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HIGH PRESSURE PHYSICS AND CHEMISTRY IN GIANT PLANETS AND THEIR SATELLITES
D. Stevenson
To cite this version:
D. Stevenson. HIGH PRESSURE PHYSICS AND CHEMISTRY IN GIANT PLANETS AND THEIR SATELLITES. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-97-C8-103.
�10.1051/jphyscol:1984819�. �jpa-00224317�
H I G H PRESSURE PHYSICS AND CHEMISTRY I N GIANT PLANETS AND T H E I R S A T E L L I T E S +
D . J . S t e v e n s o n
Division of Geoloyicul und P l a m t a r y Sciences, C a l i f o r n i a I n s t i t u t e o f Technology, Pasadena, CA 91125, U.S. A .
Resume - Les c o u c h e s p r o f o n d e s d e J u p i t e r , S a t u r n e , Uranus, Neptune a i n s i q u e l e u r s s a t e l l i t e s s o n t Q t u d i e s . Des r e s u l t a t s i m p o r t a n t s c o n c e r n a n t l a P h y s i q u e d e l a M a t i e r e Condensee s o n t e x p o s e s . La d i s c u s s i o n p o r t e s u r t o u t s u r H2, H-lie, H -H 0 , H2-CH
2 2 4 , NH3, e t CH4.5.75H 0. La P l a n e t o l o g i e b e n e f i -
c i e r a d e s dCveloppements , p r e s e n t s e t f u t u r s , t i e o r i q u e s e t e x p d r i m e n t a u x .
A b s t r a c t - The deep i n t e r i o r s o f J u p i t e r , S a t u r n , Uranus, Neptune, and t h e i r s a t e l l i t e s a r e d i s c u s s e d . The i m p o r t a n t i s s u e s r e l e v a n t t o t h e p h y s i c s o f condensed m a t t e r a r e i d e n t i f i e d . The d i s c u s s i o n emphasizes Hz, H-He, H -
H20. Hz-CHI. WH3. and CH4.5.15H20. P r e s e n t and f u t u r e t h e o r y and experiment are o f g r e a t b e n e f i t t o p l a n e t a r y s c i e n c e .
The l a s t decade h a s s e e n d r a m a t i c developments i n o u r u n d e r s t a n d i n g o f p l a n e t s , p r i m a r i l y because o f t h e d a t a r e t u r n e d by deep s p a c e missions. For example, t h e Voyager m i s s i o n c o n s i s t s o f two s p a c e c r a f t which f l e w p a s t both J u p i t e r and S a t u r n ; one o f t h o s e s p a c e c r a f t w i l l b e t h e first man-made o b j e c t t o v i s i t Uranus, i n e a r l y 1986. One o f t h e u l t i m a t e g o a l s o f t h i s e x p l o r a t i o n e f f o r t is t o e l u c i d a t e t h e o r i g i n and composition o f t h e s o l a r system. T h i s r e q u i r e s both s p a c e c r a f t d a t a and a knowledge o f t h e p r o p e r t i e s o f r e l e v a n t m a t e r i a l s a t t h e thermodynamic c o n d i t i o n s encountered w i t h i n t h e p l a n e t s and t h e i r s a t e l l i t e s . The h i g h p r e s s u r e p h y s i c s community is p r o v i d i n g a n e s s e n t i a l s e r v i c e t o p l a n e t a r y s c i e n c e i n t h i s a r e a . My g o a l i n t h i s b r i e f summary review is t o o u t l i n e t h e p r e s e n t s t a t e o f a f f a i r s and i d e n t i f y t h e a r e a s where p r e s e n t and f u t u r e experiment o r t h e o r y c a n p r o v i d e s u b s t a n t i a l advances.
A d e t a i l e d d e t e r m i n a t i o n o f t h e i n t e r n a l p r o p e r t i e s o f p l a n e t s c a n never b e achieved e x c e p t by d i r e c t sampling. The i n t e r p r e t a t i o n o f p l a n e t a r y i n t e r i o r s must r e l y h e a v i l y on "models" which s i m p l i f y , by j u d i c i o u s c h o i c e o f assumptions, t h e complex n a t u r e o f r e a l p l a n e t s . For o u r p r e s e n t d i s c u s s i o n , t h e two most i m p o r t a n t assumptions d e s e r v i n g f u r t h e r d i s c u s s i o n a r e t h o s e concerning composition and t h e i n t e r n a l d i s t r i b u t i o n o f c o n s t i t u e n t s . (Other assumptions, s u c h a s h y d r o s t a t i c e q u i l i b r i u m , are c r u c i a l t o t h e models b u t a r e o f w e l l - e s t a b l i s h e d v a l i d i t y and m e r i t l e s s concern.)
The bulk compositions o f g i a n t p l a n e t s and t h e i r s a t e l l i t e s a r e assumed t o be r e l a t e d t o Gosmio abundances ( o r , a l m o s t e q u i v a l e n t l y , t h e p r i m o r d i a l compo- s i t i o n of t h e s u n ) . The r e l a t i o n s h i p is n o t a n i d e n t i t y s i n c e p l a n e t a r y compo- s i t i o n i s a l s o determined by r e l a t i v e v o l a t i l i t y (1.e. which c o n s t i t u e n t s can condense under t h e thermodynamic c o n d i t i o n s encountered d u r i n g p l a n e t a r y f o r - mation). It i s convenient t o s u b d i v i d e t h e c o n s t i t u e n t s i n t o three c l a s s e s of m a t e r i a l s : g a s e s . ices, and rock. 'Oases' mean p r i m a r i l y hydrogen and helium,
' C o n t r i b u t i o n number 4143 o f t h e D i v i s i o n o f G e o l o g i c a l and P l a n e t a r y Sciences, C a l i f o r n i a I n s t i t u t e o f Technology, Pasadena, C a l i f o r n i a 91125.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984819
C8-98 JOURNAL DE PHYSIQUE
which comprise 0.92 and 0.08 (approximate y ) number f r a c t i o n o f a cosmic m i x t u r e . A l l o t h e r e l e m e n t s combined comprise -lo-' number f r a c t i o n . Although t h e s e e l e m e n t s a r e overwhelmingly most abundant, t h e y are a l s o the most v o l a t i l e and c a n o n l y b e i n c o r p o r a t e d i n t o a p l a n e t i n g a s e o u s form. High v o l a t i l i t y and h i g h m o l e c u l a r v e l o c i t y (implying e a s e o f e s c a p e ) p r e v e n t s t h e s e c o n s t i t u e n t s from b e i n g s i g n i f i c a n t components o n b o d i e s $ E a r t h i n s i z e . Not s u r p r i s i n g l y , however, hydrogen and helium are t h e main c o n s t i t u e n t s o f J u p i t e r and S a t u r n . The " i c e s "
mean H20, CH4. NH3, and p o s s i b l y CO, C02, N2; m a t e r i a l s which form from 0, C, and N; r e s p e c t i v e l y t h e t h i r d , f o u r t h , and s i x t h most abundant e l e m e n t s i n t h e
Universe. (The f i f t h most abundant element is Ne and is a v e r y minor component o f ' g a s e s ' ) . The "ices" a r e c a p a b l e o f c o n d e n s a t i o n ( a s s o l i d s ) i n t h e o u t e r s o l a r system, t o v a r y i n g d e g r e e s depending on d i s t a n c e from t h e Sun. Water i c e a p p e a r s t o predominate a t J u p i t e r ; t h e o t h e r i c e s a r e p r e s e n t ( b u t i n u n c e r t a i n amounts) i n more d i s t a n t bodies. The p l a n e t s Uranus and Neptune a r e predominantly i c y . The t h i r d c l a s s o f m a t e r i a l is " r o c k w ' , e s s e n t i a l l y e v e r y t h i n g o t h e r t h a n * ' g a s e s "
and "ices." The primary c o n s t i t u e n t s o f "rock1' a r e t h o s e found i n t h e E a r t h : magnesium s i l i c a t e s and i r o n ( b o t h a s m e t a l and bound up i n s i l i c a t e s o r o x i d e s ) . When models o f p l a n e t s are c o n s t r u c t e d , i t i s u s u a l t o t r e a t t h e s e t h r e e m a t e r i a l c l a s s e s a s t h e primary b u i l d i n g b l o c k s and t o assume ( w i t h minor v a r i - a t i o n s ) t h a t t h e r e l a t i v e e l e m e n t a l abundances w i t h i n a m a t e r i a l c l a s s a r e e q u a l t o t h e cosmic abundanoea. For example, t h e C:O r a t i o might be k e p t f i x e d b u t t h e H:O r a t i o might be a n a d j u s t a b l e parameter. It is i m p o r t a n t t o u n d e r s t a n d t h a t t h i s p r o c e d u r e i s a n g s s u m ~ t i o n . We do n o t u n d e r s t a n d enough a b o u t t h e e a r l y s o l a r system, o r t h e dynamics and phase mixing o f p l a n e t a r y i n t e r i o r s t o a s s e s s w i t h c o n f i d e n c e t h e v a l i d i t y o f t h i s approach. N e v e r t h e l e s s , i t y i e l d s r e s u l t s con- s i s t e n t w i t h o b s e r v a b l e p r o p e r t i e s . An o b v i o u s c o r o l l a r y o f t h i s approach is t h a t p l a n e t s and p e r h a p s s a t e l l i t e s a r e l a y e r e d . i n a c c o r d a n c e w i t h t h e r e l a t i v e den- sities o f t h e t h r e e c o n s t i t u e n t o l a s s e s : r o c k c o r e , i c e l a y e r , g a s envelope.
Although t h i s g i v e s t h e l o w e s t (1.e. most n e g a t i v e ) g r a v i t a t i o n a l e n e r g y , i t i s n o t n e c e s s a r i l y t h e p r e f e r r e d thermodynamic s t a t e if t h e l a y e r s are m i s c i b l e (e.g. i f t h e " i c e * ' can mix f u l l y w i t h t h e "gas"). It is i n t h i s a r e a , i n p a r t i c u l a r , t h a t o u r knowledge i s most l i m i t e d and where f u t u r e experiment and t h e o r y w i l l h e l p immense1 y .
It i s n o t p o s s i b l e t o d e s c r i b e t h e c o n s t r u c t i o n o f p l a n e t a r y models h e r e . The i n t e r e s t e d r e a d e r w i l l f i n d a b r i e f r e v i e w 111 o r more d e t a i l e d d i s c u s s i o n s
[2,3,41 elsewhere. For o u r p r e s e n t purpose, i t is most u s e f u l t o list t h e thermo- dynamic c o n d i t i o n s encountered:
Large S a t e l l i t e s Mercury, Mars E a r t h , Venus Uranus, Neptune S a t u r n
J u p i t e r
T a b l e I
Thermodynamic C o n d i t i o n s i n P l a n e t s and S a t e l l i t e s
'center T c e n t e r (K)
P r e s s u r e s a r e o b t a i n e d from s o l u t i o n o f t h e e q u a t i o n o f h y d r o s t a t i c e q u i l i b r i u m w i t h g i v e n a s s u m p t i o n s f o r t h e c o n s t i t u e n t s and t h e i r l a y e r i n g . Temperatures are
p l a n e t s and s a t e l l i t e s e l i m i n a t e h e a t p r i m a r i l y by c o n v e c t i o n - r e g a r d l e s s o f whether t h e y a r e s o l i d o r f l u i d ) . We w i l l now d i s c u s s b r i e f l y e a c h body ( o r c l a s s o f body) i n t h e o u t e r s o l a r system, i n o r d e r o f d e c r e a s i n g mass.
J u p i t e r is a r g u a b l y t h e b e s t understood p l a n e t i n o u r s o l a r system
( i n c l u d i n g E a r t h ) . A l l known p r o p e r t i e s a r e c o n s i s t e n t w i t h a body t h a t c o n s i s t s o f -95% cosmic abundance m i x t u r e (by mass), w i t h t h e remaining -5% (i.e. ten-twenty E a r t h masses) i n t h e form o f a dense ( p r o b a b l y r o c k ) c o r e . O f t h e cosmic envelope, roughly 70% is a m e t a l l i c hydrogen-helium-minor c o n s t i t u e n t m i x t u r e . The o u t e r m o s t 30% i s a m o l e c u l a r hydrogen-helium-minor c o n s t i t u e n t m i x t u r e . The amount o f i c e might be i n e x c e s s o f cosmic abundance s i n c e d i r e c t o b s e r v a t i o n s o f t h e atmosphere i n d i c a t e a f a c t o r of two enhancement o f methane. The t e m p e r a t u r e w i t h i n J u p i t e r is h i g h enough t h a t no first o r d e r phase t r a n s i t i o n s ( o t h e r t h a n c l o u d c o n d e n s a t i o n o f minor c o n s t i t u e n t s ) a r e encountered a t P $ 1 Mbar, and i t is u n c l e a r whether molecular -> m e t a l l i c hydrogen is a first o r d e r t r a n s i t i o n o r i m m i s c i b i l i t y o f helium o c c u r a t still g r e a t e r d e p t h s ( t h e s e i s s u e s a r e d i s c u s s e d f u r t h e r below).
S a t u r n is s u p e r f i c i a l l y r a t h e r s i m i l a r t o J u p i t e r , b u t w i t h some i m p o r t a n t d i f f e r e n c e s . It h a s a h i g h e r mass f r a c t i o n (-209b) i n t h e form o f dense ( r o c k ? ) c o r e and I R d a t a s u g g e s t s t h a t i t h a s a helium d e p l e t i o n (by - f a c t o r o f two r e l a t i v e t o cosmic) i n t h e o u t e r envelope. S a t u r n ' s lower mass a l s o means t h a t a much s m a l l e r f r a c t i o n (-one t h i r d ) o f its mass is a m e t a l l i c hydrogen-dominated mixture. The helium d e p l e t i o n s u g g e s t s l i m i t e d s o l u b i l i t y o f helium i n hydrogen
( d i s c u s s e d f u r t h e r below).
Uranus and Neptune are much l e s s w e l l understood b u t a r e c l e a r l y v e r y d i f - f e r e n t from J u p i t e r and S a t u r n . A c o n s i s t e n t model h a s a rock c o r e o f s e v e r a l E a r t h masses surrounded by a w a t e r - r i c h l a y e r which e x t e n d s p a r t way o r a l l t h e way t o a n atmosphere t h a t is known t o be hydrogen-dominated. It i s n o t known whether t h e r e is a n a b r u p t o r g r a d u a l t r a n s i t i o n from water-dominated c o n d i t i o n s , a l t h o u g h i t i s a r g u e d below t h a t a n a b r u p t ("ocean" -> atmosphere) t r a n s i t i o n is n o t r e q u i r e d . I n any e v e n t , t h e i c e component o f t h e s e p l a n e t s is t y p i c a l l y - h a l f t h e t o t a l mass, b u t w i t h a l a r g e u n c e r t a i n t y .
a s a t e l l i t e s r a n g e from 10 and Europa (rock-dominated), through Ganymede and C a l l i s t o ( w a t e r i c e and r o c k ) t o T i t a n and T r i t o n ( u n c e r t a i n composition b u t p o s s i b l y l i k e Ganymede w i t h a n a d d i t i o n o f a t l e a s t some more v o l a t i l e i c e s , e s p e c i a l l y CH4). I n t h e more i c e - r i c h s a t e l l i t e s , t h e s t r u c t u r e i s u s u a l l y b e l i e v e d t o be a rock-rich c o r e surrounded by a n i c e l a y e r t h a t is p a r t i a l l y o r e n t i r e l y s o l i d C51. The t e m p e r a t u r e (1300 K) anc! p r e s s u r e ( s e w t e n s o f k i l o b a r s ) encountered i n t h i s i c e r e q u i r e d e t a i l e d c o n s i d e r a t i o n o f t h e w a t e r i c e , NH3-H20 and CHI-H 0 phase diagrams, some a s p e c t s o f which a r e d i s c u s s e d f u r t h e r below.
T i t a n is b e l i e v e d t o have a hydrocarbon ocean 161 and T r i t o n may have l i q u i d N2 o n its s u r f a c e [71.
IWORTANT CONDENSED MATTER PROBLEMS
We t u r n from t h e s e g e n e r a l i t i e s a b o u t p l a n e t s t o a c o n s i d e r a t i o n o f t h e m a t e r i a l p r o p e r t i e s r e q u i r e d f o r a n improved u n d e r s t a n d i n g o f p l a n e t a r y i n t e r i o r s . The f o l l o w i n g Table i d e n t i f i e s t h e i m p o r t a n t p u r e c o n s t i t u e n t s and m i x t u r e s ; t h e subsequent d i s c u s s i o n summarizes t h e c u r r e n t s t a t e o f knowledge and i d e n t i f i e s t h e work needed f o r f u r t h e r p r o g r e s s .
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Table I1
P l a n e t a r y M a t e r i a l s and Problems
M a t e r i a l Thermodynamic Conditions Problem
H-He P - few mar, T - lo4 K
H20 100 kbar 5 P 5 Mbar, T 5000 K
H2-H20 same c o n d i t i o n s
H2-CH4 same c o n d i t i o n s
H2-CH4-C0 same c o n d i t i o n s
NH <-> N2, H2 P 2 1 0 kbar. T - 1000 K +
~ o % l e g a s e s same
d i a n o c i a t i o n , e l e c t r i c a l c o n d u c t i v i t y
s o l u b i l i t y , volume of mixing n a t u r e of bonding ( i o n i c ? ) , decomposition?, e l e c t r i c a l c o n d u c t i v i t y
s o l u b i l i t y , i o n i z a t i o n , d i s s o c i a t i o n , c o n d u c t i v i t y s o l u b i l i t y , d i s s o c i a t i o n d i s s o c i a t i o n , s o l u b i l i t y , c l a t h r a t e s , chemistry?
d i s s o c i a t i o n
s o l u b i l i t y i n metals, p a r t i t i o n i n g between phases, xenology
This t a b l e is not exhaustive and tends t o emphasize binary endmembers o f t h e r e a l multicomponent system. The emphasis is on t h e major c o n s t i t u e n t s , except f o r t h e e n t r y on noble g a s e s which has been included because t h e s e minor c o n s t i t u e n t s may be important t r a c e r s of i n t e r n a l processes and ( i n t h e p a r t i c u l a r c a s e o f xenon) may even have chemistry.
Molecular Hvdroaen. The behavior of H2, e s p e c i a l l y a t P 2 0.5 Mbar, is f a r from well understood. Simple models f o r d i s s o c i a t i o n and e l e c t r o n i c e x c i t a t i o n suggest a s u b s t a n t i a l change i n t h e s p e c i f i c h e a t and Gruneisen 7 , which may have a dramatic e f f e c t on t h e dynamics and e v o l u t i o n o f J u p i t e r and Saturn. Perhaps t h e most i n t e r e s t i n g r e l e v a n t experiment o r theory f o r H2 concerns t h e value o f t h e band gap energy between valence and conduction l e v e l s a s a f u n c t i o n o f p r e s s u r e , s i n c e t h i s a f f e c t s e s t i m a t e s of both thermodynamic and t r a n s p o r t p r o p e r t i e s , e s p e c i a l l y e l e c t r i c a l conductivity. The l a t t e r is important, s i n c e r e c e n t magneto- hydrodynamic c a l c u l a t i o n s [81 suggest t h a t t h e observed wind p r o f i l e s i n t h e atmos- pheres o f J u p i t e r and Saturn may be p a r t l y d e t rmlned by t h e e l e c t r i c a l conduc- t i v i t y o f H2 i n t h e r e g i o n where p' - 0.2 ..om-' and T - 5000 K . E x i s t i n g band s t r u c t u r e c a l c u l a t i o n s [91 i n d i c a t e t h a t t h e band gap i n H2 decreases t o z e r o a t P
- 1 Mbar (presumably a precursor t o t h e H2 -> monatomic m e t a l l i c hydrogen tran- s i t i o n ) . D i r e c t confirmation of t h i s behavior (e.g. o p t i c a l r e f l e c t i v i t y s t u d i e s ) or i n d i r e c t evidence o f t h i s behavior (e.g., e l e c t r i c a l c o n d u c t i v i t y a t high T ) would be highly d e s i r a b l e .
Hvdronen-Helium Mixtures. I n t h e molecular regime, an a c c u r a t e equation of s t a t e f o r HZ-He would provide a determination of t h e H2-He i n t e r a c t i o n p o t e n t i a l , e s s e n t i a l f o r c o n s t r u c t i n g equations of s t a t e f o r p l a n e t a r y i n t e r i o r s . Currently, t h i s p o t e n t i a l is estimated by t h e o r e t i c a l arguments alone. However, t h e most important problem concerns t h e s o l u b i l i t y of helium i n m e t a l l i c hydrogen. Although t h i s system is well understood i n t h e i n f i n i t e p r e s s u r e l i m i t [I01 and moderately well understood i n t h e low e l e c t r o n d e n s i t y l i m i t C111, t h e phase diagram is l e a s t well understood i n p r e c i s e l y t h e p r e s s u r e range of most i n t e r e s t (-1 t o 10 Mbars).
An a p p r o p r i a t e t h e o r e t i c a l c a l c u l a t i o n would involve e v a l u a t i o n of t h e "immersion energy": t h e energy c o s t o f i n s e r t i n g a helium atom i n t h e e l e c t r o n g a s
Water. A t high p r e s s u r e ( P - 0.2-0.6 Mbar), H20 a p p e a r s t o e i t h e r i o n i z e a s H ~ O + O H - i f t h e t e m p e r a t u r e i s h i g h (T 2 1500 K) [12,131 o r ~ p o l y m e r i z ~ ( i . e . hydrogen bond is no l o n g e r d i s t i n g u i s h a b l e from a c o v a l e n t bond) i f t h e t e m p e r a t u r e is low (T - 300 K) 1141. It i s c l e a r t h a t t h e r a n g e of p o s s i b l e b e h a v i o r is l a r g e ; f u r t h e r u n d e r s t a n d i n g o f t h i s is e s s e n t i a l . Water s h o u l d e v e n t u a l l y m e t a l l i z e and might e v e n decompose o r d i s p r o p o r t i o n a t e a t s u f f i c i e n t l y h i g h p r e s s u r e . None of t h e s e p o s s i b i l i t i e s h a s been a n a l y z e d y e t . One p o s s i b l e measurement o f i n t e r e s t would be t h e o p t i c a l band g a p . T h i s would p r o v i d e i n f o r m a t i o n on e l e c t r i c a l con- d u c t i v i t y ( e v e n though t h e s m a l l e s t band g a p may h e i n d i r e c t ) and may a i d d e t e r - m i n a t i o n o f t h e m e t a l l i z a t i o n p r e s s u r e ; b o t h v e r y i m p o r t a n t f o r improved o f Uranus and Neptune and t h e i r p r o b a b l e m a g n e t i c f i e l d s .
Water-Hvdronen. Recent low p r e s s u r e ( P ,( 3 k b a r ) measurements 1151 i n d i c a t e t h a t w a t e r and hydrogen mix i n a l l p r o p o r t i o n s f o r T 2 650 K. It is n o t known whether t h i s b e h a v i o r e x t e n d s t o v e r y h i g h p r e s s u r e s . I f i t d o e s , t h e n models f o r t h e deep atmosphere o f Uranus [ I 6 1 s u g g e s t a w a t e r c l o u d b a s e a t P - 1 t o 2 k b a r , below which a uniform HZ-H20 m i x t u r e (50-75s H20 by e x t e n d s t o i n d e f i n i t e d e p t h s . An i n t e r e s t i n g consequence o f t h i s w a t e r - r i c h , hydrogen-poor e n v i r o n m e n t is t h e f i n i t e e q u i l i b r i u m abundance o f N2 ( a t t h e e x p e n s e o f NH3) and CO ( a t t h e expense o f CH4 and H20), u n l i k e t h e d e e p a t m o s p h e r e s o f J u p i t e r and S a t u r n where CH4 and NH3 a r e t h e overwhelmingly p r e f e r r e d forms o f c a r b o n and n i t r o g e n , respec- t i v e l y . I n a n y e v e n t . i t is v e r y i m p o r t a n t t o u n d e r s t a n d t h e H20-H2 phase diagram t o e x t r e m e p r e s s u r e s . It is p o s s i b l e , f o r example, t h a t t h i s m i x t u r e is analogous t o metal-ammonia ( o r a l k a l i m e t a l - a l k a l i h a l i d e ) m i x t u r e s a t e x t r e m e p r e s s u r e ? A m e t a l l i c s t a t e s u c h a s t h i s may e x i s t a t a p r e s s u r e s u b s t a n t i a l l y l e s s t h a n t h a t needed t o m e t a l l i z e p u r e w a t e r . A s a s i d e i s s u e , i t would a l s o be o f i n t e r e s t t o know t h e s o l u b i l i t y o f He i n H20 s i n c e t h e s y s t e m o f i n t e r e s t is, i n r e a l i t y . approximated a s t h e t e r n a r y m i x t u r e HZ-He-H20.
Methane-Hydrone~, There is c l e a r e v i d e n c e t h a t many h y d r o c a r b o n s decompose ( o r c o l l a p s e ) upon s h o c k compression, p r o b a b l y i n t o g r a p h i t e and hydrogen 117,181.
It i s v e r y i m p o r t a n t t o e s t a b l i s h t h e r a n g e o f t e m p e r a t u r e and C:H r a t i o s f o r which t h i s d e c o m p o s i t i o n c a n o c c u r . It i s e q u a l l y i m p o r t a n t t o e s t a b l i s h whether a n a c t u a l p h a s e s e p a r a t i o n o c c u r s ( i m p l y i n g p o s s i b l e f o r m a t i o n o f a diamond o r l i q u i d m e t a l l i c ( 1 ) c a r b o n l a y e r i n Uranus and Neptune) o r whether a c o l l a p s e d b u t i n t i - m a t e l y mixed C-H s t r u c t u r e r e s u l t s . Hugoniot d a t a a l o n e a r e i n s u f f i c i e n t t o answer t h i s q u e s t i o n . Diamond c e l l work, p e r h a p s a t e l e v a t e d t e m p e r a t u r e s , would seem most a p p r o p r i a t e . i n i t i a l l y o n p u r e methane o r some o t h e r ( s a t u r a t e d ) hydrocarbon.
There is a l r e a d y evidence t h a t u n s a t u r a t e d bonds d o n o t p e r s i s t a t h i g h p r e s s u r e 1191.
Methane-Water. Pure methane ice d o e s n o t condense under s o l a r n e b u l a con- d i t i o n s u n t i l T ,( 20 K and i t is q u e s t i o n a b l e w h e t h e r t h e s e v e r y c o l d c o n d i t i o n s were e v e r a c h i e v e d . However, t h e n e a r l y s t o i c h i o m e t r i c c l a t h r a t e h y d r a t e
(CH4'5.75H20, i f a l l c a g e s i t e s a r e f i l l e d ) f o r m s more r e a d i l y ( a t T - 40 K i n t h e s o l a r n e b u l a , p e r h a p s a t T - 80-100 K i n t h e n e b u l a e s u r r o u n d i n g p r o t o - g i a n t p l a n e t s ) . Remarkably l i t t l e is known a b o u t c l a t h r a t e compounds a t h i g h p r e s s u r e . e x c e p t f o r t h e t e t r a h y d r o f u r a n c l a t h r a t e 1201, p r o b a b l y a poor a n a l o g f o r t h e CH4 c l a t h r a t e b e c a u s e o f i t s d i f f e r e n t s t r u c t u r e . A r e c e n t , d e t a i l e d s t a t i s t i c a l m e c h a n i c a l t r e a t m e n t o f c l a t h r a t e s 1211 p r e d i c t s t h a t t h e methane c l a t h r a t e decom- p o s e s a t P - 1 2 t o 1 4 k b a r a t a l l T 1 3 0 0 K i n t o i c e V I and s o l i d CH4. (At h i g h e r T, i t decomposes a t lower P.) T h i s may have p r o f o u n d i m p l i c a t i o n s f o r t h e t h e r m a l h i s t a r y o f T i t a n , p e r h a p s o f f e r i n g a n e x p l a n a t i o n f o r t h e o r i g i n o f CH from which t h e p r e s e n t e t h a n e - r i c h o c e a n is d e r i v e d [5,61. A h i g h p r i o r i t y exper4ment.
p r o b a b l y a c c e s s i b l e e v e n w i t h p i s t o n a p p a r a t u s , is t h e s t a b i l i t y o f methane c l a t h r a t e u n d e r p r e s s u r e .
C8-102 JOURNAL DE PHYSIQUE
A t higher T and P, it is o f i n t e r e s t t o know t h e mixing p r o p e r t i e s and chemical e q u i l i b r i a o f t h e CH4-H20 system. What a r e t h e e q u i l i b r i u m abundances o f CO, H2, ... ? Is phase s e p a r a t i o n p o s s i b l e i n t h e deep i n t e r i o r , where water i o n i z e s but CH4 remains n e u t r a l ?
Nitrogen. A t high p r e s s u r e and temperature, it i s known t h a t NH3 is a good i o n i c conductor [131, presumably because o f t h e formation of N H ~ N H ; p a i r s . How- e v e r , i t may a l s o be p o s s i b l e t o form a f i n i t e amount of N a p r o c e s s which is known t o occur a t low p r e s s u r e s i n shock t u b e s . This may g i of importance f o r t h e o r i g i n of molecular n i t r o g e n i n T i t a n and p o s s i b l y T r i t o n . The i m p o r t a n t p o i n t t o s t r e s s i s t h a t d e s p i t e t h e very high abundance o f hydrogen i n t h e o u t e r system, i t is p o s s i b l e t o encounter c o n d i t i o n s i n which s m a l l but s i g n i f i c a n t amounts o f N2 a r e produced and preserved. High p r e s s u r e and temperature p r o c e s s e s a r e a l i k e l y example. The r e c e n t l y d e t e c t e d anomalous behavior o f N2 under shock compression [221, s u g g e s t i n g d i s s o c i a t i o n o r even m e t a l l i z a t i o n , r a i s e s i n t e r e s t i n g q u e s t i o n s about t h e behavior o f N-H mixtures a t megabar pressures.
Noble Oases. These a r e i n c l u d e d h e r e because n o b l e g a s e s a r e o f t e n u s d a s t r a c e r s of p r o c e s s e s i n p l a n e t a r y i n t e r i o r s . For example, t h e o u t g a s s i n g of He f
from E a r t h i s sometimes used a s a n i n d i c a t o r o f a "primordial r e s e r v o i r " o f m a t e r i a l . An i n t e r e s t i n g q u e s t i o n f o r t h e noble g a s e s concerns t h e behavior o f xenon. T h i s element i s more chemically r e a c t i v e t h a n t h e o t h e r noble gases and is b e l i e v e d t o undergo a n i n s u l a t o r - m e t a l t r a n s i t i o n a t P - 1 ' 3 Mbar [231. It is a l s o
"depleted" i n t h e E a r t h ' s atmosphere, i n t h e s e n s e t h a t XeIKr ( f o r example) is l e s s on E a r t h t h a n i n m e t e o r i t e s . The cause o f t h i s i s unknown but may be r e l a t e d t o t h e high p r e s s u r e p r o p e r t i e s o f Xe. It i s i n t e r e s t i n g t o s p e c u l a t e whether analogous anomalies w i l l a r i s e when t h e G a l i l e o probe (containing a mass spectrom- e t e r ) e n t e r s J u p i t e r ' s atmosphere i n t h e l a t e 1980's and measures t h e noble g a s abundances .
CONCLUDING COMMENTS
The emerging p e r s p e c t i v e o f p l a n e t a r y i n t e r i o r s s u g g e s t s complexity and chemistry of a v a r i e t y seldom s u s p e c t e d a decade ago. The l i g h t molecular m a t e r i a l s which predominate i n t h e o u t e r system e x h i b i t a s t a r t l i n g r i c h n e s s of behaviors which a r e becoming i n c r e a s i n g l y e v i d e n t from shock compression and diamond a n v i l c e l l experiments. I n some r e s p e c t s , extreme p r e s s u r e c r e a t e s
s i m p l i c i t y (e.g. e v e r y t h i n g e v e n t u a l l y m e t a l l i z e s ) but p l a n e t a r y c o n d i t i o n s i n c l u d e a f a r more complicated i n t e r m e d i a t e regime where d i s s o c i a t i o n , phase s e p a r a t i o n , and semiconduction a r e p o s s i b l e , probably l i k e l y . The e x c i t i n g p r o s p e c t o f r e l a t i n g e x t e r n a l o b s e r v a t i o n s t o c o n d i t i o n s deep w i t h i n t h e p l a n e t ( t h e o u t e r p l a n e t analog o f p e t r o l o g y ) l e a d s one t o hope t h a t t h e high p r e s s u r e p h y s i c s community c o n t i n u e t o be aware of t h e important a p p l i c a t i o n s o f t h e i r work t o p l a n e t s .
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