Halley dust component

Halley dust component

Adv. Space Res. Vol. 9, No. 2. pp. (2)23—(2)27, 1989 Printed in Great Britain. All rights reserved. 0273—1177/89 S0.00 + .50 © 1989 COSPAR Copyright...

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Adv. Space Res. Vol. 9, No. 2. pp. (2)23—(2)27, 1989 Printed in Great Britain. All rights reserved.

0273—1177/89 S0.00 + .50 © 1989 COSPAR


ON THE ORIGIN OF P/HALLEY DUST COMPONENT L. M. Mukhin,* A. D. Grechinsky* and T. V. Ruzmaikina** *

Space Research institute, U.S.S.R. Academy of Sciences, Moscow,

U.S.S.R. **Q Yu. Schmidt Institute of Physics of the Earth, U.S.S.R. Academy of Sciences, Moscow, U.S.S.R.

ABSTRACT The dust impact mess—spectrometer (PUMA) was used to found that the majority of dust particles of comet Halley contain organic compounds /1—4/. The paper supports the idea of interstellar origin of thee. duet particles. INTRODUCTION Comets are prominent among the objects of the Solar system. Indeed, planets, asteroids, end meteorites hay, undergone, to a certain extent, the influence of various metamorphic processes during the evolution of the Solar system. As to comets, they are relic objects of the So— 1cr system, still keeping intact the primordial solid component, that manifested itself 4.5 to 4.6 billions yr ago in the protopisnutary disk. This is why it is the comets that are regarded as the key to understanding physical and chemical processes that occured at early stages of Solar system formation. Contact experiments intend.d to investigate the chemical composition of comet Halley’s dust component, yielded unique information which permits a radically new approach to the problem of suolution of solid matter in the Solar system and hence to the solution of some principal questionsi 1 . Where did comet Halley form? 2. Ia th. dust component of the comet Halley the interstellar mater or it is the product of hot gas condensation in the protoplanetary disk? Of prime interest for the solution of these questions are the data ebout the chemical composition of the dust. RESULTS AND DISCUSSION Experiments performed aboard the VEGA—i and 2 spacecraft /1/ showed that major portion of the comet’s dust flux are small particles (4ij.~.m). Thu dust impact mass—spectrometer (PUMA) provided data about the elemental composition of these particles /2/. It turned out that there are light elements (H, C, N, 0) in the mass—spectra of the majority of particles. There is very ground to believe that, at leest pert of light elements er. bound in the organic compounds of different degree of complexity /3/, though from time to time the hydrogen and oxygen excess can be explained by the presence of water or 0H~ group. Extremely important is the fact that micron and eubmicro~particles analyzed by PUMA , keeps their temperature within 400 to 500 K during about 10 s.c. This is also the reason why easily—boiling constituents which the organic component of dust could potentially incorporate, must have disappeared by the moment when the mass—spectrometer was counting dust particles. Thus, the organic fraction of duet is being modified during the time particles need to move from the nucleus to the spa~ cecraft. The results of the masa—spectrometry experiment do not yield explicit information about the specific composition of organic compounds. However, if we use the results of numerical calcu— letion of the cometary duet temperature /4/, one may assume that dust particles contain compounds with the decomposition temperature T.’200 C. Data on thermostability of various organic compounds do not disagree with the presence — in dust particles — of compounds of kerogen /6/,type yellow organic residuum Greenberg obtained in his experiments /5/. It is also possible that dust contains such biologically important compounds as amino acid.. (Note that the characteristic time of the thermal destruction of some amino acids is more than 10 5 sec /7/,


L. M. Mukhin et al.


which is more than the characteristic time of flight nucleus to the spacecraft.

of particles’-’lO

4 sec from the comet

On the whole, the results of mese—spectrometry experiments are explained well in the model, according to which the comet contains a great number of dust particles consisting of a silicate nucleus and a mantle made of organic compounds and voletiles. According to Greenberg the particles with such a morphology form in th. interstellar clouds /5/. However, Greenberg argues in favor of the possibility of organic compounds formation in the diffusive and molecular clouds. The possibility itself cannot serve as the proof of dust particles genesis in the interstellar medium. The interstellar origin of cometary dust can be immediately evident from studying the cometary gas isotopic composition. These date are not available now. Nonetheless the up—to—date inderect data make it possible to etate that cometary dust probably has an interstellar nature. In the given paper we discuss these date: radical difference between the mineral content of cometary gas and the meteorite matter, the possibility of survival of dust particles organic shelle during the formation of the protoplanetsry disk, and the difficulty of formation of dust particles with a silicate nucleus and an organic mantle in the protoplanetery disk itself. These facts will be analyzed below. There is a histogram of Mg/re distribution in the submicron samples of carbonaceous chon— drites Orgueil and Murchison end cometary dust in th. work /a/. The difference in histograms may be interpreted by the MgO and FeO presence in cometary dust. And according to the results /13/ it is metal oxides which consist of two atoms, exist in the interstellar medium. The fact that cometary dust particles contain the complicated organic compounds is en argument in favor of their interstellar origin. Irradiation of ice mixture CH4, H20, NH3, HCN and so on by ultraviolet, X—rays or cosmic rays is essential to formation of complicated organic compounds on dust particles. And the radiation dose, should be considerable, not less then 1010 rad 1013 erg/g. It is unlikely that the cometary matter in the Oort cloud was exposed to this radiation dose during 4.5 billions years. According to /15/ the comet lay.r’-~100 m thick is exposed to the above mentioned ionizing radiation dose. Since according to the available estimates comet Halley has made about 1000 passages near the Sun and lost in each passage the matter layer about 1 a thick, it becomes evident that during the latter passage we analyzed particles which were exposed to the radiation dose in the Oort cloud much lower then it is necessary for organic molecules formation. The necessity of exposure to great radiation doses heavily restrict organic compounds formation on dust particles in the protoplanetary disk. The disk is opaque for the ultraviolet Solar radiation. Indeed, the optical depth in the direction perpendicular to the disk plane is C:


bL~. where a is the dust particls~radius, n~is the number of duet particles in the unit volume, ~ is the disk surface density. Assuming that the characteristic value a is the same as in the interstellar mediurni a—1O~cmand nd/n ~ we find 2) (2) 10.(E/1 g cm— From this expression it follows that whether the dust concentration is normal, the disk becomes transparent along the vertical for radiation, i. a. C~ I only if 6< 6~ :0.1 g cm2 In the protoplanetary disk even with the minimum mass ~n~.-io~2 M 0, the surface density is at least two orders higher ~ up to the region of outer planets Uranus and Neptune (30 A. U.). (These density restrictions in the region of outer plenets were obtained by addition of H and He volatilea, which have been probably lost in the process of planet accumulation prior to the solar composition restoration in the zone of their formation. Thus, obtained surface density dependence on radius, proves to be proportional to R3/2). According to Yamsmoto,/9/ 3/2confined up to the boundary of this we find that the region where comet Halley formed, was between 14 and 115 A.region U. Extrapolating the density distribution 6 ~. optical R 6 ~. surface i g cm~2. Therefore, the disk depth for ultraviolet radiation is great:’C) 10. So, ultraviolet radiation is effectively absorbed in the high disk layer which contains a relatively smell portion of dust matter. At the stage of matter accretion from the disk that results in dust and gas transports in the vertical direction. ing at R distance from the Sun exists during

the molecular cloud there is a developed turbulence in mixing. Participating in the turbulent motion dust During accretion time tacc~1O5 years a particle, bein the region, transparent for the shortwave radiation

Origin of Halley’s Dust Component


0(R)= tacc~ 2Sz~p(R,z)dz





here zü(R) is determined from the condition~t(R, sed to the radiation dose



g cm~) 1. During this time it will

2lra2ta(R)/m_1014(R/i A.U.)_i~’2 rad ~ HareVc





be expo-


TQ) dV (RØ/R)

c/)l~, >\c~200nm, m is the dust particle mass.

Thus, in the zone, when comet Halley forms dust particles were exposed to the dose about twice orders lees than that necessary for the organic matter formation from the ice mixture. (Dust transport from the near Solar regions cannot be en explanation to the great amount of organics, since in these regions ice sublimation occurs). As the Sun passed through the T Ta— un stage it’s ultraviolet luminosity increases in io2 — ~ times. But using method stated in /18/, we can show that temperature of dust particles would be rather high (—160 K at 100 A. U. ) and sublimation of the ice would terminate the origin of organic. After accretion is completed and turbulence damped in the disk, duet rapidly settles down in the central plane, that does not allow its long exposure in the outer layer. The X—ray radiation with the quantum energy ~ 20 keV can play a greet role in the irradiation of dust particles. The exposure time necessary for the radiation dose is sufficient to form qrganic components, i. e. A> A 0 ~ ,018 erg/g, is A0~~

~ (5)




where I is the radiation spectral density /14/,~~is the portion of X—ray radiation, absorbed by dust particles. When the heavy component density z * 0.02 is standard for the interstellar matter end in all obtain t>102° s**3.1O1 years, longer the Solar parts of the disk should be3,fully concentrated in dus~,c~~ z : that 0.02.is With the than assumption that system lifetime. particles become larger the necessary exposure tim. grows. Note that a’-1O~cm and ~ When ~i g cm the real time of dust exposure in the protoplanetary disk is shorter than the Solar system lifetime. It is restricted by the time of dust settling in the central disk plane and of solid body formation. For the disk with the normal interstellar gas—to—dust ratio the characteristic time of dust sedimentation is equal /16/ to




A. U.)3/2 years.


Let us consider now the possibility of survival of organic compounds in the accretion process as the disk forms. According to the cork /17/ the matter flows from the edge to the center (uR<0) in the external pert of the disk within the range 0.6Rd~R~0.9Rd.This region is surrounded by the parts with up~0. Such a velocity distribution in the disk means that the element joined to the disk near it’s edge first, approaches the center, end reaching some ra— dius~0.6Rd reverses it’s direction of motion and starts to remove. Due to this velocity die— tribution, the matter in the external part of the disk never epproaced the nucleus to R
(9.C.M.6.y.R1/2/8.~ 58).R_7/8

L. M. Mukhin et a!.


where 6 se is the Stephan—Boltzmann constant, ~ is the disk s~rfacedensity. In the majority 1/2 ~ const, and since for R * of the density distribution close stationary ~V V R temperature is I A.disk U.,6—1O3 • I0~ g cm2, thenis for the tolimited values T

(400 • 500).(R/1




The velocity of the sccreting matter is v

(CM/p)h/2aa3.106.(R/1 A.




that is much higher than the sound velocity in the disk: cc


(kBT/~.mi4h/2* 105.(R/1 A. u.)_7/16 cu~f,.


Therefore, on the disk equator, where the matter falls down almost perpendicularly to the disk surface, there should be a shock wave. Heating in the accretion shock wave is most dangerous for survival of the shell organic matters on dust particles. So we consider this aspect in greater detail. Dust particle heating in the shock wave i. discussed in papers /11/, /12/. The temperature is determined by the balance of energy a dust particle acquires and radietes in the IR range. Behind the shock front the particle heating sources are: UV radiation absorption and particles collision with gas molecules. The energy acquired by a particle is equal to L,’ .~ira2 n

3 mH


0I~tv where n 0 is the perticlee density of the accreting gas, fL is its molecular weight, a is the duet particle radius, v is the accreting matter velocity. At the distances R~Rd the envelope rotation is insignificant. Gee fells down apherically—eymmetriceliy and its density may be expressed as 2~Lmp U IOhl.(IIIQ/M)1/2.(I A. U./R)3~~’2.(1o5 years/tacc) cm3 , (12) no U 11/4iTv R where taco is the full accretion time. The energy lost by a particle depends on emissivity. According to liii. theory the emissivity of a particle in the IR bend is determined by the ratio Q (a,~)


cj’(a/A). ~


where c 1 end el, A is the ~ 1; 0; 0, of a radius,

~ are the dimensionless coefficients which depend on a sort of psrticle materiwavelength. For graphite, silicate and organic compounds the coefficients are c1 a 21; 1; 2, respectively. For this Q (a,A) the energy radiated by a particle is determined by 2 a3 h 1O4~c~(~,+4)1 (Ic 5 ‘~ a 8 T 8 1/ h) 03P (14)

where c is the speed of light, h is the Planck constant. From L4 U L it follows that the temperature of graphite, silicate and made of organic compounds particles is approximately equal to 4 cm/s)(105 years/taco) ]1”5.(R/i A. u.~3’5K. (15) T~ 4OO.{(M/m~).(1O The given formulas suggest that in the shock wav, at the disk edge the temperature of micron dust particles does not reach 400 K when the disk radius is slighly more then 1 A. U. For O.1&m particles this is fulfilled if the disk radius is more than 3 A. U. It is essential that in the region at hand R>15 A. U. of planets—giants and outer planets this condition will be certainly correct. Hence in these regions the comets can form which involve interstellar duet, containing the organic matter. REFERENCES 1.

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of Halley’s Dust Component



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