Cometary Science Newsletter

July 2020
Michael S. P. Kelley (

Postdoc position: scientific data analysis on a MIDAS/Rosetta project

The Space Research Institute (IWF) of the Austrian Academy of Sciences (OeAW), Austria`s leading non-university research and science institution, is offering a POST DOC POSITION (F*M) (full-time, 40h per week) on a one year Rosetta/MIDAS project for ESA.

The MIDAS atomic force microscope [1, 2] on-board the Rosetta comet orbiter collected and scanned micrometre-sized dust particles of comet 67P/Churyumov-Gerasimenko [3]. Its dataset is a unique library containing structural information of cometary material that can be used to further our knowledge about the early solar system [4, 5]. The aim of the advertised project is to understand which properties of the MIDAS particles offer the best insight into questions about comets and our solar system.

The candidate should have a PhD in physics (or a similar area) and have good knowledge of Python (or a similar programming language). Background knowledge about, e.g., cometary dust or atomic force microscopy is beneficial but no prerequisite. The person should be a highly motivated, autonomous researcher.

If interested please find the full job advertisement at and apply no later than July 31, 2020 by sending an e-mail to the project leader Dr. Thurid Mannel (thurid.mannel(at)

References: [1] Riedler et al. 2007 in Space Sci. Rev. 128, [2] Bentley et al. 2016 in Acta Astron. 125, [3] Bentley et al. 2016 in Nature 537, [4] Blum et al. 2017 in MNRAS 469, [5] Mannel et al. 2019 in A&A 630.

The Austrian Academy of Sciences (OeAW) pursues a non-discriminatory employment policy and values equal opportunities, as well as diversity. The OeAW lays special emphasis on increasing the number of women in senior and in academic positions. Given equal qualifications, preference will be given to female applicants.

Editorial Team and SOC Announced for New Comets III Volume

COMETS III is a proposed new volume in the Space Science Series projected for publication in 2023. The Editorial Team and Scientific Organizing Committee (SOC) for COMETS III has been recently announced. Community-wide input on topics and chapters is welcome and may be communicated to any of the editors or SOC members.

Editorial Team for Comets III:

  • Karen Meech, University of Hawaii (Lead Editor)
  • Michael Combi, University of Michigan (Co-Lead)
  • Dominique Bockelee, Paris Observatory
  • Sean Raymond, University of Bordeaux
  • Michael Zolensky, Johnson Space Center

Scientific Organizing Committee (SOC) for Comets III:

  • Dennis Bodewits, Auburn University
  • Bjorn Davidsson, Jet Propulsion Laboratory
  • Cecile Engrand, University of Paris-Saclay
  • Marina Galand, Imperial College London
  • Aurelie Guilbert-Lepoutre, CNRS/University of Lyon
  • Michael S. P. Kelley, University of Maryland
  • Jean-Baptiste Vincent, DLR Institute of Planetary Research

Refereed Articles

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

Outburst and Splitting of Interstellar Comet 2I/Borisov

  • Jewitt, D. 1
  • Kim, Y 2
  • Mutchler, M. 3
  • Weaver, H. 4
  • Agarwal, J. 5
  • Hui, M 6
  1. University of California at Los Angeles
  2. Max Planck, Gottingen
  3. Space Telescope Science Institute, Maryland
  4. Johns Hopkins Applied Physics Laboratory, Maryland
  5. Technical University, Braunschweig
  6. University of Hawaii, Honolulu

We present Hubble Space Telescope observations of a photometric outburst and splitting event in interstellar comet 2I/Borisov. The outburst, first reported with the comet outbound at 2.8 AU (Drahus et al. 2020), was caused by the expulsion of solid particles having a combined cross-section about 100 sq. km and a mass in 0.1 mm sized particles about 2e7 kg. The latter corresponds to 1e-4 of the mass of the nucleus, taken as a sphere of radius 500 m. A transient “double nucleus” was observed on UT 2020 March 30 (about three weeks after the outburst), having a cross-section about 0.6 sq. km and corresponding dust mass 1e5 kg. The secondary was absent in images taken on and before March 28, and in images taken on and after April 03. The unexpectedly delayed appearance and rapid disappearance of the secondary are consistent with an origin through rotational bursting of one or more large (meter-sized) boulders under the action of outgassing torques, following their ejection from the main nucleus. Overall, our observations reveal that the outburst and splitting of the nucleus are minor events involving a negligible fraction of the total mass: 2I/Borisov will survive its passage through the planetary region largely unscathed.

The Astrophysical Journal Letters (In press)

arXiv: 2006.01242

  • Li, J. 1
  • Jewitt, D. 2,3
  • Mutchler, M. 4
  • Agarwal, J. 5
  • Weaver, H. 6
  1. Department of Earth, Planetary and Space Sciences, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095-1567, USA
  2. Department of Earth, Planetary and Space Sciences, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095-1567, USA
  3. and Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA
  4. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
  5. Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
  6. The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA

Solar system objects with perihelia beyond the orbit of Jupiter (q>5 au) are too cold for water ice to generate an appreciable coma via sublimation. Despite this, numerous high-perihelion objects (HPOs) including many comets and recently escaped Kuiper Belt objects (Centaurs) are observed to be active out at least to the orbit of Saturn (q~10 au). Peak equilibrium temperatures at 10 au (∼125 K), while far too low to sublimate water ice, are sufficient to sublimate supervolatiles such as CO and CO2 ice. Temperatures at 10 au are also high enough to trigger the rapid crystallization of exposed amorphous ice, thus constituting another possible driver of distant activity. While supervolatile ices can sublimate strongly (as r_H^-2) to at least Kuiper Belt (30au) distances, H crystallization is an exponential function of temperature that cannot be sustained much beyond ∼10 au. The heliocentric dependence of the activity thus suggests an observational test. If activity in high-perihelion objects is triggered by crystallization, then no examples of activity should be found with perihelia of q≫10 au. If, on the other hand, activity is due to free sublimation of exposed supervolatile ices, or another cause, then distant activity might be detected. We obtained sensitive, high-resolution Hubble Space Telescope observations of HPOs to search for activity beyond the crystallization zone. No examples of activity were detected in 53 objects with q>15 au, consistent with the crystallization trigger hypothesis. However, sensitivity limits are such that we cannot reject the alternative hypothesis that mass loss is driven by the sublimation of supervolatile ices. We also searched for binary companions in our sample, finding none and setting an empirical 3σ limit to the binary fraction of <8%.

The Astronomical Journal (Published)

DOI: 10.3847/1538-3881/ab7faf NASA ADS: 2020AJ....159..209L arXiv: 2003.06519

Activity of (6478) Gault during January 13 – March 28, 2019

  • Ivanova, O. 1,2,3
  • Skorov, Yu. 4
  • Luk'yanyk, I. 3
  • Tomko, D. 1
  • Husárik, M. 1
  • Blum, J. 4
  • Egorov, O. 5
  • Voziakova, O. 5
  1. Astronomical Institute of the Slovak Academy of Sciences, Slovak Republic
  2. Main Astronomical Observatory of the National Academy of Sciences of Ukraine, Ukraine
  3. Astronomical Observatory of Taras Shevchenko National University of Kyiv, Ukraine
  4. Technische Universität Braunschweig, Institute for Geophysics and Extraterrestrial Physics, Germany
  5. Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia

We present the results of photometric observations of active asteroid (6478) Gault performed at heliocentric distances from 2.46 to 2.30 au and geocentric distances from 1.79 to 1.42 au between January 15 and March 28, 2019. Observations were carried out at the 2.5-m telescope of SAI MSU (CMO) on January 15, 2019 and at the 1.3-m and 0.61-m telescopes (SPb) on February 6 and March 28, 2019, respectively. The direct images of the asteroid were obtained with the broad-band B, V and R filters. Comet-like structures were detected at all observation dates. Colour maps were built and colour variations along the tail for the observation made on January 15, 2019 were analysed. The Afρ was calculated for the R filter, The evaluated value varies from 47 to 32 cm for the period from January to the end of March, 2019. The rotational period of the body is estimated from the light curve by different methods and is about 1.79 hr. Possible mechanisms of triggering Gault's activity are discussed.

Monthly Notices of the Royal Astronomical Society (In press)

DOI: 10.1093/mnras/staa1630 NASA ADS: 2020arXiv200512030I arXiv: 2005.12030

Absolute magnitude of small cosmic dust particles

  • Zubko, E. 1
  1. Kyung Hee University, Republic of Korea.

Agglomerated debris particles are realistic model cosmic dust particles that reproduce their highly irregular and fluffy morphology. We compute the absolute magnitude of these model particles in the broad-band filters B, V, R, and I from the widely used Johnson–Cousins photometric system. These data are aimed at providing simple quantitative interpretation of brightness of a cloud of cosmic dust, such as cometary coma, zodiacal light, lunar horizon glow, etc. Using this information, number of dust particles can be estimated from the apparent magnitude of the cloud and therefore the total volume of dust. It is significant that the smallest volume of dust is achieved using submicron particles.

Monthly Notices of the Royal Astronomical Society (Published)

DOI: 10.1093/mnras/stz3316 NASA ADS: 2020MNRAS.492..810Z

On the small contribution of supermicron dust particles to light scattering by comets

  • Zubko, E. 1,2
  • Videen, G. 1,3,4
  • Arnold, J.A. 5
  • MacCall, B. 4
  • Weinberger, A.J. 5
  • Kim, S.S. 1
  1. Kyung Hee University, Republic of Korea.
  2. Far Eastern Federal University, Russia.
  3. Space Science Institute, USA.
  4. The Environmental Laboratory, USA.
  5. Carnegie Institution of Washington, USA.

We quantitatively investigate the contribution of large dust particles to the polarimetric response in comets using the light-scattering properties of model agglomerated debris particles. We demonstrate that large, supermicronsized particles have a decreasing role on the degree of linear polarization at phase angle α < 80°, and the effect of particles greater than 10 μm is minimal. At larger phase angles, they may only slightly increase the measured percent of polarization by up to 1%. Omitting the effects of these particles in modeling the observations only slightly affects the retrievals of the microphysical properties of dust in comets and could lead to a small underestimation of the index in a power-law size distribution and population of weakly absorbing dust particles.

The Astrophysical Journal (Published)

DOI: 10.3847/1538-4357/ab8ae4 NASA ADS: 2020ApJ...895..110Z

Revisiting the particle-size constraint of the 10-μm silicate feature

  • Chornaya, E. 1.2
  • Zakharenko, A.M. 1
  • Zubko, E. 3
  • Kuchmizhak, A. 1,4
  • Golokhvast, K.S. 1
  • Videen, G. 5,6
  1. Far Eastern Federal University, Russia.
  2. Institute of Applied Astronomy of RAS, Russia.
  3. Kyung Hee University, Republic of Korea.
  4. Institute of Automation and Control Processes FEB RAS, Russia.
  5. [1,5] Space Science Institute, USA.

We experimentally measure the mid-IR spectra of half-millimeter-sized olivine particles with very compact morphology and smooth surfaces. We find that the 10-μm silicate feature is present in these large, irregularly shaped samples having surface roughness that is much smaller than optical wavelengths. Based on Mie theory, which assumes the particles are spherical, this feature should not exist for such large particles. As a consequence, its presence has been taken as an indicator that the particles or grains composing such particles were micron-sized or smaller. The measurement of this feature in real, irregularly shaped particles, suggests that the assumption of sphericity may severely limit our interpretations of remote-sensing data.

Icarus (Published)

DOI: 10.1016/j.icarus.2020.113907

Polarization of disintegrating Comet C/2019 Y4 (ATLAS)

  • Zubko, E. 1
  • Zheltobryukhov, M. 2
  • Chornaya, E. 2,3
  • Kochergin, A. 2,3
  • Videen, G. 1,4
  • Kornienko, G. 2
  • Kim, S.S. 1
  1. Kyung Hee University, Republic of Korea.
  2. Institute of Applied Astronomy of RAS, Russia.
  3. Far Eastern Federal University, Russia.
  4. Space Science Institute, USA.

We observe Comet C/2019 Y4 (ATLAS) before and after its disintegration while making polarimetric measurements over a wide range of phase angles. The disintegration event was marked with a dramatic growth of the positive polarization branch that is consistent with a large relative abundance of absorbing material of up to (96.5 ± 3.4)%. This polarization spike relaxed as the carbonaceous particles are preferentially swept from the coma due to solar radiation pressure. The observations suggest that the primordial material stored within comets are extremely rich in carbonaceous material. The pristine cometary material is processed by subsequent solar interactions, forming a refractory crust on the nucleus surface. Polarimetry provides a means of measuring the volume ratio of carbonaceous material, and hence the weathering that has occurred on the comet due to these interactions. The polarimetric response of Comet C/2019 Y4 (ATLAS) appears similar to that of Comet C/1995 O1 (Hale-Bopp), except on few epochs that are similar to that of Comet C/1996 B2 (Hyakutake).

Monthly Notices of the Royal Astronomical Society (In press)

DOI: 10.1093/mnras/staa1725