It is modeled that a significant component of the jets of some comets are released as aggregate clumps, which then fragment and shed particles after release, leading to a heterogeneous innermost coma

Clark, B. C.

Economou, T. E.

Green, S. F.

Sandford, S. A.

Zolensky, M. E.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsStreaming clumps ejection model and theheterogeneous inner coma of Comet Wild 2Conference or Workshop ItemHow to cite:Clark, B. C.; Economou, T. E.; Green, S. F.; Sandford, S. A. and Zolensky, M. E. (2004). Streaming clumpsejection model and the heterogeneous inner coma of Comet Wild 2. In: 35th Lunar and Planetary Science Conference,15-19 Mar 2004, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1801.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.uk  STREAMING CLUMPS EJECTION MODEL AND THE HETEROGENEOUS INNER COMA OF COMET WILD 2.  B. C. Clark1, T. E. Economou2, S. F. Green3, S. A. Sandford4, M. E. Zolensky5, 1Lockheed Martin (ben-ton.c.clark@LMCO.com), 2Univ. Chicago, 3Open University, UK, 4NASA Ames Research Center, 5NASA Johnson Space Center.    Introduction:  The conventional concept of cometary comae is that they are dominated by fine par-ticulates released individually by sublimation of surface volatiles and subsequent entrainment in the near-surface gas.  It has long been recognized that such particulates could be relatively large, with early esti-mates that objects perhaps up to one meter in size may be levitated from the surface of the typical cometary nucleus [1].  However, the general uniformity and small average particulate size of observed comae and the relatively smooth, monotonic increases and decreases in particle density during the Giotto flythrough of comet Halley’s coma in 1986 [2] reinforced the view that the bulk of the particles are released at the surface, are fine-sized and inert.  Jets have been interpreted as geometrically constrained release of these particulates.  With major heterogeneities observed [3] during the recent flythrough of the inner coma of comet Wild 2, these views deserve reconsideration. Model:  In contrast to release of individual particles, it is modeled that a significant component of the ob-jects are released in the form of aggregate clumps of particulates.  These clumps are cemented such that they can disintegrate into their finer constituents long after release.  Furthermore, the objects may be ejected by streaming of volatiles through constrained passages to the surface as jets that form distributed sources of secondary particle release with minimal angular disper-sion from the surface point of initial release. Analogies:  An oversimplified analog is the familiar artificial firework consisting of a projectile which, after traveling some distance from the point of launch, disin-tegrates into secondary, then tertiary projectiles.  Such a firework is known as a “starburst”, and often includes a trail of small particles between splits. A common experience by investigators who have worked with mixtures of particulate materials with at least one volatile comp onent is that when placed in a vacuum, catastrophic eruptions can result.  For exa m-ple, when using certain powdered minerals to create a martian analog material, it was found during pumpdown to a low atmo spheric pressure (10 mbar) that samples of more than a few mm thickness would apparently be stable for many minutes then suddenly erupt violently [4].  This was especially true for mixtures which con-tained adsorbed volatiles, such as  montmorillonites with interlayer H2O, salts with H2O of crystallization, and fine-grained chemical powders.  To prevent this behavior, it became necessary to thoroughly de-gas the material by thermal bakeout and/or vacuum oven dry-ing for many hours before use [5].  These eruptions typically created a flared conical cavity with a deep core. Experiments with basalt powders for regolith impact experiments also produced explosive devolatilization, and the potential relevance to planetary phenomena were pointed out [6].  Similar effects have been reported anecdotally for many years by investigators working with powders placed in low-pressure systems. Investigations into the simulation of interstellar or-ganic synthesis using UV irradiation of volatile material condensed on cryogenic cold fingers have commonly experienced the phenomenon that when such materials are allowed to warm, even slightly, violent volatilization can occur as energy is released in the ice by mobilized and highly reactive ions and radicals [7]. The European artificial comet experiment, known as KOSI, found evidence of release of particles much lar-ger than the inert grains from which the mixture of inert materials and ices was formulated [8].  Mechanisms:  Solar insolation is undoubtedly the primary driving energy source for coma formation.  However, as the thermal wave penetrates, thermal en-ergy can be converted to stored mechanical energy via a buildup of gas pressure as phase transformations are stimulated in readily volatilized icy constituents.  Some phase changes are exothermic, which can further en-hance the stored energy.  In principle, chemical reac-tions may be facilitated by the sudden availability of significant amounts of constituents in the highly mo-bile gaseous form, i.e., homogeneous (gas-gas) and heterogeneous (gas-solid) reactions.  In the deep cryo-genic state in which they formed and were stored, the diverse chemical constituents of cometary nuclei are almost certainly non-equilibrated, perhaps further aug-mented by radiochemical stored energy from long-term cosmic ray exposure and the decay of K-40 in minerals. Cometary material is thought to be fragile and of low tensile strength, based upon the common of occur-rence of seemingly spontaneous comet splitting [9] and the observed nature of the final disintegration of comet Shoemaker-Levy 9 under gravity gradient forces as it approached Jupiter. There are many pathways by which deep pockets of gas may eventually find their way to the cometary surface.  To the extent that the rubble pile model  [10] of small body aggregates is cor-Lunar and Planetary Science XXXV (2004) 1801.pdf  rect, natural seams between cometesimals provide a network of interconnected voids. After 4.5 billion years of existence, the accumulated pummeling by smaller objects may have created numer-ous impact channels/tunnels in the near-surface.  If hypervelocity impact into low density material pro-duces a conical cavity analogous to the “carrot track” of particles in aerogel [11], it would provide natural channels for release and for light-trapping and focusing of solar energy.  In 1974, comet Wild 2 suffered a close encounter with Jupiter [12] that may have created zones of weakness, cracks and crevices in the nucleus. Trapped gas release can be relatively energetic and violent, with the potential for producing rubble, espe-cially if cometary mixtures are intrinsically physically heterogeneous with natural fracture zones or pre-existing voids.  Once a passageway become “active” in the release of large material, the production of addi-tional large objects may become self-reinforcing as en-trained pieces strike the sidewalls and eject additional aggregates of material.  As erosion progresses, pres-sure-relief driven exfoliation of chunks of material could also be a major contributing mechanism. Released clumps would be expected to break apart and shed subsequent to their sudden exp osure to hard vacuum and solar insolation..  Under these conditions, the more volatile portions of the clumps will be lost to the gas phase by sublimation, the rate being deter-mined by the composition of the volatiles themselves.  Extremely volatile materials, like CO-rich ices will sub-lime rapidly, while H2O-rich ices and more volatile spe-cies trapped in them, will sublime more slowly [13]  Even slower sublimation rates may occur if clumps are held together with organic ‘glues.’  Indeed, experiments in which cometary ice analogs are processed by high energy radiation (UV or particle) demonstrate that com-plex mixtures of  organic species are produced that show a range of sublimation temperatures above that of H2O ice (200-300K) [14].  In addition to fragmentation driven by volatile loss, shedding also be driven by the release of energy from phase changes and chemical reactions and space charging by solar UV photoelectric effect which could lead to electrostatic disruption.  Thus, with mild release velocities, any given clump evolves to a coherent cloud of particles with the height scale for disintegration depending on the nature of the volatiles present and the physical processes in action.  At large distances, the outer coma may take on the ap-pearance and characteristics of a more uniform region as the effect of velocity dispersions become prominent. Evidence:  As covered elsewhere [3], the coma of Wild 2 has been found to be remarkably heterogene-ous, even at distances of hundreds to perhaps thou-sands of nuclear radii.  Wild 2’s trajectory-altering en-counter with Jupiter may mean that it is a fresh arrival from the Kuiper belt storage zone [15], and hence that the new thermal environment and minimal surface ero-sion from only four perihelion passages may be activat-ing highly volatile materials that previously have been stable.  Furthermore, if the Wild surface is only now being significantly eroded, accumulated impact zones from its earliest history may be serving as conduits for channeled releases. It should be asked why the coma of comet Halley was so different.  The 4-slope model of the integrated dust fluence during the Giotto passage [2] implies, in fact, at least a bimodal component of dust, which could be a mix between classical dust emission and a clumpy source of larger fragments.  From the VEGA flythrough, evidence was given of examples of time-correlated par-ticle densities that were interpreted as particle fragmen-tation [16].  In addition, the discovery of CN jets from Halley and subsequent examples, has been interpreted [17] as release of volatiles from a distributed source, presumably the CHON particles [18].  Finally, the pul-sating jets and the post-perihelion outbursts of Halley and several other comets at large solar distances argue for the presence of buried, but pockets of highly vola-tile material apparently accessible to the thermal-wave, and the apparent spontaneous splitting of comets are further evidence of intrinsic devolatilization mecha-nisms for fragility and propensity to disintegrate [9, 19].  The few radar observations of active nuclei made so far typically indicate the presence of cm-size material near the surface.  Disaggregation of a single 1 cm particle can produce up to 1E9 particles of 10 um, or 1E12 mi-cron-sized particles. References: [1] Whipple, F.L. (1979) Halley mission model.  [2] McDonnell J.A.M. (1987) et al. Astron. As-trophys. 187, 719.  [3]  Tuzzolino A.J. et al. (2004) LPSC this conf.  [4]  Baird, A. et al. (1977) JGR, 82, 4595.  [5]  Clark, B.C. et al. (1982) JGR 87, 10059.  [6]  Hartmann, W.K, (1993), Icarus 104, 226.  [7]  d'Hendecourt, L.B. et al. (1982) Astron. Astrophys. 109, L12-L14.  [8]  Misc. (1995) Planet. Space Sci., 43;  ESA SP-302.  [9]  Sekan-ina, Z. (1997)  Astron. Astrophys. 318, L5. [10]  Weis s-man, P. (1990) Nature 320, 242n. [11]  Tsou, P. et al. (2003) JGR, 108, SRD 3.  [12]  Sekanina, Z. and D.K. Yeomans (1985) Astron. J. 85, 2335.  [13]  Sandford, S. A., & Allamandola, L. J. (1993) Astrophys. J. 417, 815-825. [14]  Bernstein, M. P. et al. (1995) Astrophys. J. 454, 327-344. [15]  Brownlee et al. (2003) JGR, 108, SRD 1.  [16] Simpson J.A. et al. (1989) Adv. Space Res. 9 , 250.  [17]  Klavetter, J. J. and M. F. A'Hearn (1994) Icarus 107, 322.  [18]  Clark, B.C. et al. (1987) Astron. Astro-phys. 187, 779.  [19]  Samarasinha, N.H. (1999) Icarus 154, 540.  Lunar and Planetary Science XXXV (2004) 1801.pdf

Streaming clumps ejection model and the heterogeneous inner coma of Comet Wild 2

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Streaming clumps ejection model and the heterogeneous inner coma of Comet Wild 2

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