The wide variation in infrared brightness temperature of Titan is explained in terms of a greenhouse effect. Radiometric observations in the infrared and microwave frequencies indicate an alternate hot atmospheric model. Methane, ammonia, hydrogen atoms, and nitrogen atoms are suggested as main constituents for the Titan atmosphere

Morrison, D.

English

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2.2 INFRARED PHOTOMETRY AND SPECTROPHOTOMETRY OF TITAN 
D. Morrison 
In t roduct ion  
I  would l i k e  t o  begin t h i s  discussion o f  radiometric measurements of  Ti tan  
with a b r i e f  h i s t o r i c a l  review. Since t h e  discovery by Kuiper (1944) o f  CH4 
bands i n  t h e  spectrum of t h i s  s a t e l l i t e ,  it has been known t h a t  Ti tan  possesses 
an atmosphere, but following t h e  d iscuss ions  by Kuiper (1944, 1952), it has been 
assumed u n t i l  r ecen t ly  t h a t  t h i s  atmosphere i s  very tenuous, with a surface  pres-  
su re  of only a few mi l l i ba r s .  Such an atmosphere, as we know from t h e  example of 
Mars, has only rel 'at ively minor e f f e c t s  on t h e  su r face  tempkrature, at: l e a s t  i n  
t h e  hemisphere fac ing t h e  Sun. Thus, it was expected t h a t  t h e  mean temperature 
on t h e  day s i d e  o f  Ti tan  would be approximately t h a t  of  a hemisphere i n  equ i l ib -  
rium with t h e  in so la t ion .  ) A t  T i tan ' s  d is tance  from the  Sun, t he  maximum such 
temperature, corresponding t o  a blackbody fac ing t h e  Sun, i s  127°K. For an 
objec t  with the  bolometric Bond albedo o f  Ti tan ,  which is ,  according t o  Younkin 
(1973), ,equal  t o  0.27, t h e  maximum equil ibrium subsolar  temperature i s  116°K. 
The corresponding disk-averaged i n f r a r e d  br ightness  temperature would be about 
0.92 times t h i s ,  o r  107°K. These temperatures a re  a few degrees lower than 
those t h a t  would have been computed a few years  ago, pr imar i ly  as  a r e s u l t  o f  
t he  h igher  bolometric albedo adopted here ;  t h e  f i r s t  authors t o  measure T i t a n ' s  
radiometric temperature i n  f a c t  expected t o  f ind  110 5 T 2 115°K (Low 1965; 
Allen and Murdock 1971; Morrison e t  a l .  1972). 
Recent observations i n  a number of a reas  have changed these  expectat ions 
and have increased t h e  s ign i f i cance  of radiometric measurements of  Ti tan .  Most 
i n f l u e n t i a l  has been the  work of Trafton (1972a, b ) ,  who discovered s p e c t r a l  
evidence f o r  H2 on Titan and presented an important reanalys is  of  Kuiper's CH4 
observations as well  as  o f  his'own. The r e s u l t  of  t h i s  work l a s  t o  suggest 
t h a t  t h e  su r face  pressure  on Titan i s  s u b s t a n t i a l l y  higher,  o f  t he  order  o f  
0.1 atm. A t  t he  same time, polar imetr ic  s tud ie s  by Veverka (1973) and Zel lner  
(1973), as  well  as  severa l  discussions o f  t he  low u l t r a v i o l e t  albedo o f  t h i s  
s a t e l l i t e ,  showed t h a t  t h e  atmosphere of Ti tan  contains o p t i c a l l y  th i ck  clouds 
and r a i sed  the  p o s s i b i l i t y  t h a t  an extens ive  and spect roscopica l ly  unobserved 
atmosphere extends t o  grea t  depths below the  cloud layer .  An atmosphere of 
t h i s  magnitude might be expected t o  moderate va r i a t ions  of surface  temperature 
s o  t h a t  t h e  in f r a red  br ightness  temperature o f  Titan would approach t h a t  of  an 
isothermal objec t  o f  i t s  albedo and d is tance  from the  Sun, which is  TB 2 84°K. 
Recent Radiometric Measurements 
With t h i s  background, we now t u r n  t o  the  i n f r a r e d  observations,  which 
have provided many su rp r i se s  during the  pas t  3 years .  The f i r s t  observers t o  
note  t h e  anomalously high in f r a red  br ightness  temperature of Titan were Allen 
and Murdock (1971), who obtained a temperature of 125 ? 2 " ~  i n a band from 10 
t o  14 vm (nominally 12.4 pm). Since they had expected a maxicinum br ightness  
temperature of %11S0K, Allen and Murdock concluded t h a t  t h e r e  was l i k e l y  t o  
be a greenhouse e f f e c t  on Titan t h a t  increased t h e  observed 12.4-micron f lux  
by a f a c t o r  of  about 2 over t h e  equil ibrium l eve l .  An e a r l i e r  br ightness  tem- 
pe ra tu re  of 132 ? 5°K i n  t h e  8- t o  14-micron b%nd published without comment by 
Low (1965) was cons is tent  with t h i s  suggestion.  
https://ntrs.nasa.gov/search.jsp?R=19740012369 2020-03-23T11:11:19+00:00Z
I 
The ac tual  magnitude of t he  change i n  br ightness  temperature with wave- 
length,  and hence of t he  pos tu la ted  greenhouse e f f e c t ,  became apparent with 
the  pub l i ca t ion  o f  addi t ional  observations i n  t h e  in f r a red .  Morrison e t  a l .  
(1972), observing i n  a broad band from 16 t o  28 tlm (nominally 20 pm) found a 
temperature o f  93 5 ~ O K ,  almost as low as  t h e  equil ibrium temperature f o r  an 
isothermal s a t e l l i t e .  They f u r t h e r  documented t h e  range of observed tempera- 
t u r e s  by quoting unpublished observations by G i l l e t t  and Forres t  t h a t  y ie lded 
a disk temperature of 134 rt: Z°K a t  11 pm and 144 f 3OK a t  8.4 pm. They assumed 
t h a t  t he  temperature i n  t h e  atmosphere inc reases  with depth and concluded t h a t  
t he  only abundqnt gas t h a t  could provide the  indica ted  grea t  opaci ty  i n  the  20- 
micron band toge the r  with an opacity decreasing from 12 t o  8 pm i s  H2 a t  su f -  
f i c i e n t  pressure  t o  induce translational-rotaGional t r a n s i t i o n s .  Further,  s ince  
t h e  bulk of t h e  r a d i a t i o n  from such a cool body i s  a t  wavelengths between about 
15 and 50 pm, they noted t h a t  t h i s  l a rge  opaci ty  i n  t h e  20-micron band would 
r e s u l t  n a t u r a l l y  i n  a greenhouse e f f e c t .  They the re fo re  argued f o r  a massive 
hydrogen atmosphere with surface  pressure  of hundreds of mi l l i ba r s  and surface  
temperatures ,150°K (see aljso Cruikshank and Morrison 1972). A t  t he  same time, 
ca l cu la t ions  by Sagan and Mullen (Sagan 1973) indica ted  t h a t  a pure hydrogen 
greenhouse on Titan i s  capable of generating surface  temperatures h igher  than 
200 '~ .  These arguments f o r  a massive greenhouse e f f e c t  have been developed 
extens ively  by Pollack (1973) and Sagan (1973) and have encouraged pub l i c  in-  
t e r e s t  i n  T i t an  as a poss ib le  abode o f  l i f e .  
A grea t  improvement i n  t h e  s p e c t r a l  r e so lu t ion  of radiometric measurements 
has now been achieved by G i l l e t t  e t  a l .  (1973), who have observed Titan a t  8 wave- 
lengths between 8 and 13 pm with a cooled Til ter-wheel  spectrometer having a reso- 
l u t i o n  A X / X  = 0.015. These narrow-band data ,  t oge the r  with seve ra l  broad-band 
measurements i n  the 8-13 pm band, reveal  s u b s t a n t i a l  s t r u c t u r e  i n  the  thermal 
spectrum, suggestive o f  l i n e  emission. Such s t r u c t u r e  was not  predic ted  by the  
greenhouse models (Pollack 1973), but it i s  cons is tent  with the  hot  atmosphere 
model r ecen t ly  proposed by Danielson e t  a l .  (1973) as an a l t e r n a t i v e  t o  the  
greenhouse models. 
One addi t ional  i n f r a red  measurement of Ti tan  has been made, a t  4.9 pm by 
Joyce e t  a l .  (1973). The mail1 cons t i t uen t s  suggested f o r  the  atmosphere of 
Ti tan  -- CHI.,, NH3, H z ,  and N2 -- have no absorption bands a t  t h i s  wavelength, 
so t h a t ,  as ide  from poss ib l e  cloud opaci ty ,  a temperature a t  4.9 pm might be 
expected t o  be characterist j2c of t h a t  near  t h e  surface  o f  t h e  s a t e l l i t e .  Joyce 
e t  a l .  d id  no t  de t ec t  4.9-micron r ad ia t ion  from Ti tan ,  but  s e t  an upper l i m i t  
--
of  190°K. Their  observation a l so  r e s t r i c t s  t h e  4.9-micron albedo of Ti tan  t o  
a value o f  l e s s  than 0.5. 
A l l  of  these  observed temperatures are  summarized i n  Table 2-1, ca te-  
gorized as  narrow-band and broad-band measurements. 
In t e rp re t a t ions  
The tempecuures shown i n  Table 2-1 c l e a r l y  show the  l a rge  amount of; 
information contained i n  the  in,frared observations,  information tha t  can be 
used t o  d i s t ingu i sh  among models. Pollack and Danielson discuss these  ,obser- 
va t ions  i n  d e t a i l  r e l a t i v e  t o  t h e i r  respect ive  models. I w i l l  simply make a 
few q u a l i t a t i v e  remarks here about t he  observations.  The most s t r i k i n g  fea- 
t u r e  of t hese  da t a  i s  t he  increase  i n  temperature from 20 um t o  8 um. This 
Table 2-la.  Titan Narrow-Band Measurements ( G i l l e t t  e t  a l .  1973) 
Table 2-lb. Titan Broad-Band Measurements 
AUTHORS 
Joyce et al. (1973) 
Gillett et al. (1973) 
Low (1965) 
Gillett et al. (1973) 
Gi 1 lett et a1 . (1973) 
Allen & Murdock (1971) 
Morrison et al. (1972) 
t rend can be understood i n  terms of e i t h e r  of  two very d i f f e r e n t  temperature 
regimes. The f i r s t  assumes, perhaps influenced by our experience with Venus 
and t h e  Jovian p l ane t s ,  t h a t  t he  temperature increases  with depth. In t h i s  
case, t h e  20-micron temperature corresponds t o  a level  high i n  the  atmosphere, 
and t h e  shorter-wavelength values t o  lower l eve l s .  Then the  opaci ty  must be 
much g rea t e r  i n  the  20-micron band, and one i s  led  na tu ra l ly  t o  pressure-induced 
H2 absorption,  a high surface  pressure ,  and a l a rge  greenhouse e f f e c t .  The sec- 
ond model, as  suggested by Danielson e t  a l .  (1973), assumes t h a t  t h e r e  i s  a tem- 
pera ture  invers ion  i n  the  atmosphere. In t h a t  case,  one seeks opacity sources 
i n  t h e  10-micron band, where t h e  hot  atmosphere i s  r ad ia t ing ,  while t he  20-micron 
temperature i s  more l i k e l y  t h a t  of  t he  surface .  Such a model does not requi re  
e i t h e r  high temperatures o r  high pressures a t  t h e  surface  o f  Ti tan .  
Perhaps t h e  most s t ra ight forward  way t o  d i s t ingu i sh  between these  models 
would be t o  measure the  surface  temperature from microwave observations,  where 
the  opaci ty  of even a massive atmosphere and clouds i s  expected t o  be very small .  
Such observations have been attempted r ecen t ly  by Briggs and Drake a t  Cornell 
with t h e  NRAO in ter ferometer  a t  a wavelength o f  3.75 cm. I have been t o l d  by 
Briggs t h a t  they  have not  succeeded i n  separa t ing  the  f lux  dens i ty  of Ti tan  from 
the  noise  and from the  much l a r g e r  f lux  dens i ty  o f  Saturn. Much of t he  d i f f i -  
cu l ty ,  as  it tu rns  ou t ,  is due t o  poor ephemerides of Ti tan .  I t  i s  r a t h e r  i r o n i c  
t h a t  we are  prevented from measuring t h e  surface  temperature o f  Titan only by an 
inadequate knowledge o f  i t s  o r b i t .  However, I f e e l  t h a t  t h i s  e f f o r t  t o  observe 
Titan a t  microwave frequencies i s  a very promising l i n e  of research ,  and t h a t  
it may well provide us with t h e  bes t  means t o  confiim o r  r e f u t e  t h e  greenhouse 
models i n  the  next  year  o r  two. 
Another highly promising avenue f o r  research l i e s  i n  obta in ing improved 
spectrophotometry i n  the  8- t o  14-micron band and i n  the  17- t o  28-micron band. 
The means now e x i s t  t o  obtain an excel lent  spectrum from 8 t o  14 pm with a reso- 
l u t ion  o f  ~ 1 0 0 ,  and i t  i s  probable t h a t  i n  another year i t  w i l l  be poss ib le  t o  
achieve a resolu t ion  o f  s20 i n  t h e  20-micron band. In the  20-micron band, va r i -  
a t ions  of br ightness  temperature with wavelength due t o  va r i a t ions  i n  t h e  opacity 
of HS and CHI, a re  predic ted  by Pol lack ' s  (1973) model, but not  by Danielson e t  a l .  
(1973). A t  t h e  s h o r t e r  wavelengths a l so ,  t hese  two models p red ic t  d i f f e r e n t  
spect ra .  
I would l i k e  t o  note  here  t h a t ,  no matter  what the  model, Titan must emit 
most of  i t s  thermal r ad ia t ion  between 15 and 50 pm and (barr ing  an i n t e r n a l  
heat  source) ,  t h e  brightness temperature over t h i s  spec t r a l  region must average 
about 84OK, the  e f f e c t i v e  temperature. Thus, I would expect no r e a l l y  b i g  sur-  
p r i s e s  from, f o r  ins tance ,  a broad-band measurement a t  35 pm. In the  10-micron 
region,  i n  con t r a s t ,  only a small f r a c t i o n  of t h e  t o t a l  thermal energy is  emit- 
t ed ,  even a t  br ightness  temperatures of 150°K. As a consequence, these  tempera- 
t u re s  a re  not  cofistrained t o  be near  t he  e f f e c t i v e  temperature of Ti tan ,  and 
t h e i r  wavelength va r i a t ions  can y i e l d  valuable information on the  temperature 
s t r u c t u r e  of t he  atmosphere. 
Summary 
In summary, t he  wide va r i a t ion  i n  in f r a red  br ightness  temperature o f  Titan 
has, together  with t h e  spectroscopic and polar imetr ic  s tud ie s ,  been l a rge ly  re-  
sponsible f o r  t he  grea t  i n t e r e s t  i n  Ti tan .  The o r i g i n a l  explanation of t hese  
temperatures was i n  terms o f  a  greenhouse e f f e c t ,  but t h a t  i n t e r p r e t a t i o n  i s  
now being challenged. Further radiometric observations,  both i n  the  inf rared  
and a t  microwave frequencies,  appear t o  provide the  bes t  prospects f o r  d i s -  
t inguishing among competing models within t h e  next year o r  two. 
Pollack: I  be l ieve  I  can give some f u r t h e r  input on the  prospects f o r  addi t ional  
observations.  G i l l e t t  i s  planning t o  make much more extensive spectrophotometric 
measurements i n  the  8- t o  13-micron region,  and I  agree t h a t  could he lp  e l iminate  
some of t he  ambiguitZes between Danielson and myself. In addi t ion ,  I  think it 
i s  very important t o  search f o r  s t r u c t u r e  due t o  pressure-induced bands a t  longer 
wavelengths. In t h a t  regard,  Houck and I  a r e  hoping t o  make use of t h e  NASA 
C-141 airborne te lescope t o  take  a  look i n  t h e  16- t o  60-micron region.  
Sagan: A few days ago I  t a lked  t o  Briggs and Drake about t h e i r  microwave obser- 
va t ions  and asked them s p e c i f i c a l l y  i f  they could 'set an upper l i m i t  now t o  t h e  
f lux  dens i ty  of Titan.  I  was t o l d  they could c e r t a i n l y  exclude br ightness  tem- 
pera tures  of 300°K o r  higher,  but t h a t  without a  r e f ined  ephemeris, they  were 
no t  su re  how much lower they would be able  t o  go. 
I  would a l s o  l i k e  t o  ask i f  t he re  a re  any plans f o r  high a l t i t u d e  observa- 
t i o n s  i n  t h e  5- t o  8-micron region,  from a i r c r a f t ,  balloons,  o r  perhaps space- 
c r a f t ?  I t  i s  j u s t  t e r r i b l y  exc i t i ng ,  t h i s  increase  i n  temperature as  t h e  wave- 
length ge t s  s h o r t e r  and s h o r t e r ,  and it c e r t a i n l y  would be i n t e r e s t i n g  t o  get  
beyond t h e  atmospheric cut -of f  a t  8  um. 
Morrison: I  don ' t  know of any plans,. Since Titan i s  f a i n t  and i s  so  c lose  t o  
Saturn,  a  subs t an t i a l - s i zed  te lescope and very good point ing  w i l l  be required.  
Perhaps something could be done from the  C-141 i n  t h a t  wavelength range. But 
we should remember t h a t ,  even i f  t he  br ightness  temperature i s  going up, the  
ac tua l  i n f r a r e d  f lux  dens i t i e s  a re  dropping very f a s t  as one gets  t o  wavelengths 
s h o r t  of  8 pm. 
Danielson: I  would think t h a t  s h o r t e r  than 8  um you would p r i n c i p a l l y  be exam- 
i n i n g  the  CHI, i n  t h e  Ti tan  atmosphere. You would l ea rn  something about t he  
temperature s t r u c t u r e  i n  t h e  upper atmosphere, but I  don' t  th ink  you would be 
learning anything more fundamental than t h a t .  


Infrared photometry and spectrophotometry of Titan

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