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AMORPHOUS ICE IN ASTROPHYSICS
J. Klinger
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, suppl6ment a u n012, Tome 46, d 6 c e m b r e 1985 page C8-657
AMORPHOUS I C E
I N
A S T R O P H Y S I C SJ. K l i n g e r
Laboratoire de GZacioZogie e t Gbophysique de Z'Environnement,
B.P. 96, 38402Saint Martin drHdres Cedex, France
Resume - L e s c o n d i t i o n s e x t r c m e s d a n s I e s q u e l l e s s e condense l a m a t i s r e e x t r a t e r r e s t r e f a v o r i s e n t l a f o r m a t i o n d e s o l i d e s a m o r p h e s . Le p r e s e n t a r t i c l e e s t c o n s a c r e 2i l ' e a u s o l i d e a m o r p h e q u i a 6 t e i d e n t i f i e e d a n s l e m i l i e u i n t e r s t e l l a i r e e t q u i s e m b l e j o u e r u n r a l e d a n s l ' 6 v o l u t i o n d e s comstes. Sa p r e s e n c e e s t egalement p r o b a b l e d a n s l e s p a r t i c u l e s d e s a n n e a u x d e S a t u r n e e t d a n s l e s c a l o t t e s p o l a i r e s d e c e r t a i n s s a t e l l i t e s g l a c e s d e s p l a n e t e s g e a n t e s . A b s t r a c t - The e x t r e m e c o n d i t i o n s u n d e r w h i c h e x t r a t e r r e s t r i a l m a t t e r c o n d e n s e s f a v o r t h e f o r m a t i o n o f amorphous s o l i d s . T h i s p a p e r i s d e a l i n g w i t h amorphous s o l i d w a t e r t h a t h a s been i d e n t i f i e d i n t h e i n t e r s t e l l a r medium and t h a t seems t o b e i m p o r t a n t f o r t h e e v o l u t i o n o f comets. It i s l i k e l y t o occur i n t h e p a r t i c l e s o f S a t u r n ' s r i n g s and i n p o l a r c a p s o f some o f t h e i c y s a t e l l i t e s o f t h e g i a n t p l a n e t s . I
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INTRODUCTION C o n d e n s a t i o n o f m a t t e r u n d e r e x t r e m e c o n d i t i o n s l i k e t h a t o f a s t r o p h y s i c a l e n v i r o n m e n t s o f t e n o c c u r f a r from e q u i l i b r i u m . T h i s e v i d e n t l y f a v o u r s t h e f o r m a t i o n of amorphous s o l i d s . T h i s is t r u e f o r r e f r a c t o r y m a t e r i a l s l i k e s i l i c a t e s a s w e l l a s f o r m o d e r a t e l y v o l a t i l e s u b s t a n c e s .W i t h t h e improvement o f s p e c t r o s c o p i c d a t a due t o modern o b s e r v a t i o n a l t e c h n i q u e s a p a s s i o n a t i n g s u b j e c t i s opening t o p h y s i c i s t s i n t e r e s t e d
i n
amorphous s u b s t a n c e s . The f i e l d o f a m o r p h o u s s o l i d s i n s p a c e i s wide ; t h u s i t is n o t p o s s i b l e t o c o v e r t h e whole s u b j e c t by a s i n g l e p a p e r . So t h e a u t h o r d e c i d e d t o l i m i t t h i s r e v i e w t o t h e a m o r p h o u s f o r m s o f a s u b s t a n c e t h a t s i n c e t h e f i r s t h a l f o f t h i s c e n t u r y i s though t o b e r a t h e r common i n s p a c e : H20. T h i s s u p p o s i t i o n i s j u s t i f i e d by t h e h i g h cosmic abundance o f H and 0.I t i s w e l l known t h a t H 0 f o r m s a n a m o r p h o u s s o l i d b y v a p o r d e p o s i t i o n a t
2
t e m p e r a t u r e s lower t h a n 100 K. S i n c e r e c e n t t i m e s we h a v e some e v i d e n c e t h a t t h e e v o l u t i o n o f some o u t e r s o l a r system b o d i e s seems i n f a c t i n f l u e n c e d by t h e p h y s i c a l p r o p e r t i e s o f amorphous s o l i d w a t e r .
I1
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THE INTERSTELLAR MEDIUMI n t e r s t e l l a r c l o u d s c a n b e i n v e s t i g a t e d u s i n g t h e a b s o r p t i o n f e a t u r e s d e t e c t e d i n t h e l i g h t o f s t a r s l a y i n g b e h i n d t h e c l o u d . I f t h e t y p e o f t h e s t a r i s s u f f i c i e n t l y w e l l known, t h e compounds r e s p o n s i b l e f o r t h e a b s o r p t i o n s c a n b e i d e n t i f i e d .
I n 1968 a n a b s o r p t i o n i n t h e 3 pm band h a s been r e p o r t e d i n NML Cygnus / I , 2/. I n t h e m e a n t i m e a g r e a t number o f v e r y s i m i l a r a b s o r p t i o n f e a t u r e s h v e been found i n
f
3m o l e c u l a r c l o u d s ( m o l e c u l a r c l o u d s a r e i n t e r s t e l l a r c l o u d s w i t h 1 0 molecules/cm ) .
C8-65 8 J O U R N A L
DE
PHYSIQUE T h e s e a b s o r p t i o n f e a t u r e s w e r e t h o u g h t t o b e d u e t o s o l i d H20. B u t t h i s i n t e r p r e t a t i o n l e f t s e v e r a l problems open :-
The w e a k n e s s o f t h e a b s o r p t i o n f e a t u r e s u g g e s t e d t h a t o n l y a b o u t 7 o f t h e a b s o r b i n g m a t t e r was i c e . T h i s was l e s s t h a n p r e d i c t e d /3/. -, Mie c a l c u l a t i o n s f o r g r a i n s o f 0.1 and 1 pm i n s i z e u s i n g t h e s p e c t r a l p a r a m e t e r s o f c r y s t a l l i n e i c e gave l i n e s h a p e s v e r y d i f f e r e n t f r o m t h a t o f t h e i n t e r s t e l l a r a b s o r p t i o n s /4/.-
The l i b e r a t i o n b a n d t h a t o c c u r s f o r c r y s t a l l i n e i c e n e a r 12 pm was n o t found i n t h e i n t e r s t e l l a r a b s o r p t i o n s .The q u e s t i o n o f t h e n a t u r e o f t h e s t u f f producing t h e 3 pm a b s o r p t i o n was a t l e a s t p a r t i a l l y answered b y Leger e t a l . /5/. These a u t h o r s compared t h e i n f r a r e d s p e c t r u m n e a r 3 pm o f amorphous s o l i d w a t e r o b t a i n e d by v a p o r d e p o s i t i o n a t l i q u i d n i t r o g e n e t e m p e r a t u r e t o t h e a b s o r p t i o n f e a t u r e of t h e Becklin-Neugebauer p o i n t s o u r c e w h i c h i s s i t u a t e d i n t h e O r i o n n e b u l a . It t u r n e d o u t t h a t t h e low wavelength wing o f t h e i n t e r s t e l l a r a b s o r p t i o n c o u l d b e f i t t e d q u i t e s a t i s f a c t o r i l y b y t h e l a b o r a t o r y d a t a . As t h e a b s o r p t i o n f e a t u r e f o r amorphous i c e a r e a s was f l a t t e r t h a n t h a t of c r y s t a l l i n e i c e t h e p r o p o r t i o n o f i n t e r s t e l l a r i c e i s p r o b a b l y a f a c t o r o f 1 . 5 h i g h e r t h a n e s t i m a t e d p r e v i o u s l y .
As i t i s t h e c a s e f o r t h e 0
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H s t r e t c h i n g band, t h e 12 pm l i b e r a t i o n band i s weaker f o r amorphous i c e t h a n f o r c r y s t a l l i n e i c e . F u r t h e r t h i s band i s s h i f t e d t o l o n g e r w a w e l e n g t h s f o r t h e a m o r p h o u s s o l i d /5, 6/. These f a c t s p r o b a b l y e x p l a i n t h a t t h e12
p
band i s d i f f i c u l t t o f i n d i n i n t e r s t e l l a r a b s o r p t i o n s .Thus i t s e e m s e s t a b l i s h e d t h a t s o l i d H 0 i n i n t e r s t e l l a r c l o u d s i s amorphous. The 2
s h a p e o f t h e long wavelength wing t h a t c a n n o t b e f i t t e d b y l a b o r a t o r y s p e c t r a o f p u r e amorphous i c e most l i k e l y i s d u e t o t h e CH s t r e t c h i n g v i b r a t i o n o f o t h e r " i c e s " t h a t a r e p r e s e n t on t h e i n t e r s t e l l a r g r a i n s /7/. An a l t e r n a t i v e e x p l a n a t i o n c o u l d b e t h a t t h e l o n g w a v e l e n g t h w i n g is-g;? t o w a t e r i c e g r a i n s c o n t a i n i n g some NH3 w i t h g r a i n s i z e d i s t r i b u t i o n a s N a a t h e maximum g r a i n r a d i u s b e i n g
=
1.2 pm /8/. The main d i f f i c u l t y o f t h i s e x p l a n a t i o n i s t h a t i n t e r s t e l l a r g r a i n s p r o b a b l y a r e much s m a l l e r t h a n t h e maximum g r a i n r a d i u s r e q u i r e d i n o r d e r t o f i t t h e 3 pm a b s o r p t i o n l i n e . I11-
COMETS.Comets p r o b a b l y a r e t h e m o s t p r i m i t i v e b o d i e s i n t h e s o l a r system. They formed i n t h e o u t e r p a r t o f t h e p r e m o r d i a l s o l a r n e b u l a and t h e y p r o b a b l y always remained a t a t e m p e r a t u r e lower t h a n 100 K s i n c e 4.6 b i l l i o n y e a r s ; thy9 t h e y may be w i t n e s s e s o f t h e i n t e r s t e l l a r m a t t e r t h a t formed t h e p l a n e t s . Some 1 0 comets a r e t h o u g h t t o b e c o n t a i n e d i n t h e s o c a l l e d "Oort cloud". The Oort c l o u d i s a s p h e r e a r o u n d t h e s u n w i t h a r a d i u s o f a b o u t 150 000 A s t r o n o m i c a l U n i t s ( 1 AU
=
mean d i s t a n c e sun-
e a r t h=
1 5 0 m i l l i o n km).
The comets i n t h e o u t e r s h e l i o f t h e O o r t c l o u d a r e s u b j e c t t o g r a v i t a t i o n a l p e r t u b a t i o n s b y n e a r l y s t a r s . I n t h i s way some o f them a r e i n j e c t e d i n t o t h e i n n e r s o l a r system. Here t h e cometary i c e s a r e e v a p o r a t e d t h u s f o r m i n g a g a s a n d d u s t h a l o ("coma") and t a i l s o f d u s t and i o n i z e d g a s . There i s i n d i r e c t e v i d e n c e t h a t H 0 i s a major c o n s t i t u a n t o f comets :2
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M o l e c u l a r f r a g m e n t s l i k e H 0+ and OH a r e c u r r e n t l y d e t e c t e d i n cometary comae. 2-
The l i g h t c u r v e o f most comets a s a f u n c t i o n o f h e l i o c e n t e r d i s t a n c e f i t s w e l l w i t h H 0 e v a p o r a t i o n .2
It i s w e l l known t h a t amorphous s o l i d w a t e r c r y s t a l l i z e s i n an i r r e v e r s i b l e manner a t t e m p e r a t u r e s between 130 and 1 5 3 K depending on t h e c o n d i t i o n s o f f o r m a t i o n / 9 ,
lo/.
D u r i n g t h i s p h a s e t r a n s i t i o n some h e a t i s r e l e a s e d . The c r y s t a l l i z a t i o n o f amorphous i c e may b e t h e key t o some u n e x p l a i n e d b e h a v i o r o f comets :-
t h e l i g h t c u r v e s o f some comets show an asymmetry w i t h r e s p e c t t o p e r i h e l i o n , t h e comet b e i n g o f t e n l e s s b r i g h t a f t e r p e r i h e l i o n t h a n b e f o r e .I n o r d e r t o e x p l a i n t h e o u t b u r s t s P a t a s h n i c k e t a l . /11/ s u g g e s t e d t h a t t h e h e a t f r e e d d u r i n g c r y s t a l l i z a t i o n may b e t h e e n e r g y s o u r c e f o r o u t b u r s t s , t h e o u t b u r s t b e i n g t r i g g e r e d when f r e s h amorphous l a y e r s r e a c h t h e t r a n s i t i o n t e m p e r a t u r e . Due t o t h e c o u p l i n g b e t w e e n t h e r o t a t i o n a n d t h e r e v o l u t i o n o f t h e c o m e t n u c l e u s t h e o u t b u r s t o c c u r s i n an a p p a r e n t l y random manner. On t h e o t h e r h a n d t h e h e a t c o n d u c t i o n c o e f f i c i e n t o f t h e i c e i n c r e a s e s i n a n i m p o r t a n t m a n n e r d u r i n g c r y s t a l l i z a t i o n and t h e t e m p e r a t u r e dependence of t h e h e a t c o n d u c t i o n c o e f f i c i e n t i s r e v e r s e d /11/. Thus t h e t h e r m a l i n e r t i a o f c o m e t s t h a t h a v e a t h i c k c r y s t a l l i n e c r u s t s h o u l d b e h i g h e r t h a n t h a t o f comets which h a v e n o t . The development o f such a c r u s t depends on t h e o r b i t a l p a r a m e t e r s . When t h e t h e r m a l i n e r t i a o f t h e comet i s h i g h , t h e p o s t - p e r i h e l i o n a c t i v i t y should b e lower t h a n t h e p r e - p e r i h e l i o n a c t i v i t y . The p r e s e n t s t a t e m e n t s seem i n good a g r e e m e n t w i t h o b s e r v a t i o n /12, 13/.
I V
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PLANETARY RINGS AND I C Y SATELLITESMost o f t h e s a t e l l i t e s o f J u p i t e r and S a t u r n and t h e p a r t i c l e s o f S a t u r n ' s r i n g s c o u n t a i n h i g h amounts of s o l i d H 0 .
2
The p a r t i c l e s o f t h e r i n g s y s t e m o f S a t u r n a r e c o n t i n u o u s l y b r o k e n u p d u e t o p a r t i c l e - p a r t i c l e c o l l i s i o n s a n d a r e e r o d e d d u e t o s p u t t e r i n g b y e n e r g e t i c p a r t i c l e s o f S a t u r n ' s m a g n e t o s p h e r e . The p e r m a n e n t r e d i s t r i b u t i o n o f H 0 a t t e m p e r a t u r e s lower t h a n 100 K p r o b a b l y produces amorphous i c e . T h i s i s c o m p a e i b l e w i t h t h e l o w t h e r m a l i n e r t i a found when t h e r i n g p a r t i c l e s a r e e c l i p s e d by S a t u r n 1 1 4 , 15/. On t h e i c y s a t e l l i t e s o f J u p i t e r and S a t u r n v o l a t i l e s u r f a c e m a t e r i a l i s removed by s p u t t e r i n g o f e n e r g e t i c p a r t i c l e s and b y m e t e o r i t i c b o m b a r d m e n t . Some o f t h e s e s a t e l l i t e s have a s u f f i c i e n t g r a v i t a t i o n n a l f i e l d t o b e a b l e t o r e t a i n a n i m p o r t a n t f r a c t i o n o f t h e e j e c t e d m o l e c u l e s . These m o l e c u l e s c a n r e c o n d e n s e i n c o l d r e g i o n s o f t h e b o d y where t h e y form an amorphous d e p o s i t . P o l a r c a p s have indeed been found on Ganymede, one o f t h e G a l i l e a n S a t e l l i t e s o f J u p i t e r /16, 17/. The c o n d i t i o n s u n d e r which t h e s e d e p o s i t s have formed s t r o n g l y s u g g e s t t h a t t h e y a r e amorphous.
V
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CONCLUSIONI t s e e m s now e s t a b l i s h e d t h a t a m o r p h o u s H 0 o c c u r s i n m o l e c u l a r c l o u d s i n t h e i n t e r s t e l l a r medium. It i s a l m o s t c e r t a i n t h a ? amorphous s i l i c a t e s a r e p r e s e n t i n i n t e r s t e l l a r g r a i n s t o o . But v e r y f e w i s known u n t i l now on t h e o c c u r e n c e o f amorphous forms o f o t h e r m o d e r a t e l y v o l a t i l e s u b s t a n c e s . Some m o r e k n o w l e d g e c o u l d h e l p u s t o b e t t e r u n d e r s t a n d t h e mechanism o f d e p o s i t of i n s t e r s t e l l a r g a s on g r a i n s and on r e c y c l i n g o f g r a i n m a t e r i a l .
Comets seem t o b e v e r y p r i m i t i v e b o d i e s and t h u s c o u l d b e much i n f l u e n c e d by t h e p h y s i c a l b e h a v i o r o f p r e m o r d i a l a m o r p h o u s s u b s t a n c e s . Amorphous d e p o s i t s c o u l d f u r t h e r b e p r e s e n t on r i n g p a r t i c l e s on S a t u r n and i n p o l a r c a p s o f i c y s a t e l l i t e .
I n summary we c a n s a y t h a t improved knowledge o f amorphous s u b s t a n c e s c a n h e l p u s t o u n d e r s t a n d t h e e v o l u t i o n o f i n t e r s t e l l a r m a t t e r , t h e e a r l y s t a g e o f t h e s o l a r system and t h e p a r t i c u l a r e v o l u t i o n o f some o u t e r s o l a r s y s t e m b o d i e s . ACKOWLEDGEMENTS F i n a n c i a l s u p p o r t f r o m t h e F r e n c h
"
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