Cometary Science Newsletter

Issue
15
Month
June 2016
Editor
Michael S. P. Kelley (msk@astro.umd.edu)

Refereed Articles

Abstracts of articles in press or recently published. Limited to 3000 characters.

The Global Shape, Density and Rotation of Comet 67P/Churyumov-Gerasimenko from Pre-Perihelion Rosetta/OSIRIS Observations

  • Jorda, L. 1
  • and 42 co-authorsNone
  1. Laboratoire d’Astrophysique de Marseille, UMR7326 CNRS/Universite Aix-Marseille, 38 rue Frederic Joliot-Curie, 13388 Marseille Cedex 13, France

The Rosetta spacecraft reached comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) in August 2014 at an heliocentric distance of 3.6 au and was then put in orbit around its nucleus to perform detailed observations. Among the collected data are the images acquired by the OSIRIS instrument up to the perihelion passage of the comet in August 2015, which allowed us to map the entire nucleus surface at high-resolution in the visible. Stereophotoclinometry methods have been used to reconstruct a global high-resolution shape model and to monitor its rotational parameters using data collected up to perihelion. The nucleus has a conspicuous bilobate shape with overall dimensions along its principal axes of (4.34 ± 0.02) × (2.60 ± 0.02) × (2.12 ± 0.06) km. The best-fit ellipsoid dimensions of the individual lobes along their principal axes of inertia are found to be 4.10×3.52×1.63 km and 2.50 × 2.14 × 1.64 km. Their volume amounts to 66% and 27% of the total volume of the nucleus. The two lobes are connected by a “neck” whose volume has been estimated to represent ~7% of the total volume of the comet. Combining the derived volume of 18.8 ± 0.3 km3 with the mass of 9.982 ± 0.003 1012 kg determined by the Rosetta/RSI experiment, we obtained a bulk density of the nucleus of 532 ± 7 kg m−3. Together with the companion value of 535 ± 35 kg m−3 deduced from the stereophotogrammetry shape model of the nucleus (Preusker et al., Astronomy and Astrophysics 583, A33, 2015), these constitute the first reliable and most accurate determination of the density of a cometary nucleus to date. The calculated porosity is quite large, ranging approximately from 70 to 75% depending upon the assumed density of the dust grains and the dust-to-ice mass ratio. The nature of the porosity, either micro or macro or both, remains unconstrained. The coordinates of the center of gravity are not compatible with a uniform nucleus density. The direction of the offset between the center of gravity and the center of figure suggests that the big lobe has a slightly higher bulk density compared to the small one. The center of mass position cannot be explained by different, but homogenous densities in the two lobes. The initial rotational period of 12.4041 ± 0.0001 h of the nucleus persisted until October 2014. It then slightly increased to a maximum of 12.4304 h reached on 19 May 2015 and finally dropped to 12.305 h just before perihelion on August 10, 2015. A periodogram analysis of the (RA, Dec) direction of the Z-axis of the comet obtained in parallel with the shape reconstruction exhibits a highly significant minima at 11.5±0.5 day clearly indicating an excited rotational state with an amplitude of 0.15 ± 0.03 degree.

Icarus (In press)

DOI: 10.1016/j.icarus.2016.05.002

Potential Jupiter-family comet contamination of the main asteroid belt

  • Hsieh, H. H. 1,2
  • Haghighipour, N. 3
  1. Planetary Science Institute, USA
  2. Academia Sinica, Taiwan
  3. University of Hawaii, USA

We present the results of "snapshot" numerical integrations of test particles representing comet-like and asteroid-like objects in the inner solar system aimed at investigating the short-term dynamical evolution of objects close to the dynamical boundary between asteroids and comets as defined by the Tisserand parameter with respect to Jupiter, TJ (i.e., TJ=3). As expected, we find that TJ for individual test particles is not always a reliable indicator of initial orbit types. Furthermore, we find that a few percent of test particles with comet-like starting elements (i.e., similar to Jupiter-family comets) reach main-belt-like orbits (at least temporarily) during our 2 Myr integrations, even without the inclusion of non-gravitational forces, apparently via a combination of gravitational interactions with the terrestrial planets and temporary trapping by mean-motion resonances with Jupiter. We estimate that the fraction of real Jupiter-family comets occasionally reaching main-belt-like orbits on Myr timescales could be on the order of ~0.1-1%, although the fraction that remain on such orbits for appreciable lengths of time is certainly far lower. Thus, the number of JFC-like interlopers in the main-belt population at any given time is likely to be small, but still non-zero, a finding with significant implications for efforts to use main-belt comets to trace the primordial distribution of volatile material in the inner solar system. The test particles with comet-like starting orbital elements that transition onto main-belt-like orbits in our integrations appear to be largely prevented from reaching low eccentricity, low inclination orbits. We therefore find that low-eccentricity, low-inclination main-belt comets may provide a more reliable means for tracing the primordial ice content of the main asteroid belt than the main-belt comet population as a whole.

Icarus (Published)

DOI: 10.1016/j.icarus.2016.04.043 NASA ADS: 2016Icar..277...19H arXiv: 1604.08557

Physical property and dynamical relation of the circular depressions on comet 67P/Churyumov-Gerasimenko

  • W.-H. Ip 1,2,3
  • I.-L. Lai 2
  • J.-C. Lee 4
  • Y.-C. Cheng 1
  • Y. Li 3
  • Z.-Y. Lin 1
  • J.-B. Vincent 5
  • S. Besse 6
  • H. Sierks 5
  • C. Barbieri 7,8
  • P.L. Lamy 9
  • R. Rodrigo 10. 11
  • D. Koschny 6
  • H. Rickman 12, 13
  • H.U. Keller 14
  • J. Agarwal 5
  • M.F. A'Hearn 15
  • M.A. Barucci 16
  • J.-L. Bertaux 17
  • I. Bertini 8
  • D. Bodewits 15
  • S. Boudreault 5
  • G. Cremonese 18
  • V. Da Deppo 19
  • B. Davidsson 12
  • S. Bebei 20
  • M. De Cecco 21
  • M.R. El-Maarry 22
  • S. Fornasier 16
  • M. Fulee 23
  • O. Groussin 9
  • P.J. Gutierez 24
  • C. Guettler 5
  • S.F. Hviid 25
  • L. Jorda 9
  • J. Knollenberg 25
  • G. Kovacs 5
  • J.-R. Kramm 5
  • E. Kuehrt 25
  • M. Kueppers 26
  • F. La Forgia 7
  • L.M. Lara 24
  • M. Lazzarin 7
  • J.J. Lopez-Moreno 24
  • S. Lowry 27
  • S. Marchi 28
  • F. Marzari 7
  • H. Michalik 14
  • S. Mottola 25
  • G. Naletto 8,19, 29
  • N. Oklay 5
  • M. Pajola 8
  • N. Thomas 22
  • E. Toth 30
  • C. Tubiana 5
  1. Institute of Astronomy, National Central University, Taiwan
  2. Institute of Space Sciences, National Central University, Taiwan
  3. Space Science Institute, Macau University of Science and Technology, Macau
  4. Dept. of Earth Science, National Central University, Taiwan
  5. Max-Planck Institut fuer Sonnensystemforschung, Germany
  6. Research and Scientific Support Dept., ESA, The Netherlands
  7. Dept. of Physics and Astronomy, University of Padova, Italy
  8. Centro di Ateneo di Studi ed Attivita Spaziali, University of Padova, Italy
  9. Aix Marseille Universite, CNRS, LAM, France
  10. Centro de Astrobiologia, Spain
  11. Intenational Space Science Institute, Switzerland
  12. Dept. of Physics and Astronomy, Uppsala University, Sweden
  13. PAS Space Research Center, Poland
  14. Institut fuer Geophysik und extraterrestrische Physik, Technische Universitat Braunschweig, Germany
  15. Dept. of Astronomy, University of Maryland, USA
  16. LESIA-Observatoire de Paris, France
  17. LATMOS, CNRS/UVSQ/IPSL, France
  18. INAF Osservatorio Astronomico di Padova, Italy
  19. CNR-IFN UOS Padova LUXOR, Italy
  20. Dept. of Industrial Engineering, University of Padova, Italy
  21. University of Trento, Italy
  22. Physikalisches Institut, University of Bern, Switzerland
  23. INAF-Osservatirui Astronomico di Trieste, Italy
  24. Instituto de Astrofisica de Andalucia, Spain
  25. Deutsches Zentrum fuer Luft- und Raumfahrt (DLR), Institut fuer Planetenforschung, Germany
  26. European Space Astronomy Centre, Spain
  27. Centre for Astrophysics and Planetary Sciences, The University of Kent, United Kingdom
  28. Southwest Research Institute, Boulder, USA
  29. Dept. of Information Engineering, University of padova, Italy
  30. MTA CSFK Konkoly Observatory, Hungary

Aim. To characterize the circular depressions of comet 67P/Churyumov-Gerasimenko and investigate whether such surface morphology of a comet nucleus is related to the cumulative sublimation effect since becoming a Jupiter family comet.

Methods. The images from the Rosetta/OSIRIS science camera experiment are used to construct size frequency distributions of the circular depression structures on comet 67P and they are compared with those of the Jupiter family comets 81P/Wild 2, 9P/Tempel-1, and 103P/Hartley 2. The orbital evolutionary histories of these comets over the past 100,000 years are analyzed statistically and compared with each other.

Results. The global distribution of the circular depressions over the surface of 67P is charted and classified. Descriptions are given to the characteristics and cumulative size frequency distribution of the identified features. Orbital statistices of the Jupiter-family comets visited by spacecraft are derived.

Conclusions. The size frequency distribution of the circular depressions is found to have a similar power law distribution to those of 9P/Tempel 1 and 81P/Wild 2. This might imply that they could have been generated by the same process. Orbital integration calculation shows that the surface erosion histories of 81P/Wild 2, and 9P/Tempel 1 could be shorter than those of 67P, 103P/Hartley 2 and 19P/Borrelly. From this point of view, the circular depressions could be dated back to the pre-JFC phase or the transeptunian phase of these comets. The north-south asymmetry in the distribution of the circular depressions could be associated with the heterogeneous structure of the nucleus of comet 67P and/or the solar insolation history.

Astronomy & Astrophysics (Published)

DOI: 10.1051/0004-6361/201628156