Cometary Science Newsletter

Issue
45
Month
December 2018
Editor
Michael S. P. Kelley (msk@astro.umd.edu)

Refereed Articles

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

Models of Rosetta/OSIRIS 67P Dust Coma Phase Function

  • F. Moreno1
  • D. Guirado1
  • O. Muñoz1
  • I. Bertini2
  • C. Tubiana3
  • C. Güttler3
  • M. Fulle4
  • A. Rotundi5,6
  • V. Della Corte5,6
  • S. L. Ivanovski5
  • G. Rinaldi5
  • D. Bockelée-Morvan7
  • V. V. Zakharov5,8
  • J. Agarwal3
  • S. Mottola9
  • I. Toth10
  • E. Frattin2
  • L. M. Lara1
  • P. J. Gutiérrez1
  • Z. Y. Lin11
  • L. Kolokolova12
  • H. Sierks13
  • G. Naletto14,15,16
  • P. L. Lamy17
  • R. Rodrigo18,19
  • D. Koschny20
  • B. Davidsson21
  • M. A. Barucci22
  • J.-L. Bertaux17
  • D. Bodewits23
  • G. Cremonese24
  • V. Da Deppo16
  • S. Debei25
  • M. De Cecco26
  • J. Deller13
  • S. Fornasier22
  • W.-H. Ip27,28
  • H. U. Keller9,29
  • M. Lazzarin15
  • J. J. López-Moreno1
  • F. Marzari14
  • X. Shi13
  1. Instituto de Astrofísica de Andalucía, CSIC, 18008 Granada, Spain
  2. Department of Physics and Astronomy“G. Galilei,”University of Padova,Italy
  3. Max-Planck-Institut für Sonnensystemforschung, Goettingen, Germany
  4. Osservatorio Astronomico, Via Tiepolo 11, I-34143 Trieste, Italy
  5. INAF—Istituto di Astrofisica e Planetologia Spaziali, Rome, Italy
  6. Universitá degli Studi di Napoli Parthenope, Naples, Italy
  7. LESIA, Observatoire de Paris, PSL Research University, Meudon, France
  8. Laboratoire de Météorologie Dynamique, UPMC, Sorbonne Universités, Paris, France
  9. Deutsches Zentrum für Luft- und Raumfahrt (DLR), Germany
  10. MTA CSFK Konkoly Observatory of the Hungarian Academy of Sciences, Hungary
  11. National Central University, Taiwan
  12. Planetary Data System Group, Department of Astronomy, University of Maryland, USA
  13. Max Planck Institute for Solar System Research, Göttingen, Germany
  14. University of Padova, Department of Physics and Astronomy, Padova, Italy
  15. University of Padova, Italy
  16. CNR-IFN UOS Padova LUXOR, Via Trasea 7, I-35131 Padova, Italy
  17. Laboratoire Atmosphères, Milieux et Observations Spatiales, CNRS & Université de Versailles Saint-Quentin-en-Yvelines, France
  18. Centro de Astrobiología, CSIC-INTA, E-28850 Torrejón de Ardoz, Madrid, Spain
  19. International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
  20. Science Support Office, European Space Research and Technology, Noordwijk, The Netherlands
  21. Jet Propulsion Laboratory, Pasadena, CA 91109, USA
  22. LESIA, Observatoire de Paris, France
  23. Auburn University, AL 36849, USA
  24. INAF, Astronomical Observatory of Padova, Italy
  25. University of Padova, Department of Industrial Engineering, Padova, Italy
  26. University of Trento, Trento, Italy
  27. Graduate Institute of Astronomy, National Central University, Taiwan
  28. Space Science Institute, Macau
  29. Institut für Geophysik und extraterrestrische Physik, Braunschweig, Germany

The phase function of the dust coma of comet 67P has been determined from Rosetta OSIRIS images. This function shows a deep minimum at phase angles near 100°, and a strong backscattering enhancement. These two properties cannot be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly oriented aggregate particles. We show, however, that an ensemble of oriented elongated particles of a wide variety of aspect ratios, with radii r>10 μm, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed phase function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60%–70% range.

Astronomical Journal (Published)

DOI: 10.3847/1538-3881/aae526 arXiv: 1809.10424

Fine-scale structure in cometary dust tails I: Analysis of striae in Comet C/2006 P1 (McNaught) through temporal mapping

  • Price, Oliver 1,2
  • Jones, Geraint H. 1,2
  • Morrill, Jeff 3,4
  • Owens, Mathew 5
  • Battams, Karl 3
  • Morgan, Huw 6
  • Drückmuller, Miloslav 7
  • Deiries, Sebastian 8
  1. Mullard Space Science Laboratory, University College London, UK
  2. The Centre for Planetary Sciences at UCL/Birkbeck, London, UK
  3. Naval Research Laboratory, Washington, DC, USA
  4. Now at: NASA Headquarters, Washington, DC, USA
  5. University of Reading, UK
  6. Aberystwyth University, Wales, UK
  7. Brno University of Technology, Czech Republic
  8. European Southern Observatory, Germany

Striated features, or striae, form in cometary dust tails due to an as-yet unconstrained process or processes. For the first time we directly display the formation of striae, at C/2006 P1 McNaught, using data from the SOHO LASCO C3 coronagraph. The nature of this formation suggests both fragmentation and shadowing effects are important in the formation process. Using the SOHO data with STEREO-A and B data from the HI-1 and HI-2 instruments, we display the evolution of these striae for two weeks, with a temporal resolution of two hours or better. This includes a period of morphological change on 2007 January 13-14 that we attribute to Lorentz forces caused by the comet's dust tail crossing the heliospheric current sheet. The nature of this interaction also implies a mixing of different sized dust along the striae, implying that fragmentation must be continuous or cascading. To enable this analysis, we have developed a new technique - temporal mapping - that displays cometary dust tails directly in the radiation beta (ratio of radiation pressure to gravity) and dust ejection time phase space. This allows for the combination of various data sets and the removal of transient motion and scaling effects.

Icarus (Published)

DOI: 10.1016/j.icarus.2018.09.013 NASA ADS: 2019Icar..319..540P

Comet C/2013 V5 (Oukaimeden): Evidence for Depleted Organic Volatiles and Compositional Heterogeneity as Revealed through Infrared Spectroscopy

  • DiSanti, M. A. 1,2,3,4
  • Bonev, B. P. 1,3,4,5
  • Gibb, E. L. 1,4,6
  • Roth, N. X. 4,6
  • Dello Russo, N. 4,7
  • Vervack, R. J., Jr. 4,7
  1. Goddard Center for Astrobiology, NASA Goddard Space Flight Center, MD 20771, USA
  2. Solar System Exploration Division, Planetary Systems Laboratory MS 693, NASA GSFC, MD 20771, USA
  3. Visiting Astronomer, W. M. Keck Observatory, Maunakea, HI USA
  4. Visiting Astronomer, Infrared Telescope Facility, Maunakea, HI, USA
  5. Department of Physics, American University, Washington, DC, USA
  6. Department of Physics and Astronomy, University of Missouri-St. Louis, Saint Louis, MO, USA
  7. The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USA

We obtained high-resolution (λ / Δλ ~ 25,000) pre-perihelion spectra of Comet C/2013 V5 (Oukaimeden) using NIRSPEC at Keck 2 on UT 2014 September 5–6, and CSHELL at the NASA-Infrared Telescope Facility on September 11–13, altogether spanning a range in heliocentric distance Rh = 0.789 – 0.698 AU. We report water production rates, and production rates and abundance ratios relative to co-measured H2O for eight trace molecules: CO, H2CO, CH34, C2H2, C2H6, HCN, and NH3. Our measured water production rates from NIRSPEC and CSHELL observations remained relatively constant. They also were close to those from SOHO/SWAN observations that encompassed our dates, suggesting H2O production dominated by release directly from or within approximately 2,000 km of the nucleus. All trace volatiles were depleted relative to their respective median abundances among comets, excepting NH3, which was consistent with its median abundance. Most surprising were pronounced increases in abundance ratios for CH3OH (by 51% relative to simultaneously measured H2O) and especially C2H6 (by 87%) between September 5 and 6. On September 5, C2H6 was severely depleted, consistent with its lowest abundance yet measured for any comet. It also tracked the spatial profile of H2O, suggesting C2H6 was associated with a polar ice phase dominating gas production. On September 6, C2H6 was only moderately depleted and was spatially distinct from H2O, suggesting both polar- and nonpolar-dominated ice phases contributed to the activity then. Our results are consistent with a non-homogeneous volatile composition for C/2013 V5, implying differential processing of its constituent ices.

Astronomical Journal (Published)

DOI: 10.3847/1538-3881/aade87 NASA ADS: 2018AJ....156..258D

A Tale of “Two” Comets: The Primary Volatile Composition of Comet 2P/Encke Across Apparitions and Implications for Cometary Science

  • Roth, N. X. 1,8
  • Gibb, E. L. 1,2,8
  • Bonev, B. P. 2,3,8
  • DiSanti, M. A. 2,4,8
  • Dello Russo, N. 5,8
  • Vervack, R. J., Jr. 5,8
  • McKay, A. J. 4,6,8
  • Kawakita, H. 7,8
  1. Department of Physics and Astronomy, University of Missouri-St. Louis, St. Louis, MO, USA
  2. Goddard Center for Astrobiology, NASA-Goddard Space Flight Center, Greenbelt, MD
  3. Department of Physics, American University, Washington, DC, USA
  4. Solar System Exploration Division, Planetary Systems Laboratory Code 693, NASA-Goddard Space Flight Center, Greenbelt, MD, USA
  5. The Johns Hopkins University Applied Physics Laboratory, Lauel, MD, USA
  6. Universities Space Research Association/NASA Postdoctoral Program, Columbia, MD, USA
  7. Koyama Astronomical Observatory, Kyoto Sangyo University Motoyama, Kamingamo, Kita-ku, Kyoto 603-8555, Japan
  8. Visiting Astronomer, Infrared Telescope Facility, Maunakea, HI, USA

The highly favorable 2017 apparition of 2P/Encke allowed the first comprehensive comparison of primary volatile abundances in a given comet across multiple apparitions. This apparition offered opportunities to address pressing questions in cometary science, including investigating evolutionary and/or heliocentric distance (Rh) effects on volatile production, sampling the hypervolatiles CO and CH4 in an ecliptic comet, and measuring volatile release at small Rh. The faintness and frequently low geocentric velocity of ecliptic comets during most apparitions make our near-infrared observations of these hypervolatiles rare and of high scientific impact. We characterized the volatile composition of 2P/Encke on three post-perihelion dates using the iSHELL spectrograph at the NASA Infrared Telescope Facility on Maunakea, HI. We detected fluorescent emission from nine primary volatiles (H2O, CO, C2H6, CH3OH, CH4, H2CO, NH3, OCS, and HCN) and three fragment species (OH*, NH2, and CN), and obtained a sensitive upper limit for C2H2. We report rotational temperatures, production rates, and mixing ratios (abundances relative to H2O). Compared to mean abundances in comets observed to date in the near-infrared, mixing ratios of trace gases in 2P/Encke were depleted for all species except H2CO and NH3, which were “normal.” The detection of the hypervolatiles CO and CH4 is particularly notable given the paucity of measurements in ecliptic comets. We observed significant differences in primary volatile composition compared to published pre-perihelion results from 2003 at larger Rh. We discuss possible mechanisms for these differences and discuss these results in the context of findings from the Rosetta mission and ground-based studies of comets.

Astronomical Journal (Published)

DOI: 10.3847/1538-3881/aae0f7 NASA ADS: 2018AJ....156..251R

Photometric Study of Comet C/2014 S2 (PANSTARRS) After the Perihelion

  • Betzler, A.S.1,2
  • de Souza, O.F. 3
  • Betzler, L. B. S 3
  1. Centro de Ciência e Tecnologia em Energia e Sustentabilidade, Universidade Federal do Recôncavo da Bahia, Feira de Santana, Brazil
  2. Brazilian Meteor Observation Network (BRAMON), Nhandeara, Brazil
  3. Centro de Formação de Professores, Universidade Federal do Recôncavo da Bahia, Amargosa, Brazil

We analyzed the BVR photometry of comet C/2014 S2 obtained between March and June 2016, in observatories installed in Europe and the United States. Using the Lomb–Scargle periodogram, we found that the most probable periodicity deduced from the V-band magnitudes is 2.70 days, suggesting that it is the period of rotation of the nucleus of this comet is 2.70±0.07 days or 68±2 h, with a peak-to-peak light curve amplitude of 0.4±0.1 magnitudes. We verify that the absolute magnitude H0 and the activity index n differ from each other when they are calculated from the visual or CCD magnitudes. Considering the absolute magnitude Hv0= 6.0, obtained from visual magnitudes, we estimate that the lower limit of nuclear radius is 1.3 km. Analyzing the variation of magnitude R with the photometric aperture, we suggest that the coma of this object was in steady-state within the time limits of our observational interval. The coma had a mean color index B–V =0.79±0.22 , which is typical of active comets. Additionally, we have shown that the use of a variable photometric aperture, linked to geocentric distance, is probably unnecessary for the comet PANSTARRS.

Earth, Moon, and Planets (Published)

DOI: 10.1007/s11038-018-9521-5

Distant Comet C/2017 K2 and the Cohesion Bottleneck

  • Jewitt, D. 1
  • Agarwal, J. 2
  • Hui, M-T., 1
  • Li, J. 1
  • Mutchler, M. 3
  • Weaver, H. 4
  1. UCLA
  2. Max Planck Institute
  3. Space Telescope Science Institute
  4. Johns Hopkins/Applied Physics Laboratory

Distant long-period comet C/2017 K2 (PANSTARRS) has been outside the planetary region of the solar system for 3 Myr, negating the possibility that heat retained from the previous perihelion could be responsible for its activity. This inbound comet is also too cold for water ice to sublimate and too cold for amorphous water ice, if present, to crystallize. C/2017 K2 thus presents an ideal target in which to investigate the mechanisms responsible for activity in distant comets. We have used Hubble Space Telescope to study the comet in the pre-perihelion heliocentric distance range 13.8 to 15.9 AU. In this range, the coma maintains a logarithmic surface brightness gradient m = -1.010±0.004, consistent with mass loss proceeding in steady-state. The absence of a radiation pressure swept tail indicates that the effective particle size is large (radius above 0.1 mm) and the mass loss rate is of order 200 kg/s, remarkable for a comet still beyond the orbit of Saturn. Extrapolation of the photometry indicates that activity began in 2012.1±0.5, at 26 AU, where the isothermal blackbody temperature is only 55 K. This large distance and low temperature suggest that cometary activity is driven by the sublimation of a super-volatile ice (e.g.~CO), presumably preserved by K2's long-term residence in the Oort cloud. The mass loss rate can be sustained by CO sublimation from an area smaller than 2 sq. km, if located near the hot sub-solar point on the nucleus. However, while the drag force from sublimated CO is sufficient to lift millimeter sized particles against the gravity of the cometary nucleus, it is 100 to 1000 times too small to eject these particles against inter-particle cohesion. Our observations thus require either a new understanding of the physics of inter-particle cohesion or the introduction of another mechanism to drive distant cometary mass loss. We suggest thermal fracture and electrostatic supercharging in this context.

The Astronomical Journal (In press)

arXiv: 1811.07180