Greenberg, J.M.

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A fine mist of very small comet dust particles

Leiden University Scholary Publications

Ada. Space Res. Vol.4, No.9, pp.211—2l2, 1984 0273—1177/84 $0.00 + .50Printed in Great Britain. All rights reserved. Copyright © COSPARA FINE MIST OF VERY SMALLCOMET DUST PARTICLESJ. Mayo GreenbergLaboratory Astrophysics,Huygens Laboratory, Wassenaarseweg78, 2300 RALeiden, NetherlandsABSTRACTA comet nucleus considered as an aggregate of interstellar dust would produce a mist ofvery finely divided (radius 0.01 pm) particles of carbon and metal oxides accompanyingthe larger dust grains. These small particles which are very abundant in the interstellardust size spectrum would provide substantial physical effects because of their large sur-face area. They may show up strongly in particle detectors on the Halley probes. A strongbasis for serious consideration of these particles comes from the other evidence thatinterstellar dust grains are the building blocks of comets; e.g. (1) the explanation ofthe “missing” carbon in comets; (2) The S2 molecule detection which suggests that thecomet solid ice materials have been previously subjected to ultraviolet radiation (as areinterstellar grains) before aggregation into the comet; (3) the predicted dust to gasratio.INTRODUCTIONVery little consideration has been given to the possibility of cometary particles as smallas 0.01 p, radius. However, if comets are aggregated interstellar dust there is goodreason to expect that such particles ahould provide a very significant contribution to thecometary dust spectrum.Theoretical and observational evidence is accumulating to support the hypothesis thatcomets formed and remained cold after aggregation. Theoretical studies by Biermann andMichel /1/ led to the conclusion that if comets aggregated in the presolar nebula diskthey formed at a distance 1O~~A.U. from the sun. More recently, Biermann and Mestel /2/have provided reasons to expect that comets formed in a protocometary nebula within acloud fragment gravitationally bound to the presolar nebula but one in which smallerbodies, like comets, are preferentially formed without a central condensation, assuggested some time earlier by Cameron /3/. In both of the above situations comets had tohave formed directly Out of the existing molecular cloud material which is (other than I-Iand He) predominantly bound in interstellar dust.Even if comets formed as near to the sun as the Uranus—Neptunus region, it has been shownby computer simulations that they must not have formed as a result of fragmentation oflarger bodies but rather are in their original state /4/.It is plausible to assume, therefore, that the comets which occupy the Oort cloudcondensed in a cold medium and have remained very cold during the subsequent4.5 10’ years. This leads to the comet being made up of aggregated interstellar dust whosetemperature probably did not exceed 15 K so that at least most, if not all, of thevolatile components would survive in their original form. Inevitably then the lessvolatile and refractory components are imbedded in a matrix of ices of which H20 is thelargest single component /5,6,7/. This model has been shown to provide a quantitativeexplanation of the “missing carbon” in comets /8,6/, and the dust to gas ratio /6,7/.A’Hearn et al /9/ have attributed the existence of S2 in a comet spectrum to interstellardust irradiated before being incorporated into the comet nucleus.VERY SMALL INTERSTELLAR DUST GRAINS (a < 0.01 pm)It has been recognized since the discovery of far ultraviolet interstellar extinction thatthis is produced by particles smaller than 0.1 pm. It has now been shown that a basicproperty of this extinction can only be explained if all the particles responsible for thewavelength dependent extinction shortward of X 2500 A may be characterized as being ofradii < 0.01 pm /10/. The conclusion is that, for every tenth micron sized grain there are211212 J.M. Greenbergperhaps a thousand hundredth micron sized grains. This result has been further supportedin a comprehensive study of the FUV extinction by more than 100 stars deduced from TUEspectra for 142 stars /11/. Based on the amount of FUV extinction we predict that forevery tenth micron sized interstellar grain there are a thousand hundredth micron (orless) sized grains of which about a half are composed of some form of carbon and a halfare some silicate or metallic oxide composition. We thus have a bimodal distribution ofvery small metallic and dielectric grains.VERY SMALL COMETDUST GRAINS (a < 0.01 pm)No direct evidence for comet particles less than 0.01 pm seems to exist. However, oneshould expect that as the volatiles of a comet dust particle, considered as an aggregateof interstellar dust particles, evaporate they should release a cloud of small carbon andsilicate (or metallic oxide) grains which were trapped in the icy matrix. Indirectevidence is provided by detection of such particles in meteor streams /12/ which may bepresumed to have originated in comets.These particles are dynamically quite different from the more classical sized cometgrains. Electromagnetic forces become important for very small charged particles /13/. Ashas been pointed out by Wallis and Hassan /14/, in their calculations relevant toparticles a bit larger than the ones we propose here, one expects 0.01 pm dielectric(silicate) grains to suffer little radiation pressure and they may be acceleratedtransverse to the Giotto spacecraft behind its shield and impact the detectors. Since thehundredth micron size particles are orders of magnitude more abundant than the largerparticles, any detection device which is sensitive to particles of mass = gm wouldalmost certainly be saturated. This is of great concern for many of the dust experiments(see e.g. /15/).Another effect of the very small grains could be the role their large surface area playsin the chemistry of the coma. Also their strong coupling to the gas in combination withtheir mass (‘- 1/5 of the gas) could exert significant influences on the tail.There appear to be good reasons to give careful and detailed attention to the effects ofexceedingly small particles on the chemical and dynamical evolution of the coma and tailof comets.ACKNOWLEDGEMENTI should like to thank Hugo Fechtig for suggesting that I look into the possibility ofvery small comet grains.REFERENCES1. L. Biermann, L.K.W. Michel, Moon & Planets, 18, 447 (1978).2. L. Biermann, Phil. Trans. R. Soc. London, A3O3, 351 (1981).3. A.G.W. Cameron, Icarus, 18, 407 (1973).4. R. Greenberg, Paper presented at Dynamics & Origin of Comets, Rome (1984).5. J.M. Greenberg, in Comets, ed. L. Wilkening (Univ. of Arizona Press), 131 (1982).6. J.M. Greenberg, in Cometary Exploration, ed. T.J. Combosi, Hung. Acad. Sd., 23 (1983).7. J.M. Greenberg, in Asteroids, Comets, Meteors, ed. C.—I. Lagerkvist and H. Rickman,Uppsala University Press, 259 (1983).8. A.H. Delsemme, in Comets, ed. L. Wilkening, Univ. of Arizona Press, 85 (1982).9. N. A’Hearn, P. Feldman, in Proceedings of Workshop on Ices in the Solar System, Nice(1984).10. J.M. Greenberg and G. Chlewicki, Astrophys. J., 272, 563 (1983).11. C. Chlewicki, J.M. Greenberg, S. Aiello, B. Barsella, P. Patriarchi, M. Perionotto,in: Proceedings 4th European IDE Conference ESA SP—218, 507 (1984).12. S.F. Singer & J.E. Stanley, in Solid Particles in the Solar Syst~~, Reidel, 329(1979).13. T. Mukai, Astron. Astrophys. 99 1 (1981).14. M.K. Wallis and M.H.A. Hassan, in Cometary Exploration, ed. T.I. Combosi, Hung. Acad.Sci., 57, (1983).15. H. Fechtig, in Cometary Exploration, ed. T.I. Combosi, Hung. Acad. Sci., 57, 203.

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A fine mist of very small comet dust particles

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