Cometary Science Newsletter

June 2015
Michael S. P. Kelley (

Refereed Articles

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

A Further Investigation of Apparent Periodicities and the Rotational State of Comet 103P/Hartley 2 from Combined Coma Morphology and Lightcurve Datasets

  • Knight, M.M. 1
  • Mueller, B.E.A. 2
  • Samarasinha, N.H. 2
  • Schleicher, D.G. 1
  1. Lowell Observatory, USA
  2. Planetary Science Institute, USA

We present an analysis of Kitt Peak National Observatory and Lowell Observatory observations of comet 103P/Hartley 2 obtained from August through December 2010. The results are then compared with contemporaneous observations made by the EPOXI spacecraft. Each ground-based dataset has previously been investigated individually; the combined dataset has complementary coverage that reduces the time between observing runs and allows us to determine additional apparent periods at intermediate times. We compare CN coma morphology between ground-based datasets, making nine new measurements of apparent periods. The first five are consistent with the roughly linearly increasing apparent period during the apparition found by previous authors. The final four suggest that the change in apparent period slowed or stopped by late November. We also measure an inner coma lightcurve in both CN and R-band ground-based images, finding a single-peaked lightcurve which repeats in phase with the coma morphology. The apparent period from the lightcurve had significantly larger uncertainties than from the coma morphology, but varied over the apparition in a similar manner. Our ground-based lightcurve aligns with the published EPOXI lightcurve, indicating that the lightcurve represents changing activity rather than viewing geometry of structures in the coma. The EPOXI lightcurve can best be phased by a triple-peaked period near 54-55 hr that increases from October to November. This phasing reveals that the spacing between maxima is not constant, and that the overall lightcurve shape evolves from one triple-peaked cycle to the next. These behaviors suggest that much of the scatter in apparent periods derived from ground-based datasets acquired at similar epochs are likely due to limited sampling of the data.

The Astronomical Journal (In press)

NASA ADS: arXiv:1505.03039 arXiv: 1505.03039

First Extreme and Far Ultraviolet Spectrum of a Comet Nucleus: Results from 67P/Churyumov-Gerasimenko

  • S. A. Stern 1
  • L.M. Feaga 2
  • E. Schindhelm 1
  • A. Steffl 1
  • J. Wm. Parker 1
  • P.D. Feldman 3
  • H.A. Weaver 4
  • M.F. A’Hearn 2
  • J. Cook 1
  • J.-L. Bertaux 5
  1. Southwest Research Institute, 1050 Walnut St, Boulder, CO 80302, USA.
  2. University of Maryland, College Park, MD 20742, USA.
  3. Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218, USA.
  4. Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA.
  5. LATMOS/IPSL, CNRS/INSU, Univ. Versailles St-Quentin, 11 Bd d'Alembert, 78280 Guyancourt, France.

We used the Alice spectrograph onboard the Rosetta comet orbiter spacecraft to observe the surface of comet 67P/Churyumov-Gerasimenko in the extreme and far ultraviolet (EUV/FUV) from 700-2050 Å in mid-August 2014. These observations were before significant EUV/FUV coma signatures were observed by Alice. The resulting coadded spectrum has high signal to noise and reveals: (1) a very FUV- dark surface with (2) a blue spectral slope and (3) no evidence of significant H2O ice absorption in the FUV. We fit the measured reflectance spectrum with a model including 99.5% tholins, 0.5% H2O-ice, and a neutral darkening agent. Since we could not find any natural material with sufficiently low EUV/FUV reflectance, we interpret the low I/F as evidence of a fluffy, light-trapping surface. We interpret the blue spectral slope as consistent with a surface consisting primarily of tholins, though it may alternatively be the result of Rayleigh scattering by fine particles in the regolith.

Icarus (In press)

DOI: 10.1016/j.icarus.2015.04.023 NASA ADS: 2015Icar..256..117S

The Cometary Composition of a Protoplanetary Disk as Revealed by Complex Cyanides

  • Karin I. Oberg 1
  • Viviana V. Guzman 1
  • Kenji Furuya 2
  • Chunhua Qi 1
  • Yuri Aikawa 3
  • Sean M. Andrews 1
  • Ryan Loomis 1
  • David J. Wilner 1
  1. Harvard-Smithsonian Center for Astrophysics
  2. Kobe University
  3. Leiden Observatory, Leiden University

Observations of comets and asteroids show that the Solar Nebula that spawned our planetary system was rich in water and organic molecules. Bombardment brought these organics to the young Earth's surface, seeding its early chemistry. Unlike asteroids, comets preserve a nearly pristine record of the Solar Nebula composition. The presence of cyanides in comets, including 0.01% of methyl cyanide (CH3CN) with respect to water, is of special interest because of the importance of C-N bonds for abiotic amino acid synthesis. Comet-like compositions of simple and complex volatiles are found in protostars, and can be readily explained by a combination of gas-phase chemistry to form e.g. HCN and an active ice-phase chemistry on grain surfaces that advances complexity. Simple volatiles, including water and HCN, have been detected previously in Solar Nebula analogues - protoplanetary disks around young stars - indicating that they survive disk formation or are reformed in situ. It has been hitherto unclear whether the same holds for more complex organic molecules outside of the Solar Nebula, since recent observations show a dramatic change in the chemistry at the boundary between nascent envelopes and young disks due to accretion shocks. Here we report the detection of CH3CN (and HCN and HC3N) in the protoplanetary disk around the young star MWC 480. We find abundance ratios of these N-bearing organics in the gas-phase similar to comets, which suggests an even higher relative abundance of complex cyanides in the disk ice. This implies that complex organics accompany simpler volatiles in protoplanetary disks, and that the rich organic chemistry of the Solar Nebula was not unique.

Nature (Published)

DOI: 10.1038/nature14276 NASA ADS: 2015Natur.520..198O arXiv: 1505.06347