Cometary Science Newsletter

March 2021
Michael S. P. Kelley (

Research opportunities in cosmic dust/heliosphere science, Switzerland

There are two research opportunities in cosmic dust science at the ETH in Zürich, Switzerland.

The Astrophysical Dust Group at the Institute for Particle Physics and Astrophysics (IPA), ETH Zürich, in Switzerland, is looking for highly motivated candidates to apply for a postdoctoral researcher position, and a PhD position, within the framework of the ERC funded project “The heliosphere and the dust: characterisation of the solar and interstellar neighbourhood”.

The projects include cosmic dust data analysis, modelling of the interstellar dust transport through the heliosphere, and applying the results to other astrospheres and the solar system in the past. For more details and submission of the applications, see:
Postdoc position:
PhD position:

Application deadline: March 31, 2021.

For questions (no applications!), please contact Veerle Sterken:

ESA Archival Research Visitor Programme

To increase the scientific return from its space science missions, the European Space Agency (ESA) welcomes applications from scientists interested in pursuing research projects based on data publicly available in the ESA Space Science Archives (

The ESA Archival Research Visitor Programme is open to scientists, at all career levels, affiliated with institutes in ESA Member States and Collaborating States. Early-career scientists (within 10 years of the PhD) are particularly encouraged to apply. PhD students are also welcome to apply through their supervisors.

During their stay, visiting scientists will have access to archives and mission specialists for help with the retrieval, calibration, and analysis of archival data. In principle, all areas of space research covered by ESA science missions can be supported.

Residence lasts typically between one and three months, also distributed over multiple visits. Research projects can be carried out at ESAC (Madrid, Spain) and at ESTEC (Noordwijk, Netherlands). To offset the expenses incurred by visitors, ESA covers travel costs from and to the home institution and provides support for lodging expenses and meals.

Applications received before 30 April 2021 will be considered for visits in autumn/winter (2021/2022).

For further details, including areas of research and contact information, please refer to:

or write to the programme coordinator (Guido De Marchi) at

Refereed Articles

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

The Formation of Bilobate Comet Shapes through Sublimative Torques

  • Safrit, T. K. 1
  • Steckloff, J. K. [1][2]3
  • Bosh, A. S. 1
  • Nesvorny, D. 4
  • Walsh, K. 4
  • Brasser, R. 5
  • Minton, D.A. 6
  1. Massachussetts Institute of Technology, Cambridge, MA, USA
  2. University of Texas at Austin, Austin, TX, USA
  3. Planetary Science Institute, Tucson, AZ, USA
  4. Southwest Research Institute, Boulder, CO, USA
  5. Earth Life Science institute, Tokyo Institute of Technology, Tokyo, Japan
  6. Purdue University, West Lafayette, IN, USA

Recent spacecraft and radar observations found that ~70% of short-period comet nuclei, mostly Jupiter-family comets (JFCs), have bilobate shapes (two masses connected by a narrow neck). This is in stark contrast to the shapes of asteroids of similar sizes, of which ~14% are bilobate. This suggests that a process or mechanism unique to comets is producing these shapes. Here we show that the bilobate shapes of JFC nuclei are a natural byproduct of sublimative activity during their dynamical migration from their trans-Neptunian reservoir, through the Centaur population, and into the Jupiter family. We model the torques resulting from volatile sublimation during this dynamical migration, and find that they tend to spin up these nuclei to disruption. Once disrupted, the rubble pile-like material properties of comet nuclei (tensile strengths of ~1–10 Pa and internal friction angles of ~35°) cause them to reform as bilobate objects. We find that JFCs likely experienced rotational disruption events prior to entering the Jupiter family, which could explain the prevalence of bilobate shapes. These results suggest that the bilobate shapes of observed comets developed recently in their history (within the past ~1–10 Myr), rather than during solar system formation or collisions during planet migration and residency in the trans-Neptunian population.

Planetary Science Journal (Published)

DOI: 10.3847/PSJ/abc9c8 NASA ADS: 2021PSJ.....2...14S arXiv: 2011.01394

Testing Short-term Variability and Sampling of Primary Volatiles in Comet 46P/ Wirtanen

  • Khan, Y. 1, 10
  • Gibb, E. L. 1, 10
  • Bonev, B. P. 2, 10
  • Roth, N. X. 3, 4, 10
  • Saki, M. 1, 10
  • DiSanti, M. A. 4, 10
  • Dello Russo, N. 5, 10
  • Vervack, R. J., Jr. 5, 10
  • McKay, A. J. 2, 4, 10
  • Combi, M. R. 6
  • Shou, Y. 6
  • Cordiner, M. A. 4, 7
  • Kawakita, H. 8, 10
  • Fougere, N. 6
  • Protopapa, S. 9
  1. Department of Physics and Astronomy, University of Missouri-St. Louis, One University Boulevard, St. Louis, MO, USA
  2. Department of Physics American University 4400 Massachusetts Avenue NW Washington, DC 20016, USA
  3. Universities Space Research Association Columbia, MD, USA
  4. Solar System Exploration Division Code 690, NASA-Goddard Space Flight Center, Greenbelt, MD, USA
  5. Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA
  6. Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA
  7. Department of Physics, Catholic University of America, Washington DC, USA
  8. Koyama Astronomical Observatory Kyoto Sangyo University Motoyama, Kamigamo, Kita-ku, Kyoto, 603-8555, Japan
  9. Southwest Research Institute, Boulder, CO, USA
  10. Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawai’i under contract NNH14CK55B with the National Aeronautics and Space Administration

The exceptionally favorable close approach of Jupiter-family comet 46P/Wirtanen in 2018 December enabled characterization of its primary volatile composition with exceptionally high spatial resolution and sensitivities using the iSHELL spectrograph at the NASA Infrared Telescope Facility on Maunakea, HI. We sampled emissions from H2O, HCN, C2H2, NH3, C2H6, and CH3OH on UT 2018 December 21 using two instrumental settings that spanned the 2.9–3.6 μm spectral region. We also obtained a sensitive 3σ upper limit for H2CO and for the rarely studied molecule HC3N. We report rotational temperatures, production rates, and mixing ratios (relative to H2O as well as to C2H6). We place our results in context by comparing them with other comets observed at near-IR wavelengths. We also compare our results with those obtained using the NIRSPEC-2 spectrograph on Keck II on UT December 17 and 18 and with results obtained from iSHELL on other dates during the same apparition. Within 1–2σ uncertainty, production rates obtained for all molecules in this work were consistent with those obtained using NIRSPEC-2 except H2O, indicating low-level variability on a timescale of days. Mixing ratios with respect to H2O in 46P/Wirtanen were consistent with corresponding values from NIRSPEC-2 within the uncertainty with the exception of CH3OH, which yielded a higher ratio on December 21. Our measurements afforded a high temporal resolution that spanned ∼ 2/3 of the rotational period of 46P/Wirtanen, enabling us to test short-term variability in the production rates of H2O and HCN due to rotational effects. Both H2O and HCN production rates showed similar temporal variability, resulting in nearly constant HCN/H2O.

The Planetary Science Journal (Published)

DOI: 10.3847/PSJ/abc95c

Carbonyl Sulfide (OCS): Detections in comets C/2002 T7 (LINEAR), C/2015 ER61 (PanSTARRS), and 21P/Giacobini–Zinner and stringent upper-limits in 46P/Wirtanen

  • Mohammad Saki 1,9
  • Erika L. Gibb 1,2,9
  • Boncho P. Bonev 2,3,9
  • Nathan X. Roth 4,5,9
  • Michael A. DiSanti 2,6,9
  • Neil Dello Russo 7,9
  • Ronald J. Vervack, Jr. 7,9
  • Adam J. McKay 3,6,9
  • Hideyo Kawakita 8,9
  1. Department of Physics & Astronomy, University of Missouri-St. Louis, One University Blvd., St. Louis, MO, USA
  2. Goddard Center for Astrobiology, NASA-Goddard Space Flight Center, Greenbelt, MD, USA
  3. Department of Physics, American University, Washington, DC, USA
  4. Solar System Exploration Division, Astrochemistry Laboratory Code 691, NASA-Goddard Space Flight Center, Greenbelt, MD, USA
  5. Universities Space Research Association, Columbia, MD, USA
  6. Solar System Exploration Division, Planetary System Laboratory Code 693, NASA-Goddard Space Flight Center, Greenbelt, MD, USA
  7. Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA
  8. Koyoma Astronomical Observatory, Kyoto Sangyo University Motoyama, Kamingamo, Kita-ku, Kyoto 603-8555, Japan
  9. Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawai’i under contract NNH14CK55B with the National Aeronautics and Space Administration

Carbonyl sulfide (OCS) is one of the sulfur-bearing molecules detected in different astronomical environments, including comets. The present-day sulfur chemistry in comets may reveal much about the origin of these ices and their subsequent processing history. Cometary sulfur molecules such as H2S, H2CS, SO2, SO, CS, CS2, S2, and NS have been detected in many comets. However, OCS, the only sulfur-bearing species with fluorescence emission lines at infrared wavelengths, is under-represented in comet volatile studies, having been reported in only six comets so far. We targeted OCS with the NASA Infrared Telescope Facility in comets 46P/Wirtanen, 21P/Giacobini–Zinner, and C/2015 ER61 (PanSTARRS) in 2017–2018 using the high-resolution iSHELL spectrograph, and in C/2002 T7 (LINEAR) in 2004 using the heritage CSHELL spectrograph. In comet C/2015 ER61, the OCS abundance was similar to those measured in bright comets such as comets C/2012 S1 (ISON) and C/1996 B2 (Hyakutake), whereas in C/2002 T7 it was relatively depleted. Our OCS measurement in 21P/Giacobini–Zinner is the first definitive detection of this molecule in a Jupiter-family comet from a ground based facility and is close to the average OCS abundance determined in comet 67P/Churyumov–Gerasimenko by the Rosetta mission. Our 3σ upper limit for comet 46P/Wirtanen is the lowest reported OCS abundance in any comet. We present production rates and mixing ratios (with respect to H2O) for these comets and place our results in the context of comets measured to date.

Astronomical Journal (Published)

DOI: 10.3847/1538-3881/aba522 NASA ADS: 2020AJ....160..184S