-accretion of fragments after an energetic impact onto a larger parent body. We point to the primordial signatures exhibited by 67P/Churyumov-Gerasimenko and other comet nuclei as critical tests of the collisional evolution.

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Rosetta and Philae find comet 67P/Churyumov-Gerasimenko not magnetised

Measurements made by Rosetta and Philae during the probes multiple landings on Comet 67P/Churyumov-Gerasimenko show that the comets nucleus is not magnetised.
Studying the properties of a comet can provide clues to the role that magnetic fields played in the formation of Solar System bodies almost 4.6 billion years ago. The infant Solar System was once nothing more than a swirling disc of gas and dust but, within a few million years, the Sun burst into life in the centre of this turbulent disc, with the leftover material going into forming the asteroids, comets, moons and planets.
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