Synthesis of organic compounds in interstellar dust and their transport to earth via comets

Adv. Space Res. Vol. 9, No. 6, pp. (6)15-.(6)23. 1989 Printed in Great Britain. All rights reserved.

0273—117789 $0.00 + .50 Copyright © 1989 COSPAR

SYNTHESIS OF ORGANIC COMPOUNDS IN INTERSTELLAR DUST AND THEIR TRANSPORT TO EARTH VIA COMETS J. Mayo Greenberg Laboratory of Astrophysics, University of Leiden, P.O. Box 9504, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands

ABSTRACT It now appears that the chemical evolution of the pre-solar system Interstellar dust ensures that a major fraction of comets is in the form of complex organic molecules at least partially of a preblotic nature and that the submioron interstellar dust preserves its chemical integrity as a result of’ forming a very tenuous low density comet structure whose solid matter occupies — 1/5 of the total volume. This low density micro structure further provides a physical basis for comets bringing a significant fraction of the original interstellar organic molecules to the earth unmodified by the impact event. Finally, the evidence for a large number of comet collisions with the early earth ensured that the major organic molecular budget on the earth’s surface was “continuously” supplied along with water well before 3.8 billion years ago which Is the earliest date for life. INTRODUCTION If the age of the earth is no more than - 14.55 Gyr and we accept a cool-off time of — 0.14 Gyr, and a fully active and extensive life as of 3.8 Gyr ago /1/, there was at most about 0.3 to 0.14 Gyr available for life to have evolved from simple precursor organic molecules. It is hardly likely that the earth’s primitive atmosphere could have generated prebiotic material in the high concentrations provided by comet impacts and furthermore comets could have done this many times over. Therefore the composition of comets is probably of great significance in the seeding of organics on the primitive earth. It is well recognized that many comets as well as asteroids have impacted the earth /2/. The moon’s surface bears obvious evidence for the latter, and the co~tinuous appearance of’ new comets from the Oort cloud implies a certain probability, — 10° yr~ for comet collisons with the earth even now. It is suggested that in the early stages of the earth’s evolution large numbers of comets added material to the solar system atmosphere and oceans /3/. The principal aim in this paper is to make some estimates both as to the chemical distribution as well as to the amount of such material which an individual comet brought to the earth. This involves first assuming a chemical and morphological model for comets and secondly answering the question of what happens to such bodies when they strike a planet at - 50 km The basic ingredients of all objects in a star or planetary system are derived from the interstellar medium. How direct the connection between them depends on the degree by which the interstellar components have been modified or metamorphosed. This dependence is largely a function of the temperature prevailing before, during or after the birth process of the object relative to the volatility of the various interstellar dust components. Of’ all the objects of the solar system the comets are presumed to be the most primitive; i.e., they are believed to represent most closely the pre—solar system environs. The first

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J. M. Greenberg

question is how cold it was where comets were born. If the comets were born in the primitive solar accretion disc at distances of’ the order of’ the outer planets, say between Uranus and Neptune, and we assume that the temperature distributions were as predicted by Cameron /14/ then the maximum would be about - 60 K which is too low to evaporate H 20 ice. Ruzmaikina and Maeva /5/ suggest an even lower temperature, T < 37 K. If turbulent transport plays an important role /6/ then some of the constituents which were evaporated closer to the center including even such refractories as silicates, but certainly H20 would have percolated outwards and recondensed but not in their original configuration. In such a case, completely presolar material may not necessarily exist even where temperatures never exceeded - 50 K. However it appears, in general, that such turbulent mixing has not been important. 14 A.U., as suggested by Biermann and Michel /7/hand, then the comet ingredients be unlikely On the other if comets were born atwould a distance of 1O to have been at temperatures > 16 K /8/. If their birthplace was even further out, in a gravitationally connected (to the presolar nebula) cloud fragment with its own accretion disk /9,10/, or, as suggested by Clube and Napier /11/ in any dense molecular clouds, the temperatures to be considered are not greater than — 15 K. PRECOMETARY INTERSTELLAR DUST Comparison of observed infrared absorption spectra with spectra in the laboratory gives clear evidence that the inner mantle is an (as yet) not completely but at least partially understood and identified complex organic mixture or polymer /12,13,114/, and that the outer mantle contains not only H 20 but also Co /151, OCN /16/ and a sulfur containing molecule (probably OCS) as well as H2S /17/. Evidence also exists for the presence of H2CO /18/. The basic processes which govern the evolution of the core—mantle grains in molecular clouds are: accretion of the condensibles which include 0, C, N and S; ultraviolet photoprocessing of the accreted ices at the prevailing grain temperatures CT - 10—15 K; and grain explosions which replenish molecules in the gas phase. Considerations of the gas phase ion molecule reactions plus grain surface interactions and explosions leads to the predominance of H20 in the outer grain mantles /19,20/. It is as a result of the photoprocessing of the H20 rich mantles that there occurs a buildup of complex organic material /21,114/ which is tough enough and refractory enough for a substantial fraction to survive the destructive processes which occur during the grain’s existence in the diffuse clouds. Along the way to producing comple organic molecules, the effect of ultraviolet photoprocessing is to modify the molecular composition of the grain mantles. In the laboratory it is seen for example, that CO is converted into CO2 /22/. Another product of the photoprocessing is the diatomic sulfur molecule S2 as demonstrated in the laboratory from irradiation of low temperature CT 10 K) dirty ices containing H2S /23/. Laboratory investigations of grain mantles have been shown to have a direct application to observations of interstellar grains. A recent analysis of the infrared spectrum of’ a deeply imbedded protostellar object has led to the definite identification in the predominantly H20 ice mantle of the OCN ion. The abundance of cyanate ion is shown to be - 1%. This, in itself is an interesting result because it is the first identification of an ion in interstellar grains /16/. However, from the point of view of dust — comets — and the origin of life it presents an entirely new possibility; the presence of pyrimidines in Interstellar dust and, by inference, the suggestion that these basic building blocks of prebiotic chemistry were brought very early to the earth by comets. The reason to suggest that pyrimidines form a substantial part of the dust is that we also know that cyano—actylene is a constituent of the gas /214/ and that accretion of cyanoacytylene on the dust may lead to chemical combination with the cyanate ion to form cytosine and “since cytosine hydrolyzes quite readily to uracil”, these reactions constitute a prebiotic synthesis of’ the pyrimidines /25/. The direct evidence for organic residues in space was finally exhibited in the observations

Organic Compounds in Interstellar Dust of a 3.11 pm feature

seen towards objects

itt the galactic

center.

(6)17 The first

such observation

was of SgrAW /26/ and, since then, better observations of this feature taken with higher resolution have confirmed its presence in a number of different galactic center sources /27,28,29/. Laboratory produced residues have been succesful in recreating the essential shape of the 3.14 pm feature which consists of contributions from the C—H stretch in CH 2 and CH3 groups in complex organic molecules /30/. The spectral correspondence between the laboratory residue and the galactic center supported quantitatively by the measured strength of the laboratory 3.14 pm feature /31/.

is

1g cm’~3s~.The to The production rate of the O.R. in the laboratory, when ~caled to determinetime the required production rate in the produce molecular observed clouds, density gives of ~O.R. /31/ dp0 2~x~1O’~26g R /dt - 1.5 cm’~3 x Is 10”- 11 x 10~yr. This is of the order of molecular cloud lifetimes and shows that the O.R. grain mantles are produced at a rate which can maintain it against destruction. The evolutionary picture of dust which is emerging is a cyclic one in which the particles find themselves alternately in diffuse clouds and in molecular clouds. A small silicate core captured within a molecular cloud gradually builds up an inner mantle of organic refractory material which has been produced by photoprocessing of the volatile ices. Within the dense clouds critical densities lead to star formation and subsequent ejection of’ some of’ the cloud material back into the surrounding space. Much of this material finding itself in a very tenuous low density environment, expands to the diffuse cloud phase. Dust particles in the diffuse medium are subjected to numerous destructive processes which rapidly erode refractory material. It is important to note that without their organic refractory mantles the silicate cores could not survive. The rate of destruction of pure silicate grains leads to a maximum lifetime of TSU —j 8_sr w~ichconverts to a mass loss rate of dpsi 3x 10 1/dt — —5 x 10 g cm 5 5gabundance cm3 s•’~ which is Assuming a mass loss rate from M stars of 1 M0 yr’~ and a full cosait~c Silicate production 100 times lower leads th to a production rate for silicates of dpsi1/dt < 10 3 s’~ is adequate thethemantle material in O.R. the 14an the destruction rate. Ontoth replenish other hand, production rate lost of the diffus~ phase of dp0 R ~cloud /dt ~o 1 g even ctii if the O.R. is somewhat less tough than the silicates /21/. Therefore silicate core-organic refractory mantle grains survive the diffuse cloud phase to reenter the molecular cloud phase. The mean st~r production rate of 1-2 M 1 implies an interstellar medium turnover time - 5 x 10’ yr so that this is the absolute 0 yrmaximum lifetime of a dust particle no matter how resi~tant t~ destruction. If’ we use a mean molecelar cloud—diffuse cloud period of 2 x 100 yr (100 years in each) then a typical grain anywhere in space will have undergone at least 20 cycles so that, for example, the typical diffuse cloud dust particle age is > io9 yr and consists of a mix of particles which have undergone a wide variety of

photoprocess ing. Table 1 shows a listing of molecules which may be shown from observations and laboratory spectra to be definitely present in interstellar grains. The relative proportions of the various chemical constituents are estimated in Table 2 for precometary grains in the latest stage of cloud contraction. The H 20 content is estimated by assuming that it is about 70% of the volatile grain mantles as seen in molecular clouds /32,12/.

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J. M. Greenberg Table 1

Molecules directly observed in interstellar grains and/or strongly inferred from laboratory spectra and theories of’ grain mantle evolution. Molecule

Comment

H

20

0

NH3

0

H2S

0

CO H2CO

0 0

OC~(

0

OCS

0

CO2

I

H2

-

K2

I

K2

H2

CH4

I

complex organic

0

“silicate”

0

C,B

“carbonaceous”

0

B

I

HC3N O I

I

observed, inferred,

I

M1 C.

-

inner mantle, core

M2

-

H1

K2

outer mantle,

B

-

small bare,

Table 2 Mass distributions of’ the principal chemical constituents of precometary dust. Component

Mass fraction

Silicates Carbonaceous Organic residue

0.14

H2O C~ Other molecules and radicals (H2CO, NH3, CO2 CM11, OCS,H2~,OCN, HC3N, HCO...)

+ (O.06)* (0.06) * 0.19

0.38 0.05 ‘

0.13

*Itema in parentheses correspond to very small particles (a < 0.01 pm)

In forming the nucleus we assume that first clumps of’ grains form and then clumps of clumps and so on until finally we reach the size of the comet nucleus. The question is, does this lead to a fully compact structure or does it lead to an open structure. The evidence we have for an open structure is the low density of meteors which are known to be comet debris /53,514,55/. If we should start with the interstellar dust tightly packed and then remove all

Organic Compounds in Interstellar Dust

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the volatiles (along with the trapped super small particles) the resulting of 3 /36/.mean It density is however the remaining core—organic refractory grains skeleton is about 0.5 g cm’ observed that meteors (which are what is left after the original cometary volatiles have evaporated) have a characteristic density of — 0.1 g cm’3 /37/.. We see therefore that the original dust was not fully packed and, in fact, about 20% (0.1/0.5) of the original comet nucleus was filled with dust — the remaining 80% being empty space /36/. A dust packing factor of about 0.2 is suggested by the mean comet density of p 0.25 g cm’~3derived from the tidal splitting of P/Comet Brooks 2 by Sekanina and Yeomans /38/. EVOLUTION OF THE NUCLEUS IN THE OORT CLOUD AND LATER Comets, once formed, become objects of the Oort cloud circulating about the sun in near insterstellar space. Insofar as further ultraviolet processing is concerned, even l4~billion years can lead only to changes in the outer few microns (!) of the nucleus. Furthermore, the temperature of the nucleus, barring possible internal heating by radionuclides, is even lower than the 10-15 K of’ interstellar grains. However, cosmic ray protons are another matter and their effects have been considered by several authors /39,110,141/. Their principal effect is to change the chemical composition of the material they penetrate in a way similar to the changes induced by ultraviolet processing of dust in interstellar space. The question is how significant is this additional processing? It had already been shown by Moore and Donn /140/ that only the outer fraction of a meter of the nucleus could be chemically modified by the MeV cosmic ray protons so that the higher energy ones must first be slowed during penetration before they are effective. A recent calculation of the radiation dosimetry of a cometary nucleus by cosmic ray protons with energies ranging from 1 Hey to 1010 0eV and including all secondary cascade effects will be used as a comparison with the ultraviolet radiation dosimetry of’ interstellar grains /112/. According to the new calculation the absorbed dose after k~billion years at about 1 m depth in a comet of density 1 g cm’3 is 1011 rad which is qul 1 6teradsmall on a compared core—mantle in its with grain the typical ultravioletThus lifetime. interstellar the ultraviolet photoprocessing dose on grains dose ofduring 6 x i0only one of the several billion years they spend before aggregation is about 2.5 x 1o16 rad which is 100,000 times more than the cosmic ray dose at the surface after aggregation and even more in the interior. Since the outer few meters of a comet are lost already in its first apparition there is no reason to expect to find evidence of cosmic ray effects inside a comet like Halley which has been around many times. The existence of a crust as observed over - 90% of’ the surface of comet Halley is probably due to evaporation of volatiles from the outer layers and perhaps fall— back of previously ejected dust rather than the effect of cosmic rays /43/. The next question is the modification of’ the nucleus by internal heating after entering the inner solar system. Heat conduction of porous structures is substantially lower than that of solid material. First of all we start with the fact that the grain ices are amorphous which means that their thermal conductivity is already a few orders of magnitude lower than crystalline ice /l414/• Next I have estimated the thermal conductivity of the fluffy bird’s nest structure by letting each grain have about 14—5 grains touching it and by letting each contact surface be about 2 x 1O’~5cm2(limited by local heat dissipation after collisions). Using a mean grain area of 2 x 1O~3cm2(fully “grown” pre—solar grain) means that only about s x io2 of the surface of’ each grain can conduct heat so that the net heat conductivity of’ a comet is expected to be 5 x 10’~ that of’ crystalline ice. This leads to a very high temperature gradient at the nucleus surface and at the same time, because of the very open structure, a very high evaporation rate which leads to efficient cooling. A detailed calculation incorporating both of these effects has not yet been done but it appears possible that while the surface of such a comet would be hotter than had been anticipated the amount of internal heating may still be low. The diffusion of vapor into the interior is limited by the very small pore size (recondensation occurs quickly) so that the morphological structure of the aggregated dust tends to be self preserving.

J. Ni. Greenberg

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COMET IMPACTS What happens to a comet when it impacts a planet is difficult to answer with great confidence particularly if’ we are trying to determine precisely what fraction of’ it evaporated or pyrolyzed by the intese shock generated heat. The comet structure postulated in the previous section possesses features at various scales which must play a role on the effects of the shock of impact at speeds — 50 km s~. We have 3) shown that the packing of a comet nucleus must be in the range 0.1 to 0.2 ~ 0.3 g cm so that the assumption of a mean comet density ~C 0.1 made by Lin /145/ for considering comet impacts was not entirely unreasonable. At such low densities the impact with the atmosphere will already be expected to disintegrate a considerable outer fraction of the comet before it impacts the planet. It has been shown /116/ that the strong heating by shock compression of porous bodies can lead to sharp anomalies on the Hugoniot curve. Whereas, for compact materials, the pressure rises with decreasing volume (compression), for increasing porosity (decreasing packing) the pressure rises more and more steeply and finally one reaches the anomalous state in which the volume increase with increasing pressure. This leads to the generation of a rarefaction shock rather than a compression shock. Another factor which must play an important role in the effect of the shock is that on the smallest (interstellar grain) scale, the composition of the grains is chemically heterogeneous and the sound speed in the more volatile components is substantially less than in the refractory components. The shock dissipates more energy in the low sound velocity components, which therefore provide an energy sink, so that they are quickly evaporated with less heat going into the refractories. In a manner of speaking, each grain may act like a free surface unloading material in the direction of the initial motion of the shock. In general, boundaries between the hierarchy of sizes of cometesimals /147/ treated in this way would provide fragile dividing layers in •the comet nucleus structure leading to splitting of the comet into many fragments following a similar hierarchy. Thus rather than the shock leading to complete evaporation of the comet, this distribution of the total energy allows for a great deal of the energy being dissipated by unloading at the enormously increased surface area. The energy being taken up by comet fragments could be radiated and ablated away so that if this scenario is correct we could believe that a substantial fraction of the comet material preserves its chemical and particulate integrity, rather than being totally evaporated. COMET CONTRIBUTION TO THE EARTH As early as 1961 Oró /118/ suggested that comets could have supplied part or all of the initial inventory of organic matter for chemical evolution. Chang /149/ stated that if the hydrogen/carbon ratio in comets is whitin an order of magnitude of values between 90 and L4 then comets must have provided a major fraction of the volatiles of the present atmosphere and oceans and bound in the biosphere and the crust. Following Greenberg /8/ it may be shown that the interstellar dust model predicts H/C for comet volatiles alone to be (H/C) 9.1/.28x3.7 — 9 while including refractories, leads to (H/C) 3. Both of these values are well within the bounds given by Chang.Vanysek /50/ has shown that by assuming an enhancement of D in comets as due only to molecules Xli other than H 2O, then in the comet model derived in Table 2 a ratio of’ XD/XH ~ leads to an upper limit of 0/H 2 x 1O~ in comets which is similar to the ratio DIM for the water in the Earth’s ocean and an order of magnitude higher than in the interstellar medium. Even though, in the current epoch, the number of comet impacts is too small to contribute much to the earth’s volatiles (see e.g. Zimbelman /51/ and references therein), in the past, whether or not periodic comet showers have occurred, there must have been an early period of’ large numbers of comet impacts. For example, Chyba /52/ used the late impact record on the moon to arrive at an estimate of 1025 g of material accreted on the earth. If 10% of this was by comets then during the period 3.9 Gyr to 14.3 Gyr ago about 140% of the present ocean mass was deposited along with an

Organic Compounds in Interstellar Dust

(6)2 1

almost equal mass of organics. Let

us now consider the contribution of one comet. Using a mean density ~C 0.3 and 7.5 km gives a total mass 51Tgm x 1017 gm. Thus one large comet like comet H 211 gmHalley (mt. supplies — 0.3 x 5 x 1017 — 1.5 x lO Phys. Tables) which is equivalent to about 20.1O~10 The current km comets. oceanItmass is interesting is — 1.3 x ioto note that the quantity of complex organic matter accompanying this amount of water is — .21 x 1.2 x 5 x 1O17x ~ — .5 x 1o211gm which is about a million times the current biomass of the earth. If it is indeed true that comets provided the early oceans and therefore also the initial inventory of organic matter for chemical evolution then permitting even a small fraction of the organic refractories, in the form of amino acids and pyrimidine to survive intact, they would have given an enormous impetus to prebiotic evolution. —

A corrolary of the possibility that comets were responsible for life’s origins is, of course, the possibility that comets were responsbile for extinction and concomittant evolutionary jumps. The dust injected into the earth’s atmosphere by one comet can provide a very substantial extinction of sunlight if the fragments are small enough. An upper limit to the extinction is obtained by reducing the comet to its orginal interstellar dust non—volatile constituents. The number density of 0.1 pm grains in a p — 0.1 porous comet is - (0.1) R~/a~ — io32~ Distributed evenly within the earth’s atmosphere this leads to an colu~in density of’ 1O~2/ll,TR~ 2 x io13 cm’2 and an optical depth in the ultraviolet — 10 (I). Since much smaller optical depths may be sufficient to trigger an ice age the effect of one comet could be catastrophic even allowing only a small surviving fraction of’ comet debris with or without considering the additional debris from the impacted earth. Without periodic comet showers, the mean freqency of earth collisions is — 1 x io8 x number of’ new comets per year, which is of the order of unity /53/. This about one half the value resulting from taking the ratio of the projected area of the earth to the area of its orbit. To obtain this value I have multiplied the probability derived by ~imbelman per passage (1.33 x 1O”~)with comet /514/, During a comet shower as large as 2 x 10 as postulated by Davis et al. /55/ the number of comet collisions could be as large as 20. This number is based on the presence of N 0 1Q13 comets in the Oort cloud which is a rather high estimate compared with Weissman’s value of N0 — 1-2 x io12 /53/ for the outer Oort cloud although comparable with his estimate of the number of’ comets in an “inner Oort cloud” between 20 and 30 thgusand A.U. The mean frequency of shower comets based on 20 per 26 million years is 80 x 10 yr 1 which is — 100 times as large a~the general average frequency of ~omet impacts. This frequency of shower comets of 80 tc~ 100 extended over the full 14.6 x 10’ yrs leads to a total H20 input of (at most) 14 x 1O of the current ocean mass so that the presumption of an exceedingly high initial comet flux is required if the comets were to have contributed substantially to the earth’s water. On the other hand, the same shower frequency could have easily provided, over an initial period of — 5 x io8 years, an inventory of complex organic molecules comparable to or greater than the current biomass.

CONCLUDING REMARKS Evidence for very low temperature comet aggregation appears more and more to support the theory that interstellar dust grains in pristine form are the basic building blocks of comets. The mean density of such material compared with meteors leads to the conclusion that have strong implications on the albedo, nongravitatlonal and the comets are 3, probably as fluffy as freshly fallen snow.the The implied comet forces densities of physics p ~ 0.3 gofcmcomet impacts. The assumption that comets have the same chemical composition as fully accreted interstellar dust confirm the idea that comets contributed very substantially not only to the earth’s water but also to its early surface complex molecular composition. Presumptive evidence now exists that not only amino acids but also pyrimidines constituted a part of the interstellar prebiotic chemical composition of comets brought to the early earth.

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J. M. Greenberg

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