Comments on ice particles as major scatterers of infrared radiation from Venus atmospher

Cheyney, H.

Hansen, J. E.

English

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4 t 
COMMENTS ON THE PAPER BY D.G.  REA AND B . T .  O'LEARY 
ON THE COMPOSITION O F  THE VENUS CLOUDS 
James E.  Hansen and Howard C h e y n e y  
I n s t i t u t e  f o r  Space S t u d i e s  
G o d d a r d  Space P l i g h t  C e n t e r ,  NASA 
N e w  York, New York 
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2 
In a recent paper Rea and O'Leary (1968) examined 
the cases for and against ice clouds on Venus and con- 
cluded ". . . . that there is no direct evidence for H20 
ice crystals, and furthermore, if they do exist on Venus 
and dominate the near-infrared scattering process, they 
must be submicron in diameter." We believe that at 
least the latter of these two conclusions is not correct, 
because clouds cornposed of ice particles a few microns 
in diameter do not generally cause absorption features 
at 1.5 and 2.02 as strong as assumed by Rea and O'Leary. 
Their conclusion was based in part on laboratory reflec- 
tivity spectra obtained by Zander (1966) but we believe 
that Zander's reflectivity spectra are representative of 
particles an order of magnitude larger than suggested in 
Zander's publication. 
Rea and O'Leary examined spectra of Venus obtained 
by Kuiper (1962) with an earth-based telescope for wave- 
lengths 1 . 4 ~  5 A < 2.311 and balloon observations of - 
Bottema, Plummer, Strong, and Zander (1964, 1965) for 
1 . 8 ~  < A < 3.411. They showed that the absorption near - -. 
1.5 and 2.011 is due largely to C 0 2 ;  hence, except for 
the reflectivity minimum near 3 ~ ,  there is no signifi- 
cant evidence for the broad band absorption features 
that would be expected for ice in the near-infrared. 
3 
Rea and O'Leary illustrated spectral reflectivities of ice 
clouds having particle diameters Q 10-100~ and these 
showed very strong absorption features at 1.5 and 2 . 0 ~ .  
Although the strength of these features would diminish for 
smaller particles, it was shown that the features were 
nearly as strong in laboratory measurements by Zander 
(1966) in which the particles were reported as having 
diameters primarily in the range 1~ - < d - < 3 ~ .  From this 
they argued that either ice particles are not the princi- 
pal scatterers of the infrared radiation on Venus, or, 
if they are, the particle diameters must be less than 1~ 
and probably less than .lp. Since the planet-wide exist- 
ence of submicron ice particles is unlikely they concluded 
that the major scatterers of radiation on Venus are almost 
certainly not composed of H20 ice. 
However, solutions of the radiative transfer problem 
for diffuse reflection from ice clouds have been made 
(Hansen and Cheyney, 1968) for a size distribution of 
spherical particles and compared to Zander's observations. 
The optical constants for ice were taken from the tables 
compiled by Irvine and Pollack (1968); from these the 
absorption and scattering cross sections and the phase 
function were calculated for each particle, weighted for 
the mixture, and employed in the transfer problem to 
4 
obtain the complete angular distribution of reflected 
light. When the size distribution specified in Zander's 
(1966) article (diameters of 1-3pj was employed, the 
absorption features at 1.5 sr,d 2.0; were found to be much 
too weak to be compatible w L ~ h  the laboratory observations; 
however, it was learned from Zander (1968) that a signifi- 
cant number of the 1 - 3 p  parL;cles wzre cluscere6 into 
tightly packed (void space  2 2 0 % )  aggregates as large as 
15p. Since the cor.;?or.enLs of an aqgregate do noc scatter 
incoherently it is a better a2proximation to treat each 
agpegate as a single large particle, and a relatively 
small nunber of these may completely change the scattering 
properties of a unit volume. According to Zander (1968) 
20% of the particles were aggregates (with an aggregate 
counted as being one partic;e) a;id -the distribucion of the 
aggregates peaked at d n, l o 4 .  PIhen che complece distribu- 
tion including aggregates was used in the radiative trans- 
fer problem a close fit was obtained with Zander's observed 
curve. 
Frorn the apparent discrepancy between Sagan and 
Pollack I s (1967) calculations and Zander I s  laboratory 
spectra, Rea and O'Leary suggested that Sagan and Pollack's 
29proxinate radiative transfer solution may be in serious 
error, but our results for a narrow size distribution are 
5 
compatible with the single particle calculations of Sagan 
and Pollack. 
can not cause an order of magnitude error in the particle 
sizing. 
Certainly their two stream approximation 
The primary point that we wish to make is that with 
micron sized ice particles as the major scatterers the 
reflectivity minima near 1 . 5  and 2 . 0 ~  are not generally 
as large as assumed by Rea and O'Leary and hence Kuiper's 
ground based observations may be compatible with clouds 
of such particles. For e x a ~ ~ p i e ,  w i t h  d particle size 
distribution having diameters 111 < d < 3~ with the max- 
imum of the distribution at 1 . 7 ~  we obtain the reflectiv- 
ities in Table 1 
- - 
Table 1 
n . - 1 - - . - 2  --l 
A ( P I  
3 p l I e L  I L d l  
h -+ 1 . 5 2  1 . 7  1 . 8 5  2 . 0  2 . 1 5  3 . 0  3 . 2  3 . 4  Albedo at 
h = 1.8511 
T = 8 . 2 5  . 2 4  . 2 3  . 1 8  . 18  . 0 0 3  . 0 2  .13 4 0 %  
T = 1 6  . 4 5  . 4 6  . 4 5  .35 . 3 7  . 003  . 0 2  . 1 4  5 6 %  
T = 3 2  . 6 0  . 6 7  . 6 8  . 47  .55 . 0 0 3  . 0 2  . 1 4  7 0 %  
where the reflectivity is the ratio of the calculated 
intensity to the intensity of a Lambert surface with 
albedo unity an6 7 is the optical thickness of the ice 
clouds at A = .95p.  The corcputations were made for the 
angles of incidence and emergence in Zander’s experiment 
and hence the results are not identical to the integrated 
light from Venus at a given phase angle, but Rea and 
O’Leary’s conclusions were a l s o  based on Zander’s exper- 
iment. The table illustrates that with a finite cloud of 
micron-sized ice particles the absoqtion features at 1.5 
and 2 . 0 ~  are weak arid hence the spectroscopic observations 
must be accurate in order to establish or rule out the 
existence of ice clouds. 
We do not maintain that this represents positive 
evidence for ice clouds on Venus; we do believe, however, 
that the arguments against having clouds of micron-sized 
ice crystals are not yet conclusive. 
7 
i 
ACKNOWLEDGMENTS 
We would like to thank Dr. R. Jastrow, director of 
the Institute for Space Studies, for his hospitality and 
provision of excellent facilities. 
We would like to thank Drs. J. Pollack, W. Plumer, D. Rea 
and P. Thaddeus for helpful discussions and Dr. R. Zander 
for valuable correspondence. During the course of this 
research J. E. 3 .  held a National 3esearch Council Post- 
doctoral Research Associateship supported by the National 
Aeronautics and Space Administration and H. C. was 
supported by grant NsG 399/33 - 016 - 013 through the 
Department of Meteorology and Oceanography, New York 
University. 
8 
REFERENCES 
Bottema, M., W. Plummer, J. Strong, and R. Zander, 
"Composition of t h e  clouds of Venus", Astrophys. J., 
140, 1640, 1964. 
- 
-
Bottema, M., W. P l u r r m e r ,  J. Strong, and R. Zander, "The 
composition of the Venus clouds and implications for 
model atmospheres'', J. Geophys. Res., 70, 4401, 1965. - -- 
Hansen, J., and H. Cheyney, "Theoretical spectral scattering 
of ice clouds in the near infrared", to be published, 1968. 
Irvine, W.M., and J. 3. 3ollack, "Infrared optical properties 
of water and ice s?heres", -LO be publisheci in Icarus, 1968. 
Kuiper, G. 3 . ,  " lhotoneyry  cf thz i~5rareZ speccrum of Venus, 
1 - 2.5 aicrons", Cornrrui;. Lunar Planetary Lab., 1, 83, 1962. - 
Rea, D.G., and O'Leary, B.T., "On -the composition of the Venus 
clouds", J. Geophys. Res., 73, 665, 1968. - - - 
Sagan, C., and J. 3. Pollack, "Anisotropic nonconservacive 
scattering and the clouds of Venus1', J. Geophys. Res., 
72, 469, 1967. 
- -
~ 
Zander, Ii., "Spectral scattering properties of ice clouds 
and hoarfrost'', J. Geophys. Res., 71, 375, 1966. - - -
Zander, R., private communication, 1968. 


Comments on the paper by D. G. Rea and B. T. O'Leary on the composition of the Venus clouds

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