The low temperature organic chemistry of Titan's geofluid

Adv. Space Res. Vol. 15, No. 3, pp. (3)321-(3)333, 1995 Copyright Q 1994 COSPAR Printed in Great Britain. All rights reserved. 0273-l 177/95 $7.00 + om

THE LOW TEMPERATURE ORGANIC CHEMISTRY OF TITAN’S GEOFLUID F. Raulin, P. Bruston, P. Paillous and R. Sternberg EPCOS-LPCE, URA 1404 du CNRS, University Paris - Val de Marne, F-94010 CrCteil Cedex, France

ABSTRACT Organic chemistry on Titan and prebiotic chemistry on Earth involve the same N-containing organics : nitriles and their oligomers. Couplings of their chemistry in the three parts of Titan’s geofluid (atmosphere, aerosols and surface) seem to play a key role in the organic chemical evolution of the planet. If liquid water was present on Titan, then a prebiotic chemistry, involving eutectics, similar to that of the early Earth, may have occurred. However, liquid water is currently absent and a prebiotic chemistry based only on N-organics may be evolving now on Titan. The other consequence of the low temperatures of Titan is the possible formation of organics unstable at room temperature and very reactive. So far, these compounds have not been sytematically searched for in experimental studies of Titan’s organic chemistry. C4N2 has already been detected on Titan. Powerful reactants in organic chemistry, CH2N2, and CH3N3, may be also present. They exhibit spectral signatures in the mid-IR strong enough to allow their detection at the 10-100 ppb level. They may be detectable on future IR spectra (IS0 and Cassini) of Titan. INTRODUCTION Obviously, there are close similarities between the so-called ptebiotic chemistry, which, on Earth, gave rise to the first living systems, and Titan’s organic chemistry /1,2/. Several of the key ingredients of terrestrial prebiotic chemistry /3/, mainly HCN and HC3N, are present in Titan’s atmosphere, both in the gas and aerosol phases (Table 1, /4/). If oceans even partly cover Titan’s surface, these compounds will be also among the organic solutes dissolved in noticeable quantities in the methane-ethane-nitrogen solvent /5/. But the exobiological implications of Titan’s chemistry go far beyond these analogies. Titan provides a unique example of a prebiotic reactor at a planetary size, with an atmosphere having nearly optimal composition for gas-phase prebiotic syntheses : N2-CH4. Such chemistry involves the three parts of Titan’s environment : - gas phase : the main source of organics (Table 1); - aerosols : carriers of these organics from the atmosphere to the surface (Fig. 1); - surface : solid, or liquid, allowing the accumulation of these organics, and, partly, their further evolution. Studying the couplings of the involved organic processes in these different parts, should provide information on extraterrestrial prebiotic chemistry and, by analogy, on terrestrial prebiotic processes, but with a main difference : the low temperature, In this paper, we study the possible implications of Titan’s low temperature on its prebiotic-like chemistry.

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F. Radio PI al.

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Table 1 : Chemical Composition of Titan’s Stratosphere (adapted from /4/).

Stratosphere* Mixing Ratio (E=Equ.; N=North Pole )

Compounds

Nitrooen N2 Argo\ Ar Methane CH4 Hydrogen H2

Production In Simulation Experiments **

Plausible Prebiotic Source of:

0.81- 0.99 ? 0 - 0.10 0.005 - 0.04 0.002- 0.006

Ethane C2H6

1.3 x lo5

Maj.

Acetylene C2H2

2.2 x10-6

Maj.

Propane C3H8 Ethylene C2H4 Propyne C3H4

7.0 x1o-7 9.0 x10-8 4.4 x 10-9

* ++ +

-> CH oligom. -> CH oligom.

Diacetylene C4H2

1.7 x 1o-8 1.4x 1o-g

+

-> CH oligom.

-> CH oligom.

2.2 x 1o-8

N-ON 1.6 x 1O-7

E

6.0 x 1O-7

N

7.0 x 10-8

N

++

Cyanogen C2N2 Acetonrtrile CH3CN Dicyano4ac;tylene

4.5 x 10-9 Detected Solid Pha.

N

+ +t

N

-> CN oligom.

Carbon dioxide CO2

1.4 x 10:;

E

)- *

N# T

) Possible source of

Carbon monoxide CO

c7.0 x 10 6.0 x lo5 -6 c4.0 x 10

Hydrogen cyanide HCN Cyanpcy

lene

Maj

fv

* N : 0.1 mbar or (#) 1.5 rnbar; T = troposphere * Mean relative abundance : Maj.= major pfoduct >> ++ >> +

-> CHN oligom. amino. ac., purines pyrimid. -> CHN oligom. pyrimidines -> Condens. Ag.

) 0-organics on ) Titan

Organic Chemistry of Titan’s Geofluid w.

The ChemicalCompositionof Titan’sAerosolsin the Stratosphere : MicrophysicalModelingof theEvolutionof theParticlefrom 100 to 50 km Altitudes.(cf. /l, 21,andrefs.included).

.*

CRUST

Ng t c2H2 LAYER

F. Raulin et al.

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LIQUID WATER OR NO LIQUID WATER ?

Titan may have known periods during its evolution where liquid water was present in noticeable quantities /6/. This may have provided the full environment required for prebiotic chemistry to evolve even closer to the chemistry of living systems, including the formation of amino-acids, polypeptides and some of the building blocks of polynucleotides . This could have been possible even at temperatures significantly lower than 0°C through the formation

of eutectics. One of the best example is the HCN-H20 eutectic /7/: this binary system remains liquid at temperatures lower than -20°C, for HCN mole fractions around 70 to 80% . At such temperatures, the reaction of polymerisation of HCN (assuming a pH close to the optimum value, that of the pKa of HCN/CN- at the corresponding temperature) is slow, but not negligeable. For instance, the rate determining process of this polymerisation is the dimerisation, which gives rise to HCN tetramer, through the dimer formation. The activation energy of this reaction is about 82 KJ mole-l and its rate constant is 8.2x10-6 mole-l 1 s-l at 61°C /8/. From the extrapolation of these data, it should be only 6.5~10-~~ mole-l 1 s-l at -20°C. As shown on Fig. 2, this is enough to allow a 50% completed polymerisation from a 20 molar aqueous solution of HCN within about 5 years (only), at -2O”C! But at -100°C the required time would be about 3~10~ years ! Although such processes could have been largely increased by the occurrence of dissolved ammonia /9/ (likely to have been also present, if liquid water was them), the extremely low temperature is a large handicap. Fig. 2 : Half-Life of Some Prebiotic Reactions (HCN polymerisation - from 20 M HCN aqueous solutions - and hydrolysis of nitrile into acid, through amide, from dilute aqueous solutions) as a Function of Temperature. -,-,--a-

l-l -87--&--#---,-I-#-

RCONH2 __--> RCQH

4 HCN ---> (HCNM

-

Temperature

YZ

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Organic Chemistry of Titan’s Geofloid

It is known that the hydrolysis of HCN polymers can give rise to amino-acids and purine and pyrimidine bases. The mechanisms involved in these chemical processes are still not well understood, nor are their kinetics. Nevertheless, we can considere analogous reactions, such as the hydrolysis of the nitrile chemical structure (-CN) into the acidic one (-C02H). which is involved in the Strecker amino-acid synthesis /lo/. For instance (Fig. 2). the half-life of hydrolysis of aminoacetonitrile (H2N-CH2-CN) in the corresponding amide ( H2N-CH2-CONH2) is 1 year at 25 “C; and 40 years at 0 “C /lo/; the half life of hydrolysis of the amide into the amino-acid is 9 and 300 years, respectively, at those temperatures. Extrapolation of those data at -20 and -60 ‘C gives half-times of about 1000 years and 9.6 millions of years, respectively, for the nitrile/amide reaction, and 10,000 years and 39 millions of years for the amide/amino-acid reaction (Fig. 2). Thus, indeed, terrestrial-like prebiotic chemistry period may have existed on Titan, but only if long enough periods (tens of thousand of years) with relatively moderate temperatures (higher than about -5O’C) have existed. . .

.

.

Withou&@ud Water 7c

Or Prebrotx C&m&v

Even in the absence of liquid water, and of the chemical processes requiring this solvent, Titan’s prebiotic chemistry may have evolved far enough to provide complex organic chemical systems, close to the chemistry of life. Indeed, most of the terrestrial biomolecules can be replaced, at least theoretically, by “ammono” analogs (Fig. 3) where 0 atoms are replaced by N atoms /l l/. For instance, C=O and OH groups can be systematically replaced by C=NH and NH2 groups respectively. Indeed, the C=NH group (imine function) does exist and its chemical properties are very close to those of the carbonyl group C=O. In addition, several cl-aminoamidines have been synthesized and some of the HCN oligomers may involve the structure of “ammono” peptides. Such compounds may have been formed on Titan surface, either at the beginning of its history, if ammonia was abundant, or more recently, if oceans are present. How far chemical evolution can go within the framework of such an exotic pseudobiochemistry has to be determined. Fin. 3 : Some (terrestriaI) biomolecules and their “amono” analogues. (from/l I/). Terrestrial

biomolecule

ltAmmono’V analog

0

II NH2 -CH (R) -C-OH

NH2- CH(R) --2 eH,,

a-aminoacid

a-aminoamidine

0

f;”

II -NH-CH (R) -C-NH-CH peptide

H6 Ribose

(R ’ ) -

-NH-CH(R)-C-NH-CH(R’)V1ammonolV peptide

bH

“Ammonott

ribose

x

I

<

M

<

<

<< <

2 mbar 1.5 MeV (a)

Protons

High:

M <

+

t

<

<<

M

<

I

<

<<

M

(a)

<<

M

<

zl <

CC

<

- 1 atm

F

r’ooraers or more

(a)

-1atm

r

reveral <<

sev. <<

M

<
<

<<

<<

<<

CC

M << << << << < << <<

c<

M M M <
rson ,991

several <<

(e

0.24 mbar

several <<

M M < < < < <<

M

&al

Tba

J 17 mbar

of Titan’s atmosphere.

M << << << << <<

several <<

several << M << < < < < cc <<

<<

M < cc )< ) <<

M

z$L & al 1975

26 mbar

Corona

I Discharges

<<

<<

<<

CC

IG << < <

<

XzL &al 1983

H2 escape

1.2 bar

Electric Ar c

compared to the currently known composition

c< :

< <<

M

M

1 CC

cc <

Electrons

lergy Particules

.t* less abundant than M by : <: : one 01 r of magnitude;

solid n

I

--I--

Detected

2 300 n 16 n

-t----

42 n

62n

M cc

al 1981

0.5 bar loo-200Nn

-s-j-T

13 000 90

(if n) IOrtll Dole

uv

Table 2 : Organic Products of Titan’s Simulation Experiments

M : major prod1

C2H5CN CH2CHCN CHCCN C3H5CN CH3C2CN cqN2 other nitriles CH3N3 other Norgmics

C2N2 CH3CN

HCN

zz”* C6H6 C6H2, C8H2 other hydrocarbons

c2H2 C3H8 C3H6 CH3C2H CH2CCH2

C2H6 C2H4

(Ref.1

PRODUCT

TITAN equator or,

Organic Chemistry of Titan’s Geofluid

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With or without liquid water, Titan’s organic chemistry presents a great opportunity: to understand the role of water in exobiology and the processes of chemical evolution toward Life, in the Universe. A REFERENCE PREBIOTIC LABORATORY Exoerimental Simulations vs Observations of Titan Without speculating too far, mom practically, Titan already offers a practical way to test our theoretical and experimental approaches to planetary organic chemistry. Several experiments have been carried out to simulate the chemical evolution of a N2-CH4 atmosphere, and the organic syntheses from such a medium /12- 16/. Comparison of their results with the currently known composition of Titan’s atmosphere shows a quite satisfactory agreement. In particular, the experiments by Raulin et al. simulating H2 escape /12, 13/, and the very recent experiments by Thompson et al /15/ provide a list of organic compounds with their relative abundance which is, for most of the main products, qualitatively and quantitatively close to the currently available list of minor species in Titan’s atmosphere (Table 2; /16/J. It has been shown, from the study of their IR spectra /17-19/, that several of the compounds which have been detected in these experiments but not in Titan, could be nevertheless present in Titan’s atmosphere at the ppb or even fraction of ppm level. For most of them, including the nitriles and several of the hydrocarbons, such as benzene, this is typically the level of expected concentration in Titan’s atmosphere, from theoretical or experimental simulations. The main differences between observations of Titan and the results from these experiments are : the abundance of ethane relative to the other products and the detection in Titan of C4N2 /20/. The problem with ethane can be easily explained, since most of the experiments use medium or high energy electrons, protons or gamma rays, instead of photons. Ethane. as the main product of methane photolysis, must be photochemically synthesized in Titan’s atmosphere. This is confirmed by the few experiments done with UV light. However, the other point is more critical, and shows one of the strong limitations of the experiments which have been carried out, so far : the temperature problem. All these experiments use temperature conditions much higher than Titan’s temperature. Moreover, the analytical techniques developped to analyse the products are usually not compatible with the quantitative analysis of compounds of low stability at room temperature. HC3N. for instance may experience some thermal degradation in the chromatographic column, if GC or GC-MS techniques are used. This is particularly important in the case of polyacetylenes (such as C6H2 and CgH2 included in kinetic modelings of Titan’s atmosphere /l/), dicyanopolyacetylenes (such as C4N2), and, most of all, diazo- and azido- compounds (such as diazomethane, CH2N2. and methylazide, CH3N3). In addition, one should point out that such compounds have not been specifically searched for during experiments simulating Titan’s organic chemistry. These are probably the main reasons why C4N2 has not been reported yet in simulation experiments. The Case of Thermallv Unstable Oreaniq Several of these organics, only stable at low temperatures, could be formed in Titan’s atmosphere. The presence of butynedinitrile (dicyanodiacetylene, C4N2) has already been reported in condensed phase /20/. Methylazide, CH3N3, has been tentatively detected in the most recent simulation experiment /15/. It is also, with other azides (HN3 and C2H5N3). the product of y-irradiation of solutions of H2. CI-Q and C2H6 in liquid nitrogen /21/. The formation of diazomethane may be expected from simple reactions such as /22/z --- (M) --> lCH2+N2 WPCt Both compounds are very volatile and very unstable at room temperature : they are explosive and their use requires great care. Diazomethane is a yellow gas, very toxic; its boiling point is -23°C. It is frequently used in organic syntheses, in spite of its unstability. In particular, it allows methylation reaction, on 0, N, S and C atoms, cycloaddition reactions, and cycle extension. Its structure can be represented by the mesomeric forms :

JASR 15:3-V

F. Raulin et al.

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<_________> Hz - NEN! H2C=N=N> Methylazide (azidomethane) CH3N3. is a gas, very explosive. Its boiling point is 18 “C . It is also used in organic synthesis. It gives methylamine by reduction, imines by rearrangement, and triazoles and other heterocycles by cycloaddition. The azido group is almost linear, with terminal N=N bond shorter than that of the N-N group bonded to C. Its structure can be represented by the mesomeric forms : <_________> H3C - N-NENI H3C-N=N=N> Fig. 4: Infrared spectrum of : a) : diazomethane (from /25/, 1 = 10 cm, P = 13 mbar)); b) ; methyl azide (from /26/, 1 xP not given, deduced value : 645 mbar cm)

1.

.I..

:;:

2000

1600

1200

I

I ,

800

400 cm-’

P 2

50

f i AZIDOMETHANE 0 LOO0

1000

2000

1600 FRECUENCY

1200

800

LOO

KM-‘1

The IR spectra of these two compounds have already been studied in detail, athough the absolute intensities of their bands have not been published. We have recently systematically reconsidered the literature data, in order to estimate these intensities, from the experimental spectra already available . For CH2N2, three independent publications /23-25/ are available with spectra and associated information allowing the calculation of absolute band intensities. Figure 4a shows, as an example, one of the published IR spectra /25/. From these data we have selected the most intense IR bands of diazomethane and estimated their absolute intensity. The results are plotted on Table 3. Diazomethane exhibits several intense signatures in the IR. The most intense are 2vg around 850 cm-l, vg+vg around 950 crn-ltvq around 1170 cm-l, v3 around 1400 cm-*, v2 around 2100 cm-l, 2~4 around 2300 cm-l and vt around 3070 cm-*. Similarly, fig. 4b shows one of the most recently published IR spectra of methylazide (but given without indication of the cell length, nor on the sample pressure) /26/. From this paper and with the help of the only publications including the necessary data (spectra and related indications) /27-28/, we have determined the most intense bands of methylazide, and estimate their absolute intensities. The results are plotted on table 4. The most intense bands are vro around 250 cm-l,% around 665 cm-l, vs arOyId910Cm-1.VgarOUnd 1300Cm -l,v5 at 1416 cm-l and the combination band ~1+~4+~12 at 1478 cm .

S 58.6

V

849

Crawfordet al, 1950

1282

1177

853

V

8.1---

14.9

34.2

S

Pierson et al, 1956

1170

852.3

V

-=-xR---

549

S

30.0

54.9

S’ *

Vogt & Winnewisser, 1984

8.1

Table 3 : IR Bands of CH2N2

9.4

49.2 _ 15

S +

Average

22.5

V, wavenumber in cm-l and S, calculated absolute band intensity in at.rns2cm-l base e *S’ : corrected band intensity assuming P = 130 mbar instead of the published value (13 mbar).

3v9

v4

v8

v5+v6

2v9

Band

74.4

17.9

16.3

40.1

294

61.7 17.4

1272

910 1132

299

54.9 17.1

3.7

31.3

16.1

4.5

18.4

Table 4 : IR Bands of CH3N3

V, wavenumber in cm-l and S, calculated absolute band intensity in atmm2cm-l base e. * values note considered ** IxP not given; assumed value : 645 mbar cm-l (by comparison of absorbance at 664 cm-l with Mantica & Zerbi spectrum)

1478

79.7

‘5+‘4+‘12

1478

1472

v12

1270 1455

110.6 *

v4

v6

910 1128 1416

1301

v7

55.8 108.5 *

v5

920 1130

v8

775

WV15

14.3 666

559

v9

3.6 560

557

v14

S 410

V

Nielsen & Sjogren ** 1987 S V

?

s

Mantica & Zerbi 1960

250

V

Eyster1940 & Gillette

VlO

Band

77.1 T 2.6

17.5

16.3

40.1

296 ~3

+ 4.2 57-5 _ 1.7 17.3 r 0.2

3.7

30.1 T 1.2

+ 4.8 11.3 _ 7.7

4.5

18.4

S

Average

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Organic Chemistry of Titan’s Geofluid

Both compounds thus exhibit intense IR signatures in the spectral region of Titan observed by the Voyager IRIS instrument (about 1400-200cm-l). Before higher resolution and sensitivity spectra of Titan become available, from these IRIS spectra (Pig. 5) it is already possible to derive upper limits of abundance of the two studied N-organ&, compatible with the absence of signature on the IRIS spectra. $? $??;(A) / fo) (&(A) / ho) (%o / &o(T)) where Iv(A) and Ivo represent respectively the contribution of the compound A and that of a reference compound in Titan’s radiance at the wavenumbersv and vo, f(A) and fo are their mole fraction, Sv(A) and Svo are the absolute intensity of their IR bands, centmd at the wavenumbersv and vo, respectively, and BV(T) and Bvo(T) arethe values of the Planck function at temperature T and at wavenumbers v and vo. Using the vg band of HC3N (vo = 500 cm-l; Svo = 32.1 cm-2 arm-1, base e; Ivo = 0.16 erg cmm2sr l/cm-l) as a reference, its mean stratosphericmole fraction (3.5x10-8).the IRIS noise level in this wavenumber range (about I&8), and the valuesof the Planck function at 130-150 K /17-19/, the resulting upper limits, for each band of the studied compounds, are plotted on Table 5a ( CH2N2) and Table 5b (CH3N3). The obtained values indicate that diazomethane and methylazide may be present in Titan’sstratosphere.with a mole fraction as high as about 33 ppb and 1.5 ppb, respectively. m: Infrared spectrum of Titan from the IRIS-Voyagerdata (average of 26 spectra taken at grazing incidence near the North Pole) /4/. The wavenumberpositions of the signatures of diazomethane (a) and azidomethane (b) are indicated by an arrow.

I

b

200

600

WAVENUMBER

1000

CM-1

1400

(3)332

F. Raulin et al.

: Maximum Mole Fraction of Diazomethane (a) and Azidomethane (b)

I&I&

Compatible With Titan’s IRIS Spectra, Based on IR Band Intensity.

CH2N2

(a) :

Centre cm-l

Band

852.3

2vg

CH2N2 ~ mole fraction

SV cmc2 atm-l 49.2

3.3x10-8

“5+“6

950

42.9

Y

“a

1109

4.0

7

“4

1170

22.5

3%

(W :

I

1280

I

8.1

1.3x10-6 ,

1.1x10-5

CH3N3

Band “10 ? “14 A_ “8 “I “6

Centre cm-l

CH3N3

SV

mole fraction

250

cm-2 atrn- l 18.4

410

4.5

_

560

11.3

U

666

30.1

7

910

57.5

T

1132

17.3

1272

299

1.5x10-9

4.1x10-7 8.4x10-8

One should point out that these calculations are based on absolute band intensities estimated or determined at room temperature. However, recent studies on the variation of the absolute IR band intensity with temperature, carried out between room and low temperatures for benzene /29/ and cyanoacetylene /30/ show that SV varies only slightly with T. However, the band intensity presently used for diazomethane and methylazide are only estimated from literature spectra. Thus new spectroscopic experimental works, specifically devoted to band intensity determination for these compounds, as well as others such as C6H2 and C8H2, are still needed. Some are in progress in our laboratory, including in the UV range at low temperature /31/.

CONCLUSIONS Review of the experimental work which has been developed so far to study Titan’s chemistry, shows that there is now a need for low temperature experimental simulations, including the use of analytical procedures compatible with the quantitative analysis of compounds only stable at low temperature, such as direct MS, or fast CC, eventually coupled to MS, or fast spectroscopic techniques, such as FTIR, with very sensitive detectors (MCT). Several organic compounds of low stability at room temperature could be present in Titan’s atmosphere and surface. Several of them have strong spectral signatures; they could be detected, even before the encounter of the Cassini-Huygens mission with

Organic Chemistry of Titan’s Geofluid

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the Satumian system, by remote sensing techniques, mainly in the mid- and far- IR regions. Tbis is the case for diazomethane with its 2vg (852.3 cm-l) and v +vg (950 cm-l) bands and methylazide, with its vto (250 cm-l), vt4 (560 cm-l) and vg (666 cm- ?) bands. If these compounds are indeed present in Titan’s geofluid, then a very complex organic chemitry, involving these highly reactive compounds can be expected in Titan’s geofluid. ACKNOWLEDGMENTS This work has been supported by the Centre National d’Etudes Spatiales and the Programme National de Plant?tologie.

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141-148 (1991); F. Raulin et al, J. British

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26 27 28 29 30 31

included.. - C.J. Nielsen & C.E. Sjogren, J. Mol. Struct. 150, 361-379 (1987). - E.H. Eyster & R.H. Gillette, J. Chem. Phys. 8, 369-377 (1940). - E. Mantica & G. Zerbi, Gazz. Chim. Ital. 90, 53-68 (1960). - M. Khlifi et al, J. Mol. Spectrosc. 154, 235-239 (1992). - M. Khlifi et al, J. Mol. Spectrosc. ,155, 77-83 (1992). - P. Bmston et al, J. Geophys. Res. 91 (E2), 17,513-17517 (1991).

(1964), and refs.