Cometary Science Newsletter

Issue
21
Month
December 2016
Editor
Michael S. P. Kelley (msk@astro.umd.edu)

Comet Notes

Brief observational reports or other notes related to specific comets. Limited to 1000 characters. The CSN is not intended to replace telegram services or other breaking news outlets.

4*P Coma Morphology Campaign

As you may know three comets (41P/Tuttle-Giacobini-Kresak, 45P/Honda-Mrkos-Pajdusakova in early 2017, and 46P/Wirtanen in late 2018) will have close approaches (0.08–0.15 AU) to Earth in the next two years — a relatively rare occurrence.

Similar to the Comet ISON Coma Morphology Campaign, we are organizing this 4*P Coma Morphology Campaign and are requesting the participation of both professional and amateur astronomers. The goal is to achieve science facilitated by a multi-longitudinal observing campaign.

Please look at http://www.psi.edu/41P45P46P for more information related to the campaign.

Nalin Samarasinha, Beatrice Mueller, Matthew Knight, Tony Farnham, and Walt Harris

Refereed Articles

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

Evidence for geologic processes on comets

  • Sunshine, Jessica M. 1
  • Thomas, Nicolas 2
  • El-Maarry, Mohamed Rammy 2
  • Farnham, Tony L. 1
  1. University of Maryland
  2. University of Bern

Spacecraft missions have resolved the nuclei of six periodic comets and revealed a set of geologically intriguing and active small bodies. The shapes of these cometary nuclei are dominantly bilobate reflecting their formation from smaller cometesimals. Cometary surfaces include a diverse set of morphologies formed from a variety of mechanisms. Sublimation of ices, driven by the variable insolation over the time since each nucleus was perturbed into the inner Solar System, is a major process on comets and is likely responsible for quasi-circular depressions and ubiquitous layering. Sublimation from near-vertical walls is also seen to lead to undercutting and mass wasting. Fracturing has only been resolved on one comet but likely exists on all comets. There is also evidence for mass redistribution, where material lifted off the nucleus by subliming gases is deposited onto other surfaces. It is surprising that such sedimentary processes are significant in the microgravity environment of comets. There are many enigmatic features on cometary surfaces including tall spires, kilometer-scale flows, and various forms of depressions and pits. Furthermore, even after accounting for the differences in resolution and coverage, significant diversity in landforms among cometary surfaces clearly exists. Yet why certain landforms occur on some comets and not on others remains poorly understood. The exploration and understanding of geologic processes on comets is only beginning. These fascinating bodies will continue to provide a unique laboratory for examining common geologic processes under the uncommon conditions of very high porosity, very low strength, small particle sizes, and near-zero gravity.

Journal of Geophysical Research - Planets (Published)

DOI: 10.1002/2016JE005119

Observational Constraints on Water Sublimation from 24 Themis and 1 Ceres

  • Adam McKay1
  • Dennis Bodewits2
  • Jian-Yang Li2
  1. University of Texas at Austin, Austin, TX
  2. University of Maryland, College Park, MD

Recent observations have suggested that there is water ice present on the surfaces of 24 Themis and 1 Ceres. We present upper limits on the H2O production rate on these bodies derived using a search for [OI]6300 Angstrom emission. For Themis, the water production is less than 4.5 x 1027 mol/s, while for Ceres our derived upper limit is 4.6 x 1028 mol/s. The derived limits imply a very low fraction of the surface area of each asteroid is active (< 2 x 10-4), though this estimate varies by as much as an order of magnitude depending on thermal properties of the surface. This is much lower than seen for comets, which have active areas of 10-2 - 10-1. We discuss possible implications for our findings on the nature of water ice on Themis and Ceres.

Icarus (In press)

DOI:  10.1016/j.icarus.2016.09.032 arXiv: 1609.07156

SOHO Comets: 20-Years and 3,000 Objects Later

  • Battams, K. 1
  • Knight, M.M. 2
  1. U.S. Naval Research Laboratory, USA
  2. University of Maryland, USA

We present a summary of the more than 3,000 sungrazing and near-Sun comets discovered in coronagraph images returned by the Solar and Heliospheric Observatory (SOHO), since its launch in December 1995. We address each of the four main populations of objects observed by SOHO: Kreutz (sungrazing) group, Meyer group, Marsden and Kracht (96P-Family) group, and non-group comets. Discussions for each group include basic properties, discovery statistics, and morphological appearance. In addition to updating the community on the status of the discoveries by SOHO, we also show that the rate of discovery of Kreutz sungrazers has likely remained static since approximately 2003, and report on the first likely fragmentation pair observed within the Meyer group.

Philosophical Transactions of the Royal Society A (In press)

NASA ADS: 2016arXiv161102279B arXiv: 1611.02279

Mid-infrared spectra of comet nuclei

  • Kelley, M. S. P. 1
  • Woodward, C. E. 2
  • Gehrz, R. D. 2
  • Reach, W. T. 3
  • Harker, D. E. 4
  1. Departement of Astronomy, University of Maryland, USA
  2. Minnesota Institute for Astrophysics, University of Minnesota, USA
  3. Universities Space Research Association, Stratospheric Observatory for Infrared Astronomy, USA
  4. Center for Astrophysics and Space Sciences, University of California, San Diego, USA

Comet nuclei and D-type asteroids have several similarities at optical and near-IR wavelengths, including near-featureless red reflectance spectra, and low albedos. Mineral identifications based on these characteristics are fraught with degeneracies, although some general trends can be identified. In contrast, spectral emissivity features in the mid-infrared provide important compositional information that might not otherwise be achievable. Jovian Trojan D-type asteroids have emissivity features strikingly similar to comet comae, suggesting that they have the same compositions and that the surfaces of the Trojans are highly porous. However, a direct comparison between a comet and asteroid surface has not been possible due to the paucity of spectra of comet nuclei at mid-infrared wavelengths. We present 5–35 μm thermal emission spectra of comets 10P/Tempel 2, and 49P/Arend-Rigaux observed with the Infrared Spectrograph on the Spitzer Space Telescope. Our analysis reveals no evidence for a coma or tail at the time of observation, suggesting the spectra are dominated by the comet nucleus. We fit each spectrum with the near-Earth asteroid thermal model (NEATM) and find sizes in agreement with previous values. However, the NEATM beaming parameters of the nuclei, 0.74 to 0.83, are systematically lower than the Jupiter-family comet population mean of 1.03±0.11, derived from 16- and 22-μm photometry. We suggest this may be either an artifact of the spectral reduction, or the consequence of an emissivity low near 16 μm. When the spectra are normalized by the NEATM model, a weak 10-μm silicate plateau is evident, with a shape similar to those seen in mid-infrared spectra of D-type asteroids. A silicate plateau is also evident in previously published Spitzer spectra of the nucleus of comet 9P/Tempel 1. We compare, in detail, these comet nucleus emission features to those seen in spectra of the Jovian Trojan D-types (624) Hektor, (911) Agamemnon, and (1172) Aneas, as well as those seen in the spectra of seven comet comae. The comet comae present silicate features with two distinct shapes, either trapezoidal, or more rounded, the latter apparently due to enhanced emission near 8 to 8.5 μm. The surfaces of Tempel 2, Arend-Rigaux, and Hektor best agree with the comae that present trapezoidal features, furthering the hypothesis that the surfaces of these targets must have high porosities in order to exhibit a spectrum similar to a comet coma. An emissivity minimum at 15 μm, present in the spectra of Tempel 2, Arend-Rigaux, Hektor, and Agamemnon, is also described, the origin of which remains unidentified. The compositional similarity between D-type asteroids and comets is discussed, and our data supports the hypothesis that they have similar origins in the early Solar System.

Icarus (In press)

DOI: 10.1016/j.icarus.2016.11.029 NASA ADS: 2016arXiv161109389K arXiv: 1611.09389

The 67P/Churyumov-Gerasimenko observation campaign in support of the Rosetta mission

  • Snodgrass, C.1
  • and 119 co-authorsNone
  1. The Open University, UK

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.

Phil. Trans. Royal Soc. A (In press)