Cometary Science Newsletter

July 2015
Michael S. P. Kelley (

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.

A Martian Meteor Shower from Comet C/2013 A1 (Siding Spring)

In October 2014, comet C/2013 A1 (Siding Spring) passed Mars with a minimum distance near 140,000 km. The encounter was the subject of an interplanetary observing campaign, with 30 separate observation plans submitted, many of which were successful. Perhaps most important were the observations from Mars-orbiting spacecraft. They were afforded an exceptional opportunity to observe this event using contemporary instrumentation. The first results of those observations, from MAVEN, Mars Express, and Mars Reconnaissance Orbiter, concern the aftermath of the resulting meteor shower. They have been published in Geophysical Research Letters and abstracts are included in this month's CSN issue (Schneider et al. 2015, Gurnett et al. 2015, Benna et al. 2015, Restano et al. 2015). In addition, Pasquale Tricarico wrote an overview of the encounter and the four articles, including some lessons learned (Tricarico et al. 2015).

- Mike Kelley

The Closest Approach of Comet 252P/LINEAR 12 to Earth

Comet 252P/LINEAR 12 will make an historic close approach to Earth. Passing with a minimum distance of 0.0357 AU (5.3 million km, with an uncertainty of order 10,000 km; JPL HORIZONS) on 2016 March 21, we will have an opportunity to study the nucleus and inner coma of what is either a very small or a weakly-active comet. Such close approaches are infrequent, with perhaps only a dozen known beforehand over the past few decades.

The encounter is best observed in the Southern Hemisphere, with a solar elongation of 90° and declinations near -80° to -90° on the night of closest approach. We estimate the nucleus of 252P has a radius upper-limit of ~0.9 km, and a peak Afρ (a proxy for dust production) of order magnitude 2 cm. The comet is available to the Northern Hemisphere before and after closest approach, with the most favorable epoch starting about a week after perigee (Dec>-25°, solar elongation >100°, Δ>0.05 AU).

The Hubble Space Telescope has a program to characterize the comet during closest approach with broadband imaging distributed over 6 orbits (PI Jian-Yang Li). With 40 mas pixels, the WFC3/UVIS camera will have a projected linear scale of 1 km/pixel. Contact Jian-Yang Li ( if you are interested in coordinating observations or assisting with the recovery effort.

- Mike Kelley and Jian-Yang Li

Refereed Articles

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

Prompt Emission by OH in Comet Hyakutake

  • Michael F. A'Hearn 1
  • K. S. Krishna Swamy 2,4
  • Dennis D. Wellnitz 1,5
  • Roland Meier 3,5,6
  1. Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
  2. Tata Institute of Fundamental Research (Ret'd), Homi Bhabha Road, Colaba, Mumbai, 400005 India
  3. Varian Medical Systems Imaging Laboratory GmbH, Täfernstraße 7, CH5405 Baden-Dättwil, Switzerland
  4. Visiting Scientist, University of Maryland.
  5. Visiting Astronomer, Kitt Peak National Observatory. KPNO is operated by AURA, Inc. under contract to the National Science Foundation.
  6. At University of Maryland at the time of the observations.

Laboratory studies predict that the photo-dissociation of H2O by solar Lya photons in the comae of comets should lead to a small percentage of OH in high rotational states of the A 2Sigma+ electronic state. These states should promptly emit a near-ultraviolet (near-UV) photon in a transition to the X 2Pi state. From there, the radicals decay to the lowest rotational states by direct rotational transitions and via ro-vibrational cascade in the 1-0 vibrational band, all within the X 2Pi state. Normally in Earth-based observations the lines are extremely weak compared to the fluorescence of OH in sunlight. Since the prompt emission rate is directly proportional to the column density of water, whereas the fluorescent emission of OH is proportional to the column density of OH, the lines due to prompt emission are relatively strongest very close to the nucleus, a region not often accessible from Earth. We report here the first spectrally resolved detection of near-UV prompt emission by cometary OH, as seen in the bright comet Hyakutake (C/1996 B2), on the day of its closest approach to Earth. All the expected doublets are seen, in both the P and Q branches, up to rotational quantum number N′ = 22 with relative strengths in good agreement with the laboratory results. The fluxes of the lines are in agreement with what one expects from the various measurements of the water production rate of the comet, including that deduced from the OH fluorescent lines in these spectra.

The Astronomical Journal (In press)

DOI: 10.1088/0004-6256/150/1/5

Measurements of the Near-Nucleus Coma of Comet 67P/Churyumov-Gerasimenko with the Alice Far-Ultraviolet Spectrograph on Rosetta

  • Paul D. Feldman 1
  • Michael F. A'Hearn 2
  • Jean-Loup Bertaux 3
  • Lori M. Feaga 2
  • Joel Wm. Parker 4
  • Eric Schindhelm 4
  • Andrew J. Steffl 4
  • S. Alan Stern 4
  • Harold A. Weaver 5
  • Holger Sierks 6
  • Jean-Baptiste Vincent 6
  1. JHU, USA
  2. UMd, USA
  3. LATMOS, France
  4. SwRI, USA
  6. MPIS, Germany

Aims. The Alice far-ultraviolet spectrograph onboard Rosetta is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/Churyumov-Gerasimenko and to determine their spatial distribution and evolution with time and heliocentric distance.

Methods. Following orbit insertion in August 2014, Alice made observations of the inner coma above the limbs of the nucleus of the comet from cometocentric distances varying between 10 and 80 km. Depending on the position and orientation of the slit relative to the nucleus, emissions of atomic hydrogen and oxygen were initially detected. These emissions are spatially localized close to the nucleus and spatially variable with a strong enhancement above the comet's neck at northern latitudes. Weaker emission from atomic carbon and CO were subsequently detected.

Results. Analysis of the relative line intensities suggests photoelectron impact dissociation of H2O vapor as the source of the observed H I and O I emissions. The electrons are produced by photoionization of H2O. The observed C I emissions are also attributed to electron impact dissociation, of CO2, and their relative brightness to H I reflects the variation of CO2 to H2O column abundance in the coma.

Astronomy & Astrophysics (In press)

DOI: 10.1051/0004-6361/201525925 arXiv: 1506.01203

Comet 67P/Churyumov-Gerasimenko: Constraints on Its Origin from OSIRIS Observations

  • Rickman, H. 1
  • Marchi, S. 2
  • and 39 co-authors
  1. P.A.S. Space Research Center, Warsaw, Poland and Dept. of Physics & Astronomy, Uppsala Univ., Sweden
  2. Southwest Res. Inst., Boulder CO, USA

Context. One of the main aims of the ESA Rosetta mission is to study the origin of the solar system by exploring comet 67P/Churyumov-Gerasimenko at close range.

Aims. In this paper we discuss the origin and evolution of comet 67P/Churyumov-Gerasimenko in relation to that of comets in general and in the framework of current solar system formation models.

Methods. We use data from the OSIRIS scientific cameras as basic constraints. In particular, we discuss the overall bi-lobate shape and the presence of key geological features, such as layers and fractures. We also treat the problem of collisional evolution of comet nuclei by a particle-in-a-box calculation for an estimate of the probability of survival for 67P/Churyumov-Gerasimenko during the early epochs of the solar system.

Results. We argue that the two lobes of the 67P/Churyumov-Gerasimenko nucleus are derived from two distinct objects that have formed a contact binary via a gentle merger. The lobes are separate bodies, though sufficiently similar to have formed in the same environment. An estimate of the collisional rate in the primordial, trans-planetary disk shows that most comets of similar size to 67P/Churyumov-Gerasimenko are likely collisional fragments, although survival of primordial planetesimals cannot be excluded.

Conclusions. A collisional origin of the contact binary is suggested, and the low bulk density of the aggregate and abundance of volatile species show that a very gentle merger must have occurred. We thus consider two main scenarios: the primordial accretion of planetesimals, and the re-accretion of fragments after an energetic impact onto a larger parent body. We point to the primordial signatures exhibited by 67P/Churyumov-Gerasimenko and other comet nuclei as critical tests of the collisional evolution.

Astronomy & Astrophysics (In press)

arXiv: 1505.07021

Spectrophotometric Properties of the Nucleus of Comet 67P/Churyumov-Gerasimenko from the OSIRIS Instrument Onboard the ROSETTA Spacecraft

  • Fornasier, S. 1,2
  • Hasselmann P.H. 1
  • Barucci, M.A. 1
  • Feller, C. 1,2
  • Besse, S. 4
  • and 49 co-authors
  1. LESIA, Observatoire de Paris, CNRS, UPMC Univ Paris 06, Univ. Paris-Diderot, 5 Place J. Janssen, 92195 Meudon Pricipal Cedex, France
  2. Univ Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris Cedex 13, France
  3. Observatorio Nacional, General Jose' Cristino 77, Sao Cristovao, Rio de Janeiro, Brazil
  4. Research and Scientific Support Department, European Space Agency, 2201 Noordwijk, The Netherlands

The Rosetta mission of the European Space Agency has been orbiting the comet 67P/Churyumov-Gerasimenko (67P) since August 2014 and is now in its escort phase. A large complement of scientific experiments designed to complete the most detailed study of a comet ever attempted are onboard Rosetta. We present results for the photometric and spectrophotometric properties of the nucleus of 67P derived from the OSIRIS imaging system, which consists of a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). The observations presented here were performed during July and the beginning of August 2014, during the approach phase, when OSIRIS was mapping the surface of the comet with several filters at different phase angles (1.3—54 deg.). The resolution reached up to 2.1 m/px. The OSIRIS images were processed with the OSIRIS standard pipeline, then converted into I/F radiance factors and corrected for the illumination conditions at each pixel using the Lommel-Seeliger disk law. Color cubes of the surface were produced by stacking registered and illumination-corrected images. Furthermore, photometric analysis was performed both on disk-averaged photometry in several filters and on disk-resolved images acquired with the NAC orange filter, centered at 649 nm, using Hapke modeling. The disk-averaged phase function of the nucleus of 67P shows a strong opposition surge with a G parameter value of -0.13±0.01 in the HG system formalism and an absolute magnitude H_v(1,1,0) = 15.74±0.02 mag. The integrated spectrophotometry in 20 filters covering the 250-1000 nm wavelength range shows a red spectral behavior, without clear absorption bands except for a potential absorption centered at ~290 nm that is possibly due to SO2 ice. The nucleus shows strong phase reddening, with disk-averaged spectral slopes increasing from 11%/(100 nm) to 16%/(100 nm) in the 1.3-54 deg. phase angle range. The geometric albedo of the comet is 6.5±0.2% at 649 nm, with local variations of up to ~16% in the Hapi region. From the disk-resolved images we computed the spectral slope together with local spectrophotometry and identified three distinct groups of regions (blue, moderately red, and red). The Hapi region is the brightest, the bluest in term of spectral slope, and the most active surface on the comet. Local spectrophotometry shows an enhancement of the flux in the 700-750 nm that is associated with coma emissions.

Astronomy & Astrophysics (In press)

DOI: 10.1051/0004-6361/201525901 arXiv: 1505.06888

A New Analysis of Spitzer Observations of Comet 29P/Schwassmann-Wachmann 1

  • Schambeau, C. A. 1
  • Fernandez, Y. R. 1
  • Lisse, C. M. 2
  • Samarasinha, N. 3
  • Woodney, L. M. 4
  1. Department of Physics, University of Central Florida, Orlando, FL
  2. Johns Hopkins University Applied Physics Laboratory, Laurel, MD
  3. Planetary Sciences Institute, Tucson, AZ
  4. Department of Physics, California State University San Bernardino, San Bernardino, CA

We present a new analysis of Spitzer observations of comet 29P/Schwassmann-Wachmann 1 taken on UT 2003 November 21, 23, and 24, similar to a previous investigation of the observations (Stansberry et al. 2004), but using the most recent Spitzer data pipeline products and intensive image processing techniques. Analysis of images from the IRAC 5.8 & 8.0 μm bands and the MIPS 24.0 & 70.0 μm bands resulted in photometry measurements of the nucleus after a suite of coma modeling and removal processes were implemented. SW1 was not identified in the 5.8 μm image from the previous work so its incorporation into this analysis is entirely new. Using the Near Earth Asteroid Thermal Model (Harris 1998) resulted in a nucleus radius measurement of R = 30.2+3.7-2.9 km and an infrared beaming parameter value of η = 0.99 +0.26. We also measured an infrared geometric albedo, p5.8 = 0.5 ± 0.5. Extrapolating a 0.04 V-band albedo and using a normalized reflectivity gradient S′ = 14.94 ± 1.09 [% (1000 Å)−1] (Duffard et al. 2014) we recover an infrared albedo of p5.8 = 0.31 in the near infrared consistent with the value recovered from thermal modeling. The dust composition extracted from IRS spectra are very comet-like, containing mainly amorphous ferromagnesian silicates (but with a minority of crystalline silicates as well), water ice, and metal sulfides.

Icarus (In press)

arXiv: 1506.07037

MAVEN IUVS Observations of the Aftermath of the Comet Siding Spring Meteor Shower on Mars

  • Nicholas M. Schneider 1
  • and 16 co-authors
  1. LASP, U. Colorado

We report the detection of intense emission from magnesium and iron in Mars' atmosphere caused by a meteor shower following Comet Siding Spring's close encounter with Mars. The observations were made with the Imaging Ultraviolet Spectrograph, a remote sensing instrument on the Mars Atmosphere and Volatile EvolutioN spacecraft orbiting Mars. Ionized magnesium caused the brightest emission from the planet's atmosphere for many hours, resulting from resonant scattering of solar ultraviolet light. Modeling suggests a substantial fluence of low-density dust particles 1–100 µm in size, with the large amount and small size contrary to predictions. The event created a temporary planet-wide ionospheric layer below Mars' main dayside ionosphere. The dramatic meteor shower response at Mars is starkly different from the case at Earth, where a steady state metal layer is always observable but perturbations caused by even the strongest meteor showers are challenging to detect.

Geophysical Research Letters (Published)

DOI: 10.1002/2015GL063863

An Ionized Layer in the Upper Atmosphere of Mars Caused by Dust Impacts from Comet Siding Spring

  • Gurnett, D. A. 1
  • Morgan, D. D. 1
  • Persoon, A. M. 1
  • Granroth, L. J. 1
  • Kopf, A. J. 1
  • Plaut, J. J. 2
  • Green, J. L.3
  1. Dept. of Physics and Astronomy, University of Iowa, Iowa City, IA 52242, USA
  2. Jet Propulsion Laboratory, Pasadena, CA 91109, USA
  3. NASA Headquarters, Washington, DC 20546, USA

We report the detection of a dense ionized layer in the upper atmosphere of Mars caused by the impact of dust from comet Siding Spring. The observations were made by the ionospheric radar sounder on the Mars Express spacecraft during two low-altitude passes approximately seven and fourteen hours after closest approach of the comet to Mars. During these passes an unusual transient layer of ionization was detected at altitudes of about 80 to 100 km with peak electron densities of (1.5 to 2.5) x 105 cm-3, much higher than normally observed in the Martian ionosphere. From comparisons to previously observed ionization produced by meteors at Earth and Mars, we conclude that the layer was produced by dust from the comet impacting and ionizing the upper atmosphere of Mars.

Geophysical Research Letters (Published)

DOI: 10.1002/2015GL063726

Metallic Ions in the Upper Atmosphere of Mars from the Passage of Comet C/2013 A1 (Siding Spring)

  • Benna M. 1,2
  • Mahaffy, P. R. 1
  • Grebowsky, J. M. 1
  • Plane, J. M. C. 4
  • Yelle, R. V. 3
  • Jakosky, B. M. 5
  1. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
  2. CSST, University of Maryland Baltimore County, Baltimore, Maryland, USA
  3. Department of Planetary Sciences, University of Arizona, Tucson, Arizona, USA
  4. Faculty of Mathematics and Physical Sciences, University of Leeds, Leeds, UK
  5. Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA

We report the first in situ detection of metal ions in the upper atmosphere of Mars resulting from the ablation of dust particles from comet Siding Spring. This detection was carried out by the Neutral Gas and Ion Mass Spectrometer on board the Mars Atmosphere and Volatile Evolution Mission. Metal ions of Na, Mg, Al, K, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn, and possibly of Si, and Ca, were identified in the ion spectra collected at altitudes of ~185 km. The measurements revealed that Na+ was the most abundant species, and that the remaining metals were depleted with respect to the CI abundance of Na+. The temporal profile and abundance ratios of these metal ions suggest that the combined effects of dust composition, partial ablation, differential upward transport, and differences in the rates of formation and removal of these metal ions are responsible for the observed depletion.

Geophysical Research Letters (Published)

DOI: 10.1002/2015GL064159

Effects of the Passage of Comet C/2013 A1 (Siding Spring) Observed by the Shallow Radar (SHARAD) on Mars Reconnaissance Orbiter

  • Marco Restano 1
  • Jeffrey J. Plaut 2
  • Bruce A. Campbell 3
  • Yonggyu Gim 2
  • Daniel Nunes 2
  • Fabrizio Bernardini 1
  • Anthony Egan 4
  • Roberto Seu 1
  • Roger J. Phillips 5
  1. Dipartimento DIET, Università di Roma "La Sapienza", Rome, Italy
  2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
  3. Smithsonian Institution, Washington, District of Columbia, USA
  4. Space Operations Department, Southwest Research Institute, Boulder, Colorado, USA
  5. Planetary Science Directorate, Southwest Research Institute, Boulder, Colorado, USA

The close passage of Comet C/2013 A1 (Siding Spring) to Mars provided a unique opportunity to observe the interaction of cometary materials with the Martian ionosphere and atmosphere using the sounding radar SHARAD (SHAllow RADar) aboard Mars Reconnaissance Orbiter. In two nightside observations, acquired in the 10 h following the closest approach, the SHARAD data reveal a significant increase of the total electron content (TEC). The observed TEC values are typical for daylight hours just after dawn or before sunset but are unprecedented this deep into the night. Results support two predictions indicating that cometary pickup O+ ions, or ions generated from the ablation of cometary dust, are responsible for the creation of an additional ion layer.

Geophysical Research Letters (Published)

DOI: 10.1002/2015GL064150

High-Resolution Spectra of Comet C/2013 R1 (Lovejoy)

  • P. Rousselot 1
  • A. Decock 2
  • P. P. Korsun 3
  • E. Jehin 4
  • I. Kulyk 3
  • J. Manfroid 4
  • D. Hutsemékers 4
  1. University of Franche-Comté, Observatoire des Sciences de l’Univers THETA, Institut UTINAM - UMR CNRS 6213, BP 1615, 25010 Besançon Cedex,France
  2. LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
  3. Main Astronomical Observatory of NAS of Ukraine, 27 Akademika Zabolotnoho Street, 03680 Kyiv, Ukraine
  4. Département d’Astrophysique, de Géophysique et d’Océanographie, Université de Liège, Allée du Six Août, B-4000 Liège, Belgium

Context. High-resolution spectra of comets permit deriving the physical properties of the coma. In the optical range, relative production rates can be computed, and information about isotopic ratios and the origin of oxygen atoms can be obtained.

Aims. The main objective of the work presented here was to obtain information about the chemical composition of comet C/2013 R1 (Lovejoy), a bright and long-period comet that passed perihelion (0.81 au) on 22 December 2013.

Methods. We used the HARPS-North echelle spectrograph at the 3.5 m telescope TNG to obtain high-resolution spectra of comet C/2013 R1 (Lovejoy) in the optical range immediately after its perihelion passage during four consecutive nights in the period December 23 to 26, 2013.

Results. Our results demonstrate the ability of HARPS-North to efficiently obtain cometary spectra. Very faint emission lines, such as those of 15NH2, have been detected, leading to a rough estimate of the 14N/15N ratio in NH2. The 12C/13C ratio was measured in the C2 lines and is equal to 80±30. The oxygen lines were studied as well (green to red line intensity ratios and widths), confirming that H2O is the main parent molecule that photodissociates to produce oxygen atoms. This suggests that this comet has a high CO2 abundance. Relative production rates for C2 and NH2 were computed, but we found no significant deviation from a typical NH2/C2 ratio.

Astronomy & Astrophysics (In press)

Sublimation-Induced Orbital Perturbations of Extrasolar Active Asteroids and Comets: Application to White Dwarf Systems

  • Dimitri Veras 1
  • Siegfried Eggl 2
  • Boris T. Gänsicke 1
  1. Department of Physics, University of Warwick, Coventry CV4 7AL, UK
  2. IMCCE Observatroire de Paris, Univ. Lille 1, UPMC, 77 Av. Denfert-Rochereau, 75014 Paris, France

The metal budgets in some white dwarf (WD) atmospheres reveal that volatile-rich circumstellar bodies must both exist in extrasolar systems and survive the giant branch phases of stellar evolution. The resulting behaviour of these active asteroids or comets which orbit WDs is not well-understood, but may be be strongly influenced by sublimation due to stellar radiation. Here we develop a model, generally applicable to any extrasolar system with a main sequence or WD star, that traces sublimation-induced orbital element changes in approximately km-sized extrasolar minor planets and comets traveling within hundreds of au. We derive evolution equations on orbital timescales and for arbitrarily steep power-law sublimation dependencies on distance, and place our model in a Solar system context. We also demonstrate the importance of coupling sublimation and general relativity, and the orbital consequences of outgassing in arbitrary directions. We prove that nongravitational accelerations alone cannot result in orbit crossing with the WD disruption radius, but may shrink or expand the orbit by up to several au after a single pericentre passage, potentially affecting subsequent interactions with remnant debris and planets. Our analysis suggests that extant planets must exist in polluted WD systems.

Monthly Notices of the Royal Astronomical Society (In press)

NASA ADS: 2015arXiv150607174V arXiv: 1506.07174