Cometary Science Newsletter

Issue
8
Month
November 2015
Editor
Michael S. P. Kelley (msk@astro.umd.edu)

Comet Notes

Brief observational reports or other notes related to specific comets. Limited to 1000 characters. The CSN is not intended to replace telegram services or other breaking news outlets.

C/2013 UQ4 (Catalina)

Dr. Ivanova (ivanovaleksandra@gmail.com; sandra@mao.kiev.ua) observed Comet C/2013 UQ4 (Catalina) from 2014-06-02 through 2014-08-25 with broad band Cousins filters (BVRI). Preliminary analysis of the observations reveals significant short-term variations of the photometric color in the innermost coma (~3000 km). The color slope (measured with B and R filters) was found to decrease from +11.2% per 0.1 μm (red color) to -3.5% per 0.1 μm (blue color) over some 20 hours. However, the wide-band filters make it difficult to discriminate origin of the phenomenon, as the measured flux is governed simultaneously with dust and gaseous emission. We are searching for any complementary data that may help to estimate relative contribution of gas and dust in Comet UQ4 and invite scientist(s) holding such data for collaboration on our analysis of the observations. For further discussion, please, contact directly Dr. Ivanova or Dr. Evgenij Zubko (evgenij.s.zubko@gmail.com).

Drs. Ivanova and Zubko

Refereed Articles

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

The NEOWISE­-Discovered Comet Population and the CO+CO2 production rates.

  • James M. Bauer 1,2
  • Rachel Stevenson 1
  • Emily Kramer 1
  • A. K. Mainzer 1
  • Tommy Grav 3
  • Joseph R. Masiero 1
  • Yan R. Fernández 4
  • Roc M. Cutri 2
  • John W. Dailey 2
  • Frank J. Masci 2
  • Karen J. Meech 5,6
  • Russel Walker 7
  • C. M. Lisse 8
  • Paul R. Weissman 1
  • Carrie R. Nugent 1
  • Sarah Sonnett 1
  • Nathan Blair 2
  • Andrew Lucas 2
  • Robert S. McMillan 9
  • Edward L. Wright 10
  • and the WISE and NEOWISE Teams 1,2,3,10
  1. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 183-401, Pasadena, CA 91109
  2. Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA
  3. Planetary Science Institute, Tucson, AZ
  4. Department of Physics, University of Central Florida, Orlando, FL
  5. Institute for Astronomy, University of Hawaii, Manoa, HI
  6. NASA Astrobiology Institute, Institute for Astronomy, University of Hawaii, Manoa, HI
  7. Monterey Institute for Research in Astronomy, Marina, CA
  8. Applied Physics Laboratory, Johns Hopkins University, Laurel, MD
  9. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ
  10. Department of Physics and Astronomy, University of California, Los Angeles, CA

The 163 comets observed during the WISE/NEOWISE prime mission represent the largest infrared survey to date of comets, providing constraints on dust, nucleus sizes, and CO+CO2 production. We present detailed analyses of the WISE/NEOWISE comet discoveries, and discuss observations of the active comets showing 4.6 μm band excess. We find a possible relation between dust and CO+CO2 production, as well as possible differences in the sizes of long and short period comet nuclei.

Astrophysical Journal (In press)

arXiv: 1509.08446

Size and albedo distributions of asteroids in cometary orbits using WISE data

  • Licandro, J. 1,2
  • Ali-Lagoa, V.1,2
  • Tancredi, G3
  • Fernandez, Y.4
  1. Instituto de Astrof ısica de Canarias (IAC), C/Via Lactea s/n, 38205 La Laguna, Tenerife, Spain.
  2. Departamento de Astrofısica, Universidad de La Laguna (ULL), E-38205 La Laguna, Tenerife, Spain.
  3. Departamento de Astronomıa, Facultad de Ciencias, Igua 4
  4. 225, 11400 Montevideo, Uruguay.
  5. Physics Department, University of Central Florida, P. O. Box 162385, Orlando, FL 32816.2385, USA

We study the distributions of effective diameter (D), beaming parameter (η), and visible geometric albedo (p_V) of asteroids in cometry orbits (ACOs) populations, derived from NASA's Wide-field Infrared Explorer (WISE) observations, and compare these with the same, independently determined properties of the comets. The near-Earth asteroid thermal model (NEATM) is used to compute the D, p_V and η. We obtained D and p_V for 49 ACOs in Jupiter family cometary orbits (JF-ACOs) and 16 ACOs in Halley-type orbits (Damocloids). We also obtained η for 45 of them. All but three JF-ACOs (95% of the sample) present a low albedo compatible with a cometary origin. The p_V and η distributions of both ACO populations are very similar. For the entire sample of ACOs, the mean geometric albedo is p_V = 0.05 ± 0.02, (p_V = 0.05 ± 0.01 and p_V =0.05 ± 0.02 for JF-ACOs and Damocloids, respectively) compatible with a narrow albedo distribution similar to that of the Jupiter family comets (JFCs), with a p_V ~ 0.04. The η = 1.0 ± 0.2. We find no correlations between D, p_V , or η. We compare the cumulative size distribution (CSD) of ACOs, Centaurs, and JFCs. Although the Centaur sample contains larger objects, the linear parts in their log-log plot of the CSDs presents a similar cumulative exponent (β = 1.85 ± 0.30 and 1.76 ± 0.35, respectively). The CSD for Damocloids presents a much shallower exponent β = 0.89 ± 0.17. The CSD for JF-ACOs is shallower and shifted towards larger diameters with respect to the CSD of active JFCs, which suggests that the mantling process has a size dependency whereby large comets tend to reach an inactive stage faster than small ones. Finally, the population of JF-ACOs is comparable in number that of JFCs, although there are more tens-km JF-ACOs than JFCs.

Astronomy and Astrophysics (In press)

NASA ADS: 2015arXiv151002282L arXiv: 1510.02282

Comet Science with the James Webb Space Telescope

  • Michael S. P. Kelley 1
  • Charles E. Woodward 2
  • Dennis Bodewits 1
  • Tony L. Farnham 1
  • Murthy S. Gudipati 3,4
  • David E. Harker 5
  • Dean C. Hines 6
  • Matthew M. Knight 7
  • Ludmilla Kolokolova 1
  • Aigen Li 8
  • Imke de Pater 9
  • Silvia Protopapa 1
  • Ray W. Russell 10
  • Michael L. Sitko 11
  • Diane H. Wooden 12
  1. Department of Astronomy, University of Maryland, College Park
  2. Minnesota Institute for Astrophysics, University of Minnesota
  3. NASA Jet Propulsion Laboratory
  4. Institute for Physical Sciences and Technology, University of Maryland
  5. Center for Astrophysics and Space Sciences, University of California, San Diego
  6. Space Telescope Science Institute
  7. Lowell Observatory
  8. Department of Physics and Astronomy, University of Missouri
  9. Department of Astronomy, University of California, Berkeley
  10. The Aerospace Corporation
  11. Space Science Institute
  12. NASA Ames Research Center

The James Webb Space Telescope (JWST), as the largest space-based astronomical observatory with near- and mid-infrared instrumentation, will elucidate many mysterious aspects of comets. We summarize four cometary science themes especially suited for this telescope and its instrumentation: the drivers of cometary activity, comet nucleus heterogeneity, water ice in comae and on surfaces, and activity in faint comets and main-belt asteroids. With JWST, we can expect the most distant detections of gas, especially CO2, in what we now consider to be only moderately bright comets. For nearby comets, coma dust properties can be studied with their driving gases, measured simultaneously with the same instrument or contemporaneously with another. Studies of water ice and gas in the distant Solar System will help us test our understanding of cometary interiors and coma evolution. The question of cometary activity in main-belt comets will be further explored with the possibility of a direct detection of coma gas. We explore the technical approaches to these science cases and provide simple tools for estimating comet dust and gas brightness. Finally, we consider the effects of the observatory's non-sidereal tracking limits, and provide a list of potential comet targets during the first 5 years of the mission.

Publications of the Astronomical Society of the Pacific (In press)

NASA ADS: 2015arXiv151005878K arXiv: 1510.05878

The Photodissociation of Formaldehyde in Comets

  • Feldman, P. D. 1
  1. Johns Hopkins University, Baltimore, MD 21218

Observations of comets in the 905-1180 Å spectral band made with the Far Ultraviolet Spectroscopic Explorer in 2001 and 2004 show unusual features in the fluorescent emissions of CO and H2. These include emission from a non-thermal high-J rotational population of CO and solar Lyman-alpha induced fluorescence from excited vibrational levels of H2, both of which are attributed to photodissociation of formaldehyde. In this paper we model the large number of observed H2 lines and demonstrate the dependence of the pumping on the heliocentric velocity of the comet and the solar line profiles. We also derive the rotational and vibrational populations of H2 and show that they are consistent with the results of laboratory studies of the photodissociation of H2CO. In addition to the principal series of H I and O I, the residual spectrum is found to consist mainly of the Rydberg series of C I multiplets from which we derive the mean carbon column abundance in the coma. Fluorescent emissions from N I and N2 are also searched for.

Astrophysical Journal (Published)

DOI: 10.1088/0004-637X/812/2/115 NASA ADS: 2015ApJ...812..115F

An improved model for interplanetary dust fluxes in the outer solar system

  • Poppe, A. R. 1
  1. Space Sciences Lab., U. C. Berkeley, CA, USA

We present an improved model for interplanetary dust grain fluxes in the outer solar system constrained by in-situ dust density observations. A dynamical dust grain tracing code is used to establish relative dust grain densities and three-dimensional velocity distributions in the outer solar system for four main sources of dust grains: Jupiter-family comets, Halley-type comets, Oort-Cloud comets, and Edgeworth-Kuiper Belt objects. Model densities are constrained by in-situ dust measurements by the New Horizons Student Dust Counter, the Pioneer 10 meteoroid detector, and the Galileo Dust Detection System (DDS). The model predicts that Jupiter-family comet grains dominate the interplanetary dust grain mass flux inside approximately 10 AU, Oort-Cloud cometary grains may dominate between 10 and 25 AU, and Edgeworth-Kuiper Belt grains are dominant outside 25 AU. The model also predicts that while the total interplanetary mass flux at Jupiter roughly matches that inferred by the analysis of the Galileo DDS measurements, mass fluxes to Saturn, Uranus, and Neptune are at least one order-of-magnitude lower than that predicted by extrapolations of dust grain flux models from 1 AU. Finally, we compare the model predictions of interplanetary dust oxygen influx to the giant planet atmospheres with various observational and photochemical constraints and generally find good agreement, with the exception of Jupiter, which suggests the possibility of additional chemical pathways for exogenous oxygen in Jupiter's atmosphere.

Icarus (Published)

DOI: 10.1016/j.icarus.2015.10.001

CONSERT suggests a change in local properties of 67P/Churyumov-Gerasimenko’s nucleus at depth

  • Ciarletti V. 1
  • Levasseur-Regourd A.C. 2
  • Lasue J. 3
  • Statz 4
  • Plettemeier 4
  • Rogez Y. 5
  • Hérique A. 5
  • Kofman 5
  1. UVSQ (UPSay); UPMC (Sorbonne Univ.); CNRS/INSU; LATMOS-IPSL, France
  2. UPMC (Sorbonne Univ.); UVSQ (UPSay); CNRS/INSU; LATMOS-IPSL, France
  3. Université de Toulouse; UPS-OMP; IRAP;France
  4. Technische Universität Dresden, Germany
  5. IPAG, France

After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko, the Rosetta mission provided the first opportunity of performing measurements with the CONSERT tomographic radar in November 2014. CONSERT data were acquired during this first science sequence. They unambiguously showed that propagation through the smaller lobe of the nucleus was achieved. While the ultimate objective of the CONSERT radar is to perform the tomography of the nucleus, this paper focuses on the local characterization of the shallow subsurface in the area of Philae’s final landing site, specifically determining the possible presence of a permittivity gradient below the nucleus surface. A number of electromagnetic simulations were performed with a ray-tracing code to parametrically study how the gradient of the dielectric constant in the near-subsurface affects the ability of CONSERT to receive signals. At the 90 MHz frequency of CONSERT, the dielectric constant is a function of porosity, composition, and temperature. The dielectric constant values considered for the study are based on observations made by the other instruments of the Rosetta mission and on laboratory measurements made on analog samples. The obtained simulated data clearly show that if the dielectric constant were increasing with depth, it would have prevented the reception of signal at the CONSERT location during the first science sequence. We conclude from our simulations that the dielectric constant most probably decreases with depth. Such a decrease within the sounded volume of the nucleus, could be explained by a dust/ice ratio lower at depth, an increase of porosity with depth or a higher ratio of CO2/H2O at depth.

Astronomy and Astrophysics (In press)

DOI: 10.1051/0004-6361/201526337

Far-UV phase dependence and surface characteristics of Comet 67P/Churyumov-Gerasimenko as observed with Rosetta Alice

  • Feaga, L. M. 1
  • Protopapa, S. 1
  • Schindhelm, E. 2
  • Stern, S. A. 2
  • A'Hearn, M. F. 1
  • Bertaux, J.-L. 3
  • Feldman, P. D. 4
  • Parker, J. Wm. 2
  • Steffl, A. J. 2
  • Weaver, H. A. 5
  1. Dept of Astronomy, University of Maryland, College Park, MD
  2. Southwest Research Institute, Boulder, CO
  3. LATMOS, CNRS/UVSQ/IPSL, Guyancourt, France
  4. Dept of Physics and Astronomy, Johns Hopkins University, Baltimore, MD
  5. Johns Hopkins University Applied Physics Laboratory, Laurel, MD

Aims. The Alice far-ultraviolet (FUV) spectrograph onboard Rosetta has, for the first time, imaged the surface of a comet, 67P/Churyumov-Gerasimenko (67P), in the FUV. With spatially resolved data, the nucleus properties are characterized in the FUV, including phase dependence, albedo, and spectral slope. Regional measurements across the nucleus are compared to discern any compositional variations.

Methods. Hapke theory was utilized to model the phase dependence of the material on the surface of 67P. The phase dependence of 67P was derived from a subset of data acquired at various phase angles in November 2014, within 50 km of the comet such that the nucleus was spatially resolved. The derived photometric correction was then applied to a different subset of spatially resolved data sampling several distinct geographical regions on the nucleus acquired in August–November 2014 under similar viewing geometries.

Results. In the FUV, the surface of 67P is dark, blue sloped, has an average geometric albedo of 0.054 ± 0.008 at 1475 Å near the center of the Alice bandpass, and is mostly uniform from region to region, with the exception of the Hatmehit region, which is slightly more reflective. These results are consistent with the suggestion made by the Rosetta OSIRIS and VIRTIS teams that the surface of 67P is covered with a homogeneous layer of material and that surface ice is not ubiquitous in large abundances. The modeled Hapke parameters, specifically the single scattering albedo (w) and the asymmetry factor (ζ), are determined to be 0.031 ± 0.003 and -0.530 ± 0.025 near the center of the Alice bandpass at 1475 Å. These parameters are consistent with measurements of other comet nuclei that have been observed by flyby missions in the visible and the near-infrared regimes.

Astronomy and Astrophysics (In press)

DOI: 10.1051/0004-6361/201526671

Large-scale dust jets in the coma of 67P/Churyumov-Gerasimenko as seen by the OSIRIS instrument onboard Rosetta

  • L.M. Lara1
  • S. Lowry2
  • J.B. Vincent3
  • P.J. Gutiérrez1
  • A. Rozek2
  • F. La Forgia4
  • N. Oklay3
  • H. Sierks3
  • C. Barbieri4,5
  • P. L. Lamy6
  • R. Rodrigo7,8
  • D. Koschny9
  • H. Rickman10,16
  • H. U. Keller11
  • and 20 co-authorsNone
  1. Instituto de Astrofisica de Andalucia-CSIC, Glorieta de la Astronomia,
  2. 18008 Granada, Spain e-mail: lara@iaa.csic.es
  3. Centre for Astrophysics and Planetary Science, School of Physical
  4. Sciences, The University of Kent, Canterbury CT2 7NH, United
  5. Kingdom
  6. Max-Planck Institut fuer Sonnensystemforschung, Justus-von-
  7. Liebig-Weg, 3 37077 Goettingen, Germany
  8. Department of Physics and Astronomy G. Galilei, University of
  9. Padova, Vic. Osservatorio 3, 35122 Padova, Italy

Context. During the most recent perihelion passage in 2009 of comet 67P/Churyumov–Gerasimenko (67P), ground–based observations showed an anisotropic dust coma where jet–like features were detected at 1.3 AU from the Sun. The current perihelion passage is exceptional as the Rosetta spacecraft is monitoring the nucleus activity since March 2014, when a clear dust coma was already surrounding the nucleus at 4.3 AU from the Sun. Subsequently, the OSIRIS camera also witnessed an outburst in activity between April 27 and 30, and since mid–July, the dust coma at rh ~3.7–3.6 AU preperihelion is clearly non–isotropic, pointing to the existence of dust jet–like features.

Aims. We aim to ascertain on the nucleus surface the origin of the dust jet–like features detected as early as in mid–July 2014. This will help to establish how the localized comet nucleus activity compares with that seen in previous apparitions and will also help following its evolution as the comet approaches its perihelion, at which phase most of the jets were detected from ground–based observations. Determining these areas also allows locating them in regions on the nucleus with spectroscopic or geomorphological distinct characteristics.

Methods. Three series of dust images of comet 67P obtained with the Wide Angle Camera (WAC) of the OSIRIS instrument onboard the Rosetta spacecraft were processed with dierent enhancement techniques. This was made to clearly show the existence of jet–like features in the dust coma, whose appearance toward the observer changed as a result of the rotation of the comet nucleus and of the changing observing geometry from the spacecraft. The position angles of these features in the coma together with information on the observing geometry, nucleus shape, and rotation, allowed us to determine the most likely locations on the nucleus surface where the jets originate from.

Results. Geometrical tracing of jet sources indicates that the activity of the nucleus of 67P gave rise during July and August 2014 to large-scale jet–like features from the Hapi, Hathor, Anuket, and Aten regions, confirming that active regions may be present on the nucleus localized at 60 deg northern latitude as deduced from previous comet apparitions. There are also hints that large–scale jets observed from the ground are possibly composed, at their place of origin on the nucleus surface, of numerous small-scale features.

Astronomy and Astrophysics (Published)

DOI: 10.1051/0004-6361/201526103

ROSINA/DFMS and IES observations of 67P: Ion-neutral chemistry in the coma of a weakly outgassing comet

  • S. A. Fuselier 1,2
  • and 34 co authorsNone
  1. Space Science Division, Southwest Research Institute, San Antonio, TX, USA
  2. Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA

Context. The Rosetta encounter with comet 67P/Churyumov-Gerasimenko provides a unique opportunity for an in situ, up-close investigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the Sun.

Aims. Observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within the thin coma.

Methods. Observations from late October through mid-December 2014 show the continuous presence of the solar wind 30 km from the comet nucleus. These and other observations indicate that there is no contact surface and the solar wind has direct access to the nucleus. On several occasions during this time period, the Rosetta/ROSINA/Double Focusing Mass Spectrometer measured the low-energy ion composition in the coma. Organic volatiles and water group ions and their breakup products (masses 14 through 19), CO+, and CO+2 (masses 28 and 44) and other mass peaks (at masses 26, 27, and possibly 30) were observed. Secondary ions include H3O+ and HCO+ (masses 19 and 29). These secondary ions indicate ion-neutral chemistry in the thin coma of the comet. A relatively simple model is constructed to account for the low H3O+/H2O+ and HCO+/CO+ ratios observed in a water dominated coma. Results from this simple model are compared with results from models that include a more detailed chemical reaction network.

Results. At low outgassing rates, predictions from the simple model agree with observations and with results from more complex models that include much more chemistry. At higher outgassing rates, the ion-neutral chemistry is still limited and high HCO+/CO+ ratios are predicted and observed. However, at higher outgassing rates, the model predicts high H3O+/H2O+ ratios and the observed ratios are often low. These low ratios may be the result of the highly heterogeneous nature of the coma, where CO and CO2 number densities can exceed that of water.

Astronomy and Astrophysics (Published)

DOI: 10.1051/0004-6361/201526210

Dust Impact Monitor (SESAME-DIM) Measurements at Comet 67P/Churyumov-Gerasimenko

  • Krueger, H. 1
  • Seidensticker, K.J. 2
  • Fischer, H.-H. 3
  • Albin,T. 1,4
  • Apathy, I.5
  • Arnold, W. 6,7
  • Flandes, A. 8,1
  • Hirn, A. 5,1
  • Kobayashi, M. 9
  • Loose, A. 1
  • Peter, A. 5
  • Podolak, M. 10
  1. Max-Planck-Institut fuer Sonnensystemforschung, Goettingen, Germany
  2. Deutsches Zentrum fuer Luft- und Raumfahrt, Germany
  3. Deutsches Zentrum fuer Luft- und Raumfahrt, Koeln, Germany
  4. Carl von Ossietzky University, Oldenburg, Germany
  5. MTA Centre for Energy Research, Budapest, Hungary
  6. Saarland University, Saarbruecken, Germany
  7. Universitaet Goettingen, Goettingen, Germany
  8. Universidad Nacional Autonoma de Mexico, Mexico
  9. Chiba Institute of Technology, Japan
  10. Tel Aviv University, Israel

The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae carries the Dust Impact Monitor (DIM) on board, which is part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME). DIM employs piezoelectric PZT sensors to detect impacts by sub-millimeter and millimeter-sized ice and dust particles that are emitted from the nucleus and transported into the cometary coma. The DIM sensor measures dynamical data like flux and the directionality of the impacting particles. Mass and speed of the particles can be constrained assuming density and elastic particle properties. DIM was operated during three mission phases of Philae at the comet: (1) Before Philae's separation from Rosetta at distances of about 9.6 km, 11.8 km, and 25.3 km from the nucleus barycenter. In this mission phase particles released from the nucleus on radial trajectories remained undetectable because of significant obscuration by the structures of Rosetta, and no dust particles were indeed detected. (2) During Philae's descent to its nominal landing site Agilkia, DIM detected one approximately millimeter-sized particle at a distance of 5.0 km from the nucleus' barycenter, corresponding to an altitude of 2.4 km from the surface. This is the closest ever dust detection at a cometary nucleus by a dedicated in-situ dust detector. (3) At Philae's final landing site, Abydos, DIM detected no dust impact which may be due to low cometary activity in the vicinity of Philae, or due to shading by obstacles close to Philae, or both. Laboratory calibration experiments showed that the material properties of the detected particle are compatible with a porous particle having a bulk density of approximately 250 kg m^-3. The particle could have been lifted off the comet's surface by sublimating water ice.

Astronomy and Astrophysics (In press)

arXiv: 1510.01563

The positive-polarization of cometary comae

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

We examine the dispersion of the degree of linear polarization P in comets at phase angle ~90° where the maximum amplitude of positive polarization P_max occurs. The range of polarization observed in comets is from 7% up to more than 30%, and this cannot be explained through depolarization by gaseous emissions. Instead, we suggest that the observed dispersion of P results from different properties in cometary dust. We simulate the spectral polarimetric observations of comets using model agglomerated debris particles. The vast majority of observations can be reproduced with a mixture of weakly absorbing and highly absorbing agglomerated debris particles, which obey the same power-law size distribution. Within this extremely simple approach, polarization at side-scattering angles in a given comet is governed by the relative abundance of weakly and strongly absorbing particles. We find that in comets with the highest polarization, the weakly absorbing particles appear in proportions of only 14–23% by volume; whereas, in comets with the lowest polarization P_max, their abundance is much greater, 82–95%. We conclude that the polarization at side-scattering angles unambiguously measures the relative abundance of Mg-rich silicates and refractory organics or amorphous carbon in comets. We put forth a hypothesis that low P_max could be an indicator for presence of a well-developed refractory surface layer covering cometary nucleus.

Planetary and Space Science (In press)

DOI: 10.1016/j.pss.2015.09.020

Inventory of the volatiles on comet 67P/Churyumov-Gerasimenko from Rosetta/ROSINA

  • Le Roy, L.1
  • Altwegg, K. 1, 2
  • Balsiger, H. 2
  • Berthelier, J.-J. 3
  • Bieler, A. 4
  • Briois, C. 5
  • Calmonte, U. 2
  • Combi, M. R. 4
  • De Keyser, J. 6
  • Dhooghe, F. 6
  • Fiethe, B. 7
  • Fuselier, S. A. 8, 9
  • Gasc, S. 2
  • Gombosi, T. I. 4
  • Hässig, M. 8, 2
  • Jäckel, A. 2
  • Rubin, M. 2
  • Tzou, C.-Y. 2
  1. Center for Space and Habitability, University of Bern, Switzerland
  2. Physikalisches Institut, University of Bern, Switzerland
  3. LATMOS/IPSL-CNRS-UPMC-UVSQ, France
  4. Department of Atmospheric, Oceanic and Space Science, University of Michigan, USA
  5. Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, France
  6. BIRA-IASB, Belgian Institute for Space Aeronomy, Belgium
  7. Institute of Computer and Network Engineering (IDA), TU Braunschweig, Germany
  8. Space Science Division, Southwest Research Institute, USA
  9. Department of Physics and Astronomy, University of Texas at San Antonio, USA

Context. The ESA Rosetta spacecraft (S/C) is tracking comet 67P/Churyumov-Gerasimenko in close vicinity. This prolonged encounter enables studying the evolution of the volatile coma composition.

Aims. Our work aims at comparing the diversity of the coma of 67P/Churyumov-Gerasimenko at large heliocentric distance to study the evolution of the comet during its passage around the Sun and at trying to classify it relative to other comets.

Methods. We used the Double Focussing Mass Spectrometer (DFMS) of the ROSINA experiment on ESA's Rosetta mission to determine relative abundances of major and minor volatile species. This study is restricted to species that have previously been detected elsewhere.

Results. We detect almost all species currently known to be present in cometary coma with ROSINA\ DFMS. As DFMS measured the composition locally, we cannot derive a global abundance, but we compare measurements from the summer and the winter hemisphere with known abundances from other comets. Differences between relative abundances between summer and winter hemispheres are large, which points to a possible evolution of the cometary surface. This comet appears to be very rich in CO2 and ethane. Heavy oxygenated compounds such as ethylene glycol are underabundant at 3 AU, probably due to their high sublimation temperatures, but nevertheless, their presence proves that Kuiper belt comets also contain complex organic molecules.

Astronomy and Astrophysics (In press)

Comet C/2012 S1 (ISON) coma composition at ~4 au from HST observations

  • Zubko, E. 1
  • Videen, G. 2,3
  • Hines, D. C. 3, 4
  • Shkuratov, Yu. 1
  • Kaydash, V. 1
  • Muinonen, K. 5,6
  • Knight, M. M. 7
  • Sitko, M. L. 8
  • Lisse, C. M. 9
  • Mutchler, M. 4
  • Wooden, D. H. 10
  • Li, J.-Y. 11
  • Kobayashi, H. 12
  1. Institute of Astronomy, V.N. Karazin Kharkov National University, Ukraine
  2. Space Science Institute, USA
  3. U.S. Army Research Laboratory, USA
  4. Space Telescope Science Institute, USA
  5. Department of Physics,University of Helsinki, Finland
  6. Finnish Geospatial Research Institute, Finland
  7. Lowell Observatory, USA
  8. Johns Hopkins University, Applied Physics Laboratory, USA
  9. University of Cincinnati, USA
  10. NASA Ames Research Center, USA
  11. Planetary Science Institute, USA
  12. Nagoya University, Japan

We analyze the first color and polarization images of Comet ISON (C/2012 S1) taken during two measurement campaigns of the Hubble Space Telescope (HST) on UTC 2013 April 10 and May 8, when the phase angles of Comet ISON were α≈13.7° and 12.2°, respectively. We model the particles in the coma using highly irregular agglomerated debris particles. Even though the observations were made over a small range of phase angle, the data still place significant constraints on the material properties of the cometary coma. The different photo-polarimetric responses are indicative of spatial chemical heterogeneity of coma in Comet ISON. For instance, at small projected distances to the nucleus (<500 km), our modeling suggests the cometary particles are composed predominantly of small, highly absorbing particles, such as amorphous carbon and/or organics material heavily irradiated with UV radiation; whereas, at longer projected distances (>1000 km), the refractive index of the particles is consistent with organic matter slightly processed with UV radiation, tholins, Mg−Fe silicates, and/or Mg-rich silicates contaminated with ~10% (by volume) amorphous carbon. The modeling suggests low relative abundances of particles with low material absorption in the visible, i.e., Im(m)≤0.02. Such particles were detected unambiguously in other comets in the vicinity of nucleus through very strong negative polarization near backscattering (P≈−6%) and very low positive polarization (P≈3–5%) at side scattering. These materials were previously attributed to Mg-rich silicates forming a refractory surface layer on the surface of cometary nuclei ( Zubko et al., 2012). The absence of such particles in Comet ISON could imply an absence of such a layer on its nucleus.

Planetary and Space Science (In press)

DOI: 10.1016/j.pss.2015.08.002

GIADA: shining a light on the monitoring of the comet dust production from the nucleus of 67P/Churyumov-Gerasimenko

  • V. Della Corte1
  • A. Rotundi1,2
  • M. Fulle3
  • ,E. Gruen4
  • ,P. Weissman5
  • R. Sordini1
  • M. Ferrari1
  • S. Ivanovski1
  • F. Lucarelli2
  • M. Accolla6
  • V. Zakharov7
  • E. Mazzotta Epifani8,9
  • J. J. Lopez-Moreno10
  • J. Rodriguez10
  • L. Colangeli11
  • P. Palumbo2,1
  • E. Bussoletti2
  • J. F. Crifo12
  • F. Esposito8
  • S. F. Green13
  • P. L. Lamy14
  • J. A. M. McDonnell13,15,16
  • V. Mennella8
  • A. Molina17
  • R. Morales10
  • F. Moreno10
  • J. L. Ortiz10
  • E. Palomba1
  • J. M. Perrin12,18
  • F. J. M. Rietmeijer19
  • R. Rodrigo20,21
  • J. C. Zarnecki21
  • M. Cosi22
  • F. Giovane23
  • B. Gustafson24
  • M. L. Herranz10
  • J. M. Jeronimo10
  • M. R. Leese13
  • A. C. Lopez-Jimenez10
  • N. Altobelli25
  1. IAPS, INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy
  2. Università degli Studi di Napoli “Parthenope”,
  3. Osservatorio Astronomico di Trieste, INAF,
  4. Max-Planck-Institut fuer Kernphysik,
  5. Planetary Science Section, JPL
  6. Osservatorio Astronomico di Catania, INAF,
  7. LESIA, Obs. de Paris, CNRS,
  8. Osservatorio Astronomico di Capodimonte, INAF
  9. Osservatorio Astronomico di Roma, INAF,
  10. IAA (CSIC),
  11. ESA, (ESTEC),
  12. Laboratoire Atmosphères, Milieux, Observations Spatiales, CNRS/
  13. Planetary and Space Sciences, Department of Physical Sciences,
  14. Laboratoire d’Astrophysique de Marseille,
  15. The University of Ken
  16. UnispaceKent,
  17. Departamento de Fisica Aplicada, Universidad de Granada
  18. Observatoire de Haute Provence OSU Pythéas
  19. Dept. of Earth and Planetary Science,Universityof New Mexico
  20. INTA-CSIC
  21. ISSI
  22. Selex-ES
  23. Virginia Polytechnic Institute
  24. University of Florida
  25. ESA-ESAC

Context. During the period between 15 September 2014 and 4 February 2015, the Rosetta spacecraft accomplished the circular orbit phase around the nucleus of comet 67P/Churyumov-Gerasimenko (67P). The Grain Impact Analyzer and Dust Accumulator (GIADA) onboard Rosetta monitored the 67P coma dust environment for the entire period.

Aims. We aim to describe the dust spatial distribution in the coma of comet 67P by means of in situ measurements. We determine dynamical and physical properties of cometary dust particles to support the study of the production process and dust environment modification.

Methods. We analyzed GIADA data with respect to the observation geometry and heliocentric distance to describe the coma dust spatial distribution of 67P, to monitor its activity, and to retrieve information on active areas present on its nucleus. We combined GIADA detection information with calibration activity to distinguish different types of particles that populate the coma of 67P: compact particles and fluffy porous aggregates. By means of particle dynamical parameters measured by GIADA, we studied the dust acceleration region.

Results. GIADA was able to distinguish different types of particles populating the coma of 67P: compact particles and fluffy porous aggregates. Most of the compact particle detections occurred at latitudes and longitudes where the spacecraft was in view of the comet’s neck region of the nucleus, the so-called Hapi region. This resulted in an oscillation of the compact particle abundance with respect to the spacecraft position and a global increase as the comet moved from 3.36 to 2.43 AU heliocentric distance. The speed of these particles, having masses from 10-10 to 10-7 kg, ranged from 0.3 to 12.2 m s-1. The variation of particle mass and speed distribution with respect to the distance from the nucleus gave indications of the dust acceleration region. The influence of solar radiation pressure on micron and submicron particles was studied. The integrated dust mass flux collected from the Sun direction, that is, particles reflected by solar radiation pressure, was three times higher than the flux coming directly from the comet nucleus. The awakening 67P comet shows a strong dust flux anisotropy, confirming what was suggested by on-ground dust coma observations performed in 2008.

Astronomy and Astrophysics (Published)

Optical and Near-Infrared Polarimetry for a Highly Dormant Comet 209P/LINEAR

  • Kuroda, D.1
  • Ishiguro, M.2
  • Watanabe, M.3
  • Akitaya, H.4
  • Takahashi, J.5
  • Hasegawa, S.6
  • Ui, T.4
  • Kanda, Y.4
  • Takaki, K.4
  • Itoh, R.4
  • Moritani, Y.4,7
  • Imai, M.3
  • Goda, S.3
  • Takagi, Y.5
  • Morihana, K.5
  • Honda, S.5
  • Arai, A.5,8
  • Hanayama, H.1
  • Nagayama, T.9
  • Nogami, D.10
  • Sarugaku, Y.7
  • Murata, K.11
  • Morokuma, T.7
  • Saito, Y.12
  • Oasa, Y.13
  • Sekiguchi, K.1
  • and Watanabe, J.1
  1. National Astronomical Observatory of Japan, Japan
  2. Seoul National University, Korea
  3. Hokkaido University, Japan
  4. Hiroshima University, Japan
  5. University of Hyogo, Japan
  6. Japan Aerospace Exploration Agency, Japan
  7. The University of Tokyo, Japan
  8. Kyoto Sangyo University, Japan
  9. Kagoshima University, Japan
  10. Kyoto University, Japan
  11. Nagoya University, Japan
  12. Tokyo Institute of Technology, Japan
  13. Saitama University, Japan

We conducted an optical and near-infrared polarimetric observation of the highly dormant Jupiter-Family Comet, 209P/LINEAR. Because of its low activity, we were able to determine the linear polarization degrees of the coma dust particles and nucleus independently, that is Pn=30.3+1.3-0.9% at α=92.2° and Pn=31.0+1.0-0.7% at α=99.5° for the nucleus, and Pc=28.8+0.4-0.4% at α=92.2° and Pc=29.6+0.3-0.3% at α=99.5° for the coma. We detected no significant variation in P at the phase angle coverage of 92.2°–99.5°, which may imply that the obtained polarization degrees are nearly at maximum in the phase-polarization curves. By fitting with an empirical function, we obtained the maximum values of linear polarization degrees Pmax=30.8% for the nucleus and Pmax=29.6% for the dust coma. The Pmax of the dust coma is consistent with those of dust-rich comets. The low geometric albedo of Pv=0.05 was derived from the slope-albedo relationship and was associated with high Pmax. We examined Pmax-albedo relations between asteroids and 209P, and found that the so-called Umov law seems to be applicable on this cometary surface.

The Astrophysical Journal (In press)

arXiv: 1510.05122

  • David Jewitt 1
  1. Department of Earth, Planetary and Space Sciences, UCLA

Most comets are volatile-rich bodies that have recently entered the inner solar system following long-term storage in the Kuiper belt and the Oort cloud reservoirs. These reservoirs feed several distinct, short-lived 'small body' populations. Here, we present new measurements of the optical colors of cometary and comet-related bodies including long-period (Oort cloud) comets, Damocloids (probable inactive nuclei of long-period comets) and Centaurs (recent escapees from the Kuiper belt and precursors to the Jupiter family comets). We combine the new measurements with published data on short-period comets, Jovian Trojans and Kuiper belt objects to examine the color systematics of the comet-related populations. We find that the mean optical colors of the dust in short-period and long-period comets are identical within the uncertainties of measurement, as are the colors of the dust and of the underlying nuclei. These populations show no evidence for scattering by optically-small particles or for compositional gradients, even at the largest distances from the Sun, and no evidence for ultrared matter. Consistent with earlier work, ultrared surfaces are common in the Kuiper belt and on the Centaurs, but not in other small body populations, suggesting that this material is hidden or destroyed upon entry to the inner solar system. The onset of activity in the Centaurs and the disappearance of the ultrared matter in this population begin at about the same perihelion distance (about 10 AU), suggesting that the two are related. Blanketing of primordial surface materials by the fallback of sub-orbital ejecta, for which we calculate a very short timescale, is the likely mechanism. The same process should operate on any mass-losing body, explaining the absence of ultrared surface material in the entire comet population.

The Astronomical Journal (In press)

arXiv: 1510.07069

Permittivity measurements of porous matter in support of investigations of the surface and interior of 67P/Churyumov-Gerasimenko

  • Y. Brouet 1
  • A. C. Levasseur-Regourd 2
  • P. Sabouroux 3
  • P. Encrenaz 4
  • N. Thomas 1
  • E. Heggy 5
  • and W. Kofman 6
  1. University of Bern, Physics Institute, Sidlerstrasse 5, 3012 Bern, Switzerland
  2. UPMC (Sorbonne Univ.); UVSQ (UPSay); CNRS/INSU; LATMOS-IPSL, BC 102, 4 place Jussieu, 75005 Paris, France
  3. Aix–Marseille Université, CNRS, Centrale Marseille, Institut Fresnel, UMR 7249, Campus universitaire de Saint-Jérôme,
  4. avenue Escadrille-Normandie-Niemen, 13013 Marseille, France
  5. Observatoire de Paris, LERMA, 61 avenue de l’Observatoire, 75014 Paris, France
  6. University of Southern California, Viterbi School of Engineering, 1042 Downey Way, Los Angeles, CA 90089-1112, USA
  7. UJF-Grenoble 1/CNRS-INSU, IPAG, UMR 5274, 38041 Grenoble, France

Permittivity measurements on porous samples of volcanic origin have been performed in the 0.05–190 GHz range under laboratory conditions in support of the Rosetta mission to comet 67P/Churyumov–Gerasimenko, specifically with the MIRO radiometric experiment and CONSERT radar experiment.

The samples were split into several subsamples with different size ranges covering a few μm to 500 μm. Bulk densities of the subsamples were estimated to be in the 800 to 1500 kg/m3 range. The porosities were in the range of 48% to 65%. From 50 MHz to 6 GHz and at 190 GHz, permittivity has been determined with a coaxial cell and with a quasi-optical bench, respectively.

Without taking into account the volume-scattering effect at 190 GHz, the real part of the permittivity, normalized by the bulk density, is in the range of 2.1 to 2.6. The results suggest that the real part of the permittivity of an ice-free dust mantle covering the nucleus is in the 1.5−2.2 range at 190 GHz. From these values, a lower limit for the absorption length for the millimeter receiver of MIRO has been estimated to be between 0.6 and 2 cm, in agreement with results obtained from MIRO in September 2014. At frequencies of interest for CONSERT experiment, the real part of the permittivity of a suspected ice-free dust mantle should be below 2.2. It may be in the range of 1.2 to 1.7 for the nucleus, in agreement with first CONSERT results, taking into account a mean temperature of 110 K and different values for the dust-to-ice volumetric ratio. Estimations of contributions of the different parameters to the permittivity variation may indicate that the porosity is the main parameter.

Astronomy and Astrophysics (In press)

DOI: 10.1051/0004-6361/201526099