Asteroids, Comets, Meteors 1991, pp. 613 615 _"_J '_-_ -- 7_ Lunar and Planetary Institute, Houston, 1992 613 THE ROLE OF ORGANIC POLYMERS IN THE STRUCTURE OF COMETARY q)UST- V. Vanysekl_ H. Boehnhardt2, and H. Fechtig 3 N 9 3/_J1_25 6 1 Institute of Astronomy, Charles University, Prague, Czechoslovakia 2 MBP Software & Systems ESOC, Dortmund, and Dr.-Remeis-Sternwarte, Bamberg, FRG 3 Max-Planck-lnstitute fuer Kernphysik, Heidelberg, FRG Abstract Several phenomena observed in P/Halley and other comets indicate additional fragmentation of dust particles or dust aggregates in cometary atmospheres. The disintegration of dust aggregates may be explained by sublimation of poIymerized formaldehyde - POM - which play a role as binding material between submicron individual particles. Introduction In situ obtained data concerning the size and composition of the dust in Comet Halley indicate that a high percentage of the dust particles are com- posite grains containing organic species among the may be also polymerized molecules. One of the very first proposed candidate for polymers in cometa- ry material was formaldehyde in form of polyoxymethylene or POM (Vanysek and Wickramasinghe 1975). After the discovery of repeating mass spectral pattern at 45, 61, 75, 91 and 105 ainu in inner coma of comet Halley by the PICCA experiment on board of the space craft GIOTTO (Mitchell e_ al. 1986, 1987) the possible presence of POM or similar compounds in comets was wide- ly discussed (Huebner 1987, Huebner et aL 1989 and reference therein). Also within the frame of the model of agglomerated grains for cometary dust the possible role of organic polymers as gluing material between the indi- vidual building blocks of submicron size was outlined (Boehnhardt et al. 1990}. In here outlined study this problem is newly reviewed in regard to some recently published laboratory results. Fragmentation process The fragmentation of polyoxymethylene has been studied experimentally by mass spectrometry of sublimated POM (M_511er and Jackson 1990) and by mass spectroscopy of sputtered POM by protons (Moore and Tanab4 1990). These experiments show that polymeric forms can be produced from the pure solid by sublimation or by bombardment of POM or a silicate-POM Composite by pro- tons of several hundreds keY energy. The mass distribution of fragments corresponds with the spectrum obtained by space experiment. The intensity of peaks on the laboratory spectrograms (Moore and Tanab4 1990), decreases with increasing mass as does the PICCA data. Typical masses of fragments produced by laboratory experiments are summarized in Table 1. However, the sputtering mechanism initiated by energetic protons seems to be much less effective in the interplanetary environment and the yield may be 10 -smaller as requires the estimated number density of formaldehyde mono- mers in the inner coma of P/Halley at the time of GIOTTO space craft fly-by {Mitchell et aL1987). Thus, the only acceptable process remains the subli- mation from solids. 614 Asteroids, Comets, Meteors 1991 Table 1 Mass of fragmented (H2C0) n X ainu n X amu n X 30 1 90 3 47 1 OH 91 3 H 60 2 105 3 CH 3 61 2 H 119 3 HCO 73 2 CH 1_2! 4 :H 74 2 CH 2 131 3 C0...CH 75 2 CH 3 133 4 CH 89 2 HCO 135 4 CH4 POM may grow into 10ng chains resulting in relatively stable solids with melting point about 400 to 500 K. Nevertheless, the growing process is terminated by the saturation. If we assume, that equilibrium state could be reached in the interstellar dense clouds as well in the primordial solar nebula about the mass between 120 to 150 ainu, i.e approximately with mass -_ monomer mass x n , where n = 4 to 5, then sucha .c0mpound must be reKarded ii: rather as a heavy molecule tha_n as solids, But in such a State ca}_-b_=_'a-6_- -_::_ .... -::::: creted and preserved in the CHON particles, On the other h_d, the_formal -: _ dehyde monomers easily polymerize on the silicate solids and form the sili- cate-POM composite, which may serve as a binding material between indivi- dual submicron particles in fluffy dust grains. The disintegration of such a structures by heating may be very efficient, because at heliocentric distances about 0.5 AU the submicrons grains achieve temperatures above the = melting point for POM. Very small fragments of POM may. be heated by abs0-r- ption of UV photons by thermal impulse up to 1000 K and disintegrated:imm _L: _ : diately to monomers of completely dissociated: However, at low temperatures : : only few POM binding "bridges" between individual particles may contribute to the stability of dust aggregates. For fragile dust aggregates, where4are only few bridges, the estimated tensile strength_ should be in range 10 to 10--dyn cm-, which, is the lower limit for submicron particles which may:: withstand the fragmentation by electrostatic charging in the cometary plasma (Boehnhardt 1986, Boehnhardt and Fechtig 1987). The disappearance of the POM binding bridges is followed by disintegrat_on_ of the dust aggrega_-: :: tes and increasing number density of submicron particles in_ _arger=:_[is_ -_-_:: :: ....... : ces from the cometary nucleus. There are many phenomena which indicate the- : fragmentation of cometary dust. Boehnhardt et a11. (1990) discussed in regard to the POM role following of them: Changes _pf mass distribution of dust with the nuclear distance, identical boundaries of:particles witH:_ery= :: : different masses, appearance of clusters and packets of dust grains-in coma, missing smallest grains in the dust jet while high count rates of these particles in general coma background, and production of CHON parti- cles in comm The sublimation of "gluing " material of POM type between individual submicron particles may be able to explain both the fragmenta- tion of particle aggregates and additional gas production in the coma of : .... : P/Halley. From the CHONparticles may_be_ieased poiymerized m6]e_u _ _:::: les, which however occure_ni_; in shor_erchai_:: :_: : -_:_'.:: : : : : : _ z i E 615 References Asteroids, Comets, Meteors 1991 Boehnhardt H. (1986) ESA SP-250 Vo].II. pp 207 Boehnhardt H., and Fechtig, H. (1987) Astron. A_trophys. 187, 827 Boehnhardt H. Fechtig H. and Vanysek V. (1990} Astron. Astrophys., 231, 543 Huebner W.F.(1987) Science 237, 628 Huebner W.F.Boice D.C. and Korth A. (1989) Adv. Space Res. 9(2}, 29 Mitchell D.L., Lin R.P., Anderson K.A., Carlson C.W., Curtis D.W. Korth A., Richter A.K., Reme H., Sauvaud J.A., d'Uston C., Mendis D.A. (1986) ESA SP-250, Vol I. , 203 pp. Mitchell D.L., Lin R.P., Anderson K.A., Carlson C.W., Curtis D.W. Korth A., Richter A.K., Reme H., Sauvaud J.A., d'Uston C., Mendis D.A. (1987) Science, 273, 626 Moore M.H. and Tanab4 T. (1990) Asr_. J. 365, L39 MSller D.L., and Jackson,W.M.(1990) Icarus 86, 189 Vanysek V. and Wickramasinghe N.C. (1975) _ _ Sci. 33. L19 i x = = = _= i= l