* Astronomy

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RE: Comet 67P/Churyumov-Gerasimenko

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Title: The 67P/Churyumov-Gerasimenko observation campaign in support of the Rosetta mission
Author: C. Snodgrass, M. F. A'Hearn, F. Aceituno, V. Afanasiev, S. Bagnulo, J. Bauer, G. Bergond, S. Besse, N. Biver, D. Bodewits, H. Boehnhardt, B. P. Bonev, G. Borisov, B. Carry, V. Casanova, A. Cochran, B. C. Conn, B. Davidsson, J. K. Davies, J. de León, E. de Mooij, M. de Val-Borro, M. Delacruz, M. A. DiSanti, J. E. Drew, R. Duffard, N. J. T. Edberg, S. Faggi, L. Feaga, A. Fitzsimmons, H. Fujiwara, E. L. Gibb, M. Gillon, S. F. Green, A. Guijarro, A. Guilbert-Lepoutre, P. J. Gutiérrez, E. Hadamcik, O. Hainaut, S. Haque, R. Hedrosa, D. Hines, U. Hopp, F. Hoyo, D. Hutsemékers, M. Hyland, O. Ivanova, E. Jehin, G. H. Jones, J. V. Keane, M. S. P. Kelley, N. Kiselev, J. Kleyna, M. Kluge, M. M. Knight, R. Kokotanekova, D. Koschny, E. Kramer, J. J. López-Moreno, P. Lacerda, L. M. Lara, et al. (54 additional authors not shown)

We present a summary of the campaign of remote observations that supported the European Space Agency's Rosetta mission. Telescopes across the globe (and in space) followed comet 67P/Churyumov-Gerasimenko from before Rosetta's arrival until nearly the end of mission in September 2016. These provided essential data for mission planning, large-scale context information for the coma and tails beyond the spacecraft, and a way to directly compare 67P with other comets. The observations revealed 67P to be a relatively `well behaved' comet, typical of Jupiter family comets and with activity patterns that repeat from orbit-to-orbit. Comparison between this large collection of telescopic observations and the in situ results from Rosetta will allow us to better understand comet coma chemistry and structure. This work is just beginning as the mission ends -- in this paper we present a summary of the ground-based observations and early results, and point to many questions that will be addressed in future studies.

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Title: Cold and warm electrons at comet 67P
Author: A. I. Eriksson, I. A. D. Engelhardt, M. Andre, R. Bostrom, N. J. T. Edberg, F. L. Johansson, E. Odelstad, E. Vigren, J.-E. Wahlund, P. Henri, J.-P. Lebreton, W. J. Miloch, J. J. P. Paulsson, C. Simon Wedlund, L. Yang, T. Karlsson, R. Jarvinen, T. Broiles, K. Mandt, C. M. Carr, M. Galand, H. Nilsson, C. Norberg

Strong electron cooling on the neutral gas in cometary comae has been predicted for a long time, but actual measurements of low electron temperature are scarce. We present in situ measurements of plasma density, electron temperature and spacecraft potential by the Rosetta Langmuir probe instrument, LAP. Data acquired within a few hundred km from the nucleus are dominated by a warm component with electron temperature typically 5--10 eV at all heliocentric distances covered (1.25 to 3.83 AU). A cold component, with temperature no higher than about 0.1 eV, appears in the data as short (few to few tens of seconds) pulses of high probe current, indicating local enhancement of plasma density as well as a decrease in electron temperature. These pulses first appeared around 3 AU and were seen for longer periods close to perihelion. The general pattern of pulse appearance follows that of neutral gas and plasma density. We have not identified any periods with only cold electrons present. The electron flux to Rosetta was always dominated by higher energies, driving the spacecraft potential to order -10 V. The warm (5--10 eV) electron population is interpreted as electrons retaining the energy they obtained when released in the ionisation process. The sometimes observed cold populations with electron temperatures below 0.1 eV verify collisional cooling in the coma. The cold electrons were only observed together with the warm population. The general appearance of the cold population appears to be consistent with a Haser-like model, implicitly supporting also the coupling of ions to the neutral gas. The expanding cold plasma is unstable, forming filaments that we observe as pulses.

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Collapsing cliff reveals comet's interior

Rosetta scientists have made the first compelling link between an outburst of dust and gas and the collapse of a prominent cliff, which also exposed the pristine, icy interior of the comet.
Sudden and short-lived outbursts were observed frequently during Rosetta's two-year mission at Comet 67P/Churyumov-Gerasimenko. Although their exact trigger has been much debated, the outbursts seem to point back to the collapse of weak, eroded surfaces, with the sudden exposure and heating of volatile material likely playing a role.
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Title: Close-up images of the final Philae landing site on comet 67P/Churyumov-Gerasimenko acquired by the ROLIS camera
Author: S.E. Schröder, S. Mottola, G. Arnold, H.-G. Grothues, R. Jaumann, H.U. Keller, H. Michaelis, J.-P. Bibring, I. Pelivan, A. Koncz, K. Otto, E. Remetean, F. Souvannavong, B. Dolives

After coming to rest on the night side of comet 67P/Churyumov-Gerasimenko, the ROLIS camera on-board Rosetta's Philae lander acquired five images of the surface below the lander, four of which were with the aid of LED illumination of different colours. The images confirm that Philae was perched on a sloped surface. A local horizon is visible in one corner of the image, beyond which we can see the coma. Having spent a full day on the surface Philae was commanded to lift and rotate, after which a final, sixth, LED image was acquired. The change in perspective allowed us to construct a shape model of the surface. The distance to the foreground was about 80 cm, much larger than the nominal 30 cm. This caused stray light, rather than directly reflected LED light, to dominate the image signal, complicating the analysis. The images show a lumpy surface with a roughness of apparently fractal nature. Its appearance is completely different from that of the first landing site, which was characterized by centimeter to meter-sized debris (Mottola et al., 2015). We recognize neither particles nor pores at the image resolution of 0.8 mm per pixel and large colour variations are absent. The surface has a bi-modal brightness distribution that can be interpreted in terms of the degree of consolidation, a hypothesis that we support with experimental evidence. We propose the surface below the lander to consist of smooth, cracked plates with unconsolidated edges, similar to terrain seen in CIVA images.

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Title: Mt. Wendelstein Imaging of the Post-Perihelion Dust Coma of 67P/Churyumov-Gerasimenko in 2015/2016
Author: Hermann Boehnhardt (1), Arno Riffeser (2), Matthias Kluge (2), Christoph Ries (2), Michael Schmidt (2), Ulrich Hopp (2) ((1) MPI for Solar System Research, (2) University Observatory at the LMU Munich)

Comet 67P/Churyumov-Gerasimenko (67P) was imaged with the 2m telescope at Mt. Wendelstein Observatory in the Alps. Coma and tail monitoring was performed during 51 nights between 22 August 2015 and 9 May 2016. The images through r and i Sloan Digital Sky Survey (SDSS) filters show the dust distribution around the comet, while images in the SDSS g filter indicate also the presence of coma gas in early September 2015. The dust color of 67P implies intrinsic reddening of 9 %/100nm. After maximum shortly after perihelion passage the dust activity decreased with a heliocentric exponent of 4.1 to 4.2 from late September 2015 until May 2016. The opposition surge during early 2016 can be explained by a linear light scattering phase function (beta ~ 0.04) or an asteroid-like HG-type phase function (G ~ 0.15). The radial brightness profile indicates a 'quasi-steady-state' dust coma from late September to the end of 2015. Dust fragmentation during about a month after perihelion may be responsible for radial coma profiles with slopes below unity, while dust accumulation due to very slow dust expansion velocity may result in steeper than unity profiles during 2016. Three fan-shape dust structures are characterized in the coma of 67P. A short dust ejection event on 22 -23 August 2015 has produced a dust arc-let and jet feature in the coma. In September 2015 the appearance of cometary dust tail is dominated by young dust produced around perihelion. The older dust dominates the tail appearance as of mid November 2015.

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Title: How primordial is the structure of comet 67P/C-G? Combined collisional and dynamical models suggest a late formation
Author: M. Jutzi, W. Benz, A. Toliou, A. Morbidelli, R. Brasser

There is an active debate about whether the properties of comets as observed today are primordial or, alternatively, if they are a result of collisional evolution or other processes. We investigate the effects of collisions on a comet with a structure like 67P/C-G. We develop scaling laws for the critical specific impact energies required for a significant shape alteration. These are then used in simulations of the combined dynamical and collisional evolution of comets in order to study the survival probability of a primordially formed object with a shape like 67P/C-G. The effects of impacts on comet 67P/C-G are studied using a SPH shock physics code. The resulting critical specific impact energy defines a minimal projectile size which is used to compute the number of shape-changing collisions in a set of dynamical simulations. These simulations follow the dispersion of the trans-Neptunian disk during the giant planet instability, the formation of a scattered disk, and produce 87 objects that penetrate into the inner solar system with orbits consistent with the observed JFC population. The collisional evolution before the giant planet instability is not considered here. Hence, our study is conservative in its estimation of the number of collisions. We find that in any scenario considered here, comet 67P/C-G would have experienced a significant number of shape-changing collisions, if it formed primordially. This is also the case for generic bi-lobe shapes. Our study also shows that impact heating is very localized and that collisionally processed bodies can still have a high porosity. Our study indicates that the observed bi-lobe structure of comet 67P/C-G may not be primordial, but might have originated in a rather recent event, possibly within the last 1 Gy. This may be the case for any kilometer-sized two-component cometary nuclei.

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Comet 67P/ChuryumovGerasimenko is much younger than previously thought

Based on computer simulations, Astrophysicists at the University of Bern conclude that the comet Chury did not obtain its duck-like form during the formation of our solar system 4.5 billion years ago. Although it does contain primordial material, they are able to show that the comet in its present form is hardly more than a billion years old.
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Title: The perihelion activity of comet 67P/Churyumov-Gerasimenko as seen by robotic telescopes
Author: Colin Snodgrass, Cyrielle Opitom, Miguel de Val-Borro, Emmanuel Jehin, Jean Manfroid, Tim Lister, Jon Marchant, Geraint H. Jones, Alan Fitzsimmons, Iain A. Steele, Robert J. Smith, Helen Jermak, Thomas Granzer, Karen J. Meech, Philippe Rousselot, Anny-Chantal Levasseur-Regourd

Around the time of its perihelion passage the observability of 67P/Churyumov-Gerasimenko from Earth was limited to very short windows each morning from any given site, due to the low solar elongation of the comet. The peak in the comet's activity was therefore difficult to observe with conventionally scheduled telescopes, but was possible where service/queue scheduled mode was possible, and with robotic telescopes. We describe the robotic observations that allowed us to measure the total activity of the comet around perihelion, via photometry (dust) and spectroscopy (gas), and compare these results with the measurements at this time by Rosetta's instruments. The peak of activity occurred approximately two weeks after perihelion. The total brightness (dust) largely followed the predictions from Snodgrass et al. 2013, with no significant change in total activity levels from previous apparitions. The CN gas production rate matched previous orbits near perihelion, but appeared to be relatively low later in the year.

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Comet 67P