Cometary Outbursts Research Articles

Long-term activity and outburst of comet C/2013 A1 (Siding Spring) from narrow-band photometry and long-slit spectroscopy

In this paper, we present a unique data set of more than one year's worth of regular observations of comet C/2013 A1(Siding Spring) with TRAPPIST in Chile, along with low-resolution spectra obtained with the ESO/VLT FORS 2 instrument. The comet made a close approach to Mars on October 19, 2014 and was then observed by many space and ground-based telescopes. We followed the evolution of the OH, NH, CN, $\mathrm{C_3}$, and $\mathrm{C_2}$ production rates as well as the $Af\rho$ parameter as a proxy for the dust production. We detected an outburst two weeks after perihelion, with gas and dust production rates being multiplied by a factor five within a few days. By modelling the shape of the CN and $\mathrm{C_2}$ radial profiles, we determined that the outburst happened around on November 10 around 15:30 UT ($\pm$ 5h) and measured a gas ejection velocity of $1.1\pm0.2$ km/s. We used a thermal evolution model to reproduce the activity pattern and outburst. Our results are consistent with the progressive formation of a dust mantle explaining the shallow dependence of gas production rates, which may be partially blown off during the outburst. We studied the evolution of gas composition, using various ratios such as CN/OH, $\mathrm{C_2}$/OH, or $\mathrm{C_3}$/OH, which showed little or no variation with heliocentric distance including at the time of the outburst. This indicates a relative level of homogeneity of the nucleus composition.

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

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 (∼57d), 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 10‒200kPa. The thermodynamics of enthalpy heating are described and heats of solution are calculated from gas–liquid solubility data yielding −6kJmol−1 for CO in CH4, and −15kJmol−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 >∼80kPa 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 20kJmol−1 and 13kJmol−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 ∼1kPa, and temperatures as low as 50‒65K 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 95K and accounts for activity of 29P and other distant comets up to rh=∼15AU. A similar mechanism can operate at ∼150 to 200K 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 cometary outbursts triggering secondary events, as observed for 17P, 29P and 41P.

Search for satellites near comet 67P/Churyumov-Gerasimenko using Rosetta/OSIRIS images

The European Space Agency Rosetta mission reached and started escorting its main target, the Jupiter-family comet 67P/Churyumov-Gerasimenko, at the beginning of August 2014. Within the context of solar system small bodies, satellite searches from approaching spacecraft were extensively used in the past to study the nature of the visited bodies and their collisional environ- ment. Aims. During the approaching phase to the comet in July 2014, the OSIRIS instrument onboard Rosetta performed a campaign aimed at detecting objects in the vicinity of the comet nucleus and at measuring these objects’ possible bound orbits. In addition to the scientific purpose, the search also focused on spacecraft security to avoid hazardous material in the comet’s environment. Methods. Images in the red spectral domain were acquired with the OSIRIS Narrow Angle Camera, when the spacecraft was at a distance between 5 785 km and 5 463 km to the comet, following an observational strategy tailored to maximize the scientific outcome. From the acquired images, sources were extracted and displayed to search for plausible displacements of all sources from image to image. After stars were identified, the remaining sources were thoroughly analyzed. To place constraints on the expected displacements of a potential satellite, we performed Monte Carlo simulations on the apparent motion of potential satellites within the Hill sphere. Results. We found no unambiguous detections of objects larger than ∼ 6 m within ∼ 20 km and larger than ∼ 1 m between ∼ 20 km and ∼ 110 km from the nucleus, using images with an exposure time of 0.14 s and 1.36 s, respectively. Our conclusions are consistent with independent works on dust grains in the comet coma and on boulders counting on the nucleus surface. Moreover, our analysis Article number, page 1 of 9 shows that the comet outburst detected at the end of April 2014 was not strong enough to eject large objects and to place them into a stable orbit around the nucleus. Our findings underline that it is highly unlikely that large objects survive for a long time around cometary nuclei.

A model of cometary outbursts: a new simple approach to the classical question

Cometary outbursts, i.e. a sudden increase in brightness, have aroused astronomers’ curiosity since the 1920s (Hughes). In 1927, Comet 29P/Schwassmann–Wachmann 1 was discovered, probably at the maximum of one of its outbursts. Many authors have tried to explain the source of cometary outbursts. They formed various models and hypotheses, but none of them have been definitely confirmed and generally accepted. In the present article, we propose a new model for the outburst activity of comets. This model is based on an idea of Ipatov & A’Hearn. This idea is a conclusion from the analysis of time variations in velocity of particles ejected from Comet 9P/Tempel after collision with the Deep Impact impactor. According to this, there are large cavities a few metres below the surface of comet nuclei. These cavities contain material under high gas pressure. In suitable conditions, the surface layers over the cavities can be thrown away and the deeper layers of the nucleus exposed. Finally, an outburst of the comet may be observed. The possible scenario in terms of outbursts is given. The amplitude of the jump in comet luminosity is also calculated. Numerical simulations were carried out for a wide range of cometary physical parameters, which are suitable for comets belonging to the class of Jupiter-family comets. They provide values of jumps in comet brightness that are consistent with those observed during the outbursts of many comets.

REFINED ROTATIONAL PERIOD, POLE SOLUTION, AND SHAPE MODEL FOR (3200) PHAETHON

(3200) Phaethon exhibits both comet- and asteroid-like properties, suggesting it could be a rare transitional object such as a dormant comet or previously volatile-rich asteroid. This justifies detailed study of (3200) Phaethon's physical properties, as a better understanding of asteroid-comet transition objects can provide insight into minor body evolution. We therefore acquired time-series photometry of (3200) Phaethon over 15 nights from 1994 to 2013, primarily using the Tektronix 2048x2048 pixel CCD on the University of Hawaii 2.2-m telescope. We utilized light curve inversion to: (1) refine (3200) Phaethon's rotational period to P=3.6032+/-0.0008 h; (2) estimate a rotational pole orientation of lambda=+85+/-13 degrees and beta=-20+/-10 degrees; and (3) derive a shape model. We also used our extensive light curve dataset to estimate the slope parameter of (3200) Phaethon's phase curve as G~0.06, consistent with C-type asteroids. We discuss how this highly oblique pole orientation with a negative ecliptic latitude supports previous evidence for (3200) Phaethon's origin in the inner main asteroid belt as well as the potential for deeply buried volatiles fueling impulsive yet rare cometary outbursts.

A two-dimensional model of crevasses formed by cometary activity

Abstract Current images clearly show the surface of cometary nuclei to be highly structured as well as the active regions to be distributed unevenly and limited to certain areas, while the remaining surface stays dormant. The dominant part of the emitted volatiles seems to come from crevasses and ice pockets embedded in more or less non-volatile material, describing the scenario discussed in this paper and implemented by a numerical model. We consider a crack in the cometary surface, initially filled with 95% amorphous water ice containing inclusions of more volatile species (e.g. CO2, CO) and 5% crystalline water ice, sometimes covered by a dust matrix. The thermal evolution of such a system at low temperature conditions in response to diurnal irradiation by the Sun has been studied. It is found that there is little interaction between the progress of the crystallisation front and the erosion process due to sublimation at the surface. Moreover, our results indicate that in most circumstances the gas flux due to surface sublimation is orders of magnitude higher than the additional gas release caused by crystallisation of the amorphous ice. In view of these results the often heard suggestion that the crystallisation process of the originally amorphous ice could be a major reason for the frequently observed cometary brightness outbursts appears at least debatable.

P/2010 A2 LINEAR

P/2010 A2 is an object on an asteroidal orbit that was observed to have an extended tail or debris trail in January 2010. In this work, we fit the outburst of P/2010 A2 with a conical burst model, and verify previous suspicions that this was a one--time collisional event rather than an sustained cometary outburst, implying that P/2010 A2 is not a new Main Belt Comet driven by ice sublimation. We find that the best--fit cone opening angle is about 40 to 50 degrees, in agreement with numerical and laboratory simulations of cratering events. Mapping debris orbits to sky positions suggests that the distinctive arc features in the debris correspond to the same debris cone inferred from the extended dust. From the velocity of the debris, and from the presence of a velocity maximum at around 15 cm/s, we infer that the surface of A2 probably has a very low strength (<1 kPa), comparable to lunar regolith.

NEAR-NUCLEUS PHOTOMETRY OF OUTBURSTING COMET 17P/HOLMES

Comet 17P/Holmes underwent the largest cometary outburst in recorded history on UT 2007 Oct. 23, releasing massive quantities of dust and gas. We used the Canada-France-Hawaii telescope to obtain wide-field images of 17P/Holmes on 15 dates over a period of 3 months following the outburst and employ them here to examine the subsequent activity of the nucleus and the nature of the ejecta closest to the nucleus. Through aperture photometry we observed the in- ner coma (within 2500 km of the nucleus) to fade from an apparent magnitude of 11.7 mag to 17.6 mag, corresponding to absolute magnitudes of 8.1 mag and 12.4 mag, between UT 2007 Nov. 6 and 2008 Feb. 12. A second much smaller outburst occurred on UT 2007 Nov. 12, three weeks after the original outburst, suggesting that the nucleus remained unstable. The surface brightness profile of the inner coma was consistently shallow relative to the expected steady-state profile, and showed a persistent brightness enhancement within ~ 5000 km of the nucleus. We propose that sublimating ice grains created an ice grain halo around the nucleus, while fragmenting grains were responsible for the shallow surface brightness profile.

The comet disintegration and meteor streams

Possibility of disintegration of proto-comet nucleus of sungraser comets in three zones of Solar System predicted by one of authors is considered. Testing of parameters of 118 split comets confirms the basic idea. Results of the statistical analysis of comet outbursts gave us additional argument in favor of this assumption. Almost twenty years have passed since, as a result of the search for host phases of isotopically unusual noble gases, the first discovery in 1987 of surviving pre-solar minerals (diamond and silicon carbide) in primitive meteorites. These were followed by others (graphite, refractory oxides, silicon nitride, and finally silicates) in the years since. Pre-solar grains occur in even higher abundance than in meteorites in interplanetary dust particles (IDPs). The result is a kind of ‘new astronomy’ based on the study of pre-solar condensates with all the methods available in modern analytical laboratories.

On liquid phases in cometary nuclei

In this paper we review the relevant literature and investigate conditions likely to lead to melting of H2O ice, methanol (CH3OH) ice, ethane (C2H6) ice and other volatile ices in cometary nuclei. On the basis of a heat balance model which takes account of volatiles loss, we predict the formation of occasional aqueous and hydrocarbon liquid phases in subsurface regions at heliocentric distances, rh of 1–3AU, and 5–12AU, respectively. Low triple-point temperatures and low vapour pressures of C2H6, C3H8, and some higher-order alkanes and alkenes, favour liquid phase formation in cometary bodies at high rh. Microporosity and the formation of a stabilization crust occluding the escape of volatiles facilitate liquid-phase formation. Characteristics of the near-surface which favour subsurface melting include; low effective surface emissivity (at low rh), high amorphous carbon content, average pore sizes of ∼10μm or less, presence of solutes (e.g. CH3OH), mixtures of C2–C6 hydrocarbons (for melting at high rh), diurnal thermal cycling, and slow rotation rate. Applying the principles of soil mechanics, capillary forces are shown to initiate pre-melting phenomena and subsequent melting, which is expected to impart considerable strength of ∼104Pa in partially saturated layers, reducing porosity and permeability, enhancing thermal conductivity and heat transfer. Diurnal thermal cycling is expected to have a marked effect on the composition and distribution of H2O ice in the near-surface leading to frost heave-type phenomena even where little if any true melting occurs. Where melting does take place, capillary suction in the wetted zone has the potential to enhance heat transfer via capillary wetting in a low-gravity environment, and to modify surface topography creating relatively smooth flat-bottomed features, which have a tendency to be located within small depressions. An important aspect of the “wetted layer” model is the prediction that diurnal melt–freeze cycles alter the mixing ratio vs. depth of solutes present, or of other miscible components, largely through a process of fractional crystallization, but also potentially involving frost heave. Wetted layers are potentially durable and can involve significant mass transport of volatile materials in the near-surface, increasing in extent over many rotations of the nucleus prior to and just after perihelion passage, and causing stratification and trapping of the lowest-melting mixtures at depths of several metres. A possible mechanism for cometary outbursts is proposed involving a heat pulse reaching the liquid phase in the deepest wetted zone, leading to supersaturation and triggering the sudden release under pressure of dissolved gases, in particular CO2, CO, CH4 or N2, contained beneath a consolidated near-surface layer. This study indicates that liquid water can persist for long periods of time in the near-surface of some intermediate-sized bodies (102–103km radius) within protoplanetary discs.

Space observations of comets during solar flares: A possible explanation for comet brightness outbursts

Problems connected with mechanisms for comet brightness outbursts as well as for gamma-ray bursts remain open. Meantime, calculations show that irradiation of a certain class of comet nuclei, having high specific electric resistance, by intense fluxes of energetic protons and positively charged ions with kinetic energies more than 1 MeV/nucleon, ejected from the Sun during strong solar flares, can produce a macroscopic high-voltage electric double layer with positive charge in the subsurface zone of the nucleus, during irradiation times of the order of 10–100 h at heliocentric distances around 1–10 AU. The maximum electric energy accumulated in such layer will be restricted by the electric discharge potential of the layer material. For comet nuclei with typical radii of the order of 1–10 km the accumulated energy of such natural electric capacitor is comparable to the energy of large comet outbursts that are estimated on the basis of ground based optical observations. The impulse gamma and X-ray radiation together with optical burst from the comet nucleus during solar flares, anticipated due to high-voltage electric discharge, may serve as an indicator of realization of the processes above considered. Multi-wavelength observations of comets and pseudo-asteroids of cometary origin, having brightness correlation with solar activity, using ground based optical telescopes as well as space gamma and X-ray observatories, during strong solar flares, are very interesting for the physics of comets as well as for high energy astrophysics.

COLLISIONAL EXCAVATION OF ASTEROID (596) SCHEILA

We observed asteroid (596) Scheila and its ejecta cloud using the Swift UV-optical telescope. We obtained photometry of the nucleus and the ejecta, and for the first time measured the asteroid's reflection spectrum between 290 - 500 nm. Our measurements indicate significant reddening at UV wavelengths (13% per 1000 {\AA}) and a possible broad, unidentified absorption feature around 380 nm. Our measurements indicate that the outburst has not permanently increased the asteroid's brightness. We did not detect any of the gases that are typically associated with either hypervolatile activity thought responsible for cometary outbursts (CO+, CO2+), or for any volatiles excavated with the dust (OH, NH, CN, C2, C3). We estimate that 6 x 10^8 kg of dust was released with a high ejection velocity of 57 m/s (assuming 1 {\mu}m sized particles). While the asteroid is red in color and the ejecta have the same color as the Sun, we suggest that the dust does not contain any ice. Based on our observations, we conclude that (596) Scheila was most likely impacted by another main belt asteroid less than 100 meters in diameter.

The development of brightness outburst of comet Holmes – model and observations

In this paper the development of brightness outburst of comet 17Р/Holmes is considered, detected in October 2007. Authors consider the comet disintegration process as isotropic ejection of solid fraction of comet nucleus at any point along the orbit of celestial body under consideration with a set ejection velocities range. Modeling results allow to reveal the trends in orbital elements changes of ejected fragments and to construct the general picture of space fulfillment by orbits of studied object's disintegration products at a calculation time span, to check the possibility of comet and meteor showers connection. This technology was applied for consideration of comet Holmes life cycle. By using the obtained orbital elements of the comet, authors modeled the comet nucleus disintegration for all the 17 revolutions and got the possible meteoroid complex for comet 17Р/Holmes.

Possible mechanism of brightness outbursts of comets

The mechanism of brightness outbursts of comets based on selective absorption of solar ultraviolet radiation by hydrogen atoms in the cometary head is considered. Due to this process, influence of the radiation on parent and daughter molecules in the near-nucleus region of the cometary head is different. As a result, under certain physical conditions in the cometary coma, the electronic-temperature increase may cause an outburst in the brightness of the comet.

Cometary outbursts - a search for a cause of the comet 17P/Holmes outburst

Periodic comet 17P/Holmes underwent a very large outburst of brightness on 2007 October 24. In this paper, the probable causes of this event have been reviewed. Consideration focuses on the four potential mechanisms as follows: (a) the polymerization of hydrogen cyanide HCN; (b) the crystallization of amorphous water ice; (c) thermal stresses and (d) stresses created by icebound CO. The main characteristics of cometary outbursts have been calculated and discussed as the values of released energy and jumps of brightness caused by the considered mechanisms. The obtained results are consistent with the ones observed during the outburst of comet 17P/Holmes. The main conclusion of this paper is as follows. The hypothesis concerning the stresses related to CO inclusions appears to be the most probable cause of the comet 17P/Holmes outburst according to our present knowledge about the physical and chemical structure of comets.

SuperWASP observations of the 2007 outburst of Comet 17P/Holmes

We present wide-field imaging of the 2007 outburst of Comet 17P/Holmes obtained serendipitously by SuperWASP-North on 17 nights over a 42-night period beginning on the night (2007 October 22-23) immediately prior to the outburst. Photometry of 17P's unresolved coma in SuperWASP data taken on the first night of the outburst is consistent with exponential brightening, suggesting that the rapid increase in the scattering cross-section of the coma could be largely due to the progressive fragmentation of ejected material produced on a very short timescale at the time of the initial outburst, with fragmentation timescales decreasing from t(frag)~2x10^3 s to t(frag)~1x10^3 s over our observing period. Analysis of the expansion of 17P's coma reveals a velocity gradient suggesting that the outer coma was dominated by material ejected in an instantaneous, explosive manner. We find an expansion velocity at the edge of the dust coma of v(exp) = 0.55+/-0.02 km/s and a likely outburst date of t_0=2007 October 23.3+/-0.3, consistent with our finding that the comet remained below SuperWASP's detection limit of m(V)~15 mag until at least 2007 October 23.3. Modelling of 17P's gas coma indicates that its outer edge, which was observed to extend past the outer dust coma, is best explained with a single pulse of gas production, consistent with our conclusions concerning the production of the outer dust coma.

2007 OUTBURST OF 17P/HOLMES: THE ALBEDO AND THE TEMPERATURE OF THE DUST GRAINS

Based on optical and infrared observations, we study the albedo and the temperature of the dust grains associated with the spectacular 2007 outburst of Jupiter-family comet 17P/Holmes. We found that the albedo at the solar phase angle ~16° was 0.03-0.12. While the color temperature around 3-4 μm was 360 ± 40 K, the color temperature at 12.4 μm and 24.5 μm was ~200 K, which is consistent with that of a blackbody. We studied the equilibrium temperature of the dust grains at 2.44 AU and found that the big discrepancy in the temperature was caused by the heterogeneity in particle size, that is, hotter components consist of submicron absorbing grains whereas colder components consist of large (1 μm) grains. The contemporaneous optical and mid-infrared observations suggest that the albedo and the temperature could decrease within ~ 3 days after the outburst and stabilized at typical values of the other comets. We estimated the total mass injected into the coma by the outburst on the basis of the derived albedo and the optical magnitude for the entire dust cloud, and found that at least 4 × 1010 kg (equivalent to a few meter surface layer) was removed by the initial outburst event. The derived mass suggests that the outburst is explainable by neither the exogenetic asteroidal impact nor water ice sublimation driven by solar irradiation, but by an endogenic energy source. We conclude that the outburst was triggered by the energy sources several meters or more below the nuclear surface.

TRANSIENT FRAGMENTS IN OUTBURSTING COMET 17P/HOLMES,

We present results from a wide-field imaging campaign at the Canada-France-Hawaii Telescope to study the spectacular outburst of comet 17P/Holmes in late 2007. Using image-processing techniques we probe inside the spherical dust coma and find sixteen fragments having both spatial distribution and kinematics consistent with isotropic ejection from the nucleus. Photometry of the fragments is inconsistent with scattering from monolithic, inert bodies. Instead, each detected fragment appears to be an active cometesimal producing its own dust coma. By scaling from the coma of the primary nucleus of 17P/Holmes, assumed to be 1.7 km in radius, we infer that the sixteen fragments have maximum effective radii between ~ 10 m and ~ 100 m on UT 2007 Nov. 6. The fragments subsequently fade at a common rate of ~ 0.2 mag/day, consistent with steady depletion of ices from these bodies in the heat of the Sun. Our characterization of the fragments supports the hypothesis that a large piece of material broke away from the nucleus and crumbled, expelling smaller, icy shards into the larger dust coma around the nucleus.

Continuous monitoring of comet Holmes from before the 2007 Outburst

The outburst and subsequent brightness evolution of comet 17P/Holmes has been observed using the MMT Observatory’s All-Sky Camera (Pickering 2006) on Mt. Hopkins near Tucson, Arizona, USA. The comet was picked up at the limiting visual magnitude of 5.5 on October 24.38 and tracked by the camera continuously until sunrise four hours later. During this time the comet brightened to visual magnitude 3.5. Comet Holmes was next observed just after sunset on October 25.23 at visual magnitude 2.5, where it remained approximately constant over the next three days. The comet then began to dim slowly and was followed into the early months of 2008 with periods of dense time coverage.

The destruction of cometary grains and changes in the luminosity of comets

AbstractOne explanation of the sudden changes in the brightness of comets is proposed based on the author's earlier suggestions involving the fragmentation of cometary grains. Within the inner coma, a core‐mantle model of the structure of grains is assumed. The proposed mechanism is a combination of electrostatic stress and thermodynamical fragmentation of the cometary grains water‐ice mantle. It has been shown that the vapour pressure of volatile inclusions placed in the waterice mantle of grains can increase sufficiently to cause their fragmentation. It takes place before grains can completely sublime into the vacuum away. Numerical calculations have been carried out for a large range of values of probable physical characteristics of cometary material. The proposed approach yields increases in cometary brightness consistent with observations of typical cometary outbursts. It is concluded that this approach can provide an explanation of the sudden change in activity of comets for a wide range of heliocentric distances (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)