Calibration of water absorption bands in near infrared regio

Cruikshank, D. P.

Kuiper, G. P.

English

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NO. 98 CALIBRATION OF WEAK 1.4 AND 1.9 I.L H a 0  ABSORPTIONS 
by G. P. KUIPER AND D. P. CRUIKSHANK 
November 11, 1967 
ABSTRACT 
This paper summarizes laboratory measurements of the intensity of the 1.4 and 1.9 p water-vapor bands in support of 
airborne observations of the planets Venus and Mars. 
he Venus observations and moon comparisons 
proximate calibration of the 1.4 and 1.9 p bands of 
water vapor so that the observed absorptions may 
be converted to abundance. The calibrations must 
match the astronomical spectra in resolution in order 
that the measured absorptions refer to the same 
features. Only a few laboratory spectra were ob- 
tained with the interferometer used in the observa- 
tions of Venus and the moon before it was returned 
to Cambridge, Mass. These are reproduced in Fig- 
ure 1. As noted in the legend, the spectra were ob- 
tained at the ambient pressure of 70 cm Hg so that 
the absorption depths may be enhanced by pressure 
broadening relative to the 200 mb atmospheric rec- 
ords. Record c is anduly noisy; records CI and 6 are 
a good match f .r the astronomical records. 
All othe. calibrations were made subsequent to 
the return of the interferometer, using the A-spec- 
trolrxer of LPL with a very wide slit so as to simu- 
late the band resolution used in the interferometer. 
Two sets of records were obtained, one at the ambi- 
ent laboratory pressure (70.5 cm) and one at the 
reduced pressure of about 15 cm, corresponding to 
the 200 mb atmospheric level from which the plane- 
tary observations were made. Since the water vapor 
has a much smaller scale height than the atmosphere 
at large, even at that level (Comm. LPL No. 93, 
T described in Comm. LPL No. 95 require an a p  Table l), the pressure of 200 mb or 15 cm is con- sidered more representative than the average of 
100 mb. 
Figure 2 reproduces the records obtained in 
laboratory air at p = 70 cm with the water-vapor 
content determined with a wet- and dry-bulb ther- 
mometer and a variable air path. Each band is 
characterized by its three branches, the depths of 
which can be measured withcut much ambiguity 
and should not be dependent on the precise value 
of the spectral resolution. The results of these depth 
measures are shown graphically in Figure 3. The 
central (6) branch of tne 1.9 p band has been 
omitted since it cannot be used on Venus, owing 
to the strong COa band 012 and the solar Paschen 
a line, which together obliterate the H 2 0  absorption. 
The curves of growth in Figure 3 have a nearly 
parabolic shape for amounts in excess of 8-10 p 
of water vapor, i.e., the band intensity increases 
approxiiiiately as the square root of the abundance. 
The 72 p points of Figure 2, though outside the 
diagram, were measured and used in drawing the 
dashed curves of Figure 3. Even for smaller amounts 
some curvature is ?resent. Since pressure broadening 
extends the linear part of the curve of growth, it 
follows that pressure effects are present even for 
these smaller amounts of water vapor, which is not 
surprising in view of the width and spacing of the 
19 1 
https://ntrs.nasa.gov/search.jsp?R=19690003310 2020-03-12T08:10:15+00:00Z
192 G .  P. K U I P E R  A N D  D.  P. CRUIKSHANK 
i 
I I La--- -1l--ii_l__li---1-ILI I I I I I ' I 
.ow 5mo 6000 ?mo w-4 
Fig. 1 Sample laboratory spectra of 1.4 and 1.9 p bands of HrO with Mertz interferometer. 
C A L I B R A T I O N  O F  WEAK 1.4 A N D  1.9 p H20 A B S O R P T I O N S  
Q 
Q 
9 -  * S F  
m SO w a n  
IN mn AT L - 10 CY ?' 
I I A'' 
0 
10 b- o,--- 
0 
193 
0 10 20 30 4 0 ~  c q o i  
Fis. 3 Curves of growth of five absorption fea- 
tures of water vapor id:ntified in Fig. 2 using 
two pressures: ( 1 ) lower, measures made at 
200 mb (solid dots and full-drawn line); (2) 
upper, measures at ambient laboratory pressure, 
p = 70 cm, based in part on Fig. 2. 
194 G. P. K ~ I P E R  ~ N D  .  P. C R U I K S H A N K  
unresolved rotational lines responsible for the bands 
(in other words, the individual rotational lines are 
no longer weak even for a shallow band). 
The calibrations at p = 15 cm Hg were made 
with a single-pass tube, 6.76 m long, equipped with 
pump and manometer; the water-vapor content of 
the ambient laboratory air was progressively in- 
creased by boiling water and measured at the time 
the air was admitted to the tube. 
Normally three records were obtained for each 
of the following amounts of water vapor per meter 
in the ambient laboratory air: 3.7 p, 4.3 p, 4.3 p 
(on a second day), 9.7 p, and 12.5 p. The tube 
pressures were not quite the same for the 1.4 and 
1.9 p bands. The percentage absorptions were de- 
rived and plotted in Figure 3. The comparatively 
small additional abundance due to the 37 cm labo- 
ratory air pat!. was multipiied by an cxtra factc 
1.5 to allow approximately for its higher pressure. 
It is seen that pressure af;xts different branches 
differently, with the intensity ratio at a given abscissa 
reaching d p , / p 2  = 2.2 in some cases. The pressure 
effect is small in the a branch of the 1.4 p band. 
Calibrations with different resolution will be 
published as needed in future programs. 
Acknowledgments. Dr. A. B. Binder assisted in 
the early phases of the laborata 71 work. The plane- 
tary spectroscopic program is supported by the 
National Aeronautics and Space Administration 
GrantNsG 161-61. 


Calibration of weak 1.4 and 1.9 mu water-vapor absorptions

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