Cometary Science Newsletter

February 2019
Michael S. P. Kelley (

Simulating cometary spectra (gases, dust, ices) with the Planetary Spectrum Generator (PSG)

The Planetary Spectrum Generator (PSG, has been expanded to accurately model parent-daughter photodissociation processes, and dust emissions employing the Afρ formalism. Specifically, PSG can now model several key species active in the optical (e.g., OH, C2, CN, CO, CH and NH) at high/mid/low spectral resolutions, beyond the already available model of parent species (e.g., H2O, C2H6, CH4, CH3OH). These fluorescence efficiencies were computed employing state-of-the-art spectroscopic databases following a rigorous solar fluorescence treatment, and include the Swings effect.

The three-dimensional orbital calculator of PSG automatically ingests ephemeris information from the JPL/Horizons system, permitting to obtain realistic viewing geometrical parameters. This combined with a broad range of observatory/instrument models (e.g., HST, Keck, IRTF, JWST, SOFIA) allows to obtain realistic estimates of sensitivities for any comet/small-body. The retrieval module of PSG permits to upload astronomical data, and to derive cometary abundances of volatiles (e.g., OH, H2O, CH3OH), dust and surface ices (e.g., water ice).

To try it, please visit:, load any of the cometary templates (e.g., “Comet at IR”, “comet in the optical”), and click on “Generate Spectra”. The demo videos can be helpful to better understand the functioning of the web interface and PSG (

Conference Announcements

Announcements for cometary conferences or workshops. Limited to 2000 characters.

Call for workshop contributions: Europlanet Cometary Plasma Science Workshop

Helsinki | Finland | March 28-29, 2019

We welcome contributions to the Europlanet sponsored cometary plasma science workshop hosted by the Rosetta Plasma Consortium (RPC) on March 28-29, 2019 in Helsinki, Finland.

The scope of the workshop covers all cometary plasma physics related topics, including the comet 67P/Churyumov-Gerasimenko and the Rosetta mission, other comets and missions as well as remote sensing, theory and laboratory studies.

The organizer of the workshop is the Aalto University (Espoo, Finland) and it will be held at the Finnish Meteorological Institute at the Kumpula Campus in Helsinki.

For further information and registration:

Deadline for the registrations is March 1, 2019.

Local Organizing Committee:
Esa Kallio, Aalto University
Riku Jarvinen, Aalto University
Markku Alho, Aalto University

Refereed Articles

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

The Discus Comet: C/2014 B1 (Schwartz)

  • Jewitt, D. 1
  • Kim, Y. 2
  • Luu, J. 3
  • Graykowski, A. 1
  1. UCLA Earth, Planetary and Space Sciences
  2. Max Planck Institute for Solar System
  3. Arctic University of Tromso

Long period comet C/2014 B1 (Schwartz) exhibits a remarkable optical appearance, like that of a discus or bi-convex lens viewed edgewise. Our measurements in the four years since discovery reveal a unique elongated dust coma whose orientation is stable with respect to the projected anti-solar and orbital directions. With no tail and no trail, the limited influence of radiation pressure on the dust coma sets a lower limit to the effective particle size >0.1mm, while the photometry reveals a peak coma scattering cross-section 27,000 km2 (geometric albedo 0.1 assumed). From the rate of brightening of the comet we infer a dust production rate 10 kg/s at 10 AU heliocentric distance, presumably due to the sublimation of supervolatile ices, and perhaps triggered by the crystallization of amorphous water ice. We consider several models for the origin of the peculiar morphology. The disk-like shape is best explained by equatorial ejection of particles from a nucleus whose spin vector lies near the plane of the sky. In this interpretation, the unique appearance of C/2014 B1 is a result of a near equality between the rotation-assisted nucleus escape speed (1 to 10 m/s for a 2 to 20 kilometer-scale nucleus) and the particle ejection velocity, combined with a near-equatorial viewing perspective. To date, most other comets have been studied at heliocentric distances less than half that of C/2014 B1, where their nucleus temperatures, gas fluxes and dust ejection speeds are much higher. The throttling role of nucleus gravity is correspondingly diminished, so that the disk morphology has not before been observed.

The Astronomical Journal (In press)

arXiv: 1901.01438

Near-UV and optical spectroscopy of comets using the ISIS spectrograph on the WHT

  • Hyland, M.G. 1
  • Fitzsimmons, A. 1
  • Snodgrass, C. 2,3
  1. Astrophysics Research Centre, School of Mathematics and Physics, Queen's University Belfast, BT7 1NN, UK
  2. Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
  3. School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

We present an analysis of long-slit cometary spectroscopy using the dual-arm ISIS spectrograph on the 4.2 m WHT. Eleven comets were observed over two nights in 2016 March and we detected the OH (0-0) emission band at 3085 Å in the spectra of five of these comets. Emission bands of the species NH, CN, C3, C2, NH2 and [OI] were also detected. We used Haser modelling to determine molecular production rates and abundance ratios for the observed species. We found that our average abundances relative to OH and CN were generally consistent with those measured in previous studies.

Monthly Notices of the Royal Astronomical Society (In press)

arXiv: 1901.03284

ALMA Autocorrelation Spectroscopy of Comets: The HCN/H13CN Ratio in C/2012 S1 (ISON)

  • Cordiner, M. A. 1,2
  • Palmer, M. Y. 1,2
  • de Val-Borro, M. 1,2
  • and 11 co-authors
  1. NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
  2. Department of Physics, Catholic University of America, Washington, DC 20064, USA

The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful tool for high-resolution mapping of comets, but the main interferometer (comprised of 50 × 12 m antennas) is insensitive to the largest coma scales due to a lack of very short baselines. In this Letter, we present a new technique employing ALMA autocorrelation data (obtained simultaneously with the interferometric observations), effectively treating the entire 12 m array as a collection of single-dish telescopes. Using combined autocorrelation spectra from 28 active antennas, we recovered extended HCN coma emission from comet C/2012 S1 (ISON), resulting in a fourteen-fold increase in detected line brightness compared with the interferometer. This resulted in the first detection of rotational emission from H13CN in this comet. Using a detailed coma radiative transfer model accounting for optical depth and non-local thermodynamic equilibrium excitation effects, we obtained an H12CN/H13CN ratio of 88 ± 18, which matches the terrestrial value of 89. This is consistent with a lack of isotopic fractionation in HCN during comet formation in the protosolar accretion disk. The possibility of future discoveries in extended sources using autocorrelation spectroscopy from the main ALMA array is thus demonstrated.

The Astrophysical Journal Letters (Published)

DOI: 10.3847/2041-8213/aafb05

Gas Jet Morphology and the Very Rapidly Increasing Rotation Period of Comet 41P/Tuttle-Giacobini-Kresak

  • Schleicher, D.G. 1
  • Knight, M.M. 2
  • Eisner, N.L. 1,2,3
  • Thirouin, A. 1
  1. Lowell Observatory, USA
  2. University of Maryland, USA
  3. University of Sheffield, UK

We present results from our 47-night imaging campaign of Comet 41P/Tuttle-Giacobini-Kresak conducted from Lowell Observatory between 2017 February 16 and July 2. Coma morphology revealed gas jets, whose appearance and motion as a function of time yielded the rotation period and other properties. All narrowband CN images exhibited either one or two jets; one jet appeared as a partial face-on spiral with clockwise rotation while the second jet evolved from a side-on corkscrew, through face-on, and finally corkscrew again, with only a slow evolution throughout the apparition due to progressive viewing geometry changes. A total of 78 period determinations were made over a 7-week interval, yielding a smooth and accelerating rotation period starting at 24 hr (March 21 & 22) and passing 48 hr on April 28. While this is by far the fastest rate of change ever measured for a comet nucleus, the torque required is readily within what can exist given likely properties of the nucleus. If the torque remained constant, we estimate that the nucleus could have stopped rotating and/or began to tumble as soon as only two months following perihelion, and will certainly reach this stage by early in the next apparition. Working backwards in time, Tuttle-Giacobini-Kresak would have been rotating near its rotational break-up velocity 3-4 orbits earlier, suggesting that its extreme 7-magnitude outburst observed in 2001 might have been caused by a partial fragmentation at that time, as might the pair of 1973 8-magnitude outbursts if there had been an earlier spin-down and spin-up cycle.

Astronomical Journal (In press)

NASA ADS: arXiv:1901.04565 arXiv: 1901.04565