An average of 51 Voyager 1 IRIS spectra of Jupiter's North Tropical Zone was analyzed to infer the abundance, vertical extent, and size distribution of the particles making up the ammonia cloud in this region. It is assumed that the cloud base coincides with the level at which 100% saturation of ammonia vapor occurs. The vertical distribution of particulates above this level is determined by assuming a constant total ammonia mixing ratio and adjusting the two phases so that the vapor is saturated throughout the cloud. A constant scaling factor then adjusts the base number density. A radiative transfer program is used that includes the effects of absorption and emission of all relevant gases as well as anisotropic scattering by cloud particles. Mie scattering from a gaussian particle size distribution is assumed. The vertical thermal structure is inferred from a temperature retrieval program that utilizes the collision induced S(0) and S(1) molecular hydrogen lines between 300 and 700.cm, and the 1304.cm methane band

Conrath, Barney J.

Samuelson, Robert E.

Shaffer, William A.

English

NASA Technical Reports Server

N87- 17609
STUDY OF THE AMMONIA ICE CLOUD LAYER IN THE NORTH TROPICAL ZONE OF
JUPITER FROM THE INFRARED INTERFEROMETRIC EXPERIMENT ON VOYAGER
William A. Shaffer*, Robert E. Samuelson, and Barney J. Conrath
NASA/GoddardSpace Flight Center
An average of 51 Voyager 1 IRIS spectra of Jupiter's North Tropical
Zone has been analyzed to infer the abundance, vertical extent, and
size distribution of the particles making up the ammonia cloud in
this region. It is assumed that the cloud base coincides with the
level at which 100% saturation of ammonia vapor occurs. The vertical
distribution of particulates above this level is determined by
assuming a constant total ammonia mixing ratio (gas plus condensate)
and adjusting the two phases so that the vapor is saturated through-
out the cloud. A constant scaling factor then adjusts the base
number density.
A radiative transfer program is used that includes the effects of
absorption and emmission of all relevant gases as well as anisotropic
scattering by cloud particles. Mie scattering from a gaussian
particle size distribution is assumed. The vertical thermal structure
is inferred from a temperature retrieval program that utilizes the
collision induced S(O) and S(1) molecular hydrogen lines between 300
and 700 cm -1, and the 1304 cm -I methane band.
A total column abundance of = 2 x 10 -6 g cm -2 is inferred for the
condensate. Acceptable solutions for mean particle radii range from
0.5 to 3.5 _m. These values are considerably smaller than those
preferred by Marten et al. (1981, Icarus 46, 233-248) and Orton et al.
(1982, Icarus 52, 94-i16), though are comparable to the 3 _m mean
particle radius---derived by Sato and Hansen (1979, J. Atmos. Sci. 36,
1133-1167). Several possible reasons for this apparent discrepancy
are suggested.
We have attempted to determine a particulate size of the ammonia cloud on
Jupiter's North Tropical Zone from fitting IRIS spectra. In Fig. I, we
plotted two spectra, and the dashed line was obtained by averaging 55 South
Pole spectra. The first thing that one notices in the 300-700 wavenumber
region is that there is agreement between the two spectra, which indicates a
similarity in the tropospheric thermal structure. The second thing that one
notices is that there are differences in the 1200-1300 wavenumber regions,
indicating differences in the stratospheric structure. Since the tropospheric
thermal structures are similar, the differences in the spectra at 200 wave-
numbers and 1150 wavenumbers are due primarily to the cloud structure. We
have attempted, by fitting these two windows by a cloud model, to obtain the
cloud properties in the North Tropical Zone.
*NRC Research Associate
64
https://ntrs.nasa.gov/search.jsp?R=19870008176 2020-03-20T12:56:56+00:00Z
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Figure I. Two Voyager IRIS spectra obtained for large emission
angles. The solid line is the average of 46 North Tropical Zone
spectra and the dashed line is the average of 55 South Pole Spectra.
In order to save computer time, we chose one wavenumber value in each of the
windows to perform the fitting. After we obtained a fit in each of the windows
simultaneously using the two wavenumber values, the resulting spectrum was
calculated. This was found to be sufficient to obtain a good fit in the entire
spectrum. The spectrum we analyzed was not the solid line in Fig. i. The
spectrum analyzed was obtained by averaging 51 North Tropical Zone spectra for
small emission angles (Fig. I was obtained for large emission angles).
The first step was to obtain a temperature profile from fitting the S(0) and
S(1) hydrogen lines and the methane band at 1304 wavenumbers. The cloud model
we used assumed one cloud layer with a fixed bottom and a variable, well-
defined cloud top. The cloud base was found to be at 700 millibars from the
Clauslus-Clapeyron equation. The particulate number density profile was deter-
mined by assuming that the excess of the assumed ammonia mixing ratio over the
saturation mixing ratio went entirely into particulate formation. We used a
scale factor of the number density profile to get the number density appropriate
65
for a given particle size. In our cloud model we solved for three parameters:
particle size, the location of the cloud top and the number density scale factor.
The radiative transfer program used to calculate the synthetic spectra included
absorption, thermal emission, and anisotropic Mie scattering by particulates
(assuming a Gaussian distribution of particulates about an assumed mean particle
radius). The program used a doubling and adding technique for inhomogeneous
atmospheres. We also included phosphine, acetylene, and deuterated methane in
the calculations.
Calculating the brightness temperature at the two points in the windows, 210.1
wavenumbers and 1167.9 wavenumbers, for various particle sizes and number
density scale factors, we obtained the family of curves shown in Fig. 2. Each
point on a curve corresponds to a particular number density scale factor. For
this set of curves, the cloud top was arbitrarily fixed at 530 millibars. The
tendency in these curves is to move down as the particle size decreases. For
example, the curves for 30 to 2 _m particle size move down. At 2 _m they bottom
out. For particle sizes smaller than 2 _m the curves move up again. Note, for
example, the curves for 1.0 and 0.5 _m.
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brightness temperatures calculated
with no cloud
4/_m
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0.5/J brightness
I_m temperatures
2_
I I
140 145
Calculated Brightness Temperatures at 210.1 cm"l
Figure 2. The brightness temperature at 1167.9 wavenumbers as a
function of the brightness temperature at 210.1 wavenumbers for a
cloud top at 530 mb and various mean particle radii. The straight
line contains the points representing a cloud base particulate number
density of zero and the Voyager data values.
66
There are two points of interest added to Fig. 2. The first point was obtained
by calculating the brightness temperatures assuming no cloud in the radiative
transfer program. This point also corresponds to a number density scale
factor of zero for all particle sizes. So all the particle size curves share
this point. The second point is the point corresponding to the Voyager data
values. This point by definition corresponds to a good fit of the data. If
one of the particle size curves intersects at this point, for that number
density scale factor, we have a good fit in the two windows simultaneously.
If we construct a line intersecting these two points, then one notices some-
thing about the family of curves. That is, there are two classes of curves.
One class of curves is single-rooted; that is, they have one intersection
point with the straight line, the intersection point being the no-cloud point
or number density scale factor equal to zero. The second class of curves is
double-rooted. These curves have two intersection points, the no-cloud point
and somepoint further downthe straight llne. In the case of single-rooted
particle size curves, no simultaneous fit was possible in the two windows.
For example, for a 30 micron particle size, an opacity that is sufficient to
fit at 210.1 wavenumbersis always too small at 1167.9 wavenumbers. For
double-rooted particle sizes a simultaneous fit was possible.
An intersection of the particle curve above the Voyager data values indicates
too little opacity in the cloud model. The opacity can be increased by raising
the cloud top until the curve intersects at the Voyager data values. The
amount that the cloud top must be raised depends upon the distance between the
intersection point and the Voyager data values. An intersection that occurs
below the Voyager data values indicates too muchopacity in the cloud model,
and the opacity can be decreased by lowering the cloud top. The cloud top can
be lowered until the curve intersects at the Voyager data values. The amount
that the cloud top needs to be lowered depends on the distance between the
intersection point and the Voyager data values.
We conclude from Fig. 2 that acceptable values of the meanparticle radius
which will fit both windows simultaneously are larger than 0.5 _m, since 0.5 _m
is single-rooted, and smaller than 3.5 _m, since that would also be single-
rooted.
In Fig. 3, we've used a 3 _m particle size to calculate the synthetic spectrum
shown in the dashed line. For this spectrum, the cloud top was placed at 530
millibars and the cloud masswas 2 x 10-6 g cm-2. The data spectrum is shown
with a solid line. An excellent fit is achieved in the two windows, and in the
non-window regions in-between, the fit is also good.
Fig. 4 shows an example of the sensitivity of the synthetic spectrum to as-
sumedparticle size. The solid line is the Voyager data, and one of the
dashed lines is the synthetic spectrum for a 3 _mparticle size from Fig. 3.
The other dashed line was obtained assuminga 5 _mparticle size. This spec-
trum was obtained by fitting at 210.1 wavenumbersand simultaneously getting
the closest possible fit at 1167.9 wavenumbers. Unlike the 3 _mparticle
size, for the 5 _mparticle size it is not possible to fit both windows simul-
taneously. Fig. 4 shows that in going from a 3 _m to a 5 _mparticle size,
the synthetic spectrum in the second windowblows up and a simultaneous fit is
no longer possible.
67
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130
120
110 i i i i l , i ,
200 400 600 800 1000 1200
WAVE NUMBER (CM-1)
Figure 3. A synthetic spectrum obtained for a cloud with mean particle
radius of 3.0 microns, cloud top at 530 mb and cloud mass of 2 x 10 -6 g
cm -2 (shown by dashed line). The solid line is the North Tropical Zone
IRIS spectrum.
Now, our results indicating small particle sizes, between 0.5 and 3.5 _m, are
contrary to what others have obtained, such as Marten et al. (1981). For
example, Marten et al. found that the particle size should be larger than 30
_m and was probably about I00 #m. The reason for the discrepancy between what
we have obtained and what they obtained may be due to the region of Jupiter's
atmosphere studied. Marten et al. studied the Equatorial Zone, while we have
studied the North Tropical Zone. Another reason for the discrepancy between
our results and theirs may be due to the cloud model used. We assumed a
definite cloud top with a sharp cloud cutoff. Marten et a2. assumed a scale
height with no cloud top so that the cloud essentially continued on to the top
of the atmosphere, although its density was very small. Our results, however,
do agree with those obtained by Sato and Hansen (1979), who examined the
near-lnfrared spectrum of Jupiter's North Tropical Zone. We have not consid-
ered in this study the possible dependence of the synthetic spectrum on the
variation of the cloud base level; nor have we considered different particu-
late number density profiles or different particle size distributions that are
non-Gaussian. These are presently under investigation. Thank you.
68
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Figure 4. A synthetic spectrum obtained from an attempted fit using
a 5.0 micron mean particle radius (dotted-dashed line), showing the
best fit possible for an acceptable fit at 210.1 wavenumbers. The
dashed llne is the 3.0 micron mean particle radius synthetic spec-
trum. The solid llne is the North Tropical Zone IRIS spectrum.
REFERENCES
Kunde, V., R. Hanel, W. Magulre, D. Gautier, J. P. Baluteau, A. Marten,
A. Chedin, N. Husson, and N. Scott (1982). The tropospheric gas
composition of Jupiter's North Equatorial Belt (NH3, PH3, CH3D , GeH4, H20 )
and the Jovian D/H isotopic ratio. Astrophgs. J. 263, 443-467.
Marten, A., D. Rouan, J.-P. Baluteau, D. Gautier, B. J. Conrath, R. Hanel, V.
Kunde, R. Samuelson, A. Chedin, and N. Scott (1981). Study of the ammonia
ice cloud layer in the equatorial region of Jupiter from the infrared
interferometrlc experiment on Voyager. Icarus 46, 233-248.
Sato, M., and J. E. Hansen (1979). Jupiter's atmospheric composition and
cloud structure deduced from absorption bands in reflected sunlight.
Atmos. Sci. 36, 1133-1167.
Jo
69
DR. WEST: You mentioned that 1 _m particles were in the range of acceptable
fits to your data. I showed a slide showing that for Mie scattering, 1 _m
particles show strong resonant features near 1060 wavenumbers. Is that not
added in your calculations, or how do you avoid that?
DR. SHAFFER: I believe that the small particle feature that you showed was
for 0. I _m (not i _m) which was outside of our acceptable range. No such
feature is evident in the spectrum corresponding to the 3 _m particle size.
DR. ROSSOW: Since you derive a number density and a particle size at the base
of the cloud, you can calculate the cloud mass density, which should be of
order the saturation vapor density at the base of the cloud. Since you as-
sumed a temperature and pressure level, do all those numbers agree? Did you
check to see if they do?
DR. SHAFFER: For the synthetic spectrum shown, that is the 3 _m particle
size, I believe that only about 60% of the available saturated ammonia actua-
ly goes into cloud formation at the base. However, the apparent excess might
be explained by uncertainties in the assumed ammonia mixing ratio profile, or
the retrieved temperature profile at this pressure level.
DR. ORTON: To what extent do your results depend on what your assumption was
for the distribution of ammonia gas? To what extent did it affect the 210.1
cm -I and 1150 cm -I results?
DR. SHAFFER: We don't expect that it would have much effect on the results,
since the particulate number density is not constrained by the ammonia mixing
ratio. We didn't try different ammonia profiles. We just used the profile
obtained by Kunde et al. (1982).
DR. ORTON: The study that Bob West referred to in '83 used 245 cm -I because
it was sufficiently far between ammonia line manifolds, and it was pretty far
away from ammonia gas absorption.
DR. BEZARD: The spectral region around 1150-1200 cm -I is very sensitive to
the cut-off of the Lorentz line shape that you assume. It is sensitive to the
far wing shape of the lines of the v4 methane band because it's a region of
weak gaseous absorption. What did you assume for that line shape, and did you
investigate the influence of the cut-off in your calculations?
DR. SHAFFER: The cut-off was I00 wavenumbers.
DR. OWEN: Thank you. I think we should move on.
70


Study of the ammonia ice cloud layer in the north tropical zone of Jupiter from the infrared interferometric experiment on Voyager

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