Cometary Science Newsletter

April 2016
Michael S. P. Kelley (

One Year of the Cometary Science Newsletter

In April 2015, the first Cometary Science Newsletter was published. Twelve months later, 63 reader-contributed and abstracts and 1 PhD dissertation abstract have been shared, as well as conference announcements, requests for collaboration, and several other informational items. With nearly 150 subscribers and an average of 8 items per month, the Newsletter is an easy mechanism to communicate within the cometary community. To that end, if you have suggestions for new content, or ways that may improve the website, presentation, or item submission, please contact the editor.

Thank you for helping to make a successful first year!

Mike Kelley, Editor

Letters to the Editor

Limited to 2000 characters

Cryovolcanoes and the Nucleus of Comet 29P/S-W1

Readers may be interested to know that three new Icarus papers concerning 29P/Schwassmann-Wachmann 1 show that its outbursts are seasonal, the rotation period is ~57 d, and volatile ices in its subsurface are prone to melting such that a crust forms and subsurface pressures of 20-100 kPa develop. Chemical species must exist in all three states of matter (viz. gas, liquid & solid) within 29P, and some species must separate out in response to diurnal oscillations in subsurface temperature. At 65‒95 K, liquids rich in CH4 are predicted in which CO gas (and potentially N2 and O2) readily dissolves. The physical chemistry of volatiles are described including the generation of heat via liberation of ‘enthalpy of solution’ when gases dissolve deep within cometary nuclei, far below the usual thermal skin depth, leading to sintering and the formation of gas-laden cryomagma. Eruptions occur when insolation softens surficial waxy hydrocarbons, dislodging the crust, releasing pressure and rendering CO-laden methane supersaturated. Dissolved gas undergoes rapid exsolution and an outburst develops. A similar process involving CO2 gas-exsolution from cryomagma rich in CH3OH/H2O can also operate at 150-200 K in Jupiter-family comets such as 17P/Holmes.

Comet 29P is arguably the most bizarre small body in terms of intrinsic nature and behaviour. Being in a near-circular orbit beyond Jupiter, this centaur would make an interesting target for a space probe to visit and study.

Richard Miles

Conference Announcements

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

First Announcement: Symposium Comets 2016

An international conference on cometary science entitled "Comets: A new vision after Rosetta/Philae" will be held November 14 - 18, 2016, at the Abattoirs Museum in Toulouse, France. The conference is principally organized by the teams involved in the Rosetta mission, the European Space Agency (ESA), the French Space Agency (CNES) and the Institute of Research in Astrophysics and Planetology (IRAP).

This is the latest in a series of international meetings intended to promote the exchange of knowledge and ideas among cometary scientists, with a view to integrate them in a comprehensive understanding of comets after the in-depth studies of Rosetta, Philae and other space missions and ground-based observations. In recognition of the broad scope, interdisciplinary nature, and strong international interest in this topic, we welcome the participation of any interested scientist with relevant theoretical, numerical, experimental, or observational experience. One goal of this meeting is to generate a comprehensive global understanding of comets that will serve as an important resource for future studies. Reports on the discussions of the conference will be published at a later time.

Please go to the following link to send us your indication of interest in the meeting and to receive updates from the SOC and LOC.

Refereed Articles

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

Rotationally induced surface slope instabilities responsible for CO₂ activity of Comet 103P/Hartley 2

  • Steckloff, J.K. 1
  • Graves, K. 1
  • Hirabayashi, T. 1
  • Melosh, H.J. 1,2
  • Richardson, J.E. 3
  1. Purdue University, Dept. of Earth, Atmospheric, and Planetary Sciences, West Lafayette, IN
  2. Purdue University, Dept. of Physics and Astronomy, West Lafayette, IN
  3. Arecibo Observatory, Arecibo, Puerto Rico

Comet 103P/Hartley 2 has diurnally controlled, CO2-driven activity on the tip of the small lobe of its bilobate nucleus. Such activity is unique among the comet nuclei visited by spacecraft, and suggests that CO2 ice is very near the surface, which is inconsistent with our expectations of an object that thermophysically evolved for ~45 million years prior to entering the Jupiter Family of comets. Here we explain this pattern of activity by showing that a very plausible recent episode of rapid rotation (rotation period of ~11 [10-13] hours) would have induced avalanches in Hartley 2’s currently active regions that excavated down to CO2-rich ices and activated the small lobe of the nucleus. At Hartley 2’s current rate of spindown about its principal axis, the nucleus would have been spinning fast enough to induce avalanches ~3-4 orbits prior to the DIXI flyby (~1984-1991). This coincides with Hartley 2’s discovery in 1986, and implies that the initiation of CO2 activity facilitated the comet’s discovery. During the avalanches, the sliding material would either be lofted off the surface by gas activity, or possibly gained enough momentum moving downhill (toward the tip of the small lobe) to slide off the tip of the small lobe. Much of this material would have failed to reach escape velocity, and would reimpact the nucleus, forming debris deposits. The similar size frequency distribution of the mounds observed on the surface of Hartley 2 and chunks of material in its inner coma suggest that the 20-40 meter mounds observed by the DIXI mission on the surface of Hartley 2 are potentially these fallback debris deposits. As the nucleus spun down (rotation period increased) from a period of ~11 hours to 18.34 hours at the time of the DIXI flyby, the location of potential minima, where materials preferentially settle, migrated about the surface, allowing us to place relative ages on most of the terrains on the imaged portion of the nucleus.

Icarus (In press)

DOI: 10.1016/j.icarus.2016.02.026 arXiv: 1602.08518

The CO₂ Abundance in Comets C/2012 K1 (PanSTARRS), C/2012 K5 (LINEAR), and 290P/Jager as Measured with Spitzer

  • McKay, A.J.1
  • Kelley, M.S.P.2
  • Cochran, A.L.1
  • Bodewits, D.2
  • DiSanti, M.A.3
  • Dello Russo, N.4
  • Carey M. Lisse4
  1. University of Texas Austin
  2. University of Maryland
  3. NASA Goddard Space Flight Center
  4. Johns Hopkins Applied Physics Laboratory

Carbon dioxide is one of the most abundant ices present in comets and is therefore important for understanding cometary composition and activity. We present analysis of observations of CO2 and [O I] emission in three comets to measure the CO2 abundance and evaluate the possibility of employing observations of [O I] emission in comets as a proxy for CO2. We obtained NIR imaging sensitive to CO2 of comets C/2012 K1 (PanSTARRS), C/2012 K5 (LINEAR), and 290P/Jäger with the IRAC instrument on Spitzer. We acquired observations of [O I] emission in these comets with the ARCES echelle spectrometer mounted on the 3.5-m telescope at Apache Point Observatory and observations of OH with the Swift observatory (PanSTARRS) and with Keck HIRES (Jäger). The CO2/H2O ratios derived from the Spitzer images are 12.6 ± 1.3% (PanSTARRS), 28.9 ± 3.6% (LINEAR), and 31.3 ± 4.2% (Jäger). These abundances are derived under the assumption that contamination from CO emission is negligible. The CO2 abundance for PanSTARRS is close to the average abundance measured in comets at similar heliocentric distance to date, while the abundances measured for LINEAR and Jäger are significantly larger than the average abundance. From the coma morphology observed in PanSTARRS and the assumed gas expansion velocity, we derive a rotation period for the nucleus of about 9.2 h. Comparison of H2O production rates derived from ARCES and Swift data, as well as other observations, suggest the possibility of sublimation from icy grains in the inner coma. We evaluate the possibility that the [O I] emission can be employed as a proxy for CO2 by comparing CO2/H2O ratios inferred from the [O I] lines to those measured directly by Spitzer. We find that for PanSTARRS we can reproduce the observed CO2 abundance to an accuracy of ∼20%. For LINEAR and Jäger, we were only able to obtain upper limits on the CO2 abundance inferred from the [O I] lines. These upper limits are consistent with the CO2 abundances measured by Spitzer.

Icarus (Published)

DOI: 10.1016/j.icarus.2015.11.004 arXiv: 1510.02165

HCN observations of comets C/2013 R1 (Lovejoy) and C/2014 Q2 (Lovejoy)

  • E.S. Wirström 1
  • M.S. Lerner 1
  • P. Källström 2
  • A. Levinsson 3
  • A. Olivefors 2
  • E. Tegehall 2
  1. Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden
  2. Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
  3. Department of Applied Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden

HCN J=1-0 emission from the long-period comet C/2013 R1 (Lovejoy) was observed from the Onsala Space Observatory on multiple occasions during the month before its perihelion passage on December 22, 2013. We report detections for seven different dates, spanning heliocentric distances (R_h) decreasing from 0.94 to 0.82 au. Estimated HCN production rates are generally higher than previously reported for the same time period, but the implied increase in production rate with heliocentric distance, Q_{HCN} proportionate to R_h^{-3.2}, represent well the overall documented increase since it was first observed at R_h=1.35. The implied mean HCN abundance relative to water in R1 Lovejoy is 0.2%. We also report on a detection of HCN with the new 3 mm receiver system at Onsala Space Observatory in comet C/2014 Q2 (Lovejoy) on January 14, 2015, when its heliocentric distance was 1.3 au. Relative to comet C/2013 R1 (Lovejoy), the HCN production rate of C/2014 Q2 (Lovejoy) was more than 5 times higher at similar heliocentric distances, and the implied HCN abundance relative to water 0.09%.

Astronomy & Astrophysics (In press)

arXiv: 1602.07078

Anatomy of outbursts and quiescent activity of Comet 29P/Schwassmann–Wachmann

  • Miles, R. 1
  • Faillace, G.A. 1
  • Mottola, S. 2
  • Raab, H. 3
  • Roche, P. 4
  • Soulier, J.-F. 5
  • Watkins, A. 1
  1. British Astronomical Association, UK
  2. DLR, Institute of Planetary Research, Berlin, Germany
  3. Astronomical Society of Linz, Austria
  4. Cardiff University, UK
  5. Des Etoiles Pour Tous, Saint Martin du Boschet, France

Detailed morphological and photometric characterisation of Comet 29P in the optical region is presented comprising: (a) multi-filter observations of the outburst coma in 2010–2012 with the 2.0-m Faulkes Telescopes (FT); (b) high-cadence, high-precision photometry in May–September 2014; and (c) HST observations in March 1996 (WFPC2/F702W filter). Outbursts appear to be explosive in that: the rise to maximum light is short-lived; the expanding coma fits a model in which ejecta are produced in a singular event and expand into space with uniform velocity; and the motion of condensations within the outburst coma indicate a common onset time and origin. The bright outburst of 2010 February 2 generated a dust coma exhibiting expansion speeds up to 0.257 ± 0.013 km s−1 consistent with acceleration of cometary grains close to the nucleus driven by sublimating CO ice and N2 ice at 24 ± 6 K. Material ejected ∼1 d after this outburst exhibited a V–R colour gradient: redder towards the main outflow, bluer in the opposite sense; potentially arising from differences in spectral emission (from gas), light scattering (particle size), and spectral reflectance (composition). B–V, V–R and R–I colour images revealed colour/compositional differences in near-nucleus structures and the evolution of the expanding coma, which brightened by ∼30% within 5–6 days of the outburst. Broadband photometry indicated a general reddening coma with time (change in B–V from +0.76 to +0.83; V–SDSS-r′ from 0.25 to +0.31 in 14 d). SDSS-r′–SDSS-i′ photometry indicated gradual spectral attenuation at >700 nm. Asymmetric, fan-shaped comae, characteristic of 29P, potentially form when the expanding cloud from an outburst is shielded by the nucleus. Rotational-gradient filtered HST and FT images show unusual 2-fold and 4-fold symmetry involving oppositely-directed radial outflows moving at up to 0.15 km s−1: possibly an indication of material escaping from fissures along the perimeter of a crustal ‘plate’ when dislodged by pressure build-up in the subsurface. Pairs of outbursts separated in time by 52–65 d took place in 2010, 2011, and 2012 exhibiting similar coma outflow patterns indicative of outbursts repeating a second time from the same source, and suggesting a nuclear rotation period of 59 ± 4 (s.e.) d. The escape velocity of the nucleus is sufficiently high (0.013–0.023 km s−1) that a significant fraction of ejecta falls back onto its surface, the action of which, we suggest, re-forms the crust and may trigger outbursts from nearby sites (e.g. triple events of February 2010, and May 2014). A short-lived (<1 d) anomalous brightening of 0.36 ± 0.12 mag observed on 2014 July 21 during quiescence may have arisen from an especially weak mini-outburst in which most of the ejected material failed to reach escape velocity. During quiescence in 2014, Comet 29P fluctuated in brightness over time-scales of 2–10 d by up to ±0.25 mag, probably via local jet activity ...

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

Icarus (In press)

DOI: 10.1016/j.icarus.2015.11.019

Discrete sources of cryovolcanism on the nucleus of Comet 29P/Schwassmann–Wachmann and their origin

  • Miles, R. 1
  1. British Astronomical Association, UK

Evidence for long-lived sources of cryovolcanism on the nucleus of the Comet 29P/Schwassmann–Wachmann has been found from a study of its times of outburst (t0) and the morphological development of inner coma structures. Analysis of data from the Minor Planet Center observations archive spanning 2002–2014 and other observations have yielded 64 outburst times of mainly well-observed events with a median timing uncertainty of 0.40 d. Outbursts comprise largely (i) isolated explosive events; or (ii) multiple outbursts occurring typically within 5–15 d of each other. On rare occasions, a form of continuous or gradually increasing activity is manifest, appearing to be the result of a series of mini-outbursts. Quasi-periodicity in t0 is manifested as an excess of outbursts every 52–60 d, along with a paucity of events every ∼30 d and ∼90 d. Seasonal changes in activity are evident from the temporal analysis of the outburst data. An unambiguous periodicity of 57.6 ± 0.4 d has been found in the times of 26 outbursts during 2010–2014, with all active sources at that time localised within a longitude span of ∼135–150°. Cluster analysis of t0 data for 2002–2010 and 2010–2014, and HST imaging from 1996 confirm and refine the apparent periodicity, indicating that outbursts appear to be grouped in longitude centred on at least 6 circumferential locations. Sources of activity generally persist for at least 10–20 yr, and some appear discrete in nature, able to re-outburst after a single day–night cycle. Given that outbursts are triggered by solar heating, the analysis yields a value for the mean solar day of 57.71 ± 0.06 d, equivalent to a sidereal rotation period of 57.09 ± 0.06 d, assuming the more probable prograde direction of spin. A novel outburst mechanism is outlined in which some cometary ices, principally solid CH4, confined under pressure (>12 kPa) beneath a stabilisation crust, begin to melt and absorb supervolatile gases, mainly CO and N2. These gases liberate considerable heat (5–7 kJ mol−1) via their enthalpy of solution inducing further melting deep within the nucleus where direct insolation heating is absent. This gas-solute process is most active near the solid–liquid interface, where the solvent temperature is lowest and gas solubility is highest. An outburst occurs when insolation heating of the crust above a gas-laden subsurface reservoir softens paraffinic hydrocarbons and causes a crustal plate to dislodge under the accumulated gas pressure, the sudden release of which provokes the explosive ex-solution of dissolved gases, principally CO, propelling entrained dust and debris into space. Fissures reseal as the plate sinks back under the gravitational influence of the large nucleus and the adhesive, waxy hydrocarbon fraction solidifies, permitting a new outburst cycle to begin. A detailed account of the gas ex-solution mechanism is the subject of a partner paper (Miles, R. [2015]. Icarus).

Icarus (In press)

DOI: 10.1016/j.icarus.2015.11.011

Heat of solution: A new source of thermal energy in the subsurface of cometary nuclei and the gas-exsolution mechanism driving outbursts of Comet 29P/Schwassmann‒Wachmann and other comets

  • Miles, R. 1
  1. British Astronomical Association, UK

This paper is a continuation of Miles et al. (2015) [Icarus] and Miles (2015b) [Icarus], which detail new observations of Comet 29P/Schwassmann‒Wachmann, characterise its rotational period (∼57 d), and identify the presence of discrete sources of outburst on its nucleus: the latter ruling out amorphous-to-crystalline H2O ice transitions as the cause of its outbursts. Summary data are presented for 29P and a further 16 non-fragmenting comets which exhibit outbursts of >2 magnitudes. A comprehensive physicochemical mechanism is postulated to account for major outbursts based on melting of cometary ices and the exothermic dissolution of gases, especially CO and CO2 at pressures of 10‒200 kPa. The thermodynamics of enthalpy heating are described and heats of solution are calculated from gas–liquid solubility data yielding −6 kJ mol−1 for CO in CH4, and −15 kJ mol−1 for CO2 in CH3OH close to their freezing point. Heats of solution are ∼6 times greater (per mole) than the enthalpy of fusion of the pure CH4 and CH3OH ices, enabling gas pressures of >∼80 kPa to continually melt these ices. Supervolatile O2 and N2 gases may also participate by dissolving exothermically in liquid CH4 and other hydrocarbons potentially reaching high mixing ratios. H2S and NH3 gases dissolve exothermically in CH3OH liberating up to 20 kJ mol−1 and 13 kJ mol−1, respectively, and all three hydrophilic species facilitate sintering of H2O ice in the near-surface of comets. Localised melting and consolidation is favoured in slowly-rotating cometary nuclei of intermediate dust/gas ratios, at pressures of ∼1 kPa, and temperatures as low as 50‒65 K where O2 and N2 are abundant. Nyctogenic processes on the night-time side of the nucleus restock desiccated surface layers, reseal the crust, enabling fractionation of solutes in sub-crustal liquid phases via fractional sublimation/distillation of non-polar, hydrophobic CH4 and other hydrocarbons; and by fractional crystallisation of polar, hydrophilic phases rich in aqueous CH3OH and other organic oxygenates, e.g. CH2O, able to form low melting point eutectic mixtures. A generalised outburst mechanism is described involving the containment of gases as solutes in cryomagma beneath consolidated surface crustal regions. Disruption of the crust and associated pressure loss render the cryomagma supersaturated, and the concomitant explosive exsolution of gases provokes a cometary outburst. The CO gas-exsolution mechanism operates at ∼65 to 95 K and accounts for activity of 29P and other distant comets up to rh = ∼15 AU. A similar mechanism can operate at ∼150 to 200 K driven by CO2 in aqueous CH3OH and may account for rare outbursts of Jupiter-family comets such as 17P/Holmes. At least 10–15% of all periodic comets may be subject to gas-exsolution outbursts, the majority of which are weak and go undetected. Possible surface morphologies of the nucleus of Comet 29P are discussed. The mechanism may also explain the phenomenon of strong…

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

Icarus (In press)

DOI: 10.1016/j.icarus.2015.12.053