I N S T I T U T D ' A E R O W O M I E S P A T I A L E D E , B E L G I Q U E 3 - Avenue Circulaire
B • 1180 BRUXELLES
A E R O N O M I C A A C T A
A - N° 260 - 1983
Ultraviolet absorption measurements in the atmosphere of Mars
by
Paul C. SIMON and G. BRASSEUR
B E L G I S C H I N S T I T U U T V O O R R U I M T E - A E R Ö N O M I E
3 - R i n g l a a n
B • 1180 BRUSSEL
F O R E W O R D
T h i s paper h a s been p r e s e n t e d at the W o r k s h o p on the planet M a r s held at the U n i v e r s i t y of Leeds ( G r e a t B r i t a i n ) on A u g u s t 27, 1982. It will be p u b l i s h e d b y the E u r o p e a n S p a c e A g e n c y ( E S A special Publication S P - 1 8 5 ) in the p r o c e e d i n g s of the w o r k s h o p .
A V A N T - P R O P O S
Cet article résume une communication présentée à la r é u n i o n s c i e n - tifique s u r la Planète M a r s qui s ' e s t tenue le 27 août 1982 à Leeds ( G r a n d e B r e t a g n e ) . Il sera publié par l ' A g e n c e Spatiale E u r o p é e n n e ( E S A Spécial Publication S P - 1 8 5 ) d a n s les C o m p t e - R e n d u s de la r é u n i o n .
R
V O O R W O O R D
Dit artikel werd v o o r g e d r a g e n op de v e r g a d e r i n g v a n de w e r k - g r o e p o v e r de planeet M a r s die. g e h o u d e n werd aan de U n i v e r s i t e i t v a n Leeds ( G r o o t - B r i t t a n i ë ) op 27 a u g u s t u s 1982. Het zal g e p u b l i c e e r d worden in de mededelingen ( E S A Special Publication S P - 1 8 5 ) v a n de w e r k g r o e p .
V O R W O R T
Dieser A u f s a t z stellt die Z u s a m m e n f a s s u n g einer Mitteilung w ä h r e n d der W i s s e n s c h a f t k i c h e n T a g u n g ü b e r die Planet M a r s am 27. A u g u s t 1982 in Leeds ( G r o s s - B r i t a n n i e n ) da. Er wird in den T a g u n g s b e r i c h t e n veröffentlicht ( E S A Special Publication S P 185).
U L T R A V I O L E T A B S O R P T I O N M E A S U R E M E N T S I N T H E A T M O S P H E R E O F M A R S
b y
Paul C . S I M O N a n d G . B R A S S E U R
A b s t r a c t
T h e f e a s a b i l i t y of the o b s e r v a t i o n of ozone a n d c a r b o n d i o x i d e in the martian atmosphere has been s t u d i e d for the case of an o r b i t i n g limb s o u n d i n g a b s o r p t i o n spectrometer w o r k i n g in the 120-320 nm, w a v e - l e n g t h r a n g e . Vertical profiles of ozone can be m e a s u r e d u p to 40 km.
S e a s o n n a l a n d latitudinal v a r i a t i o n s at lower altitude can be also o b s e r v e d . C a r b o n d i o x i d e c o n c e n t r a t i o n s can be determined u p to 150 km of altitude.
Résumé
La possibilité d ' o b s e r v e r l'ozone et le d i o x y d e de c a r b o n e d a n s l'atmosphère de M a r s , à l'aide d ' u n spectromètre d ' a b s o r p t i o n placé à b o r d d ' u n e s o n d e , est étudiée. Il s ' a v è r e q u e la t e n e u r d'ozone peut être mesurée a u - d e s s o u s de 40 km d ' a l t i t u d e . Il en est de même de la v a r i a t i o n selon la latitude et la s a i s o n de l'ozone d a n s la partie inférieure de l'atmosphère martienne. La concentration d u d i o x y d e de c a r b o n e peut être déterminée j u s q u ' à 150 km d ' a l t i t u d e .
S a m e n v a t t i n g
De m o g e l i j k h e i d om ozon en k o o l s t o f i o x i d e in de a t m o s f e e r v a n M a r s w a a r t e nemen door middel v a n een a b s o r p t i e s p e c t r o m e t e r aan b o o r d v a n een r u i m t e s o n d e w o r d t b e s t u d e e r d . H i e r u i t b l i j k t d a t v e r t i c a l e o z o n - p r o f i e l e n t o t op een hoogte v a n o n g e v e e r 40 km k u n n e n g e m e t e n w o r d e n . D i t g e l d t e v e n e e n s v o o r de b r e e d t e - en s e i z o e n s v e r a n d e r i n g e n op l a g e r e h o o g t e s . De c o n c e n t r a t i e v a n k o o l s t o f i o x i d e k a n b e p a a l d w o r d e n t o t op een hoogte v a n o n g e v e e r 150 km.
Z u s a m m e n f a s s u n g
Es w i r d d i e M ö g l i c h k e i t u n t e r s u c h t Ozon u n d K o h l e n d i o x y d in d i e A t m o s p h ä r e d e r P l a n e t M a r s v o n einem R a u m f a h r z e u g e aus z u b e o - b a c h t e n . Man kam z u r F e s t s t e l l u n g dass v e r t i k a l e O z o n q u e r s c h n i t t e bis z u e i n e r Höhe v o n 40 km gemessen w e r d e n k o n n t e n . Dies ist a u c h d e r Fall f ü r B r e i t e - u n d J a h r e s z e i t e n ä n d e r u n g e n a u f n i e d r i g e r e n H ö h e n . Die K o n z e n t r a t i o n d e r K o h l e n d i o x y d k a n n bis z u e i n e r Höhe v o n e t w a 150 km gemessen w e r d e n .
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1 . I N T R O D U C T I O N
A l t h o u g h c a r b o n d i o x i d e is-^the major c o n s t i t u e n t in t h e M a r t i a n a t m o s p h e r e , t r a c e species such as molecular o x y g e n , c a r b o n m o n o x i d e , w a t e r v a p o r , ozone a n d odd h y d r o g e n r a d i c a l s p l a y a c r u c i a l r o l e in t h e chemical a n d photochemical p r o c e s s e s . ( S e e r e f . 1 f o r a r e v i e w of t h e c h e m i s t r y in t h e M a r t i a n lower a t m o s p h e r e ) .
O z o n e , f o r e x a m p l e , has b e e n d e t e c t e d f o r t h e f i r s t time b y an u l t r a v i o l e t s p e c t r o m e t e r a b o a r d M a r i n e r 7 ( r e f . 2 ) a n d has b e e n m a p p e d f r o m M a r i n e r 9 ( r e f . 3 ) . A t t h e p r e s e n t t i m e , no d e t a i l e d v e r t i c a l d i s t r i b u t i o n of O^ is a v a i l a b l e b u t o b s e r v a t i o n s of t h e ozone column show a l a r g e v a r i a b i l i t y w i t h time a n d l a t i t u d e . I n f a c t , o v e r most of t h e p l a n e t , t h e t o t a l amount of ozone is smaller t h a n t h e d e t e c t i o n limit 15 - 2 of t h e s p e c t r o m e t e r s ( 3 pm-atm o r 8 x 10 molecules cm ) b u t in t h e v i c i n i t y of t h e p o l a r c a p , t h e c o n c e n t r a t i o n i n c r e a s e s c o n s i d e r a b l y d u r i n g t h e w i n t e r a n d r e a c h e s an i n t e g r a t e d v a l u e of a b o u t 60 (jm-atm
17 - 2 ( 1 . 6 x 10 molecules cm ) .
T h e e n h a n c e m e n t of ozone in t h e w i n t e r p o l a r r e g i o n a p p e a r s to be c o n n e c t e d w i t h a r a p i d d e c r e a s e in t h e w a t e r v a p o r c o n t e n t w h i c h is d r i v e n m a i n l y b y a v a r i a t i o n in t h e local t e m p e r a t u r e . I n f a c t , in t h e M a r t i a n lower a t m o s p h e r e , a d e c r e a s e of 10 K in t h e t e m p e r a t u r e r e d u c e s t h e s a t u r a t i o n w a t e r v a p o r p r e s s u r e b y a f a c t o r of 10. Most of t h e w a t e r v a p o r is t h e r e f o r e c o n d e n s e d in w i n t e r a n d t h e o d d h y d r o g e n s o u r c e ( a n d c o n s e q u e n t l y t h e odd o x y g e n loss) is s u p p r e s s e d . T h e H^O b u d g e t seems also to be r e l a t e d to t h e amount of a i r - b o r n e d u s t a n d is t h e r e f o r e s e n s i t i v e to d u s t s t o r m s .
A n u n d e r s t a n d i n g of t h e k e y aeronomic processes o c c u r i n g in t h e lower a t m o s p h e r e of M a r s r e q u i r e s simultaneous o b s e r v a t i o n s of o z o n e , w a t e r v a p o r , t e m p e r a t u r e a n d p r e s s u r e p r o f i l e s . Because of t h e lack of t h e s e d a t a , t h e k n o w l e d g e of t h e chemical composition of t h e M a r t i a n
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atmosphere as a function of the altitude has still to be b a s e d on theoretical models ( r e f s . 4 - 8 ) . T h e s e models w h i c h h a v e to account f o r the s u r p r i s i n g low O ^ and C O a b u n d a n c e s show the importance played b y o d d h y d r o g e n radicals and r e q u i r e either h i g h water v a p o r content or large vertical e x c h a n g e coefficients in o r d e r to fit the available o b s e r v a t i o n s .
T h e p u r p o s e of this paper is to show the feasability of an ozone a n d c a r b o n dioxide profile measurement b y a b s o r p t i o n s p e c t r o s c o p y in the ultraviolet. S i n c e ozone is entirely of photochemical o r i g i n , the measurement of its vertical profile s h o u l d permit an ideal validation of the c u r r e n t photochemical t h e o r y of the Martian atmosphere.
2. T H E K E P L E R P R O J E C T
A M a r s g e o p h y s i c a l o r b i t e r , called Kepler mission, has been p r o p o s e d to the E u r o p e a n S p a c e A g e n c y ( E S A - A S E ) in o r d e r to s t u d y the Martian atmosphere, the planetary interior a n d s u r f a c e s a n d the interaction with the solar w i n d . A n a s s e s s m e n t s t u d y has been p u b l i s h e d ( r e f . 9) and followed b y a Phase A s t u d y which is still in p r o g r e s s . A model scientific payload has been defined i n c l u d i n g an ultraviolet spectrometer. T h i s i n s t r u m e n t is dedicated to the measurement of radiation emitted b y the Martian atmosphere and of solar radiation a b s o r b e d b y the atmosphere. T h e airglow measuremens are p r e s e n t e d in ref. 10. T h e o r b i t e r will have a p e r i a p s i s a r o u n d 150 km and a b s o r p t i o n measurements will take place d u r i n g the limb s c a n n i n g operation mode.
For that p u r p o s e , a s u n l i g h t d i f f u s e r u s e d as a periscope s h o u l d be included in f r o n t of the ultraviolet spectrometer to avoid a solar pointing s y s t e m aboard the spacecraft.
A b s o r p t i o n measurements will be performed n e a r b y the p e r i a p s i s in o r d e r to p r o v i d e information on the vertical d i s t r i b u t i o n of atmospheric c o n s t i t u e n t s , in particular ozone and c a r b o n dioxide. T h e altitude resolution is defined b y the orbiter altitude (about 200 km) and v a r i e s
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f r o m 7 . 3 km to 3 . 5 km f o r a r a y - g r a z i n g a l t i t u d e f r o m 0 to 150 km. T h e s e n s i t i v i t y of a b s o r p t i o n m e a s u r e m e n t s is e n h a n c e d w h e n an o c c u l t a t i o n t e c h n i q u e is used since t h e o p t i c a l t h i c k n e s s is i n c r e a s e d b y a f a c t o r of a b o u t 7 0 .
3 . T H E A B S O R P T I O N M E A S U R E M E N T S
I n t h e M a r t i a n a t m o s p h e r e , t h e u l t r a v i o l e t a b s o r p t i o n s p e c t r u m is d o m i n a t e d b y ozone b e t w e e n 200 a n d 300 nm a n d b y c a r b o n d i o x i d e below 200 nm. B o t h c o n s t i t u e n t s a r e c h a r a c t e r i z e d b y a b s o r p t i o n b a n d s v a r y i n g slowly w i t h t h e w a v e l e n g t h . T h e r e f o r e , h i g h r e s o l u t i o n s p e c t r a a r e not r e q u i r e d to d e t e r m i n e t h e i r v e r t i c a l p r o f i l e s f r o m solar o c c u l t a t i o n m e a s u r e m e n t s .
T h e CO^ o p t i c a l d e p t h can be e s t i m a t e d e i t h e r f r o m m o d e l s ( r e f . 4 ) o r f r o m m e a s u r e m e n t s p e r f o r m e d b y t h e V i k i n g 1 a n d 2 missions.
C o n s i d e r i n g t h e maximum v a l u e of t h e a b s o r p t i o n c r o s s - s e c t i o n a r o u n d 1 3 0 - 1 3 5 n m , a b s o r p t i o n of t h e o r d e r of 12 p e r c e n t is still e x i s t i n g at an a l t i t u d e of 150 km.
T h e c o m p u t a t i o n of t h e Og optical d e p t h can be based on t h e o r e t i c a l v e r t i c a l p r o f i l e s c a l c u l a t e d f o r i n s t a n c e b y S h i m a z a k i ( r e f . 8 ) u s i n g a photochemical 1 - D model w h i c h i n c l u d e s seasonal a n d d i u r n a l v a r i a t i o n s in t e m p e r a t u r e , w a t e r v a p o r a n d solar r a d i a t i o n ( f i g . 1 ) . T h e a d o p t e d ozone p r o f i l e c o r r e s p o n d s to t h e m i d - w i n t e r models. V a l u e s of t h e ozone c o n c e n t r a t i o n v e r s u s a l t i t u d e a r e g i v e n in t a b l e 1 w i t h t h e c o r r e s p o n d i n g a b s o r p t i o n c r o s s - s e c t i o n r e q u i r e d to g e t an o p t i c a l t h i c k n e s s equal to 1 , w h i c h c o r r e s p o n d s to t h e b e s t c o n d i t i o n s of o b s e r v a t i o n b y a b s o r p t i o n s p e c t r o s c o p y . From t h i s t a b l e , t h e w a v e - l e n g t h s of m e a s u r e m e n t s can be d e d u c e d u s i n g f o r i n s t a n c e t h e a b s o r p t i o n c r o s s - s e c t i o n of I n n a n d T a n a k a ( r e f . 1 2 ) . T h e u s e f u l w a v e - l e n g t h i n t e r v a l n e e d e d to c o v e r t h e a l t i t u d e r a n g e f r o m 0 to 40 km is i l l u s t r a t e d on f i g . 2 . T h e lower limit of d e t e c t a b i l i t y of ozone is
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o> I I
< 20
OZONE NUMBER DENSITY (cm )
Fig. 1.- Profiles of ozone densities for different seasons as calculated by Shimazaki (Ref. 8). L is the acrocentric longitude. The dashed
b
vertical line corresponds to an optical depth of 0.05 at 255 nm.
TABLE 1.- O z o n e v e r t i c a l p r o f i l e adopted in this w o r k .
A l t i t u d e 0 concentration A b s o r p t i o n cross -3
(km) (cm ) section for T f 1
(cm )
5
10
15 20 25 30 35 40
5.6 x 10 2.7 x 10 1.3 x 10 5.4 x 10' 2.6 x 10S
1.4 x 10S
6.6 x 10*
2.1 x 10f
10 10 10
6.2 x 10 1.3 x 10 2.6 x 10 6.2 x 10 1.3 x 10
•19
- 1 8 - 1 8 - 1 8
•17
- 7 -
0.07
< •
t 1 ' r
OPTICAL DEPTH
< > 0.2 3
mars
00 i I
JC £
UJ Q Z>
^ 20h
0.2
0.2«-
- 0.2«- 1
-K-1
* > 0.5 0.9 >-
-<r *-1.8
- > I* u < -
1
- > 0 . 2
-»•0.2
1
-»•0.2
1 1
••0.2
1
- » A U < -
1
• 0 . 2 -
J L _L I I i L
200 220 2 AO 260 280 WAVELENGTH (mm)
300 320
Fig. 2.- Altitude corresponding to an optical depth whose value is quoted in the figure, as a function of the wavelength, for an occultation geometry.
determined by the maximum value of the absorption cross-section around 255 nm. Considering that an absorption of 5 percent is still measurable with a good accuracy, it clearly appears from fig. 1 that the ozone profile can be measured up to 40 km during the winter. Seasonal variations can be also easily detected below 25 km if the lower profile calculated for mid-summer conditions by Shimazaki ( r e f . 8) is considered.
In conclusion, an ultraviolet spectrometer aboard a Mars orbiter, scanning the 120-310 nm wavelength range at low resolution is well adapted for determining carbon dioxide profiles up to 150 km and ozone profiles below 40 km of altitude, by solar occultation measurements in the Martian atmosphere. Seasonal and latitudinal variations ot U3 can be also measured in the lower atmosphere.
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4 . REFERENCES
1. von Z A H N , U . and H U N T E N , D . M . , 1981, Aeronomy of Mars Lower Atmosphere : Status and Prospects, T h i s volume.
2. B A R T H , C . A . and H O R D , C . W . , 1971, Mariner U l t r a v i o l e t S p e c t r o - meter: T o p o g r a p h y and Polar C a p , Science, 173, 197.
3. B A R T H , C . A . , H O R D , C . W . , S T E W A R T , A . I . , L A N E , A . L . , D I C K , M . L . and A N D E R S O N , G . P . , 1973, Mariner 9 U l t r a v i o l e t S p e c t r o - meter Experiment : Seasonal Variation of Ozone of M a r s , Science, 179, 795.
4. KONG, T . Y . and M c E L R O Y , M . B . , 1977, Photochemistry in t h e Martian Atmosphere, I c a r u s , 32, 168.
5. KONG, T. Y . and McELROY, M . B . , 1977, T h e global D i s t r i b u t i o n of 03 on Mars, Planet. Space S c i . , 25, 839.
6. K R A S N O P O L S K Y , V . A . and P A R S H E V , V . A . , 1979, Ozone and Photochemistry in the Martian Lower Atmosphere, Planet. Space S c i . , 27, 113.
7. S H I M A Z A K I , T . and S H I M I Z U , M . , 1979, T h e seasonal v a r i a t i o n of 03 density in t h e Martian Atmosphere, J. Geophys. R e s . , 84, 1269.
8. S H I M A Z A K I , T . , 1981, A model of Temporal Variation in Ozone Density in the Martian Atmosphere, Planet. Space S c i . , 29, 21.
9. ESA, SCI ( 8 2 ) 2 , 1981, K e p l e r , Assessment S t u d y .
10. G E R A R D , J . - C . , 1982, T h e Martian Airglow and Scattered S u n - light : F u t u r e Observations from a Spinning o r b i t e r , T h i s volume.
11. S E I F , A . and K I R K , D . B . , 1977, S t r u c t u r e of t h e Atmosphere of Mars in Summer at M i d - l a t i t u d e s , J. Geophys. R e s . , 82, 4364.
12. I N N , E . C . Y . and T A N A K A , Y . , 1953, Absorption Coefficient of Ozone in the Ultraviolet and Visible Regions, J. O p t . Soc. A m e r . , 43, 870.
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