Cometary Science Newsletter

Issue
14
Month
May 2016
Editor
Michael S. P. Kelley (msk@astro.umd.edu)

Refereed Articles

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

Why are Jupiter-family comets active and asteroids in cometary-like orbits inactive?

  • Gundlach, B. 1
  • Blum, J. 1
  1. Technische Universität Braunschweig, Institut für Geophysik und extraterrestrische Physik

Context: Surveys in the visible and near-infrared spectral range have revealed the presence of low-albedo asteroids in cometary like orbits (ACOs). In contrast to Jupiter family comets (JFCs), ACOs are inactive, but possess similar orbital parameters.

Aims: In this work, we discuss why ACOs are inactive, whereas JFCs show gas-driven dust activity, although both belong to the same class of primitive solar system bodies.

Methods: We hypothesize that ACOs and JFCs have formed under the same physical conditions, namely by the gravitational collapse of ensembles of ice and dust aggregates. We use the memory effect of dust-aggregate layers under gravitational compression to discuss under which conditions the gas-driven dust activity of these bodies is possible.

Results: Owing to their smaller sizes, JFCs can sustain gas-driven dust activity much longer than the bigger ACOs, whose sub-Surface regions possess an increased tensile strength, due to gravitational compression of the material. The increased tensile strength leads to the passivation against dust activity after a relatively short time of activity.

Conclusions: The gravitational-collapse model of the formation of planetesimals, together with the gravitational compression of the sub-surface material simultaneously, explains the inactivity of ACOs and the gas-driven dust activity of JFCs. Their initially larger sizes means that ACOs possess a higher tensile strength of their sub-surface material, which leads to a faster termination of gas-driven dust activity. Most objects with radii larger than 2 km have already lost their activity due to former gravitational compression of their current surface material.

Astronomy & Astrophysics (In press)

NASA ADS: 2016arXiv160309528G arXiv: arXiv:1603.09528

En route to destruction: The evolution in composition of ices in Comet D/2012 S1 (ISON) between 1.2 and 0.34 AU from the Sun as revealed at infrared wavelengths

  • DiSanti, M. A. 1,2
  • Bonev, B 1,2
  • Gibb, E. L. 1,3
  • and 9 co-authorsNone
  1. Goddard Center for Astrobiology, NASA-Goddard Space Flight Center
  2. Physics Department, Catholic University of America
  3. Department of Physics and Astronomy, University of Missouri - St. Louis

We report production rates for H2O and eight trace molecules (CO, C2H6, CH4, CH3OH, NH3, H2CO, HCN, C2H2) in the dynamically new, sun-grazing comet C/2012 S1 (ISON), using high-resolution spectroscopy at Keck 2 and the NASA-IRTF on ten pre-perihelion dates encompassing heliocentric distances (R_h) = 1.21 – 0.34 AU. Measured water production rates spanned two orders of magnitude, consistent with a long-term heliocentric power law Q(H2O) α R_h^(-3.1±0.1). Abundance ratios for CO, C2H6, and CH4 with respect to H2O were relatively constant with R_h and below their corresponding mean values measured among a dominant sample of Oort cloud comets. CH3OH was also depleted for R_h > 0.5 AU, but was closer to its mean value for Rh ≤ 0.5 AU. The remaining four molecules exhibited higher abundance ratios within 0.5 AU: for R_h > 0.8 AU, NH3 and C2H2 were consistent with their mean values while H2CO and HCN were depleted. For R_h < 0.5 AU all four were enriched, with NH3, H2CO, and HCN increasing most. Spatial profiles of gas emission in ISON consistently peaked sunward of the dust continuum, which was asymmetric anti-sunward and remained singly peaked for all observations. NH3 within 0.5 AU showed a broad spatial distribution, possibly indicating its release in the coma provided optical depth effects were unimportant. The column abundance ratio NH2/H2O at 0.83 AU was close to “typical” NH/OH from optical wavelengths, but was higher within 0.5 AU. Establishing its production rate and testing its parentage (e.g., NH3) requires modeling of coma outflow.

Astrophysical Journal (Published)

DOI: 10.3847/0004-637X/820/1/34 NASA ADS: 2016ApJ...820...34D

The compositional evolution of C/2012 S1 (ISON) from ground-based high-resolution infrared spectroscopy as part of a worldwide observing campaign

  • Dello Russo, N. 1
  • Vervack Jr., R. J. 1
  • Kawakita, H. 2
  • Cochran, A. 3
  • McKay, A. J. 3
  • Harris, W. M. 4
  • Weaver, H. A. 1
  • Lisse, C. M. 1
  • DiSanti, M. A. 5
  • Kobayashi, H. 2
  • Biver, N. 6
  • Bockelée-Morvan, D. 6
  • Crovisier, J. 6
  • Opitom, C. 7
  • Jehin, E. 7
  1. The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USA
  2. Koyama Astronomical Observatory, Kyoto Sangyo University Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan
  3. McDonald Observatory, 2515 Speedway stop C1402, Austin, TX 78712, USA
  4. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
  5. Goddard Center for Astrobiology, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
  6. LESIA, Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon, France
  7. F.R.S.-FNRS, Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 août 17, 4000 Liège, Belgium

Volatile production rates, relative abundances, rotational temperatures, and spatial distributions in the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion (λ/Δλ ~ 2.5×104) infrared spectroscopy as part of a worldwide observing campaign. Spectra were obtained on UT 2013 October 26 and 28 with NIRSPEC at the W. M. Keck Observatory, and UT 2013 November 19 and 20 with CSHELL at the NASA IRTF. H2O was detected on all dates, with production rates increasing markedly from (8.7 ± 1.5)×1027 molecules s-1 on October 26 (Rh = 1.12 AU) to (3.7 ± 0.4)×1029 molecules s-1 on November 20 (Rh = 0.43 AU). Short-term variability of H2O production is also seen as observations on November 19 show an increase in H2O production rate of nearly a factor of two over a period of about six hours. C2H6, CH3OH and CH4 abundances in ISON are slightly depleted relative to H2O when compared to mean values for comets measured at infrared wavelengths. On the November dates, C2H2, HCN and OCS abundances relative to H2O appear to be within the range of mean values, whereas H2CO and NH3 were significantly enhanced. There is evidence that the abundances with respect to H2O increased for some species but not others between October 28 (Rh = 1.07 AU) and November 19 (Rh = 0.46 AU). The high mixing ratios of H2CO/CH3OH and C2H2/C2H6 on November 19, and changes in the mixing ratios of some species with respect to H2O between October 28 to November 19, indicates compositional changes that may be the result of a transition from sampling radiation-processed outer layers in this dynamically new comet to sampling more pristine natal material as the outer processed layer was increasingly eroded and the thermal wave propagated into the nucleus as the comet approached perihelion for the first time. On November 19 and 20, the spatial distribution for dust appears asymmetric and enhanced in the antisolar direction, whereas spatial distributions for volatiles (excepting CN) appear symmetric with their peaks slightly offset in the sunward direction compared to the dust. Spatial distributions for H2O, HCN, C2H6, C2H2, and H2CO on November 19 show no definitive evidence for significant contributions from extended sources; however, broader spatial distributions for NH3 and OCS may be consistent with extended sources for these species. Abundances of HCN and C2H2 on November 19 and 20 are insufficient to account for reported abundances of CN and C2 in ISON near this time. Differences in HCN and CN spatial distributions are also consistent with HCN as only a minor source of CN in ISON on November 19 as the spatial distribution of CN in the coma suggests a dominant distributed source that is correlated with dust and not volatile release. ...

[Editor's note: The full abstract may be found online at the journal link below.]

Icarus (Published)

DOI: 10.1016/j.icarus.2015.11.030 NASA ADS: 2016Icar..266..152D

  • Gibb, E. L. 1
  • Bonev, B. P. 2,3
  • DiSanti, M. A. 3
  • Villanueva, G. L. 2,3
  • Paganini, L. 2,3
  • Mumma, M. J. 3
  1. University of Missouri - St. Louis
  2. Catholic University of America
  3. Center for Astrobiology, NASA Goddard Space Flight Center

We performed a sensitive search for HDO in comet D/2012 S1 (ISON) on 2013 November 16, 17, and 22 using CSHELL and the NASA Infrared Telescope Facility. We constrained the HDO/H2O ratio to <2.0 VSMOW (the terrestrial ocean value) at the 3σ uncertainty level from two independent measurements corresponding to different H2O outgassing rates. This represents the best constrained HDO/H2O ratio for a comet using a small (3 m) telescope and illustrates that when CSHELL is replaced with iSHELL, 3 m class telescopes are still strong contenders for detecting minor volatile species in moderately bright comets.

Astrophysical Journal (Published)

DOI:  10.3847/0004-637X/816/2/101 NASA ADS: 2016ApJ...816..101G

Comet 322P/SOHO 1: An asteroid with the smallest perihelion distance?

  • Knight, M. M. 1,2
  • Fitzsimmons, A. 3
  • Kelley, M. S. P. 1
  • Snodgrass, C. 4
  1. University of Maryland, USA
  2. Lowell Observatory, USA
  3. Queen's University Belfast, UK
  4. The Open University, UK

We observed comet 322P/SOHO 1 (P/1999 R1) from the ground and with the Spitzer Space Telescope when it was between 2.2 and 1.2 AU from the Sun. These are the first observations of any SOHO-discovered periodic comet by a non-solar observatory, and allow us to investigate its behavior under typical cometary circumstances. 322P appeared inactive in all images. Its lightcurve suggests a rotation period of 2.8+/-0.3 hr and has an amplitude greater than ~0.3 mag, implying a density of at least 1000 kg m-3, considerably higher than that of any known comet. It has average colors of g'-r' = 0.52+/-0.04 and r'-i' = 0.04+/-0.09. We converted these to Johnson colors and found that the V-R color is consistent with average cometary colors, but R-I is somewhat bluer; these colors are most similar to V- and Q-type asteroids. Modeling of the optical and IR photometry suggests it has a diameter of 150-320 m and a geometric albedo of 0.09-0.42, with diameter and albedo inversely related. Our upper limits to any undetected coma are still consistent with a sublimation lifetime shorter than the typical dynamical lifetimes for Jupiter Family Comets. These results suggest that it may be of asteroidal origin and only active in the SOHO fields of view via processes different from the volatile-driven activity of traditional comets. If so, it has the smallest perihelion distance of any known asteroid.

Astrophysical Journal Letters (In press)

NASA ADS: 2016arXiv160407790K arXiv: 1604.07790