Spectral measurements of the thermal radiation from Jupiter in the 16-40 micrometer band were analyzed under the assumption that pressure broadened H2 transitions are responsible for the bulk of the infrared opacity over most of this spectral interval. Both the vertical pressure-temperature profile and the hydrogen mixing ratio were determined. The derived value of the molecular hydrogen mixing ratio, 0.89 + or - 0.11, is consistent with the solar value, 0.86

Houck, J. R.

Pollack, J. B.

Reed, R. A.

Schaack, D.

Summers, A.

English

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iCRSR 594
Jupiter: Its Infrared Spectrum frgm 16 to 40 Microns
February 1915	
4c^
James R. Houck, David Schaack and Robert A. Reed
Center for Radiophysics and Space Research
Cornell University
.James Pollack and A. Summers
Space Science Division, Ames Research Center
(NASA-CR-146r88) 	 JUPITER: ITS INFRARFL
	 N76-15967
S'ECTRUM FRCC 10 TO 4r NICFONS (Cornell
[In iv. 	 12 p HC $3. 9,	 CSCL t 3E
[1LClaS
03/91
	 17061
James R. Houck
Page 1
Jupiter: Its Infrared Spectrum from 16 to 40 Microns	 }
r
James R. Houck
Page 2
ABSTRACT
Spectral measurements of the thermal radiation from
Jupiter in the 16-4%Lm band are analyzed under the assump-
tion that pressure broadened H2 transitions are responsible
for the bulk of the infrared opacity over most of this spec-
tral interval. Both the vertical pressure-temperature pro-
file and the hydrogen mixing ratio are determined. The
derived value of the molecular hydrogen mixing ratio, 0.89
+ 0.11, is consistent with the solar value, 0.86.
f
James R. Houck
Page 3
The abundance of hydrogen and helium in the Jovian atmos-
phere has a direct influence on a number of astronomical problems.
Jupiter's low atmospheric temperature and great mass prevents even the
lightest atoms from escaping from the top of the atmosphere. There-
fore, Jupiter as a whole is a sample of the elemental abundance at the
time of its formation. Modern theories of explosive neucleosynthesis
shorn that the majority of the helium presently in the universe was
formed during the Big Bang ( ' ) . Further, it has been shown that the
relative abundance of hydrogen and helium depends strongly on the tem-
perature and density during the early stages of the evolution. To
the extent that the Jovian atmosphere is representative of the planet
as a whole, a measure of the hydrogen mixing ratio aH = N(H2)/
2
N(H2)+N(He) where N(x) is the number density of x) in the atmosphere
is useful for the determination of the conditions during the early
stages of the Big Bang. There are, however, several effects which
may systematically distort the atmospheric value of a H .
2
Opik has suggested a model for the formation of Jupiter in
r
which first'"hydrogen snow" collects to form a core followed by the
capture of a helium rich atmosphere (2). Salpeter has suggested that
there may be internal differentiation with the helium sinking toward
the center of the planet resulting in a hydrogen rich atmosphere(3).
Observations
Jupiter was observed on the nights of 1973 November 14 and
16 and 1974 January 21 using a 31 cm telescope mounted on the NASA
Ames Lear Jet. The aircraft was flown to an altitude of 14 km (45,000
ft.). At this altitude about 3 to 10 precipitable microns of water
per airmass remain above the observer. The telescope viewed the source
James R. Houck
Page 4
at elevation angles between 14 and 280 (4.1 to 2.1 airmass). The
spectral scans were made using two different liquid Helium cooled gra-
ting spectrometers (4) . These instruments employ a 12.5 em focal
length Ebert-Fastie spectrometer with two detectors iri the focal
plane. The No. 1 instrument has a Ge:Cu photoconductor to scan the
16-28µm band while a Ge:Ga photoconductor covers 20-40µm. The resolution
of the two channels is 0.5µ and 1.0µ respectively. The No. 2 instru-
ment has two Ge:Cu photoconak^tors and covers the range 16-2811m with
a resolution of 0.5µm. The instruments operate at f/6.5 and have a
2.7 mm entrance aperture corresponding to 4.7 minutes of arc. The
chopping frequency was 48 Hz.
The instrumental response was determined by normalizing
spectra of the Moon and Mars as if they were black body radiators
at 3500K and 2400K respectively. The instrumental profile determined
in this way was used to normalize the Jupiter data. The resulting
spectrum is shown in Fig. 1. The fig-ire shows the presence of three
absorption features at 18, 23.5 and 28µm. The first and last are the
r
J = 1 and J = U rotational . ansitions of molecular hydrogen. The
23.5wnfeature is unidentified but may be due to sulphur, silicate
dust or complex hydrocarbons in the atmosphere. (5) All of these
material have strong absorption features in this wavelength region.
A complete discussion of these possibilities will be given by Pollack
et al. (6 )
Data Analysis
The observed spectrum of Jupiter contains information about
both a portion of its vertical temperature structure and its helium
to hydrogen ratio. Before describing our numerical method for de^•i_ving
fames R. Houck
Page 5
this information, we describe the basis of our ability to do so. The
brightness temperature found at a given wavelength is approximately
equal to the value of the physical temperature in the Jovian atmos-
phere at optical depth unity. As the wavelength is v rie.:, the alti-
tude where the optical depth is unity also varies. Thus, spectral ob-
servations over a range of wavelengths provide information on the tem-
perature conditions over a corresponding range of altitudes in the
atmosphere. For the spectral band measured, molecular hydrogen is the
principal source of opacity. The Jovian atmosphere is probed from
about the 0.15 atmosphere level to the 0.6 atmosphere level, a region
which includes a temperature minimum and the top of the convection zone.
In performing the analysis described below we excluded data
close to the 23µm feature. In addition to opacity due to the rotational
and translational transitions of hydrogen we allowed for opacity due
to ammonia, which is important only at the long wavelength edge of
our data. We performed calculations both without and with an optically
thick ammonia cloud present in the zone regions of Jupiter, located at
the ammonia'saturation level of the atmosphere.
In accord -i_th radio temperature measurements we assumed
that the temperature gradient equalled the adiabatic value at pres-
sures greater than the bottom altitude boundary of the region we
sense (7) . Our derived temperature profiles are consistent with this
assumption. Above our top altitude boundary we made use of the tem-
perature gradients derived by Orton from an analysis of measurements
in the region of the 7.7µm methane band.8
Our ability to gain information about the helium to hydro-
gen ratio follows from the dependence of the hydrogen pressure induced
transitions on this ratio. This dependence is illustrated in Figure
IJames R. Houck
Page 6
1, which shows the values of the absorption coefficient of hydrogen
when the rotational transitions occur in the presence of a nearby
hydrogen molecule, (H 2-H2 ) and in the presence of a nearby helium
atom (H2-He). We see from Figure 1 that the absorption coefficient
shows a different spectral variation for the (H 2-He) case than for
the (H2-H2 ) case.
The numerical method used to obtain the desired information
from the observed spectrum of Jupiter begins with an a:3su ,zr d. trial
value of the hydrogen mixing ratio, a^,
2
. For this choice of mixing
ratio, the observed spectrum is inverted to determine the pressure-
temperature structure by means of a statistical iteration technique
developed by Smith and applied to planetary atmospheres by Ohring(9,10)
This method involves starting with a trial temperature profile, com-
puting a predicted spectrum with this profile, and correcting the pro-
file on the basis of the difference between the observed and calcu-
lated flux values, the flux residuals. The improved guess is used to
repeat this process and, after a small number of iterations, the r.m.s.
of the flux residuals approaches an asymptotic value. This procedure
is repeated for other trial values of aH
	
An estimate of,the helium
2
ratio was obtained from the locations of the minimum in the -value of
the flux residual as a function of mixing ratio.
Results
Figure 2 shows the vertical temperature structure of Jupiter
determined from our spectrum for hydrogen mixing ratios of 0.8, 0.9
and 1.0. These results refer to models containing no ammonia cloud
opacity. However, very similar curves were obtained for runs with
optically thick clouds. Our results have been smoothly joined with
_=
James R. Houck
Page 7
the temperature gradients found above and below our region of sen-
sing. Figure 2 also indicates the location of the regions probed in
our measurement, at radio wavelengths, and within the 7.7µm methane
band. The temperature values found from these various determinations
are quite consistent near their boundaries. The temperature gradi-
ent becomes adiabatic at the higher pressure domain of our sensing
region, which is consistent with the temperature lapse rate implied
by the radio results. We find that the lapse rate first reaches the
adiabatic value and therefore that the convection zone begins at
approximately the 0 . 4 atmosphere pressure level. This result is in
good agreement with the theoretical predictions of Pollack and Ohring(11)^
which were based on a radiative equilibrium model. Our measurements
also suggest the presence of a temperature minimum and the start of an
inversion layer at the lower pressure levels of our region. This
aspect of the temperature profile is suggested by the small displace-
ment of the observed minimum in our brightness temperature spectrum
from the center of the J = 1 hydrogen rotational transitions. These
r
results are • in accord with Orton's analysis of measurements made :_n
a spectral region containing the 7.74m fundamental of methane.
Also the value of our temperature minimum (115 0K) is in good agree-
ment with Orton ' s value of about 1180K.
The temperature profiles shown in Figure 2 are in conflict
with the structure found from a preliminary analysis of the Pioneer
10 S band occultation experiment (12). The latter results indicate a
much warmer atmosphere than our results at pressures above several
millibars. Our infrared observations could be reconciled with the
S band data if we postulate the presence of an optically thick cloud
near the 1 millibar pressure region. However, in this case the pres-
James R. ^iouck
Page 8
sure induced opacity of hydrogen would be negligible :,,nd we would
not expect to detect the J = 0 and J = 1 rotational transitions in
our spectrum. The presence of these features in our spectrum sug-
gests that further study of'the`S band occultation results is needed.
These conclusions are also supported by the consistency of our tem-
perature profile with values obtained at shorter infrared wavelengths
~and in the radio domain, as discussed above.
For models containing no ammonia cloud opacity in the
zones of Jupiter, we find that the hydrogen mixing ratio I.s 0.85 +
0.07, while we obtain a value of 1.0+0.0+ for the cases involving
an optically thick ammonia cloud. These error bars reflect only the
formal random errors of our results. We note that the cloud free
models had a smaller value for the minimum fractional flux residual
than the latter set ( 0.039 vs o.o43 ). Combining the above two esti-
mates, we conclude that the hydrogen mixing ratio equals 0.89 + 0.11.
This range of values encompasses the solar value of 0.86.
James R. Houck, Center for Radiophysics and Space Research,
Cornell University, Ithaca, New York 1 +853
James Pollack, Space Science Division, Ames Research Center,
Moffett Field, California 9+035
David Schaack and Robert A. Reed, Center for Radiophysics and
Space Research, Cornell University, Ithaca, New York 14853
A. Summers, Space Science Division, Ames Research Center,
Moffett Field, California 94035
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James R. Houck
Page 9
REFERENCES
1. R. V. Wagoner, Science 155, 1369 (1967).
2. E. Opik, Icarus 1, 200 (1962).
3. E. E. Salpeter, Ap.J. 181, L83 (1973).
4. D. F. Schaack and J. R. Houck, in preparation (1975).
5. R. F. Knacke and R. K. Thompson, PASP 85, 341 (1973); F. F. Bentley,
L. D. Smithson, A. L. Rozek, Infrared Spectra and Characteristic
Frequencies 700-300cm-1 , Interscience (1968); J. S. Lewis and R. G.
Prinn, Science 169, 472 (1970).
6. J. B. Pollack, J. Houck, D. Schaack, R. Reed and A. Summers,
in preparation (1975).
7. S. Gulkis and R. Poynter, Phys. Earth Planet. Interiors 6, 36 (1972).
8. G. Orton, private communication (also see G. Ohring, Astrophys. J.
134,	 1027	 (19731
	
(1974)•
9. W. Smith, Appl. Optics 9, 1993 (1970) .
10. G. Ohring, Astrophys. J. 184, 1027	 (1973) .
11. J. B. Pollack and G. Ohring, Icarus 19, 34 (1973).
12. A. Kliore, D. L. Cain, G. Fjelbo, B. LV Seidel, and S.	 I. Rasool,
Science 183, 322 (1974).
13. We thank Peter Gierasch and Carl Sagan for helpful discussions;
Glenn Orton for helpful discussions concerning his results; Donald
Card for contributions to the statistical analysis of our data;
and Betty Baldwin for help with the computer programming. The out-
standing efforts of the pilots and staff of the Airborne Science
Office of Ames Research Center greatly facilitated the observations.
This work was supported by NASA grant NGR 33-010-182.
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James R. Houck
Page 10
FIGURE CAPTIONS
Figure 1. a. The brightness temperature of Jupiter is
shown for wavelengths from 16 to 401im.
b. The pressure induced absorption coefficients
for H2-He collisions are shown. The curves shown
represent the sum of both translational and rotational
transitions.
Figure 2. A plot of the derived pressure temperature profile
for the Jovian atmosphere is shown. The pressure level
most . directly measured by various observational
techniques is shown.
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`v10 	 15	 20 25 30 35 40
Wavelength (µm)
Figure 1
PRWM NG PAGE NLAM NOR' MM
0.001
U
0.01
0E0a
-
	
0.1
a
 =
 0.8
H2
	
,i
%
 =
 I.0
ol
^
H2
0vc^
-
,
	
o'00
Temperature (°K)
Figure 2
1.0 x-
100
120
	
140
16U
	
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1
	
i


Jupiter: Its infrared spectrum from 10 to 40 microns

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