The Saturn system presents exciting and unique objects for planetary aeronomy. The photochemistry of H2 and He leads to the formation of an ionosphere. Methane photolysis results in the formation of spectroscopically detectable amounts of C2H6 and C2H2 and in the case of Titan, C2H4. Density profiles of C2H6, C2H2, and PH3 should be indicative of the strength of atmospheric mixing processes

Strobel, D. F.

English

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AERON(IMY OF SATURN 
AND TITAN 
Tbe Saturn system presents exciting d unique objects for planetary aeronomy. 
The presence of hydrocarbons on Titan raises interesting possibilities for organic 
chemistry. Light gases mcy escape Titan's gravitational field but .lot the stronger 
planetary field of Sakun and thus lead to the formation of a gas- tomidd cloud 
$~ound Saturn (McDonough and Brice, i973a, b). From our limited informatioa 
Saturn's upper atmospkre appears similar in thermal structuvs, cornpodtion, and 
photochemistry to the Jovian upper atmosphere. 
THERMAL STRUCTURE 
A distinctive feature of the Saturn system i s  the thermal IR emission. On 
Snkrl.n the pronounced emission peak of CH4 at 7 .7  prn suggests a warm stratowhere, 
T r 130 K, while the same feature on Titan is indicative of temperatures r 160 K. The 
https://ntrs.nasa.gov/search.jsp?R=19790008599 2020-03-22T01:37:31+00:00Z
most plmsible interpretation of tbRss emission syctra is the presence of a 
t e m p e r e m  inversion in their stratospheres similar to the Earth's O3 layer with 
solar IR beatbg in the 3.3 (rm CAI band balanced by IR cooling in +he 7.7 pm CH4 
b d ,  tip 12.2 pm CZHs band, and the 13.7 pm C2H2 (Gillett at d ,  1969; Daniel- 
ct d, 1973). A d d i t i ~ ~  heating may be required to acMeve these observed "temper- 
atures*' armd fine absorbing particles which absorb UV a d  visible sunlight have been 
suggested (Axel, 1972; Dmielson et d ,  1973). Since these particles are v e q  small, 
they are poor emitters d will heat up and collisionally transfer their energg to 
atmospheric games. 
Strobel and Smith (1973) conc1uded that the globally averaged vertical temper- 
ature contrast in the thermospheres of Saturn and Titan was -10 K and -90 K, respec- 
tively, for mlar E W  heating only. It should be remembered that Pioneer 10 
measured a W a n  ionospheric scale hemt that implied a much warmer tbemo- 
sphere than alar EUV hew could mdntain (Fjeldbo et dl 1975). The above 
temperature contrast for Saturn's thermosphere should be regarded as a minimum 
value. 
Molecular hydrogen, the major constituent of the outer planets, has a dis- 
sociation co-um below 845 A and an ionization continuum below 804 A (Cook and 
bietzger, 1964). Discrete absorption irl the Lyman and Werner bands can lead to 
flaoresoeot dissociation of 4 (d. Field et d ,  1966; Stecher and WiUiams. 1967). 
The deposition of solar EUV radiation in a H2 atmosphere results initially in the pro- 
+ 
clctlon of primarily Ii2' ions, which react with 112 to produce B3 + H and break a 
+ 
H2 ..~nd. If He is present the production of He ions will result in the dissociation 
of HZ by ion-molecule reartias. For each H2 molecule sod He atom ionized a t  l a s t  
two H atoms will be produced. Three body recombination of H atoms is exceedingly slow 
tn the ionosphere and consequently there is a large downward flux of H atoms on 
Saturn from Ms region. For Saturn the globally averaged downward H atom flux is 
-2 x 10 cm -2 s-I (Strobel, 1973~). 
The planetary albedo at Ly-a is in simple terms a measure of the H atom 
column density above the absorbing CH4 layer. This column density is a sensitive 
function of the eddy diffusion coefficient, K, in the vicinity of the turbopause 
(Hunten, 1969; Wallace a d  Huten, 1973). The recent roc!cet measurement of 
Ly-a from the Saturn system by \Veiser et oL (19'17) gave -700 R for the disk. TBis 
is cumparable to a 2kR signal from Japiter d, based on Wallace and Hunten (19731, 
would suggest an eddy diffusion cue£ficient near the turbopause of 10 (6-7) cm2 s-l 
In addition Weiser el d (1917) detected -200 R of Ly-a in the vicinity of Saturn which 
may be indicative of a h y d q p n  atmosphere associated with the ring system. bba- 
surelnents of t!w Ly-a brrgbtness a r d  the sa%?llites will provide important infor- 
mation on processes that prodcce the gaseous bmidal clouds discussed in the 
Intnxiuction. 
The formation of m ionosphere X2-dominated atmosphere has  been most 
recently discussed by Abeyi  and W u e  (1975). Of particular Importance are 
the major sources of H + ions as a H plasma can only r e m d n e  radiatively at a 
3 -1 .?- 
slow rate (-7 x lo-* cm s ). The H, ions prwhced by the reaction II2+ + I+ 
- "  
-1 disaodatively recornhe r2piifry el  x lo-' cmn r ). T b  H + will be the major 
rapidly. This behavior is illustra%d in Fi&ure 1 for Saturn. In the lower ionosphere 
(s 125 km, Figure 1) hydroc'irbn ions, produced bp hydrogen ions (H +, H**. %+) 
reacting with CH4 and photoionizAon of CH3 radicals (Prasad and Tan, 1974). 
dominate. They dissaiatively recombioe an order of uagnitude faster than Ei3+ d 
would lead to lower electron densities (up to a fador  of 3) in this region than illus- 
trated in Figure 1. Ionospheric models have also been developed for Titan by 
# i t t e n  et el. (1977). 
PHOTOCHEMISTRY OF HYDROCARBONS 
The principal hydrocarbon In the outer solar system is CH4. C2H6 has heen 
detected in ttre atmospheres of Saturn, and Titan (Hidgway, 1!?74; Gillett and Forrest, 
1974; Gillett et al., 1973). Hydrocarbons in the presence of UV radiation can form 
polymers. Based on phdochemical models for the outer planets Strobel (1975) con- 
cluded that only a small percentage of dissociated CH4 molecules are converted to 
complex hydrocarbons. To first order a closed photochemical model can be con- 
structed; the principal reactions are schematicaily presented in Figure 2. 
Appmximately 70% of the solar photons which dissociate CHI are at Ly-e where the 
primary processes are (Rebbert and Ausloos, 1972; .4usloos, 1972) 
where ' C H ~  denotes the singlet state of CH2. 
The only chemical means (other than energetic photons) for breaking the bond 
of two C atoms is the reaction sequence: H + CZHq - C H , H + C2H5-- 2CH3. For 2 5 
this destruction to be important a large ii atom concentration is required at pressures 
-10 mbar. As a consequence there is some production of higher hydrocarbons. To con- 
serve C atoms a downward flow of C2H6 and C2H2 1s balanced by an upward flow of 
C%. It is postulated that a deep circulation is present in Saturn that transports 
higher hydrocarbons to the hot, dense interior where they undergo thermal decompo- 
sition to produce fresh CH4 which is transp9rted upwards to replenish the CH4 
destroyed in photolysis. 
Tbe photochemical model is must sensitive to the [CII~] / [H~]  mixing ratio, 
the escape rate of H atoms from the atmosphere, and the atmospheric mixing rate 
(eddy diffusion coefficient). From Figure 2 it is evident that as the [ c H ~ ] / [ H ~ ]  ratio 
increases, the production rates of C2Hq and C2H2 will increase. The fate of the CH3 
radical depends on thc [H]/[cH~] ratio and determines the rate at which CH4 i s  
recycled. Although the escape rate of H atoms from Saturn i s  negligible, it can be 
substantiai from the satellites and actually control the H atom density distribution. 
A large escape rate depresses the H concentration and results in large conversion 
rates of CH4 to CZHs and CZHZ ( >SO% for Titan (Strobel, 1974a)). -41so Large tor,- 
centrations of C2H2 will efficiently remove 1i atoms by catalytic recombination as 
illustrated in Figure 2, H + C2H2 -' C2H3, H + C2H3 - H2 + C2H2 (Strobel, 1973a). 
The moat abundant hydrocarbons produced in CH4 photolysis are CgH6 and 
C2H2. They have vertical density distributions of the form (Strobel, 1974b): 
where K = KO exp(z/Hk), Hav is the atmospheric scale height, C is an integration 
constant, and t$o is the downward flux approximately equal to the column production 
rate. Tyyical hydrocarbon densities are illustrated in Figure 3 for Saturn. C2H6 i s  
a stable molecule in a cold, reducing atmosphere. If the interior conditions do not 
require rapid downward flow, then the C2Hg density profile is given approximately 
by the first term of (2). i .e. , CZHs is mixed. For rapid downward flow the C2H6 
density profile will exhibit the scale height of K and be represented by the second 
term of Equation (2). For C2H2 some chemical removal occurs when the ratio 
[H]/[H~] is very small. A large downward flux is required to balance this loss. Con- 
sequenily its density profile is represented by the second term of Equation (2). The 
Ft'gmr 3. Hydnx& & I ~ J  pm/fej fm the Sntum:an a t m o ~ p h  u i t h  indrcated edd) d i f i o n  M l e .  Solid line. 
K a ( M )  -"='. A h a d  lrmd: K = lO5dj-. ' .  uhwe (A{) = n n m k  cicnjitj ojrrtmacphcric ga. b u m  LwnaLr) 
rondi t ioafw C,II, is P small a h n u w r d &  (see S t m k f .  1973a). 
rapid decrease in the H concentration below 300 km is due to catalytic removal by 
C2H2 (Strobel, 1973a). It should be noted that for C2H2 (and C2H6 if rapid downward 
flow is required) that the very rapid mixing characteristic of the troposphere will 
result in a substantial decrease in the number density below the tropopause (cn the 
order of the eddy diffusion coefficient ratio). 
Products of CH photolysis on Titan are removed by condensation at a cold 4 
tropopause and possibly dissolve on the cold surface. A s  a consequence a large down- 
-2 -1 
ward flux of photolysis products is anticipated (-10l0 cm s , primarily C2HB and 
C2H2). Over the age of the solar system this would represent an accumulation of 
30 kg cmW2. Photochemical models for Titan predict observable amounts of C2H6 
and C2H for slow mixing rates in the lower stratosphere (eddy diffusion coefficients 
5 3 -1 110 c m  s ) a s  illustrated in Figure 4. Their density profiles are represented by 
the second term of Equation (2), from which it follows that large densities are associ- 
ated with slow mixing. The other essential features of the model are that the C2H6 
abundance is sensitive to the eddy diffusion coefficient, the composition of Titan's 
atmosphere, and the net escape rate of H atoms from the exobase, whereas the C2H2 
abundance i s  principally sensitive to the eddy diffusion coefficient. 
Figrrrc 4. H y a h a d w  dnrritior a a finrtiim 
o j  tbe et& diJf~~sion mJ2i~nt Jbr a p ~ m  CHI 
~tnmpbnr 08 Titdn. T& dottad iiau mpwent a 
d I  with K = l d d s - '  and a negIigiQIc 
nartion rate fir CH, + CH, + CJf4 4- H. 
PHOTOCHEMISTRY OF pH3 AND NH3 
On Saturn and Titan the cold trap temperatures at the tropopause are suffi- 
ciently low and restrict the NH3 mixing ratio above this level to less than 10-12. As 
a consequence only the more abundant pH3 on S a w n  is aeronomically important. The 
Prim and Lewis (1975) model is applicable with the addition of the reaction 
H + PH3 - pH2 + H2, which effectively doubles the dissociation rate (Lee el aL , 1976; 
Strobel, 1977). One questionable feature of this model is the absence of any direct 
mechanism for partial recycling of pH3 in the photolysis region. Each absorbed UV 
photon by PHI leads to irreversible conversion to P4 {red phosphorus). 
The vertical PH3 density profile is given by 
4~~ E J  
where 1. - 3v K , J is the PII. dissociation rate and c - 2  as  discussed above. 3 
For sufficiently slo~v mixing, this expression reduces to 
-7 For Saturn c J - P  x 10 s-I and if K-lo4  cm3 s", then the effective pH3 scale height 
is-2 km, i. e .  pH3 is confined to the lower stratosphere. The vertical distribution 
of [ P H ~ ]  is thus indicative of K(z) in the 1orver.stratosphere. 
.*, 
The photochemistry of H2 and He leads to the formation of the main ionosphere 
and, through ion chemistry 311d recombination, H atoms. Observations of electron den- 
sity and ion conlposition will yield information on the chemistry and structure of the 
ionosphere. H atoms can be observed by Ly-a emission and interpretation of the 
data should yield the mixing rate near the turbopause. CH4 photolysis leads princi- 
pally to the formation of C2HG and C2HZ. C2HG is a photochemicaily stable molecule 
and, depending on the lower boundary condition, is either quasi-mixed o r  K [C H ]is 2 6 
quasi-conserved above the tropopause. Also K [C H ] is quasi-conserved in this 2 3 
region. Thus observations of thp OaHa nnd CZH6 density profiles can yield informa- 
tion on K(z) and the magnitude of the tiown~vard C2H6 flow to the interior in the case 
of Saturn. The extent of catalytic removal of H atoms by C2HZ requires a 
measurement of H density profile. The models predict [ c ~ H ~ ] / [ c ~ H ~ ]  << 1 and that 
C H can only be detected on Titan where its scale height is an order of magnitude 2 4 
larger than on Saturn. Attempts to observe C H would serve a s  a useful internal 2 4 
check on the models, 
NH3 should he frozen oat below the tropopauw on Saturn and Titan and thus 
be asronomically unimportant. On Saturn the vertical 1313 dellsity profile should be 
indicative of K(z) i n h e  lo~ver stratosphere in a similar manner a s  the NH3 profile on 
Jupiter (Strobel, 197313). 
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DISCUSSION 
S. CHANG: If a probe went into Jupiter's atmosphere o r  Saturn's atmosphere 
and actually f o u d  sigrificant amounts of ethane in the troposphere, would you conclude 
thst some process other than methar.e photochemistry is responsib!~ ? 
STROBEL: That would be one possible interpretation. The other is tk-at 
thermodynamically there were large amcvnts in Ule deep interior 'and that the mi-ung 
W ~ S  violent enough t3 carry it up tc the observd'le region. 
A. TOWNAGA: Is  there evide11c.e that the eddy diffusion coefficient is 
uizerent at the poles relative to he cqiator? LC that were the case, the ethane mixing 
ratio might vary with latitude. 
D. STROBEL: The eddy diffusion coefficient w~orks best when it  is rc.garded as  
a global average. As one tr ies  to ascribe to it  horizontal variations, I think one is 
stretching the use of the concept. Icls a convell;ent factor to use as a f irs t  estimate of 
what's going on in t k  atmosphere, bat when the s ~ b j e c t  becomes sufficiertly matura, 
one should use actual dynamical equations to calculate the tr:msport. 
J. POLLACK: Didn't Don Hunten once nickxime it  the eddy confusion 
coefficient ? 
D. HUhTEN: The ztraight answer to Alan's question is that there could be 
large variations in vertical trrnsport rates with latitlrde on a planet like Saturn, very 
large. 
D. STROBEL. Especially ii the vertically propagating wal:es are  the cause of 
the phenomena an1 depending on where tixy are  escited. 
D. HUhTEN: George SIscoe was quite interestec~ in hydrogen production rates 
md, of course, they can be derived implicitly irom these computations. The vortical 
curvcs at the bottom right of Figure 4 give a d o ~ v ~ l w ~ r d  flux of the things left ovei , 
once you've made hydrogen. The correspor.di11g hydrogen i s  escaping, and the rate is 
9 -2 -1 9 x 1 0  cm s . 
D. STROBEI,: Right.  I should emphasize that the atmosphere i s  con5 ider- 
ably larger than the solid body, and therefore c:q:tures more solar photons th'm 
you might think. 


Aeronomy of Saturn and Titan

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