Cometary Phenomena Research Articles

Deciphering cometary outbursts: linking gas composition changes to trigger mechanisms

ABSTRACT Dust and gas outbursts are recurrent cometary phenomena, playing a crucial role in shaping the coma. Proposed outburst trigger mechanisms include cliff collapse, pressure pockets, and amorphous-to-crystalline phase transition of water ice; however, the underlying processes remain inadequately understood. In this study, we analyse Rosetta/ROSINA data from multiple outbursts on comet 67P/Churyumov-Gerasimenko and present the evolution of the gas composition in the comet’s coma during outburst events. We distinguish two distinct categories of cometary outbursts on the comet: water-driven events characterized by rapid (minutes to hours) changes in coma composition, and CO2-driven events displaying a slow, prolonged (hours to days) increase in highly volatile species. We tentatively associate these different gas composition patterns with different trigger mechanisms. Exposure of fresh ice due to cliff collapse leads to a notable water enhancement, while most perihelion outbursts coincide with substantial density increases of CO2. We propose that these CO2-driven events originate from subsurface gas-filled cavities, whose walls are suggested to have been sealed by earlier refreezing of CO2 migrating from warmer spots, hence increasing the cavity pressure required to burst.

St. Albert the Great and Robert Grosseteste on the nature and causes of comets

Addressing a subject which has received very little attention, this article explores the interpretations of comets offered by St. Albert the Great (c. 1190–1280) and Robert Grosseteste (1168–1253). It shows how, despite prima facie convergences between the two 13th-century bishops concerning the nature and causation of comets, there are nonetheless several previously unobserved subtle differences between them. For Grosseteste the celestial bodies (i.e. the stars and the planets) are the primary, and indeed sole, efficient causes of cometary phenomena, serving to draw up rarefied matter to the upper atmosphere whereupon it is inflamed as it is assimilated to the celestial nature itself. For Albert, by contrast, while the celestial bodies may help to stir up combustible vapours within the atmosphere, and at times precipitate their ascension to the heavenly vault by means of their motion and conjunction, it is not always the case that a comet arises as a result of the direct efficient causality of the celestial bodies.

On the role of electron-driven processes in planetary atmospheres and comets

After the presence of ionized layers in the Earth's atmosphere was inferred, it took 50 years to quantitatively understand them. The electron density could not be accounted for until Sir David Bates first suggested (along with Sir Harrie Massey) that the main electron-loss process was dissociative recombination with molecular ions, and he and colleagues then developed a theory to predict those rates of dissociative recombination. However, electron impact processes, particularly excitation, have been considered insignificant in most situations, in both planetary and cometary atmospheres. Here we describe cases where recent calculations have shown that electron impact excitation of molecules is important, suggesting that, just as in the time of Sir David Bates, electron-driven processes remain fundamental to our quantitative understanding of atmospheric and cometary phenomena.

Structure and density of cometary nuclei

Abstract— Understanding the nature of the cometary nucleus remains one of the major problems in solar system science. Whipple's (1950) icy conglomerate model has been very successful at explaining a range of cometary phenomena, including the source of cometary activity and the nongravitational orbital motion of the nuclei. However, the internal structure of the nuclei is still largely unknown. We review herein the evidence for cometary nuclei as fluffy aggregates or primordial rubble piles, as first proposed by Donn et al. (1985) and Weissman (1986). These models assume that cometary nuclei are weakly bonded aggregations of smaller, icy‐conglomerate planetesimals, possibly held together only by self‐gravity. Evidence for this model comes from studies of the accretion and subsequent evolution of material in the solar nebula, from observations of disrupted comets, and in particular comet Shoemaker‐Levy 9, from measurements of the ensemble rotational properties of observed cometary nuclei, and from recent spacecraft missions to comets. Although the evidence for rubble pile nuclei is growing, the eventual answer to this question will likely not come until we can place a spacecraft in orbit around a cometary nucleus and study it in detail over many months to years. ESA's Rosetta mission, now en route to comet 67P/Churyumov‐Gerasimenko, will provide that opportunity.

Release and fragmentation of aggregates to produce heterogeneous, lumpy coma streams

The unpredicted heterogeneity in particle number density in the coma of Wild 2 is consistent with delayed fragmentation to produce small particles from larger aggregates initially ejected from the cometary nucleus. The resultant heterogeneous inner coma results in stochastic variations in particle number and size distribution. Fragmentation can be accelerated after aggregate release by enhanced heating and one or more additional factors such as abrupt depressurization, phase transitions, exothermic chemical reactions, centrifugal forces, and electrostatic repulsion. Certain predicted characteristics of such in‐flight disaggregation in coma particle streams correspond to known cometary phenomena.

"Periwigged Heralds": Epistemology and Intertextuality in Early American Cometography

In the winter of 1680-81 an enormous comet appeared in the nighttime skies of Europe and the Americas.1 This blazing occasioned numerous treatises, poems, pamphlets, broadsides, ballads, engravings, and woodcuts. Evaluating this cometary copia, the historian of science, Pingre, in 1783 observes:The world was inundated with writings on these phenomena, on their nature, on their significations; for there were still and [Cometomantiens] ... I believe that the languages were not even as confused at the Tower of Babel, as were the sentiments concerning this famous comet.2Pingre's belated impatience with the discursive flood of astrologers and cometomantics echoes of course the more general Cartesian objection to the confusion of words and things, a confusion that L'Academie des sciences and Royal Society dedicated themselves to eliminating. But the arduous nature of this effort in seventeenth-century Europe is exemplified by the numerous, often-conflicting interpretations of the 1618, 1665, and 1680 comets-to say nothing of the unprecedented new stars or novas of 1572 and 1604, which were often treated as cometary phenomena. In this essay, though, I will trace the contours of this epistemological confusion in the Americas-specifically in Mexico, Peru, and New England-where European astronomical, astrological, theological, and literary discourses concerning comets were repeated and imitated but also where they were mediated by cultural concerns unique to the New World. More particularly, by focusing on some of the intertextual aspects of the interpretation of the 1680 comet in the Americas, I suggest that the hermeneutic of signs which dominated in one way or another almost every Early American cometography is indicative of broader epistemological and rhetorical tensions that characterize the period.Popular belief in late seventeenth-century Europe generally held that a comet's appearance heralded God's wrath. In Germany the terrible Wunder-Komet of 1680 precipitated the publication of nearly a hundred treatises, most of which echoed the motto on a medal struck by the State Library of Zurich: star threatens evil things. Have faith! God will set things right.3 The devout comet-watcher could thus look forward to imminent salvation, while sinners were urged to seize a final chance to repent. Still, even many of the learned who treated comets mathematically also embraced some form of theological hermeneutic. Both Kepler and, later in his life, Newton wished to retain comets as actors in God's eschatological drama.4 Though others, like Galileo and Gassendi, saw the comet as a natural event without semiotic significance, the more astrologically inclined continued to see it not just as a sign but as the effective cause itself of ill, or less often, good fortune.5 Stanislaw Lubieniecki's 1667 Theatrum cometicum thus places one comet at Jerusalem's destruction and another at Christ's birth.6 In other words, the Theatrum, like most contemporary cometographies, is fueled by the logic of analogy, for a comet was generally read as a wondrous (but usually disastrous) sign linking the microcosm and macrocosm, or alternately, as a sign legible only by translating the Book of Nature with the lexicon provided by the Book of God, the Bible.7 When it came to comets, the epistemic shift that Foucault traces in The Order of Things, from a sixteenth-century symbolic or qualitative world-view to a seventeenth-century rationalist or quantitative one, was often slowly and/or incompletely achieved. Forced to speak of a comet's beard or tail, even the less mantic cometographers found themselves interrogating the analogical nature of their discourse as much as the celestial objects that excited their curiosity.Especially in France and to a lesser extent England the epistemological and rhetorical assumptions associated with the idea that comets heralded misfortune were being subject to increasing scrutiny if not sometimes also outright scorn. …

Waves and instabilities in dusty space plasmas

Astrophysical dust occurs in many circumstances, like interstellar and circumstellar media, and between and around planets and comets. Typical Solar System applications include planetary rings, asteroid zones, cometary comae and tails, and regions of Earth's lower magnetosphere. Dust grains immersed in ambient plasmas are electrically charged by various processes and interact with electromagnetic fields. Intriguing phenomena observed in the 1980s by Voyager cameras and attributed to charged dust are radial spokes in the B-ring and braids in the F-ring of Saturn. Collective effects become important when the dust intergrain distance is smaller than the plasma Debye length, and start from observations that micron-sized dust grains can have very high negative charges and in proportion even higher masses. Characteristic dust frequencies are considerably smaller than corresponding electron or ion quantities, giving rise to new low-frequency eigenmodes, which could explain some of the low-frequency noise in space and astrophysical plasmas. Repelling electrostatic forces between charged dust grains prevent planetary rings from collapsing to very thin sheets, and oscillations in transverse ring thickness give rise to resonant phenomena, held responsible for gaps in the rings of Jupiter and Saturn. Further features are connected with fluctuating dust charges, which imply highly nontrivial source and/or sink terms in the description, and those in turn lead to new electrostatic and electromagnetic instabilities. Many different papers are reviewed which discuss waves and instabilities in dusty space plasmas, both with fixed and variable dust charges, at the linear level and, at the nonlinear level, involving double layers, solitons, vortices and other waves. These studies are at present far ahead of what observations can corroborate, a situation not likely to change soon due to the paucity of coming solar system missions concerned with planetary or cometary phenomena.

On the importance of dust in cometary nuclei

The icy conglomerate model introduced by Whipple more than 40 years ago has been widely accepted in cometary science because it is able to describe numerous cometary phenomena. In this model comets are described as a conglomerate of ices and dust where the ices represent the major component. However, some recent observations seem to favour dust rich comets. The purpose of this paper is to summarize the observational facts supporting the dominance of refractories in comets and to discuss the consequences of a dust dominated nucleus for cometary physics.

The role of comets and asteroids in Solar System development: space exploration

Exploration by space missions of the near-nucleus regions of comets Halley and Grigg-Skjellerup has resulted in valuable but expensive snapshots of cometary phenomena. The ‘ground truth’ from such missions, which can be established only by this means of dedicated space exploration, provides essential inputs to models of cometary processes. It also gives calibration data for a very wide base of cometary and asteroidal observations, past, present and future. Seen as objects which are both eroded by impacts from interplanetary dust and also the progenitors of interplanetary dust, we find both asteroids and comets are needed to contribute to this population. Contrary to expectations, as new data on the asteroids and comets is analysed, we find the differences between the two classes of primordial body is very much less distinct; accounting for the interplanetary distribution and properties of dust mass requires not only both classes of object but also a distribution of mixed classes. ESA’s newly selected cometary mission Rosetta will offer a unique opportunity, during a rendezvous encounter from aphelion to perihelion, for the extended and detailed in situ observations of a target comet. It will also act as a valuable focus on the nature and role of comets in both the origin and development of the Solar System.

Comments on “gas release from comets” and related trapped-gas experiments

A recent paper on Kr trapping and release by H 2O ice is used to emphasize the many variables that influence the results of trapped-gas experiments with cometary ice analogs. A proposed linear relationship between ice thickness and gas retained beyond the amorphous-to-cubic phase change of water is questioned. The many variables involved, combined with the uncertainty of cometary structure and composition, imply that great care is needed in the application of laboratory results to cometary phenomena and that definitive conclusions are difficult to reach.

An experimental study of the sublimation of water ice and the release of trapped gases

The sublimation of Ar/H 2O, CO/H 2O, and CO 2/H 2O mixtures condensed at 23–30 K was investigated. The release of the more volatile component of the ice mixture occurred in several discrete temperature regions. Gas release, as monitored by a mass spectrometer, was correlated with changes in the infrared spectrum of the solid ice. The results are in substantial agreement with those of Bar-Nun and co-workers, who studied ice sublimation using mass spectrometry alone, and Sanford and Allamandola, who used infrared spectroscopy alone. Several different concentrations were used for all gas/H 2O mixtures, and relative peak intensities during warming were found to depend on the initial gas/H 2O ratio. The sublimation of H 2O, CO/H 2O, and CO 2/H 2O ices following irradiation by 1-MeV protons was also studied. Irradiation of CO or CO 2 containing ices resulted in CO 2 or CO release, respectively, in addition to the original species. Some implications for cometary phenomena are given.

Clathrate hydrate formation in amorphous cometary ice analogs in vacuo.

The presence of clathrate hydrates in cometary ice has been suggested to account for anomalous gas release at large radial distances from the sun as well as the retention of volatiles in comets to elevated temperatures. However, how clathrate hydrates can form in low-pressure environments, such as in cold interstellar molecular clouds, in the outer reaches of the early solar nebula, or in cometary ices, has been poorly understood. Experiments performed with the use of a modified electron microscope demonstrate that during the warming of vapor-deposited amorphous ices in vacuo, clathrate hydrates can form by rearrangements in the solid state. Phase separations and microporous textures that are the result of these rearrangements may account for a variety of anomalous cometary phenomena.

Special GRL issue Comets III

A special issue, highlighting the results obtained by the Japanese spacecraft Sakegake and Suisei, will be published in the August Geophysical Research Letters (GRL). The editor invites experimental and theoretical papers relevant to cometary phenomena.

Are cometary nuclei primordial rubble piles?

Whipple's1 icy conglomerate model for the cometary nucleus has had considerable success in explaining a variety of cometary phenomena such as gas production rates and nongravitational forces. However, as discussed here, both observational evidence and theoretical considerations suggest that the cometary nucleus may not be a well-consolidated single body, but may instead be a loosely bound agglomeration of smaller fragments, weakly bonded and subject to occasional or even frequent disruptive events. The proposed model is analogous to the ‘rubble pile’ model suggested for the larger main-belt asteroids2, although the larger cometary fragments are expected to be primordial condensations rather than collisionally derived debris as in the asteroid case. The concept of cometary nuclei as primordial rubble piles is proposed as a modification of the basic Whipple model, not as a replacement for it.

Trapping and release of gases by water ice and implications for icy bodies

The trapping and release of H 2, CO, CO 2, CH 4, Ar, Ne, and N 2 by amorphous water ice was studied experimentally under dynamic conditions, at low temperatures starting at 16°K, with gas pressure of 5 × 10 −8−10 −6 Torr. CO, CH 4, Ar, and N 2 were found to be released in three or four distinct temperature ranges, each resulting from a different trapping mechanism: (a) 30–55°K, where the gas frozen on the water ice evaporates; (b) 135–155°K, where gas is squeezed out of the water ice during the transformation of amorphous ice to cubic ice; (c) 165–190°K, where gas and water are released simultaneously, probably by the evaporation of a clathrate hydrate, and, occasionally (d) 160–175°K, where deeply buried gas is released during the transformation of cubic ice to hexagonal ice. If the third range is indeed due to clathrate formation, CO was found to form this compound. CO 2 did not form a clathrate under the experimental conditions. Excess hydrogen did not affect the occlusion of other gases. Hydrogen itself was trapped only at 16°K. Neon was not trapped at 25°K. With cubic ice, the only trapping mechanism is freezing of gas on the ice surface. No fractionation between the gas phase and the ice was observed with a mixture of CO and Ar. Massive ejection of ice grains was observed during the evaporation of the gas in three (a,c,d) out of the four ranges. The experimental results are used to explain several cometary phenomena, especially those occurring at large heliocentric distances, and are applied also to Titan's atmospheric composition and to the possible ejection of ice grains from Enceladus.

Analysis of the nucleus and circumnuclear area of comet Halley with the “I.K.S.” infrared sounder from the “Vega” flyby probes

The “Vega” Soviet flyby probes to comet Halley will carry a French infrared sounder, called “I.K.S.”. In order to assess its observing capabilities, a theoretical model of the comet infrared emission was constructed. We show how the experiment results will be used to derive the nucleus size and radiative properties, and to study the distribution of gas and dust in the inner coma and circumnuclear area. A preliminary discussion is made of the relevance of the data in instances where the cometary phenomena would be more complex than assumed in the model.

Collision of comets with meteoroids

Statistical analysis of the quantity of dust in the cometary atmosphere in relation to the direction of motion of the comet about the Sun suggests an excess of dust for the retrograde comets. This excess is analyzed in the light of Harwit's theory of the cloud of “boulders” and of Öpik's impact theory. A comparison is also made between these excesses and other cometary phenomena such as splittings and outbursts.

Laboratory Simulation of Icy Cometary Nuclei

The problems of laboratory simulation of cometary phenomena are discussed. Results are given of experimental investigations of the thermodynamic parameters of H2O ice and of frozen mixtures of electrolytes during sublimation under conditions of high vacuum and low temperature. At a distance of 1 AU from the Sun the equilibrium temperature of H2O ice is shown to be about −75 °C. Conditions are determined for the formation and disintegration of dust matrices during the sublimation of ices containing small dust particles. Dust particles are shown to leave the surface of the ice in the form of conglomerates of primary particles. The outflow velocities of the particles are found to be as high as several metres per second. The results obtained are discussed in relation to cometary phenomena.

Physico-chemical phenomena in comets—I: Experimental study of snows in a cometary environment

The existence and thennodynamic stability of the clathrate hydrate of methane is checked in the 100°K temperature range, and its dissociation pressure is measured. Contrarily to ordinary ice or snow of water, it has the morphological appearance of a peculiar powder snow made of small icy grains. Their apparent density is between 0.33 and 0.54 g cm −3. Their actual size distribution is established and has a sharp peak for grains between 0.1 and 1 mm in dia. In a vacuum simulating cometary conditions, grains are constantly stripped from the main body of snow, by gases originating from the dissociation of the hydrate. As it is likely that all known clathrate hydrates behave in the same way, it is submitted that they could build up a halo of ice grains within the inner coma of comets. This halo may play an important role in the understanding of the cometary phenomena.

The lunar atmosphere

Optical and radio estimates of the upper limit of density of the lunar atmosphere are reviewed. Properties of the lunar ionosphere in contact with the surface are analyzed theoretically and applied to an estimate of its composition and density. Considering the balance between injection and escape, the daytime average probable number density at the surface is rated at 3 to 5 × 10 5 cm −3 , with an uncertainty ratio of about 2 and with 50–70 per cent CO 2 , 45-27 per cent H 2 O anf 4-2 per cent H 2 . The electron density, in equilibrium with contact recombination at the surface, is then 200–300 cm −3 . The sources of the atmosphere are solar wind, its interaction with, and sputtering of, the surface, meteor impact degassing, and somewhat dubious “volcanic” sources. The surface electric charge is slightly negative or neutral; only when the total density falls below 3 × 10 5 , and the electron density below 200, will there be a positive charge. The photoelectric efficiency of silicates is estimated to be of the order of 1 80 , and the electron emission as depending on surface potential is estimated accordingly from the solar ultraviolet emission. Mechanisms of escape—thermal, collisional with solar wind, and ionic—are evaluated quantitatively. Thermal escape prevails for hydrogen; the other two processes for the heavier molecules. The longest escape time scale is about 2 years for xenon, provided the layer is thin (exospheric). No permanent atmosphere can exist on the moon. The conditions of static and escaping plasma equilibrium are reviewed. Conservation of space charge in an escaping plasma stream requires the establishment of turbulent electric fields which force the electrons to oscillate with thermal velocities around the mean motion determined by the momentum of the ions. The fields prevent direct escape of the photo-electrons from the lunar surface, and play a decisive role in the structure of the lunar ionosphere. From cometary phenomena and optical data for the zodiacal light the persistent and principal component of solar wind is estimated to correspond to N 6 = 50 cm −3 , v = 200 km sec , a relative electron temperature of about 400–500°K at the earth's orbit, and a translational energy of 210 eV for the protons and ∼1000°K for the electrons. Whereas other constituents of the lunar atmosphere are escaping to space, water molecules created chemically by solar wind chiefly sublimate into the cold spots of permanent shadow where lunar glaciers up to 100 meters equivalent thickness may have accumulated over the ages.