Cometary Science Newsletter

Issue
31
Month
October 2017
Editor
Michael S. P. Kelley (msk@astro.umd.edu)

Issue 31

Solar System science with the Wide-Field InfraRed Survey Telescope (WFIRST)

The Wide-Field InfraRed Survey Telescope (WFIRST) is a NASA flagship mission currently under development. This telescope, which is set to operate at the Earth-Sun L2 point, will have a 2.4-meter primary mirror and is on track to launch in 2025/2026 with a 6-year nominal mission. WFIRST will provide imaging and spectroscopic capabilities from 0.6-2.0 µm and will be a potential contemporary and eventual successor to JWST. Observations of asteroids, the giant planets and their satellites, Kuiper Belt Objects, and comets will be possible through both the Guest Investigator (GI) and Guest Observer (GO) programs.

In a community-led assessment of the capabilities of WFIRST, we have summarized how the telescope can be used for Solar System science in a white paper posted at http://arxiv.org/abs/1709.02763. Surveys of minor bodies and time domain studies of comets and variable surfaces and atmospheres are uniquely well-suited for WFIRST with its 0.28 deg2 field of view. Previous use of astrophysics assets for Solar System science and synergies between WFIRST, LSST, and JWST are discussed. We also provide a list of proposed minor modifications to the mission, including non-sidereal tracking of 60 mas/s for observations of long-period comets and a K-band filter (~2.0-2.4 µm). We are currently soliciting feedback from the larger community prior to submission of the paper. Please send comments to either B. Holler (bholler at stsci.edu) or S. Milam (stefanie.n.milam at nasa.gov).

Postdoctoral Position in Cometary Science or Astronomy at MPS, Germany

A 3-year postdoctoral position is available at the Max Planck Institute for Solar System Research in Goettingen, Germany, to work on one of the following topics:

Details of the position can be found at http://www.mps.mpg.de/5066843/job_full_offer_11485198?c=2169.

If you have any questions, please contact Jessica Agarwal (agarwal@mps.mpg.de).

PhD Position in Cometary Science at MPS, Germany

A PhD position is available at the Max Planck Institute for Solar System Research in Göttingen, Germany, to study the motion of debris in the coma of comet 67P/Churyumov-Gerasimenko in data from the Rosetta/OSIRIS camera system.

Details of the project can be found at https://www.mps.mpg.de/phd/cometary-science-rosetta-debris-dynamics.

Applications should be sent through the online application portal (https://www.mps.mpg.de/phd/application) of the International Max Planck Research School (IMPRS) for Solar System Science at the University of Göttingen (https://www.mps.mpg.de/phd).

If you have any questions, please contact Jessica Agarwal (agarwal@mps.mpg.de).

Conference Announcements

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

Water during planet formation and evolution - Zurich, February 12-16, 2018

The workshop will focus on processes governing the delivery of water to planetary bodies in solar and exoplanetary systems, during their formation and long-term evolution. The respective roles of water (ice) inheritance from the interstellar medium, condensation in the protoplanetary nebula and processing and delivery during accretion will be discussed. We will further consider in detail the implications of the late-stage impact phase and long-term consequences for the remarkable diversity of processes affecting the water budget during the evolution of a planetary body. The goal of the workshop is to critically assess the interplay of theory and observations/experiments on the effects of water on planets and establish new research directions.

All information at: https://waterzurich.github.io

Refereed Journal Papers

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

An analysis of the BVRI colors of 22 active comets

  • Betzler, A. S. 1
  • Almeida, R. S. 1
  • Cerqueira, W. J. 1
  • Araujo, L. A. 1
  • Prazeres, C. J. M. 1
  • Jesus, J. N. 1
  • Bispo, P. A. S. 1
  • Andrade, V. B. 1
  • Freitas, Y. A. S. 1
  • Betzler, L. B. S. 1
  1. Centro de Formação de Professores, Universidade Federal do Recôncavo da Bahia, Amargosa 45300-000, Brazil

Our aim was to analyze the variation of Johnson-Kron-Cousins BVRI color indexes of a sample with 22 active comets of various dynamic groups with the time, geometrical, observational and dynamical parameters. We performed photometric observations of 16 comets between 2010 and 2014, using robotic telescopes in three continents. In addition to the sample, we used data of six comets available in the literature. A statistical comparison between the distributions of color indexes was performed using the Kruskal-Wallis H-test. The color indexes of active comets can vary a few tenths up to a magnitude on time scales that range from hours to weeks. Using the B-V colors of the observed comets, we generated a relationship that correlates the cometary visual and CCD magnitudes. We did not identify any relationship between B-V and V-R colors with heliocentric distance and phase angle. The color B-V is correlated with the photometric aperture that can be described by a logarithmic function. We did not identify any differences in the distribution of B-V color among the comets analyzed at a confidence level equal to or greater than 95%. The mean color of active comets are B-R = 1.20 ± 0.24 , B-V = 0.76 ± 0.16 and V-R = 0.42 ± 0.16 . Active comets with V-R colors outside the three standard deviation interval can be considered objects with unusual physical characteristics.

Advances in Space Research (Published)

DOI: 10.1016/j.asr.2017.04.021 NASA ADS: 2017AdSpR..60..612B

Observations of Three Stellar Occultations by Comet Hale-Bopp

  • Betzler, A. S. 1
  • Betzler, L. B. S. 1
  1. Centro de Formação de Professores, Universidade Federal do Recôncavo da Bahia

On June 12 1996 UT, three 11-12 R magnitude stars were occulted by comet Hale-Bopp. The passage of these stars through the cometary atmosphere was monitored at the Observatorio do Pico dos Dias (Brazil) using a 0.6 m telescope. We detected an absorption of starlight by the comet dust implying in a drop of (6 ± 1) × 10-2 magnitude of occulted star XPM 152-0898466 at a minimum distance of (4.3 ± 0.2)× 104 km from the central condensation. This result, combined with magnitude variations of other stars, suggests the detection of a jet in the coma. From our measurement of extinction, we infer that the opacity of the nucleus surface was τ ˜ 3. The mean geometric albedo p of dust grains is (4.5 ± 0.7) × 10-3. This low albedo may suggest a difference in the grain population associated to coma and jets.

Earth, Moon, and Planets (Published)

DOI: 10.1007/s11038-017-9503-z NASA ADS: 2017EM&P..120....1B

The Rotation and Other Properties of Comet 49P/Arend-Rigaux, 1984 - 2012

  • Nora Eisner 1,2
  • Matthew M. Knight 2
  • David G. Schleicher 3
  1. Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH
  2. Department of Astronomy, University of Maryland, College Park, MD 20742, USA
  3. Lowell Observatory, 1400 W. Mars Hill Rd, Flagstaff, AZ 86001, USA

We analyzed images of comet 49P/Arend-Rigaux on 33 nights between 2012 January and May and obtained R-band lightcurves of the nucleus. Through usual phasing of the data we found a double-peaked lightcurve having a synodic rotation period of 13.450 ± 0.005 hr. Similarly, phase dispersion minimization and the Lomb-Scargle method both revealed rotation periods of 13.452 hr. Throughout the 2011/12 apparition, the rotation period was found to increase by a small amount, consistent with a retrograde rotation of the nucleus. We also reanalyzed the publicly available data from the 1984/85 apparition by applying the same techniques, finding a rotation period of 13.45 ± 0.01 hr. Based on these findings we show that the change in rotation period is less than 14 seconds per apparition. Furthermore, the amplitudes of the light curves from the two apparitions are comparable, to within reasonable errors, even though the viewing geometries differ, implying that we are seeing the comet at a similar sub-Earth latitude. We detected the presence of a short term jet-like feature in 2012 March which appears to have been created by a short duration burst of activity on March 15. Production rates obtained in 2004/05, along with reanalysis of previous results from 1984/85 imply a strong seasonal effect and a very steep fall-off after perihelion. This, in turn, implies that a single source region dominates activity, rather than leakage from the entire nucleus.

The Astronomical Journal (In press)

arXiv: 1709.06089

Gemini and Lowell Observations of 67P/Churyumov-Gerasimenko During the Rosetta Mission

  • Knight, M.M. 1,2
  • Snodgrass, C. 3
  • Vincent, J.-B. 4
  • Conn, B. 5,6
  • Skiff, B.A. 2
  • Schleicher, D.G. 2
  • Lister, T. 7
  1. University of Maryland, USA
  2. Lowell Observatory, USA
  3. The Open University, UK
  4. DLR Institute of Planetary Research, Germany
  5. Australian National University, Australia
  6. Gemini Observatory, Chile
  7. Las Cumbres Observatory, USA

We present observations of comet 67P/Churyumov-Gerasimenko acquired in support of the Rosetta mission. We obtained usable data on 68 nights from 2014 September until 2016 May, with data acquired regularly whenever the comet was observable. We collected an extensive set of near-IR J, H, and Ks data throughout the apparition plus visible-light images in g', r', i', and z' when the comet was fainter. We also obtained broadband R and narrowband CN filter observations when the comet was brightest using telescopes at Lowell Observatory. The appearance was dominated by a central condensation and the tail until 2015 June. From 2015 August onwards there were clear asymmetries in the coma, which enhancements revealed to be due to the presence of up to three features (i.e., jets). The features were similar in all broadband filters; CN images did not show these features but were instead broadly enhanced in the southeastern hemisphere. Modeling using the parameters from Vincent et al. (2013) replicated the dust morphology reasonably well, indicating that the pole orientation and locations of active areas have been relatively unchanged over at least the last three apparitions. The dust production, as measured by A(0)fρ peaked ~30 days after perihelion and was consistent with predictions from previous apparitions. A(0)fρ as a function of heliocentric distance was well fit by a power-law with slope -4.2 from 35-120 days post-perihelion. We detected photometric evidence of apparent outbursts on 2015 August 22 and 2015 September 19, although neither was discernible morphologically in this dataset.

Monthly Notices of the Royal Astronomical Society (In press)

arXiv: 1709.06552

On the reflectance of dust in comets

  • Zubko, E. 1
  • Videen, G. 2,3
  • Shkuratov, Yu. 4
  • Hines, D. C. 3,5
  1. Far Eastern Federal University, Russia
  2. US Army Research Laboratory, USA
  3. Space Science Institute, USA
  4. Institute of Astronomy, V.N. Karazin Kharkov National University, Ukraine
  5. Space Telescope Science Institute, USA

Reflectance of cometary dust is a key parameter used in the characterization of comets. In the literature, the reflectance of single-scattering cometary dust particles is widely assumed to be the same as that of the cometary nucleus. We discuss this assumption and demonstrate its inconsistency with photometric observations of comets, laboratory optical measurements, and numerical simulation of light scattering from single-scattering dust particles and particulate surfaces composed of the same particles. We estimate the reflectance of cometary dust particles using a comprehensive physical model of polarization measured in comets over wide range of phase angle and at different wavelengths in the visible. The model predicts that the reflectance of dust in comets inversely correlates with their maximum of positive polarization P_max. We find that even the darkest dust particles appearing in comets with the highest Pmax, reflect considerably more incident solar-radiation energy, up to 200%, compared to what is thought for cometary nuclei. We also find that the reflectance retrieved from polarimetry in the visible appears in good quantitative accordance with previous estimations from infrared observations of comets. Our findings suggest that the dust production of comets is currently overestimated and may require revision.

Journal of Quantitative Spectroscopy & Radiative Transfer (Published)

DOI: 10.1016/j.jqsrt.2017.07.026

The Castalia Mission to Main Belt Comet 133P/Elst-Pizarro

  • Snodgrass, C. 1
  • Jones, G.H. 2
  • Boehnhardt, H. 3
  • Gibbings, A. 4
  • Homeister, M. 4
  • and 39 co-authors
  1. The Open University, UK
  2. Mullard Space Science Laboratory, University College London, UK
  3. Max Planck Institute for Solar System Research, Germany
  4. OHB System AG, Bremen, Germany

We describe Castalia, a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBC’s activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESA’s highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these.

Advances in Space Research (In press)

DOI: 10.1016/j.asr.2017.09.011 arXiv: 1709.03405

Spatial variations of brightness, colour and polarization of dust in comet 67P/Churyumov–Gerasimenko

  • Rosenbush, V.K. 1
  • Ivanova, O. V. 1
  • Kiselev, N. N. 1
  • Kolokolova, L. O. 2
  • Afanasiev, V. L. 3
  1. Main Astronomical Observatory of the Ukrainian NANU
  2. University of Maryland
  3. Special Astrophysical Observatory of RAN

We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov–Gerasimenko performed at the 6-m telescope of the SAO RAS in the g-sdss (465/65 nm), r-sdss (620/60 nm) and R filters. Observations in November and December 2015 and April 2016 covered the range of heliocentric distance 1.62–2.72 au and phase angle 33.2°–10.4°. The comet was very active. Two persistent jets and long dust tail were observed during the whole observing period; one more jet was detected only in December. The radial profiles of surface brightness, colour and polarization significantly differed for the coma, jets and tail, and changed with increasing heliocentric distance. The dust production Afρ decreased from 162 cm at r = 1.62 au to 51 cm at r = 2.72 au. The dust colour (g–r) gradually changed from 0.8 mag in the innermost coma to about 0.4 mag in the outer coma. The spectral slope was 8.2 ± 1.7 per cent/100 nm in the 465 to 620 nm wavelength domain. In November and December, the polarization in the near-nucleus area was about 8 per cent, dropped sharply to 2 per cent at the distance above 5000 km and then gradually increased with distance from the nucleus, reaching ∼8 per cent at 40 000 km. In April, at a phase angle 10.4°, the polarization varied between –0.6 per cent in the near-nucleus area and –4 per cent in the outer coma. Circular polarization was not detected in the comet. The spatial variations of brightness, colour and polarization in different structural features suggest some evolution of particle properties, most likely decreasing the size of dust particles.

Monthly Notices of the Royal Astronomical Society (Published)

DOI: 10.1093/mnras/stx2003 NASA ADS: 2017MNRAS.469S.475R

Light Scattering by Hierarchical Aggregates

  • Kolokolova, L.[1], Nagdimunov, L.[1], Mackowski, D.2
  1. University of Maryland
  2. Auburn University

Recent in-situ studies of the environment of comet 67P/Churyumov-Gerasimenko by the dust instruments onboard the Rosetta spacecraft have indicated a complex structure of cometary dust particles. The majority of those particles appeared to be large aggregates of hierarchical structure, i.e. aggregates of particles, which, in turn, were aggregates of smaller particles. This confirmed an earlier hypothesis that dust particles in protoplanetary disks grow under hierarchical growth process. Thus, it is very likely that hierarchical aggregates are common type of natural dust particles. In this paper, we present results of computer simulations of light scattering by a variety of hierarchical aggregates to determine how their structure affects their brightness and polarization phase curves as well as photometric and polarimetric color, and albedo. The computations were done using Multi-Sphere T-Matrix method. Our results show that the type of hierarchical structure of aggregates, specified by the number of particles at each level of hierarchy, causes variations in their light-scattering characteristics, which noticeably exceed the variations caused by different configuration of monomers in the aggregates of the same hierarchical structure. Although we could not find any regularities in the brightness and polarization dependence on the structure of the aggregates, our results clearly show that not only composition or size of aggregates, but also their specific structure should be carefully examined when light scattering by cometary or any other type of dust is modeled. Specifically, we may need to reconsider modeling the cometary dust particles using simple ballistic particle-cluster and cluster-cluster aggregates.

Journal of Quantitative Spectroscopy and Radiative Transfer (Published)

DOI: 10.1016/j.jqsrt.2017.09.019

A Comet Active Beyond the Crystallization Zone

  • Jewitt, D. 1
  • Hui, M. 1
  • Mutchler, M. 2
  • Weaver, H. 3
  • Li, J. 1
  • Agarwal, J. 4
  1. UCLA
  2. Space Telescope Science Institute
  3. APL, Johns Hopkins University
  4. Max Planck Institute for Solar System

We present observations showing in-bound long-period comet C/2017 K2 (PANSTARRS) to be active at record heliocentric distance. Nucleus temperatures are too low (60 K to 70 K) either for water ice to sublimate or for amorphous ice to crystallize, requiring another source for the observed activity. Using the Hubble Space Telescope we find a sharply-bounded, circularly symmetric dust coma 10^5 km in radius, with a total scattering cross section of 105 km2. The coma has a logarithmic surface brightness gradient -1 over much of its surface, indicating sustained, steady-state dust production. A lack of clear evidence for the action of solar radiation pressure suggests that the dust particles are large, with a mean size ≥ 0.1 mm. Using a coma convolution model, we find a limit to the apparent magnitude of the nucleus V > 25.2 (absolute magnitude H > 12.9). With assumed geometric albedo pV = 0.04, the limit to the nucleus circular equivalent radius is < 9 km. Pre-discovery observations from 2013 show that the comet was also active at 23.7 AU heliocentric distance. While neither water ice sublimation nor exothermic crystallization can account for the observed distant activity, the measured properties are consistent with activity driven by sublimating supervolatile ices such as CO2, CO, O2 and N2. Survival of supervolatiles at the nucleus surface is likely a result of the comet's recent arrival from the frigid Oort cloud.

Astrophysical Journal (Published)

DOI: 10.3847/2041-8213/aa88b4