RADIATION CHEMISTRY OF CARBON AND NITROGEN COMPOUNDS IN H2O AND NH3 ICES ; RELEVANCE FOR CHEMICAL EVOLUTION IN SPACE

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RADIATION CHEMISTRY OF CARBON AND NITROGEN COMPOUNDS IN H2O AND NH3 ICES ;

RELEVANCE FOR CHEMICAL EVOLUTION IN SPACE

B. Nebeling, K. Rössler

To cite this version:

B. Nebeling, K. Rössler. RADIATION CHEMISTRY OF CARBON AND NITROGEN COM- POUNDS IN H2O AND NH3 ICES ; RELEVANCE FOR CHEMICAL EVOLUTION IN SPACE.

Journal de Physique Colloques, 1987, 48 (C1), pp.C1-693-C1-695. �10.1051/jphyscol:19871112�. �jpa-

00226254�

JOURNAL DE PHYSIQUE

Colloque C1, supplbment au n o 3, Tome 48, mars 1987

RADIATION CHEMISTRY OF CARBON AND NITROGEN COMPOUNDS IN H,O AND NH, ICES ; RELEVANCE FOR CHEMICAL EVOLUTION IN SPACE

B. NEBELING and K. ROSSLER

Institut fur Chemie (Nuklearchemief, Kernforschungsanlage Julich. D-5170 Jiilich, F. R. G.

Abstract : Radiolysis and photolysis of ice and ice-mixtures lead to the formation of new chemical ampounds. Preexisting chemical ampounds can be changed by the interaction with radicals formed in the frozen matrices and by direct radiolysis. The actual product distribution in an icy substrate is a function of radiation dose (fluence) and dose rate (flux). The reactivity of irradiated H p i c e depends on the equilibrium concentration, the nobility and reaction velocity of H*, HO* or H O ~ * radicals, electrons, etc. The nuclear recoil process (1,2) offers an elegant method to introduce carrier-free probes without disturbing the macroscopic canpsition of the matrix. The kind of products formed by the radiwctive atom is indicative for the redox properties of the ice. The a ying radiation, e.g. of protons to induce the 1 4 ~ (p,*) 1 1 ~ process in MI3 or th=p,,dp~)~k in H$, allows to mcdify the radiation load. Fig. 1 shows the typical behaviour of 1 k products in H p i c e at 77 K as a function of the deposited proton dose (2). A neutral phase at low doses is characterized by relatively constant yields of the major products CH$, CH3OH and 002. It is followed by a reducing phase at medium dose (CH3OH increase) and an oxidizing phase at higher dose (002 becomes the only product).

0 ^'-0

J

--k

I I I I I

Fig. 1 : lk nuclear r e c o i l product d i s t r i - bution i n H20-ice a t 77 K as a function of

1

dose.

I I I

1 10.'

I lo0

I

lo1 lo2

deposited proton dose . e V per target molecule

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19871112

C1-694 JOURNAL DE PHYSIQUE

A similar behaviour is observed for f r m the nuclear reaction 160(p,d)13~. The increase and decrease of the yields of the reduced or oxidized products, respectively, can be considered as a kind of "radio1 tic r clin ".

A dose behaviour of H p i c e towards 1 k and lfN anzthat y;Yhn3!ice twards 1 1 ~ is scannarized in Fig. 2. The O/H ratio of the carbon and nitrogen ccxnpunds, averaged over all products at one dose , is normalized by the O/H ratio of the target.

depostled proton dose,eV per target molecule

Fig. 2 : Averaged O/H or N.H ratio of l k and 13N nuclear recoil products in H20 and NH2-ice at 77 K.

A value of 1 signifies that products are formed statistically f r m the 0 and H components of the lattice. A value beyond 1 means oxidation, a value below reduction. The radiolytic recycling is due to the fact that H* radicals which are by far more mobile than the HO* govern the reactions. At higher radical concentrations a reaction can occur on site, the mobility becarnes less decisive, and the HO*

radicals can exert their greater reactivity. The ice becomes oxidizing. It is of interest to note that the average N/H ratio of the 1k/M-13 system figures always in the reduced regime, since HN* radicals are not as reactive as HO*, and the entire dose regime is governed by the H* radicals. An admixture of NH3 will shift the reducing phase of water ice to higher doses. This has been checked in H@/NH3

(3 : I) at 77 K, where substantial amounts of 1kH30H and ~ ~ b 3 E - i ~ were formed at doses up to 10 eV.

The findings are of importance for the chemical reactions in icy bodies in space.

The addition of rather abundant NH3 (and CH4) increases the reducing character and, thus, enables the build-up of organic mlecules at medium to high radiation doses.

In systems governed by HO* only inorganic products could be expected, such as CO, C02 and N02.

References

(1) Rijssler K. and Nebeling B., Reactions of energetic carbon and nitrogen in H@, MI3 and CH4 ices and implications in cosmic chemistry, this conference.

(2) Rijssler K., Proc. 3rd Int. Conf. Radiations Effects in Insulators, Guildford (UK) 15-19 July 1985, in press.

COMMENTS

I think it's worth pointing out that the chemical products studied by Dr. Roessler

represent only an extremely small fraction of the total chemical change occuring

within a system due to the incidence of an high energy cosmic ray. The products are

i n f a c t o n l y t h o s e which a r e formed a t t h e very end of t h e energy d e g r a d a t i o n p a t h o f t h e p a r t i c l e and which i n c l u d e t h a t p a r t i c l e i n t h e i r chemical s t r u c t u r e . During t h e d e g r a d a t i o n o f t h e i n i t i a l p a r t i c l e energy a c y l i n d r i c a l c o r e o f 10 t o 20 1 i n d i a m e t e r o f i o n i s a t i o n and e x c i t a t i o n o f t h e b u l k medium is formed. Using a r e a s o n a b l e e s t i m a t e o f t h e average energy t o form a chemical product of 100 e V one can e s t i m a t e t h a t f o r a n i n c i d e n t p a r t i c l e of i n i t i a l energy A MeV t h e r e w i l l be lo4 A times a s many molecules o f p r o d u c t s d e r i v e d from t h e bulk m a t e r i a l (eq. hydrogen and hydrogen peroxide f o r water i c e ) a s a r e formed i n c l u d i n g t h e i n i t i a l nucleus.

T h i s d o e s n o t however d i m i n i s h t h e p o t e n t i a l g r e a t i m p o r t a n c e of t h e s e minor p r o d u c t s s i n c e as was c l e a r l y shown t h e y can provide t h e more complex b u i l d i n g blocks which may l e a d t o e v e n t u a l development o f complex molecules s i m i l a r t o t h o s e found i n e a r t h l y b i o l o g i c a l systems.

Answer :

Besides t h e high energy cosmic r a y p a r t i c l e s t h e r e a r e many low energy (eV t o some 10 eV) ones by e.g. p h o t o d i s s o c i a t i o n , low energy s o l a r wind, secondary knock on p r o c e s s e s , which l e a d t o h o t r e a c t i o n s , but n o t n e c e s s a r i l y t o d e s t r u c t i o n of t h e m a t r i x .

J.F. CRIFO

Can you t e l l whether a t t h i s time t h e r e is t h e o r e t i c a l means of p r e d i c t i n g t h e r e a c t i o n s o r whether one h a s t o r e l y o n l y on experiment

Answer :

Computer s t u d i e s by t h e a i d of program MARLOWE ( b i n a r y c o l l i s i o n s i m u l a t i o n ) , c f K.

ROESSLER, ,G. EICH, i n P r o p e r t i e s and I n t e r a c t i o n s of I n t e r p l a n e t a r y Dust, R.H.

GIESE, P. LAiYiY (Eds.) REIDEL, DORDRECHT 1985, 3 5 1 / 6 .