Organic chemistry in the oceans of Titan

Adv. Space Res. Vol. 7, No. 5, pp. (5)71—(5)81, 1987 Printed in Great Britain. All rights reserved.

0273—1177/87 $0.00 + .50 Copyright © COSPAR

ORGANIC CHEMISTRY IN THE OCEANS OF TITAN F. Raulin Laboratoire Physicochimie de L’Environnement, Université Paris Val de Marne, 94010 Creteil Cedex, France

ABSTRACT On Titan, most of the organics present in the atmosphere must condense in the lower stratosphere and be solid near the surface, except methane, ethane, propane, propene and 1—butene which must be liquid and could form oceans containing large fractions of dissolved N2. The solid organics, depending on their density relatively to this liquid, can accumulate at the surface or at the bottom of the oceans. In addition, depending on their solubility in this liquid and on their atmospheric flux down to the surface, they can dissolve partly or totally in the oceans. From this stage, Chemical Evolution on Titan must have followed a way very different from the terrestrial one, involving physical chemical processes in a cryogenic apolar solvent mainly composed of CH4—C2H6—N2, in place of organic chemistry in water. Systematic study of the volumic mass and solubility of organics in such a cryogenic mixture of various compositions, at 94 K, is presented, using thermodynamic modelling. The results suggest that the oceans of Titan could be free of any icebergs’ of organic compounds. These oceans could be very rich in dissolved organics, with relatively high concentrations, in the range 1_106 M. In addition, the concentration of several of the organic solutes should be constant, buffered by a bottom layer of the corresponding compound in the solid phase. INTRODUCTION In spite of a low surface temperature (94K), several of the conditions necessary for the development of a chemical evolution towards complex organic chemistry are present on Titan. It has an atmosphere. This atmosphere, mainly composed of N2 and CH4 /1,2/, is very favourable to the synthesis of organics of prebiotic interest /3/. And very likely, it has an ocean of ethane and methane /4,5/. Several other compounds, inorganics (H2, CO, CO2) and organics : hydrocarbons (C2H6, C2H2, C2H4, C3H8, CH3C2H and C4H2) and nitriles (HCN, F-IC3N and C2N2) have already been detected in its atmosphere. From the results of laboratory experiments /6—8/ and theoretical modellings /9—12/, it seems likely that many other organics, including hydrocarbons and nitriles with higher molecular weight, and polymers of acetylene, ethylene and HCN, must be present in noticeable amounts on Titan. Because of the cold atmospheric temperatures, most of these compounds must condense in the lower stratosphere /13/. Then, they must sediment down to the bottom of the ocean. In addition, depending on their solubility in the liquid constituting the oceans and depending on their atmospheric flux down to the surface, they must dissolve partly or totally in the oceans. What are the organic solutes present in the oceans of Titan ? What is their plausible concentration ? Is there a solid layer of organics at the surface or do all the organics, for which the saturation can be reached in the liquid phase, sink ? Such questions, important for the general field of Chemical Evolution, are crucial for the preparation of a space mission to Titan, such as Cassini /14,15/. In a preliminary work /16/ we tried to answer some of these questions using thermochemical modelling based on the Preston—Prausnitz method. We considered only the case of a few of the low molecular weight organics likely to be present in Titan’s atmosphere, and a C2H6—CH4 cryogenic solvent, neglecting the presence of dissolved N2 in Titan’s oceans. In the present paper, we use the same methodology, but we considere a C2H6—CH4—N2 cryogenic solvent, and a much larger variety of organics. These include C2—C7 alkanes and cycloalkanes, C2—C4 alkenes, cyclopentene and benzene, C2—C4 alkynes, C1—C5 nitriles, pyrimidine and adenine. In addition, we try to get some information on the solubility of model polymers of Titan’s organic polymers using the Flory interaction parameter concept. Taking into account plausible fluxes of the selected organics to the surface of Titan, we deduce, for most of the selected solutes, its plausible concentration in the oceans.

(5)71

(5)72

TABLE 1

F. Raulin

Selected Compounds, their Fusion Temperature and Flux down to Titan’s Surface Relatively to Ethane.

Compounds

Formula

Fusion Temperature Tf K (a)

Relative Flux to Ethane r 5 (b)

Alkanes + +

Ethane Propane Butane 2—Methylpropane (isobutane) Pentane 2—Methylbutane (isopentane) Dimethylpropane (neopentane) Hexane 2—Methylpentane (isohexane) 2,2—Di.methylbutane 2,3—Dimethylbutane 3—Methylhexane Cycloalkanes

—

+

-

89.9 85.5 134.8 113.6 143.3 113.1 256.3 177.7 119.3 174 144.2 100

1 .024 4E—3 “

/19/

“

4E—4

(c) “

4E—5 4E—6

Cycloalkenes

Cyclopropane Cyclopentene Cyclopentane Methylcyclopentane Cyclohexane Methylcyclohexane Alkenes

C2H6 C3H8 nrC4HlO (CH3)2CHCH3 n—C5H12 (CH3)2CFIC2H5 (CH3)4C n—C6H14 (CH3)2CH(CH2)2CH3 (CH3)3CC2H5 (CH3)2CHCH(CH3)2 C2H5 CH(CH3)C3H7

145.6 137.9 179 130.7 279.6 146.4

Q

/19/

/19/

Benzene

Ethene (ethylene) Propene 2—Methylpropene (isobutene) 1—Butane 2—Butene (cis) 2—Butene (trans) 1,3—Butadiene Benzene

CH2~CH2 CH3CH~CH2 CH3CH(CH3)~CH2 CHaCH2C2H5 CH3CH=CHCH3 “

CH2~CH—CHaCH2 C6H6

104 87.9 132.6 87.8 134.1 167.6 164.1 278.7

/19/

.04 7E—5 7E—6

(d) “

(c)

/19/ /19/ 1.5E—4 (e)

: detected in the atmosphere * : asymetric carbon (a) : Ref. /18/ p. 629—665 if no other indication. (b) Adapted from Ref. /9,12/ if no other indication.

+

SELECTED ORGANICS Table 1 gives a list of compounds, essentially organics, which could be present in Titan’s oceans, mainly based on the results from laboratory simulation experiments. This list includes compounds which have already been detected in Titan’s atmosphere, and compounds which have not been detected but are obtained in simulation experiments. Some of those have also been included in kinetic models /9—12/. We have also added to this list cyclic hydrocarbons, a C7 alkane, and a C5 nitrile, chiral compounds or models of chiral compounds /17/. And we have included adenine and pyrimidine as example of a purine and a pyrimidine base. The inorganics include ammonia which could be produced (as discussed below) directly in the upper part of the ocean. The flux of to the flux experiments molecule cm factor of-.’ the case of

most of these compounds down to the surface of Titan has been estimated, relative of ethane, on the basis of theoretical modelling /9—12/ and simulation /6/ (from photochemical models /9/, the absolute flux of ethane would be 5.8 E9 2 ~2 ). When no data was available, we have assumed that the flux decreases by a 10, when the number of carbon atom in the molecule increases by 1, as observed in the C2—C4 alkanes.

Organic Chemistry in the Oceans of Titan

TABLE 1

(5)73

(continued)

Compounds

Alkynes

-

Formula

Fusion Temperature T K (a)

Relative Flux to Ethane F~ (b)

Allene

+Ethyne (acetylene) +Propyne Allene 1— Butyne +1,3—Butadiyne (diacetylene)

CHrCH CH3C2CH CH2~C=CH2 C2H5C~CH CH~CC~CH

192.4 170.5 136.9 147.3 236.6

HCN CH3CN C2N2 C2H5CN nC3H7CN (CH3)2CHCN C2H5CH(CH3)CN

259.9 229.3 245.5 180.3 161 201.5 175

CH2=CHCN CH2=CHCH2CN CH2=C(CH3)CN CH3CH~CHCN

189.5 186.7 237.2 221.5

CH~CCN CH3C~CCN

278 289

/20/ /21/

295 633

/19/

/19/ “

.2 .01 1E—4 1E—3 7E—3

(d) (c) (d)

Nitriles +Methanenitrile (hydrogen cyanide) Ethanenitrile (acetonitrile) +Ethanedinitrile (cyanogen) Propanenitrile (propionitrile) Butanenitrile (butyronitrile) 2—Methylpropanenitrile (isobutyro—) 2—Methylbutanenitrile Propenenitrile (acrylonitrile) 1—Butenenitrile (allyl cyanide) 2—Methylpropenenitr.(methacrylo—) 2—Butenenitrile (crotononitrile) +Propynenitrile (cyanoacetylene) 2-Butynenitrile (cyanopropyne)

.034 4E—4 1E—3 2E—4 2E—5 (f)

(e) (e) (c)

2E—6 4E—4 7E—5

(e) (d)

3E—3 3E—4

(c)

/19/ “

N—Heterocyclic N~’h

Pyrimidine

NH

2

6—Aminopurine (adenine) Inorganics +Carbon dioxide Water ,4nimonia

(c) (d) (e) (f)

Assuming Assuming From the Tf value

C02 H20 NH3

216.6 273 195.4

5E—5 IE—4

a factor 0.1 for each additional carbon. rs/rC2H2 Fi/FC2H2 initial rate of formation in simulation experiments /6/. of its isomer isovaleronitrile /19/.

MAIN COMPOSITION OF THE OCEANS. Lunine’s model of a C2H6—CH4—N2 ocean is the only model of Titan’s surface, proposed until now, which can explain in agreement with the Voyager data the detection of large fractions of CH4 in Titan’s atmosphere in spite of its photolysis in the stratosphere /4,5,22/. Thus the presence of such oceans at Titan’s surface seems very likely. Recent calculations on phase equilibria in N2—CH4—C2H6 systems applied to Titan /22/ indicates that equilibrium is reached for CH4 mole fraction in the gas phase ranging from .095 to 0, corresponding to a liquid phase composition varying from CH4(0.86)—N2(O.14) to C2H6(O.97)—N2(0.O3) respectively. Exploitation of these data shows that the variation of the mole fraction of N2 in the liquid ternary mixture, X3, as a function of that of CH4, X2, follpws the empirical relation X3

=

0.03

+

0.131 (X2)L202

(1)

C02 excepted, which is solid, all the other inorganics detected in the atmosphere : H2 and CO and the noble gases likely to be present, mainly Ar, are gaseous at the surface level. Like N2, these gases are partly dissolved in the oceans. An order of magnitude of their mole fraction, Xi, in the liquid phase can be estimated from their ideal solubility

(5)74

F. Raulin

TABLE 2

Mole Fraction of Dissolved Inorganic Gases in the Ocean (Order or Magnitude).

Solute I

Ar

Atmospheric~mole fraction Fl

0 —-‘0.28

Vapor pressure at 94 K Pvpi, atm

1.9

Xi

-c.,

2

CO 5

Mole Fraction in the Ocean Xi *

H

2 10

3 10

159

4 —

2 10

3.4 5

0 -~.0.22

2 .1O

1.3 10~-9 10~

From /2/.

Fi (P/Pvpi)

(2)

Where Fi is the mole fraction of solute i in the gas phase, P is the total atmospheric pressure at the surface level (1.5 bar) and Pvpi is the equilibrium vapour pressure of pure solute i at 94K. Relation (2) has been applied to H2, CO and Ar, using F! values given in Table 2. Pvpi at 94K has been calculated from Harlacher’s equation, the four parameters of which are known for these gases /18/. The obtained values of Xi (Table 2) mUst be considered as upper limits of the real mole fraction of these gases in the oceans. In the case of argon, due to the uncertainty on Fl, /2,5/, the uncertainty on Xi is very large. For CO and H2, the mole fractions in the liquid phase are small. In the following, we will neglect the presence of these three dissolved gases. The case of Argon would require a separate study. At 94K, all the organics, methane and ethane excluded, must be solid, except propane, propene and 1—butene which must be liquid. However these three hydrocarbons are very soluble in a CH6—CH4-N2 liquid mixture and we can assume that they do not give liquid phase separation. If we suppose that the fluxes of these compounds relative to C2H6 have been constant during the history of the planet, then their mole fraction, Xs, in the ocean should be Xs

=

rs Xi

(3)

Where Xl is the mole fraction of C2H6. Among these three liquid hydrocarbons only C3H8 can have an important mole fraction, and in the following we will neglect the contribution of propene and 1—butene in the main composition of the oceans. Thus this main composition can be expressed entirely as a function of a unique parameter : the mole fraction of CH4, X2, by coupling equation (1), and (3), with rs 0.024 (for propane, cf. Table 1), and equation (4): Xi

(1— X2

—

X3)/1.024

(4)

Because of the limit values of X3, X2 varies between 0 and .86. METHODOLOGY. For the other compounds of Table 1, solid at 94K, we will estimate volumic mass ~s and solubility Cs in a cryogenic mixture C2H6—CH4—N2 of various mole fractions of CH4. Then, to determine if the compound sink or float, we will compare ~s to the voluniic mass of the solvent. To estimate its concentration in the oceans, we will calculate if saturation has been reached, by taking into account its atmospheric flux, relatively to C2H6. If saturation is reached, the concentration is equal to Cs. If it is not reached, we can assume that the mole fraction of the solute follows relation (3) and can calculate from this relation the concentration of the solute in the oceans. Volumic Mass of the Solvent. The volumic mass from

~‘

of the C2H6—CH4—N2—C3H8 system has been calculated as a function of X2

Mi / (.~Xi Vi

+

VE )

(s)

with i = 1 to 4 (4=propane), Vi molar volume of constituent i, Mi = molecular weight of i and VE is the excess volume due to the mixing. VE has been estimated from available data for binary mixtures C2H6—CH4 rich in CH4 /23/ by neglecting C3H8 and N2 contribution, using the following equation /16/

Organic Chemistry in the Oceans of Titan

VE = 3.05 (0.5 — X2) 2.16 0.68 cm 3 mole Coupling of equations (5~and (6) has been used with Vi 3 mole

and V4

=

46.45,

(5)75

V2

35.89,

interpolated values at 94°K from the literature

60.90 cm

V3

(6) 38,79

/23/.

Volumic Mass and Solubility of the monomer Solutes. For each solute s, we have determined the molar volume of the subcooled liquid, Vs, and the solubility parameter Ss at 94K, using the correlation Tables of Lyckman et ml /24,25/, knowing the critical constants Tc, Pc, Vc and the acentric factor of the solute /18, 24, 26, 27/. When not available in the literature, the critical constants have been estimated by Lydersen’s method /18, p. 12/ and the acentric factor by Edminster’s relation /18, p. 20/. Volumic Mass (~s. The volumic mass of solid solutes has been estimated as ~s ,~ Vs / Ms where Ms is the molecular weight of solute s. This supposes that the solid solute is homogeneous, and is not a fluffy material. If this assumption is correct, then one can consider that our estimate is a lower limit of the volumic mass of the solid, since usually (except in the case of very polar substances like H2O), solidification occurs with a decrease of volume. Solubility. The mole fraction Xs of solute a in a saturated solution of C2H6—CH4—N2 has been calculated by the Preston—Prausnitz method, based on the regular solution theory /24, 25, 28, 29/. Activity of solute is given by In Ys Xs

(1

=

Tf/T) AHf/RTf

—

~(i

+

—

Ttr/T) AHtr/RTtr

(7)

where /\Hf and AHtr are the enthalpies of fusion and (eventually) of transition(s), and Tf and Ttr are the temperatures of fusion and transition(s), respectively. ~s can be estimated by RT ln ~‘s with

Aij

=

Vs

(~i

-

[

~ ~~(Ais

~j)

+

Aij/2)

Xi Vi

/

(8)

lij.~i.Ej

and ~1i,volume fraction of constituent z

]

Øi ,~i

(9)

i equal to

~ Xi Vi

(10)

We have applied this system of equations to a ternary solvent (1 = C2H6, 2 CH4, 3 a N2), including a solute s. Xs has been determined by iterative calculation of equations (7) to (10) for different values of X2. In these calculations, we used the values of Vi to V4 already mentioned, and ~i = 7.4, = 9.5 and ~3 5.3 (cal/cm3 )i/2 . AHf, /~Htr and Ttr have been taken from the literature /19, 24, 28, 30/. When not available, AHf has been estimated by an analog method already used /13, p. 156/. The empirical parameter lij is available in the literature, for most of the selected hydrocarbons and CO2 /18, 24, 25, 28, 29, 30/. In the case of the nitriles, we have estimated lis and 12s from the only published data /31/, and used the following values acetonitrile : lis .08, 12s = .07 , propionitrile : ha = 12s a .07, other nitriles his = 12s a .05. For all the nitriles we used 13s a 0. From the obtained value of Xs, we have calculated the molar concentration of the solute Cs (mole/l)

=

1000 Xs

/

V (cm3)

(ii)

with V equal to the molar volume of solvent used in equation (5). The same relation has been used to calculate the molar concentration of solute a, from its atmospheric flux (Table 1), when saturation is not reached. Polymers. The solubility of polymers in the cryogenic C2H6—CH4—N2 solvent at 94K has been qualitatively estimated using the Flory parameter concept /32, 33/. This parameter X has been determined from X = X e + X h ; the entropy contribution X e has been empirically fixed to the value 0.34 /33/. The enthalpy contribution X h has been calculated from Xh=(V/RT)((p-S)2 Where V is the molar volume of the solvent, previously used in equation (5); solubihity parameter of the polymer and ~ that of the solvent given by c5~

=Ejt~.~S~

(12) ~p

is the (13)

(5)76

F. Raulin

Øi

is the volume fraction of the main constituent i of the solvent, as defined by relation (10). &p has been estimated by extrapolating at 94K the value of the solubility of 0C parameter /34/. When the at inroom a variation of —0.01 (cah/cm3)1!2 not polymer available the temperature, literature, assuming Ep at room temperature has been estimated /by group contribution methods /32/. RESULTS AND DISCUSSION. Density of the Solutes Relatively to the oceans. Figure 1 gives the specific mass of the oceans, as a function of the mole fraction of methane. On the same figure is indicated the volumic mass of the selected compounds at 94K. The specific mass of the oceans varies between .485 and .65 g cm3, and that of the solute is > 0.7 g cm3 except for HCN (.675) and C2H4 (.635). In fact, as we will see below, C2H4 is very soluble in the oceans, and in spite of its high flux down to the surface (Table 1), it must be entirely dissolved. Thus all the solutes for which the saturation in the oceans could be reached, appear denser than the oceans. These calculations do not include polymers likely to be present in Titan. However, it can be noticed that the volumic mass of the

1.6 — co

1.4

2 —

Adenjne

1.2 1

CHCC2H5

—

1120 — C4H9CN —

—

C6H6

Pyrimidine

0.9

—

—

~ 0.8 C’

—C7H16

:~c6H14 ~

0.7

C2N2

C4H10

—

C~H~CN CH3CN —

—

C3H3CN

= —

C5_C7 Cyclanes

—

C4H6 ~C4H8

—.C2H2

U

— —

C3H7CN

CHCCH3

C5H12

:~

~I

—CH2CCH2

—

C3II5CN C2H)CN

—~

—HC3N

C4H2

~

—NH3

c3H6

HCN

I

—

©

C2 144

0.6

0.5.

C2H6

-

CH4

-

N2

SYStEM

94 K

0.4. _______________________________________________ I I I I I I 0 0.2 0.4 0.6 0.8 CH4

Fig. 1.

MOLE

FRACTION

Volumic mass of the oceans as a function of methane mole fraction, and volumic mass of the selected solutes at 94 K.

Organic Chemistry in the Oceans of Titan

(5)77

solutes increases with the length of the carbon chain. Extrapolation to polymers of ethylene, acetylene or HCN would give a volumic mass between .8 and 1.2 ; in agreement with the values of the literature, or that estimated by group contribution methods /32/, (assuming that the volumic mass of the polymer is roughly temperature independent). These calculations suggest that the oceans could be free of any surface layer or iceberg of solid organics, if those compounds are not fluffy materials. However there is a high probability that the atmospheric aerosols could be fluffy and might therefore float, at least for awhile, before sinking. Solute Concentration. Calculation of the saturation concentration shows that most of the alkanes, cycloalkanes and alkenes are very soluble in the cryogenic C2H6—CH4—N2 solvent, in the entire range of studied main composition (from 0% CH4 to 0% C2H6). This solvent can dissolve lO3 to 1 (isobutane) mole/h of C4—C7 alkane, and cycloalkanes, more than 1 mole/I of cyclopentene, more than 10 mole/i of ethylene, and in the range 10 2 to several 10 1 mole/l of higher alkenes. Thus, for most of these compounds, the concentration in the oceans of Titan will be constrained by their atmospheric flux and not by their solubility. C02, alkynes and specially nitriles, those later being slightly polar compounds, are less soluble and, because of their atmospheric flux, the saturation in the oceans may have been reached for most of them. The plausible concentration of most of these solutes is represented on Fig. 2 as a function of the CH4 mole fraction in the oceans. Dashed lines indicates that saturation is not reached ; thus, in that case the concentration is governed by the rs value of Table 1. This is the case for all the alkanes, and alkenes (Fig. 2.a), the concentration of which ranges from about 1 to 3 lO6 mole/i. For alkynes, allene and benzene (Fig. 2 b) the concentration is governed by the solubility of the solute, except for allene, in ethane rich solvent, and for butyne, in methane rich solvent. Calculated values for C2H2 are in agreement with the experimental values of the literature /35/. The benzene concentration is also fixed by its solubility, the range of values (2 i0~ to l0~ mole/l) is also in agreement with the reported experimental ones /36, 37/. For Ci—C3 alkylnitriles (Fig.2.c), the concentration is limited by the solubility, and ranges from 10 ~ to 10 6 (C2N2) mole/l. When the carbon chain increases, probably due to the increasing lipophily of the compounds from its alkyl group, the solubihity increases sligthly and starting from the C4 nitrile, the concentration is constrained by the atmospheric flux. This effect is not so important with the acetylenic nitriles, the lipophilic effect being decreased by the presence of the acetylenic group, as shown_on Fig. 2 d. The concentration of the selected unsaturated nitriles ranges from 10 ~ to 10 6 mole/i. On the same figure is plotted the concentration of CO2. Its value does not vary drastically with the composition of the solvent, and remains around 10 3 mole/l. This value is in agreement with the experimental ones /38/. Some highly polar compounds are also expected to be present in Titan’s oceans. Water, the presence of which in the atmosphere would explain the presence of CO and C02, could be 5 molecule cm 2 s 1 /9/. Ammonia, could be provided influx of about produced by by meteoritic the evolution of the upper 6.i0 layer of the oceans, under the action of cosmic rays. Bombardment of liquid nitrogen solutions of H2, CH4 and C2H6 produces several organics, including polymers, and NH3 /39/. Adenine can be produced from NH3—HCN liquid mixtures /40/. Although the Preston—Prausnitz method is far froe accurate for polar compounds, we have tentatively studied, by the same method, the behaviour of these solutes (we have added pyrimidine, as an example of a pyrimidic base), in order to have a rough idee of their maximum concentration. The resu!ts are shown on Fig. 3. NH3 and pyrimidine solubihity ranges from 10 4 down to few !0 6 mole/l, that of adenine from 5 10 7 to l08 and that of water from about 10 10 to 5 10 l4~ It is clear, in the case of water, that the saturation concentration must be reached in the oceans, and that it must be very low. Polymers. Several polymers could be present on Titan, formed in the atmosphere or directly in the ocean, as mentioned above, such as polyacetylenes, polyethylenes and HCN polymers. We have considered, as a test, polymers of we.ll defined structure : polyacetylene (—CHaCH)n with C~,’ul2, polyethylene (—CH2—)n with 6 ,~ 10, and, for HCN polymers, the simplest model structure (—CH=N-)n with J~....12.4 and the more complex structure proposed by Vàlker /41/, with 6..c. 13. Fig. 4 shows the variation of the Flory interaction parameter for mixtures of each of these polymers in a C2H6—CH4—N2 solvent at 94K, as a function of the CH4 mole fraction of the solvent. We have also included, for comparison, other nitrile polymers. The solubility of the polymer in the solvent requires practically X <-c.. 0.8. It can be seen that this condition could be followed only with polyethylene, and in a methane rich solvent.

JASR 7:5—F

(5)78

F. Raulin

1

I

I

I

I

1~

—~___—...~_C

I

I

I

2H4

C3H8

— -cc ‘ C

lO.i

—

c-c

C~H1o

—

U

—

b

—

~

0

._

10.

C2H~ CNCC2 145

z

.__j5H12

© X I-

_~___S_

——

i~-~.

—

— — —-. — — — —

Z

© io-~

— — — —

U

—

£j~’18I

1O~

~

io-

——

T

\

— -. E.3146

— ~

cc “ ..cc

CH2CCH2

~

\\

C4146

61¼ c

—._

C

—

7H~...-.c-

\

~ iii~iiIiiiiTc~

a

U,

106 I

O

1O~

I

I

I

0.2 0.4 0.6 CH4 MOLE FRACTION

0.8

0

I

0.2 CH4

I

I

I

I

0.4 0.6 0.8 MOLE FRACTION

I

io—~ CH2CHCN

I

CII 3CN

~~~HCH2CN CH~C(CHj)CN

C2H5CN

CH 3CNCHC

-cc

a

~

~

5 EZcH~cN~%\d

~

HC

z ioU © =

3N

U,

C2N2

d

C 1O~

I

I

0.2 CH4

I

I

0.4 MOLE

I

0.6

I

FRACTION

I

0.8

i06.

0

0.2 C144

0.4 MOLt

0.6

0.0

FRACtION

Fig. 2. Estimated concentration of the selected solutes as a function of methane mole fraction in the oceans. a: alkanes, alkenes. b: alkynes, allene, benzene. c: alkylnitriles. d: ethylenic, acetylenic nitriles and carbon dioxide. (Dashed lines indicate that saturation is not reached).

Organic Chemistry in the Oceans of Titan

10’_2.

I

I

I

I

(5)79

-

NH

3

~mid

c—c a

~

ins

1O~.

Fig. 3. Estimate of the solubility of ammonia, water, adenine and methane mole in the pyrimidine as fraction a function of the

n~

t~

= ~ C io—~ 1012

io_14

oceans.

1

0

0.2

0.4

0.6

0.8

CH 4 MOLE

FRACTION

~

/

~

Polyacrylonitri

6

I

> 13.4)

Polymethacrylonitrile

4 .

~

r (6-12.4)

Polyacet (5~a12)

3 U U I

ylene (5~~11.6) Polyethylene

z

2

(8alO)

.

©

INSOLUBLE

U U I-

z ~1 C -J U

SOLUBLE 0

I

0

I

I

0.2 CH4

I

0.4

VOLUME

I

I

0.6

I

0.8

FRACTION

Fig. 4. Estimate of the Flory interaction parameter of the system ocean — polymers for several polymers as a function of the methane mole fraction in the oceans.

(5)80

F. Raulin

CONCLUSIONS From this study, it appears that the oceans of Titan could be free of any surface layer or iceberg of solid organics. This cryogenic liquid would be very rich in organic solutes, with concentration in the range 1 to i06 mole/l. These solutes are not only aliphatic Ci—C7 hydrocarbons, but also alkenes, ahkynes, aromatics and nitriles. Alkanes and cycloalkanes excepted, the saturation would be reached for all these solutes, only a fraction of which can be dissolved in the oceans. Thus a deep layer of alkynes (specially C2H2 and its polymers), and nitriles (specially HCN and its polymers) would be present at the bottom of the oceans. It has been pointed out that the presence of a large quantity of solid acetylene on Titan would make this planet very unstable, because of the explosive character of this hydrocarbon, when it is solid or even liquid, and that consequently only its polymers might be present /42/. However the low temperature of Titan’s surface may reduce highly this instability and allow the presence in large amounts of the monomer. The only polymer slightly soluble in this milieu would be polyethylene. This bottom solid layer would constitute an important reservoir of organics, buffering their concentration in the oceans. Most of these compounds could be analyzed by in situ measurement from a landing probe. However the richness of the ocean in organics would require chemical analysis experiments of relatively high resolution, eventually able to separate chiral compounds. As an example, the maximum concentration of 2—methylbutanenitrile (in an ethane rich ocean) could be about 5 10 5 mole/i. That of a chiral cyclic compound is difficult to estimate, but this study shows that it is not limited by its solubility. The oceans could also contain other organics, which have not been included here, such as azides RN3. Those compounds, unstable at room temperature are stable at low temperature and are obtained by irradiation of liquid nitrogen solution of H2, CH4 or C2H6 /39/. Their possible formation in the conditions of Titan’s oceans would require systematic studies. However, the results of these laboratory experiments already suggest that Titan’s oceans are not static but represent a dynamic organic chemical environment, due to the chemical reactions induced at their surface by energetic particles, mainly cosmic rays of energy > ‘s.’ 30 0eV /11, 13/. From this stage, Chemical Evolution on Titan must have followed a way very different from the terrestrial one, involving organic chemistry in a cryogenic apolar solvent. ACKNOWLEDGMENT I wish to thank T. Owen for critical reading of the manuscript and useful comments. This study has been supported by CNES grants 85/CNES/i251 and 86/CNES/l245. REFERENCES 1.

D.M. Hunten, M.G. Tomasko, F.M. Flasar, R.E. Samuelson, D.F. Strobel and D.J. Stevenson, in : Saturn, ed. T. Gehrels and M.S. Matthews, Univ. Arizona Press, Tucson 1984, p. 671.

2.

0. Gautier, in : The Atmospheres of Saturn and Titan, ESA SP—241, 1985, p. 75.

3.

F. Rauiin and A.R. Bossard, Adv. Space Res. 4, n°12,75 (1984).

4.

3..]. Lunine, D.J. Stevenson and Y.L. Yung,

5.

Science 222, 1229 (1983).

3.1. Lunine, in : The Atmospheres of Saturn and Titan, ESA SP—24i, 1985 p. 83, and Ref. Included.

6.

F. Raulin, D. Mourey and 0. Toupance, Origins of Life 12, 267 (1982) ; A. Bossard, D. Mourey and F. Raulin, Adv. Space Res. 3, n°9, (1983);

and Ref. included.

7.

B.N. Khare, C. Sagan, E.T. Arakawa, F. Suits, T.A. Callcott and M.W. Williams, Icarus

8.

60, 127 (1984) and Ref. Included. F. Cerceau, F. Raulin, R. Courtin and D. Gautier, Icarus 62, 207 (1985)

9.

L.Y. Yung, M. Allen and J.P. Pinto, Astrophys. 3. Suppl. Ser. 55, 465 (1984).

10.

D.F. Strobel, in : The Atmospheres of Saturn and Titan, ESA SP—241, 1985, p. 145.

11.

L.A. Capone, 3. Dubach, 5.5. Prasad and R.C. Whitten, Icarus 55, 73 (1983).

12.

W.-]. Borucki, C.P. McKay and R.C. Whitten, Icarus 60, 260 (1984).

13.

C. Sagan and W.R. Thompson, Icarus 59, 133 (1984).

Organic Chemistry in the Oceans of Titan

(5)81

14.

F. Raulin, D. Gautier and W. Ip, Origins of Life 14, 817 (1984).

15.

J.P. Lebreton, in : The Atmospheres of Saturn and Titan, ESA SP—241, 1985, p. 225

16.

F. Raulin, in : The Atmospheres of Saturn and Titan, ESA SP—241, 1985, p. i6i.

17.

A. Brack and 0. Spach, in : The Atmospheres of Saturn and Titan, ESA SP—241, 1985, p. 189.

18.

R.C. Reid, J.M. Prausnitz and T.K. Sherwood, The properties of gases and liquids, McGraw Hill, New York, 1977.

i9.

CRC Handbook of Chemistry and Physics, 1977—78, CRC Press, Palm Beach.

20.

C. Moureu and 3.C. Bongrand, Ann. Chim. 14, 47 (i92O).

21.

R. Vessière and F. Théron, C.R. Acad. Sci. Fr. 255, 3424 (1962).

22.

R. Thompson, in : The Atmospheres of Saturn and Titan, ESA SP—241, (1985), p. iO9.

23.

J.B. Rodosevich and R.C. Miller, AIChE 3. 19, 729 (1973).

24.

G.T. Preston and J.M. Prausnitz, Ind. Eng. Chem. Process Des. Develop. 9, 264 (1970).

25.

J.M. Prausnitz, Molecular Thermodynamics of Fluid—Phase equilibria, Prentice Hall, Enghewood Cliffs, 1969.

26.

D.V.S. Jam, S. Singh and V. Gombar, Indian 3. Chem. 24, 545 (i985).

27.

3. Nath,

28.

E. Szczepaniec—Cieciak, B. Dabrowska, J.M. Lagan and Z. Wojtaszek, Cryogenics 18, 591 (1978).

29.

E. Szczepaniec—Cieciak, B. Dabrowska and J.M. Lagan, Cryogenics 17, 621 (1977).

30.

G.T. Preston, E.W. Funk and J.M. Prausnitz, 3. Phys. Chem. 75, 2345 (1971).

31.

R. Sadus and C.L. Young, Fluid Phase Equilibria 22, 225 (1985).

32.

D.W. Van Krevelen and P.J. Hoftyzer, Properties of Polymers. Correlations with chemical

md. Eng. Chem. Fundam 21, 325 (1982).

structure, Elsevier, Amsterdam, 1972. 33.

R.F. Blanks and J.M. Prausnitz, I.E.C. Fundamentals 3, 1 (1964).

34.

H. Burrell, Off. Dig. 27, 726 (1955) ; A. Jayasri and M. Yaseen, Progr. Organic coat. 7, 167 (1979).

35.

A. Neuman and R. Mann, Chemie-Ing. Tech. 41, 708 (1969).

36.

G.P. Kuebler and C. McKinley, Adv. Cryog. Eng. 19, 320 (1974).

37.

A. Neuman, R. Mann and W.D. Szaighary, KAltetechnik—Klimatisierung 24, 143 (i972).

38.

H. Cheung and E.H. Zander, Chem. Engineer. Progr. Symp. Ser. 64, 34 (1968).

39.

K. Horigome, S. Hirokami and S. Sato, Bull. Chem. Soc. Japan 51, 725 (1978) and Ref. included.

40.

H. Wakamatsu, Y. Yamada, T. Saito, I. Kumashiro and T. Takenishi, 3. Org. Chem. 3i, 2035 (1966).

41.

T.H. V~lker, Angew. Chem. 72, 379 (1960).

42.

0.5. Matteson, 3.1. Lunine, D.3. Stevenson and Y.L. Yung, Science 223, 1127 (1984).