Upper limits on liquid water in Venus atmosphere due to presence of hydrogen chlorid

Rea, D. G.

Welch, W. J.

English

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* I  c 
3 UPPER LIMITS ON LIQUID WATER IN THE SENUS ATMOSPHERE 6 - 
6 W .  J .  Welch and D. G.  Rea 
Space Sciences LabQratory,  Radio Astronomy Laboratory,  
and Department of Electrical  Engineering 
I University of California, Berkeley, California 34720 
The exciting recent  discovery of HC1 and HF in the atmosphere of 
Venus (Connes et  al. , 1967) wil l  undoubtedly have an important influence 
on future  models of the planet 's  atmosphere. The purpose of this note is 
-- 
t o  point out one consequence of the discovery, namely, that on account of 
the presence of HC1 there  cannot be much liquid water in the Venus atmos- 
phere.  
DC conductivity. 
HC1 is highly soluble in water, and the resulting acid has a high 
If there  were much of this acid in the Venus atmosphere,  
the long wavelength r ada r  signals would be strongly attenuated, whereas 
the r a d a r  signals appear to reach  the planet 's  surface with l i t t le attenuation 
(Evans,  Ingalls, Rainville, and Silva,  1966). 
A quantitative est imate  of the centimeter wavelength attenuation of the 
acid clouds follows f rom the observed abundance of the HC1 (Connes et  a l . ,  
1967) and some assumptions about the atmospheric temperature  and p res su re  
-- 
at the level of the clouds. 
6 x 
Moreover, f r o m  the s t rength of the C 0 2  lines, they concluded that CO 
probably the major  constituent in the Venus atmosphere".  We have car r ied  
out calculations for th ree  temperatures:  240°K, 273OK and 283OK, and for 
par t ia l  p re s su res  of HC1 of 10 and mb. These p r e s s u r e s  correspond 
The HCl/CO abundance rat io  was found to  be about 2 
+ ' O 0  mill ibars  (Connes e t  a l . ,  1967) and the total effective pressure  80 -- - 40 
"is 2 
-4  
t o  total  p ressures  at the supposed liquid H20 clouds of about 200 and 2000 mb 
respectively. F o r  these two vapor p r e s s u r e s  and for  the th ree  selected 
tempera tures  the concentrations of HC1 in solution and the conductivities 
of the resulting solutions Cri t ical  Tables.  
acl Q K - - ~ Q , & - J ~ ' -  5 y s  d 2 8  0 0 1
https://ntrs.nasa.gov/search.jsp?R=19670018672 2020-03-12T11:37:04+00:00Z
- 2 -  
The data for  273OK and 283OK came directly f rom the tables and the data 
for  240 K f rom an extrapolation of the tabular values. 
in the table. 
pure water  a r e  given by the famil iar  Debye formulae for a liquid in which 
there  is dipolar relaxation. 
0 The resul ts  appear 
The r ea l  and imaginary pa r t s  of the dielectric constant of 
With E = E ' t i c  ' I ,  
L 
E 03 -ho ( L f )  
1 t ( . ) 2  / 
Values for the three  parameters ,  E 
Buckley and Maryott (1957) and appear in the table. 
E 
0' 03' 
and Xc,  were  obtained f rom 
F o r  240°K the para-  
m e t e r s  were  obtained by extrapolating the tabular values in the Circular.  
In t e r m s  of the dielectr ic  constants and conductivity the attenuation of the 
solution i s  
1/ 2 
where X is in cm. 
in the table for  T = 273OK. 
This formula is  plotted in the figure for the pa rame te r s  
The results for the other tempera tures  a r e  
near ly  the same. The attenuation of pure water is a l so  shown for com- 
p a r  ison. 
The radar  observatioris of Venus, which span the wavelength interval 
3. 8 cm to 8 m, a r e  reviewed by Evans, Ingalls, Rainville, and Silva (1966). 
At 3.8 cm the radar  c r o s s  section is 1. 2'7'0 of the geometric c r o s s  section, 
at 1 2  c m  it is about 1270, and for wavelengths 20 c m  and longer it l i e s  in 
the range 15 - 180/0. The most  recently reported experiment at 23 cm (Evans, 
Brockelman, .Dupont, Hansen, and Reid, 1966) yielded a c r o s s  section of 18% 
the geometric c r o s s  section of the planet. This is a strong echo, comparable 
- 3 -  
to  the echoes that a r e  obtained from typical t e r r e s t r i a l  mater ia ls ,  and 
implies that the mean surface dielectric constant is of the o rde r  of 7 or 
more. It appears  that, although there is clearly atmospheric absorption 
a t  3.8 cm, and possibly some absorption a t  1 2  cm, at wavelengths 20 cm 
and longer the atmosphere is  essentially transparent.  We will suppose 
that the two way atmospheric absorption at  20 c m  is  not more  than 20% 
and therefore  T~~ <, 0.1.  F r o m  the calculations we find that p N = 10 cm -1 20 
in near ly  every case.  
must  be l e s s  than .01 g m / c m  e 
sity 1 g m / m  , typical of t e r r e s t r i a l  water clouds, would be just 100 m thick. 
Hence for T <, 0.1, the m a s s  of liquid water clouds 
This amount of water cloud of average den- 
20 
2 
3 
An even smaller  upper limit on the liquid water content of the Venus 
atmosphere may be obtained by an argument which is a lso based on micro-  
wave observations of the planet, but which is  a little l e s s  direct  than the 
above. Consider the microwave emission spectrum of the planet which would 
resul t  f r o m  a cool acid cloud above the warm surface. Let the surface emis-  
sion tempera ture  be T the cloud temperature T and the cloud optical depth 
S' C Y  
The brightness of a pencil of radiation leaving the surface at  an angle 5 
-1 cos p with respect  to the surface normal and passing through the cloud is 
1 
The d isk  temperature  of the planet is TD = 2 J 
of T we find 
T p dp ,  and for  smal l  values B 
0 
A 
N 
TD = T - 2 ~ ~  (Ts -Tc). 
S 
(4) 
F r o m  an  extended se r i e s  of measurements,  Drake (1964) obtained an average 
0 
disk temperature  of 622 K at 10 cm, and f rom a s imi la r  s e r i e s  of measure-  
ments  Mayer, McCullough, and Sloanaker (1963) found an average disk 
. 
- 4 -  
0 t empera ture  of 621 K a t  3 cm. ‘The absolute e r r o r  in each ser ies  is  probably 
not grea te r  than 10%. As is  evident in figure 1, an acid cloud absorbs more  
a t  3 c m  than a t  10 cm, and, if  such a cloud were present  in the Venus atmos-  
phere in grea t  abundance, the disk temperature should be relatively lower 
a t  3 c m  than at 10 cm. Considering the measurement  e r r o r s ,  let  us suppose 
that the 3 cm disk temperature  is really about 70  lower than the 10 cm disk 
0 
t empera ture  and calculate the thickness of an acid cloud which would produce 
this difference. 
same at both wavelengths ( a s  is  probable according to  the measurements)  
and suppose that an  increase in T 
10 c m  is compensated by the absorption of the water cloud. This increase 
in T is about the maximum that might be expected from the effect of an in- 
c r e a s e  in the surface emissivity a t  3 c m  over that at  1 0  cm.  We assume 
Alternatively, we may take the disk temperature  to be the 
by about 70° at 3 cm over what it is at  
S 
S 
here that the surface emissivity i s  0 .85 - 0 . 9 0  at  10 cm, a plausible value 
derived f rom the r ada r  experiments (Pollack and Sagan, 1965) at 12 cm and 
longer wavelengths. 
and the resulting increase in T 
270 K. Then 2 ( ~ ~  - - T ~ )  (Ts  -Tc) = 2 ( ~  - -T
< 0.1 .  
The emissivity can at  most  increase to  1 .  0 a t  3 cm, 
i s  about 70°K. Le t  T 
3 10 
E 620°K and Tc 
S S 
0 
10 ) (350) < * 70, and - T ~  - -T  
and p l o  = 1 2  for p - - -1 HC1 F o r  the acid cloud we find p 3  = 26 cm 
-1 -1 and p l o  = 15 c m  
* 
= mb. F o r  HC1 mb, and p 3  = 31 cm for p 
- 2  
( - r3  - - T ~ ~ )  < 0.1, the mass of liquid water clouds must  be l e s s  than 0. 007 g cm 
for  both values of pHcl. 
Liquid water clouds have been suggested for Venus to explain the micro-  
wave spectrum (Deirmendjian, 1964; Griffi th -- e t  al. , 1967). If the tempera ture  
of the liquid cloud i s  much above 273 K, the la rge  amount of water vapor that  
0 
must  l ie beneath the cloud in the lower atmosphere will produce too strong a 
specific absorption at 1. 35 cm to  fit the available data (Bar re t t  and Staelin, 
1964) .  If the cloud is  cooled to  temperatures  of the o rde r  of 240- 2 6 0 ° ~ ,  the 
- 5 -  
result ing water vapor in the lower atmosphere is consistent with the observed 
1.35 c m  disk temperatures .  Then a cloud m a s s  of 0. 2 - 0 . 3  gm/cm 2 of pure 
water  is needed to f i t  the microwave data (Basharinov and Kutuza, 1965) in 
the neighborhood of 0. 4 - 3 cm. 
attenuation near  1 cm wavelength of the HC1 solution over that of pure water, 
2 a cloud m a s s  of only 0.1 - 0. 2 g m / c m  
10  t imes  the l a rge r  upper limit obtained above. 
clouds cannot be invoked to  explain the microwave spectrum. 
With the increase by a factor of 2 in the 
is  required. However, this is  st i l l  
Consequently, liquid water 
These upper l imits  must  also be borne in mind when constructing 
atmospheric models for calculations of the greenhouse effect and interpre- 
tations of the infrared and visible spectra. 
Acknowledgment s 
This work was supported by National Science Foundation Grant GA-527, 
and National Aeronautics and Space Administration Grant N s G  101 -61 and 
Contract NASr 220. 
- 6 -  
REFERENCES 
Barre t t ,  A. H., and Staelin, D. H. 1964, Space Sci. Revs. ,  3 ,  109. 
Basharinov, A. E. ,  and Kutuza, B. G. 1965, J .  Res. NBS, 69D, 1580. 
- - 
- -
Buckley, F., and Maryott, A. A. 1957, National Bureau of Standards - 
Circular  514. 
Connes, P. , Connes, J., Benedict, W. S. ,  and Kaplan, L. D. Ap. J. 
( in press ) .  
Deirmendjian, D. 1964, Icarus,  3, 109. -
Drake, F. D. 1964, A. J., 69, 62. 
Evans, J. V. , Brockelman, R. A., Dupont, E. N. , Hansen, L. B. , and 
- -  
Reid, W. A. 1966, A. J. ,  71, 897. -- 
Evans, J. V., Ingalls, R. P., Rainville, L. P., Silva, R. R. 1966, 
A. J . ,  2, 902. 
Griffith, P. H. , Thornton, D. D., and Welch, W. J. 1967, Icarus  ( in  p re s s ) .  
Mayer, C. M. , McCullough, T. P., and Sloanaker, R. M. 1963, in 
-- 
Physics of the Planets,  Proc.  XIth Intern. Astrophys. Symposium, 
Liege, Belgium, 1962. 
Pollack, J. B. , and Sagan, C. 1965, Icarus,4, 62. -
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FIGURE CAPTION 
The absorption coefficient, p ,  as a function of wavelength, X ,  for 
pure water (a), and HC1 solutions at par t ia l  p r e s s u r e s  of HC1 of 10 -4 mb 
(b)  and mb (c). 
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Upper limits on liquid water in the Venus atmosphere

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