Cometary Science Newsletter

Issue
7
Month
October 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.

252P/LINEAR 12

Comet 252P has been recovered by Yudish Ramanjooloo and Dave Tholen as an R=23.7 mag object (MPEC 2015-S97) and MPC and JPL orbit solutions have been updated. As reported in CSN Issue #4 (The Closest Approach of Comet 252P/LINEAR 12 to Earth) this comet will pass Earth from a distance of 0.036 AU. Continued astrometry and early characterization is encouraged to help enable successful observations at closest approach.

- Mike Kelley (UMD)

Refereed Articles

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

ExploreNEOs VIII: Dormant Short-Period Comets in the Near-Earth Asteroid Population

  • M. Mommert 1,2
  • A. W. Harris 2
  • M. Mueller 3
  • J. L. Hora 4
  • D. E. Trilling 1,5,6
  • W. F. Bottke 7
  • C. A. Thomas 8
  • M. Delbo 9
  • J. P. Emery 10
  • G. Fazio 4
  • H. A. Smith 4
  1. Department of Physics and Astronomy, Northern Arizona University, Flagstaff
  2. DLR Institute of Planetary Research, Germany
  3. Netherlands Institute for Space Research, SRON, The Netherlands
  4. Harvard-Smithsonian Center for Astrophysics, Cambridge
  5. Visiting Scientist, South African Astronomical Observatory
  6. Visiting Professor, University of the Western Cape, South Africa
  7. Southwest Research Institute, Boulder
  8. ORAU/NASA Goddard Space Flight Center
  9. Universite de Nice Sophia Antipolis, CNRS, Observatoire de la Cote d'Azur, France
  10. Department of Earth and Planetary Sciences, University of Tennessee, Knoxville

We perform a search for dormant comets, asteroidal objects of cometary origin, in the near-Earth asteroid (NEA) population based on dynamical and physical considerations. Our study is based on albedos derived within the ExploreNEOs program and is extended by adding data from NEOWISE and the Akari asteroid catalog. We use a statistical approach to identify asteroids on orbits that resemble those of short-period near-Earth comets using the Tisserand parameter with respect to Jupiter, the aphelion distance, and the minimum orbital intersection distance with respect to Jupiter. From the sample of NEAs on comet-like orbits, we select those with a geometric albedo pV≤0.064 as dormant comet candidates, and find that only ∼50% of NEAs on comet-like orbits also have comet-like albedos. We identify a total of 23 NEAs from our sample that are likely to be dormant short-period near-Earth comets and, based on a de-biasing procedure applied to the cryogenic NEOWISE survey, estimate both magnitude-limited and size-limited fractions of the NEA population that are dormant short-period comets. We find that 0.3-3.3% of the NEA population with H≤21, and 9(+2/-5)% of the population with diameters d≥1 km, are dormant short-period near-Earth comets.

Astronomical Journal (In press)

arXiv: 1508.04116

The Reactivation of Main-Belt Comet 324P/La Sagra (P/2010 R2)

  • Hsieh, H. H. 1,2
  • Sheppard, S. S. 3
  1. Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan
  2. Planetary Science Institute, USA
  3. Carnegie Institution for Science, USA

We present observations using the Baade Magellan and Canada-France-Hawaii telescopes showing that main-belt comet 324P/La Sagra, formerly known as P/2010 R2, has become active again for the first time since originally observed to be active in 2010-2011. The object appears point-source-like in March and April 2015 as it approached perihelion (true anomaly of ν~300 deg), but was ~1 mag brighter than expected if inactive, suggesting the presence of unresolved dust emission. Activity was confirmed by observations of a cometary dust tail in May and June 2015. We find an apparent net dust production rate of ≲0.1 kg/s during these observations. 324P is now the fourth main-belt comet confirmed to be recurrently active, a strong indication that its activity is driven by sublimation. It now has the largest confirmed active range of all likely main-belt comets, and also the most distant confirmed inbound activation point at R~2.8 AU. Further observations during the current active period will allow direct comparisons of activity strength with 324P’s 2010 activity.

Monthly Notices of the Royal Astronomical Society (In press)

arXiv: 1508.07140

Nebular dead zone effects on the D/H ratio in chondrites and comets

  • M. Ali-Dib 1
  • R. G. Martin 2
  • J.-M. Petit 1
  • O. Mousis 3
  • P. Vernazza 3
  • J. I. Lunine 4
  1. Institut UTINAM, CNRS-UMR 6213, Observatoire de Besançon, Université de Franche-Comté, BP 1615, 25010 Besançon Cedex, France e-mail: mdib@obs-besancon.fr
  2. Department of Physics and Astronomy, University of Nevada, Las Vegas, 4505 South Maryland Parkway, Las Vegas, NV 89154, USA
  3. Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388, Marseille, France
  4. Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA

Comets and chondrites show non-monotonic behaviour of their Deuterium to Hydrogen (D/H) ratio as a function of their formation location from the Sun. This is difficult to explain with a classical protoplanetary disk model that has a decreasing temperature structure with radius from the Sun.

We want to understand if a protoplanetary disc with a dead zone, a region of zero or low turbulence, can explain the measured D/H values in comets and chondrites.

We use time snapshots of a vertically layered disk model with turbulent surface layers and a dead zone at the midplane. The disc has a non-monotonic temperature structure due to increased heating from self-gravity in the outer parts of the dead zone. We couple this to a D/H ratio evolution model in order to quantify the effect of such thermal profiles on D/H enrichment in the nebula.

We find that the local temperature peak in the disk can explain the diversity in the D/H ratios of different chondritic families. This disk temperature profile leads to a non-monotonic D/H enrichment evolution, allowing these families to acquire their different D/H values while forming in close proximity. The formation order we infer for these families is compatible with that inferred from their water abundances. However, we find that even for very young disks, the thermal profile reversal is too close to the Sun to be relevant for comets.

Astronomy & Astrophysics (In press)

arXiv: 1508.00263

Gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations

  • O. Groussin 1
  • L. Jorda 1
  • A.-T. Auger 1
  • and 49 co-authorsNone
  1. Aix Marseille Université, CNRS, LAM (Laboratoire d'Astrophysique de Marseille) UMR 7326, 13388, Marseille, France

We study the link between gravitational slopes and the surface morphology on the nucleus of comet 67P/Churyumov-Gerasimenko and provide constraints on the mechanical properties of the cometary material (tensile, shear, and compressive strengths).

We computed the gravitational slopes for five regions on the nucleus that are representative of the different morphologies observed on the surface (Imhotep, Ash, Seth, Hathor, and Agilkia), using two shape models computed from OSIRIS images by the stereo-photoclinometry (SPC) and stereo-photogrammetry (SPG) techniques. We estimated the tensile, shear, and compressive strengths using different surface morphologies (overhangs, collapsed structures, boulders, cliffs, and Philae's footprint) and mechanical considerations.

The different regions show a similar general pattern in terms of the relation between gravitational slopes and terrain morphology: i) low-slope terrains (0-20 deg) are covered by a fine material and contain a few large (>10 m) and isolated boulders, ii) intermediate-slope terrains (20-4 deg) are mainly fallen consolidated materials and debris fields, with numerous intermediate-size boulders from <1 m to 10 m for the majority of them, and iii) high-slope terrains (45-90 deg) are cliffs that expose a consolidated material and do not show boulders or fine materials. The best range for the tensile strength of overhangs is 3-15 Pa (upper limit of 150 Pa), 4-30 Pa for the shear strength of fine surface materials and boulders, and 30-150 Pa for the compressive strength of overhangs (upper limit of 1500 Pa). The strength-to-gravity ratio is similar for 67P and weak rocks on Earth. As a result of the low compressive strength, the interior of the nucleus may have been compressed sufficiently to initiate diagenesis, which could have contributed to the formation of layers. Our value for the tensile strength is comparable to that of dust aggregates formed by gravitational instability and tends to favor a formation of comets by the accrection of pebbles at low velocities.

Astronomy & Astrophysics (In press)

DOI: 10.1051/0004-6361/201526379

Evidence for Pebbles in Comets

  • Kretke, K. A. 1
  • Levison, H. F. 1
  1. Southwest Research Institute, Boulder, CO USA

When the EPOXI spacecraft flew by Comet 103P/Hartley 2, it observed large particles floating around the comet nucleus. These particles are likely low-density, centimeter- to decimeter-sized clumps of ice and dust. While the origin of these objects remains somewhat mysterious, it is possible that they are giving us important information about the earliest stages of our Solar System’s formation. Recent advancements in planet formation theory suggest that planetesimals (or cometestimals) may grow directly from the gravitational collapse of aerodynamically concentrated small particles, often referred to as “pebbles.” Here we show that the particles observed in the coma of 103P are consistent with the sizes of pebbles expected to efficiently form planetesimals in the region that this comet likely formed, while smaller pebbles are may be expected in the majority of comets, whose chemistry is often indicative of formation in the colder, outer regions of the protoplanetary disk.

Icarus (Published)

DOI: 10.1016/j.icarus.2015.08.017 arXiv: 1509.00754

Temporal morphological changes in the Imhotep region of comet 67P/Churyumov-Gerasimenko

  • O. Groussin 1
  • H. Sierks 2
  • C. Barbieri 3
  • and 46 co-authorsNone
  1. Aix Marseille Université, CNRS, LAM (Laboratoire d'Astrophysique de Marseille) UMR 7326, 13388, Marseille, France
  2. Max-Planck-Institut für Sonnensystemforschung, 37077 Göttingen, Germany
  3. Department of Physics and Astronomy, Padova University, Vicolo dell’Osservatorio 3, 35122, Padova, Italy

We report on the first major temporal morphological changes observed on the surface of the nucleus of comet 67P/Churyumov- Gerasimenko, in the smooth terrains of the Imhotep region. We use images of the OSIRIS cameras onboard Rosetta to follow the temporal changes from 24 May 2015 to 11 July 2015. The morphological changes observed on the surface are visible in the form of roundish features, which are growing in size from a given location in a preferential direction, at a rate of 5.6 – 8.1 × 10−5 m s−1 during the observational period. The location where changes started and the contours of the expanding features are bluer than the surroundings, suggesting the presence of ices (H2O and/or CO2) exposed on the surface. However, sublimation of ices alone is not sufficient to explain the observed expanding features. No significant variations in the dust activity pattern are observed during the period of changes.

Astronomy and Astrophysics (In press)

arXiv: 1509.02794

Erratum to "A distribution of large particles in the coma of Comet 103P/Hartley 2'' [Icarus 222, 634-652 (2013)]

  • Kelley, M. S. 1
  • Lindler, D. J. 2
  • Bodewits, D. 1
  • A'Hearn, M. F. 1
  • Lisse, C. M. 3
  • Kolokolova, L. 1
  • Kissel, J. 4
  • Hermalyn, B. 5
  1. Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
  2. Sigma Space Corporation, 4600 Forbes Boulevard, Lanham, MD 20706, USA
  3. Johns Hopkins University--Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
  4. Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
  5. NASA Ames Research Center / SETI Institute, MS 245-3 BLDG245, Moffett Field, CA 94035

We have corrected the radii and masses of the large particles of comet 103P/Hartley 2 presented by Kelley et al. (2013). We present revised values and conclusions in this erratum.

Icarus (In press)

DOI: 10.1016/j.icarus.2015.09.004 arXiv: 1304.4204

The Formation of Striae within Cometary Dust Tails by a Sublimation-Driven YORP-like Effect

  • Steckloff, J.K. 1
  • Jacobson, S.A. 2,3
  1. Purdue University, Department of Physics and Astronomy
  2. Observatoire de la Côte d'Azur
  3. Universität Bayreuth, Bayerisches Geoinstitut

Sublimating gas molecules scatter off of the surface of an icy body in the same manner as photons (Lambertian Scattering). This means that for every photon-driven body force, there should be a sublimation-driven analogue that affects icy bodies. Thermal photons emitted from the surfaces of asymmetrically shaped bodies in the Solar System generate net torques that change the spin rates of these bodies over time. The long-term averaging of this torque is called the YORP effect. Here we propose a sublimation-driven analogue to the YORP effect (Sublimation-YORP or SYORP), in which sublimating gas molecules emitted from the surfaces of icy bodies in the Solar System also generate net torques on the bodies. However, sublimating gas molecules carry ~104-105 times more momentum away from the body than thermal photons, resulting in much greater body torques. Previous studies of sublimative torques focused on emissions from highly localized sources on the surfaces of Jupiter Family Comet nuclei, and have therefore required extensive empirical observations to predict the resulting behavior of the body. By contrast, SYORP applies to non-localized emissions across the entire body, which likely dominates sublimation-driven torques on small icy chunks and Dynamically Young Comets outside the Jupiter Family, and can therefore be applied without high-resolution spacecraft observations of their surfaces. Instead, we repurpose the well-tested mathematical machinery of the YORP effect to account for sublimation-driven torques. We show how an SYORP-driven mechanism best matches observations of the rarely observed, Sun-oriented linear features (striae) in the tails of comets, whose formation mechanism has remained enigmatic for decades. The SYORP effect naturally explains why striae tend to be observed between near-perihelion and ~1 AU from the Sun for comets with perihelia less than 0.6 AU, and solves longstanding problems with moving enough material into the cometary tail to form visible striae. We show that the SYORP mechanism can form striae that match the striae of Comet West, estimate the sizes of the stria-forming chunks, and produce a power-law fit to these parent chunks with a power law index of -1.4 (-1.1 - -2.0). Lastly, we predict potential observables of this SYORP mechanism, which may appear as clouds or material that appear immediately prior to stria formation, or as a faint, wispy dust feature within the dust tail, between the nucleus and the striae.

Icarus (In press)

DOI: 10.1016/j.icarus.2015.09.021 arXiv: 1509.04756

Rapid Temperature Changes and the Early Activity on Comet 67P/Churyumov–Gerasimenko

  • Alí-Lagoa, V. [1], Delbo', M. [1] and Libourel, G. 1
  1. Laboratoire Lagrange, UMR7293, Université de la Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, F-06304 Nice Cedex 4, France
The so-called “early activity” of comet 67P/Churyumov–Gerasimenko has been observed to originate mostly in parts of the concave region or “neck” between its two lobes. Since activity is driven by the sublimation of volatiles, this is a puzzling result because this area is less exposed to the Sun and is therefore expected to be cooler on average. We used a thermophysical model that takes into account thermal inertia, global self-heating, and shadowing, to compute surface temperatures of the comet. We found that, for every rotation in the 2014 August–December period, some parts of the neck region undergo the fastest temperature variations of the comet’s surface precisely because they are shadowed by their surrounding terrains. Our work suggests that these fast temperature changes are correlated to the early activity of the comet, and we put forward the hypothesis that erosion related to thermal cracking is operating at a high rate on the neck region due to these rapid temperature variations. This may explain why the neck contains some ice—as opposed to most other parts of the surface—and why it is the main source of the comet’s early activity. In a broader context, these results indicate that thermal cracking can operate faster on atmosphereless bodies with significant concavities than implied by currently available estimates.

Astrophysical Journal Letters (Published)

DOI: 10.1088/2041-8205/810/2/L22 arXiv: 1509.03179

Rapid Temperature Changes and the Early Activity on Comet 67P/Churyumov–Gerasimenko

  • Alí-Lagoa, V. 1
  • Delbo', M. 1
  • Libourel, G. 1
  1. Laboratoire Lagrange, UMR7293, Université de la Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, F-06304 Nice Cedex 4, France

The so-called “early activity” of comet 67P/Churyumov–Gerasimenko has been observed to originate mostly in parts of the concave region or “neck” between its two lobes. Since activity is driven by the sublimation of volatiles, this is a puzzling result because this area is less exposed to the Sun and is therefore expected to be cooler on average. We used a thermophysical model that takes into account thermal inertia, global self-heating, and shadowing, to compute surface temperatures of the comet. We found that, for every rotation in the 2014 August–December period, some parts of the neck region undergo the fastest temperature variations of the comet’s surface precisely because they are shadowed by their surrounding terrains. Our work suggests that these fast temperature changes are correlated to the early activity of the comet, and we put forward the hypothesis that erosion related to thermal cracking is operating at a high rate on the neck region due to these rapid temperature variations. This may explain why the neck contains some ice—as opposed to most other parts of the surface—and why it is the main source of the comet’s early activity. In a broader context, these results indicate that thermal cracking can operate faster on atmosphereless bodies with significant concavities than implied by currently available estimates.

Astrophysical Journal Letters (Published)

DOI: 10.1088/2041-8205/810/2/L22 arXiv: 1509.03179

When Comets Get Old: A Synthesis of Comet and Meteor Observations of the Low Activity Comet 209P/LINEAR

  • Ye, Quan-Zhi 1
  • Hui, Man-To 2
  • Brown, Peter G. 1, 3
  • Campbell-Brown, Margaret D. 1, 3
  • Pokorný, Petr 1, 3
  • Wiegert, Paul A. 1, 3
  • Gao, Xing 4
  1. Department of Physics and Astronomy, The University of Western Ontario, London, Ontario N6A 3K7, Canada
  2. Department of Earth, Planetary and Space Sciences, University of California at Los Angeles, 595 Charles Young Drive East, Los Angeles, CA 90095-1567, U.S.A.
  3. Centre for Planetary Science and Exploration, The University of Western Ontario, London, Ontario N6A 5B8, Canada
  4. No. 1 Senior High School of Ürumqi, Ürumqi, Xinjiang, China

It is speculated that some weakly active comets may be transitional objects between active and dormant comets. These objects are at a unique stage of the evolution of cometary nuclei, as they are still identifiable as active comets, in contrast to inactive comets that are observationally indistinguishable from low albedo asteroids. In this paper, we present a synthesis of comet and meteor observations of Jupiter-family comet 209P/LINEAR, one of the most weakly active comets recorded to-date. Images taken by the Xingming 0.35-m telescope and the Gemini Flamingo-2 camera are modeled by a Monte Carlo dust model, which yields a low dust ejection speed (1/10 of that of moderately active comets), dominance of large dust grains, and a low dust production of 0.4kg·s-1 at 19 d after the 2014 perihelion passage. We also find a reddish nucleus of 209P/LINEAR that is similar to D-type asteroids and most Trojan asteroids. Meteor observations with the Canadian Meteor Orbit Radar (CMOR), coupled with meteoroid stream modeling, suggest a low dust production of the parent over the past few hundred orbits, although there are hints of a some temporary increase in activity in the 18th century. Dynamical simulations indicate 209P/LINEAR may have resided in a stable near-Earth orbit for ∼104 yr, which is significantly longer than typical JFCs. All these lines of evidence imply that 209P/LINEAR as an aging comet quietly exhausting its remaining near surface volatiles. We also compare 209P/LINEAR to other low activity comets, where evidence for a diversity of the origin of low activity is seen.

Icarus (Published)

DOI: 10.1016/j.icarus.2015.09.003 arXiv: 1509.00560

Gone in a Blaze of Glory: the Demise of Comet C/2015 D1 (SOHO)

  • Hui, M.-T. 1
  • Ye, Q.-Z. 2
  • Knight, M. M. 3
  • Battams, K. 4
  • Clark, D. 2,5
  1. Earth, Planetary and Space Sciences, UCLA
  2. Physics and Astronomy, UWO
  3. Lowell Observatory, USA
  4. Navy Research Laboratory, USA
  5. Earth Sciences, UWO

We present studies of C/2015 D1 (SOHO), the first sunskirting comet ever seen from ground stations over the past half century. The Solar and Heliospheric Observatory (SOHO) witnessed its peculiar light curve with a huge dip followed by a flareup around perihelion: the dip was likely caused by sublimation of olivines, directly evidenced by a coincident temporary disappearance of the tail. The flareup likely reflects a disintegration event, which we suggest was triggered by intense thermal stress established within the nucleus interior. Photometric data reveal an increasingly dusty coma, indicative of volatile depletion. A catastrophic mass loss rate of ~105 kg s−1 around perihelion was seen. Ground-based Xingming Observatory spotted the post-perihelion debris cloud. Our morphological simulations of post-perihelion images find newly released dust grains of size a≳10 μm in radius, however, a temporal increase in amin was also witnessed, possibly due to swift dispersions of smaller grains swept away by radiation forces without replenishment. Together with the fading profile of the light curve, a power law dust size distribution with index γ=3.2±0.1 is derived. We detected no active remaining cometary nuclei over ~0.1 km in radius in post-perihelion images acquired at Lowell Observatory. Applying radial non-gravitational parameter, A1=(1.209±0.118)×10−6 AU day−2, from an isothermal water-ice sublimation model to the SOHO astrometry significantly reduces residuals and sinusoidal trends in the orbit determination. The nucleus mass ~108--109 kg, and the radius ~50−150 m (bulk density ρd=0.4 g cm−3 assumed) before the disintegration are deduced from the photometric data; consistent results were determined from the non-gravitational effects.

Astrophysical Journal (In press)

arXiv: 1509.07606