Acceleration of individual, decimetre-sized aggregates in the lower coma of comet 67P/Churyumov–Gerasimenko

Jessica Agarwal 1 M. F. A'Hearn 2 J.-B. Vincent 1 C. Güttler 1 S. Höfner 1 H. Sierks 1 C. Tubiana 1 C. Barbieri 3 P. L. Lamy 4 R. Rodrigo 5, 6, 7 D. Koschny 8 H. Rickman 9, 10 M. A. Barucci 11 Jean-Loup Bertaux 12 I. Bertini 13 S. Boudreault 1 Gabriele Cremonese 14 Vania Da deppo 15, 14 B. Davidsson 16 S. Debei 17 M. De cecco 18 J. Deller 1 S. Fornasier 11 M. Fulle 19 A. Gicquel 1 O. Groussin 4 P. J. Gutiérrez 20 M. Hofmann 1 S. F. Hviid 21 W.-H. Ip 22 L. Jorda 4 H. U. Keller 23 J. Knollenberg 21 J.-R. Kramm 1 E. Kührt 21 M. Küppers 24 L. M. Lara 20 M. Lazzarin 3 J. J. Lopez moreno 20 F. Marzari 3 G. Naletto 13, 25, 15 N. Oklay 1 X. Shi 1 N. Thomas 26
Abstract : We present OSIRIS/NAC observations of decimetre-sized, likely ice-containing aggregates ejected from a confined region on the surface of comet 67P/Churyumov-Gerasimenko. The images were obtained in January 2016 when the comet was at 2 AU from the Sun out-bound from perihelion. We measure the acceleration of individual aggregates through a two-hour image series. Approximately 50% of the aggregates are accelerated away from the nucleus, and 50% towards it, and likewise towards either horizontal direction. The accelerations are up to one order of magnitude stronger than local gravity, and are most simply explained by the combined effect of gas drag accelerating all aggregates upwards, and the recoil force from asymmetric outgassing, either from rotating aggregates with randomly oriented spin axes and sufficient thermal inertia to shift the temperature maximum away from an aggregate's subsolar region, or from aggregates with variable ice content. At least 10% of the aggregates will escape the gravity field of the nucleus and feed the comet's debris trail, while others may fall back to the surface and contribute to the deposits covering parts of the northern hemisphere. The rocket force plays a crucial role in pushing these aggregates back towards the surface. Our observations show the future back fall material in the process of ejection, and provide the first direct measurement of the acceleration of aggregates in the innermost coma (<2km) of a comet, where gas drag is still significant.
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Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2016, 462 (Suppl. 1), pp.S78-S88. <10.1093/mnras/stw2179>
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Jessica Agarwal, M. F. A'Hearn, J.-B. Vincent, C. Güttler, S. Höfner, et al.. Acceleration of individual, decimetre-sized aggregates in the lower coma of comet 67P/Churyumov–Gerasimenko. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2016, 462 (Suppl. 1), pp.S78-S88. <10.1093/mnras/stw2179>. <insu-01367854>

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