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Stratospheric ethane on Neptune: Comparison of groundbased and Voyager IRIS retrievals
ICARUS
99,
353-362 (1992)
Stratospheric
Ethane on Neptune: Comparison and Voyager IRIS Retrievals
of Groundbased
THEODOR KOSTIUK,’ PAUL ROMANI, AND FRED ESPENAK NASA Goddard Space Flight Center, Greenbelt, Maryland 20771
AND BRUNO BBZARD Observatoire de Paris, Meudon, France
Received March 30, 1992; revised June 26, 1992
Ethane abundances and altitude distributions in Neptune’s stratosphere, retrieved from groundbased single line measurements and from the band spectrum obtained by Voyager IRIS, were compared and used to test current photochemical models. Infrared heterodyne spectroscopic measurements of the CIH, z+,RR (K = 4, J = 5) emission line doublet at 840.9763 cm-’ were taken at the NASA Infrared Telescope Facility in May-June 1989, just prior to the Voyager 2 encounter with Neptune in August of the same year. Analysis of Voyager IRIS spectra has also revealed ethane emission from the broad band centered at 822 cm-’ which includes our single line measurement (Bezard et al., 1991, J. Geophys. Res. 96, l&961-18,975). The infrared heterodyne data were reanalyzed using Voyager updated parameters and thermal profiles and model-determined ethane altitude distributions identical to those used by BCzard et al. Analysis of the line spectrum and band emission yielded consistent mole fraction distributions. Heterodyne-retrieved ethane mole fractions were -30% greater than those from IRIS, but within the experimental uncertainties (30 and 35%, respectively, for a given thermal profile). For a constant with a height ethane distribution profile an ethane mole fraction of 1.9 x 10e6 was obtained for the heterodyne data. Even though the contribution functions for the heterodyne data peak higher in the stratosphere (- 1 scale height), the retrieval uncertainties and widths of the contribution functions do not permit direct retrieval of the altitude distribution of ethane. The new nominal thermal profile and low eddy mixing in the lower stratosphere are consistent with predictions by Kostiuk et al. (1990, Icarus 88, 87-96) using preVoyager atmospheric models. The sensitivity of retrievals to changes in the thermal structure was tested. Both heterodyne and IRIS retrieved ethane mole fractions can vary up to a factor of 4 ’ Visiting astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract with the National Aeronautical and Space Administration. Presented at Neptune/Triton Conference in Tucson, Arizona, during January 6-10, 1992.
over the range of reasonable stratospheric temperature profiles (+ 10 K), with the heterodyne results being more sensitive to changes at low pressures (
INTRODUCTION
Knowledge of the abundances and distributions of hydrocarbon species in the stratosphere of Neptune is important for understanding the photochemical and transport processes, the radiative budget, the thermal structure, and haze formation in the planet’s atmosphere. Photochemical models for methane photolysis on Neptune, which include chemistry, transport, and condensation, predict the abundances and distributions of the product hydrocarbons. Direct measurements of the product species provide constraints on existing photochemical models, allowing for their improvement. Ethane (C,H,) is an important product species that has been previously detected on Neptune in emission near 12.1 pm (Macy and Sinton 1977, Gillett and Rieke 1977, Macy 1980). Volume mixing ratios for constant with height profiles of -10m6 with large uncertainty factors (-5) were derived. Using measurements of the same spectral region at 0.23 cm-’ spectral resolution, Orton et al. (1987, 1990) deduced an ethane mixing ratio of 6 x 10m6 with an uncertainty factor of 3.2 for a mole fraction profile constant above the saturation level. Kostiuk et al. (1990) tested constant and nonconstant C2H6 mole fraction profiles, derived from then-current photochemical-condensation models (Romani and Atreya 1989), using infrared heterodyne measurements of a single ethane doublet emission (RR 4,5 lines in the Z+band) at 840.9763 cm-’ (Fig. 1). The spectrum was obtained at an effective spectral resolution of 0.0033 cm-’ (100 MHz). The observations 353
0019-1035/92 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
KOSTIUK ET AL. N&I’?‘UNE
CZH6;
IRTF
I. 700
.
.
.
I
750
.
.
.
1
.
JUNE
.
800 WAWNUMBER
RR(4,S)
.
1989
.
I
850
.
.
.
..I 900
(cm-‘)
FIG. 1. Ethane line and band spectra on Neptune. The upper ethane line spectrum (histogram) was measured using infrared heterodyne spectroscopy (IRHS). The curve corresponds to the best fit to the data using the same temperature profile as Btzard et al. (1991), and the Case B ethane mole fraction profile (see Fig. 4). The retrieved ethane mole fractions are within 20% of the model input profile, while the total uncertainty, using the given thermal profile, is -30%. The ultimate accuracy of the result is dependent upon the accuracy of the thermal profile used. In the bottom plot a comparison of Voyager IRIS observed (dotted line) and calculated (solid line) spectra in the region of the vs band of C2H2 and of the vs band of CrH, (taken from BCzard et al. 1991) is presented. The observed spectrum (disk-averaged selection) consists of an average of 2920 individual spectra with a field-of-view centered between 10”and 50” S latitude and covering a large fraction of Neptune’s disk. The bar indicates the lu noise level for the selection. The synthetic spectrum was computed for C2H2 and C,H, mole fractions of 6 x lo-* and 1.5 x 10m6respectively, assuming uniform distributions above the condensation levels, Case E. Note that the heterodyne bandwidth is narrower than the thickness of the vertical line indicating the 840.9763 cm-’ position on the IRIS band spectrum.
were made May 29-June 1, 1989 at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. For a nominal Neptune temperature profile (taken from Romani and Atreya 1989), it was determined that in order to fit the observations, 2 to 5.8 times more ethane was required in the 0.02- to 2-mbar pressure region than photochemical models predicted. A mole fraction value of 3 x 10M6near
the 0.27-mbar pressure region was derived. The cumulative uncertainty, excluding that associated with the temperature profile, was 30%. It was concluded that better agreement with models could be obtained if the eddy mixing was reduced in the lower stratosphere and/or the stratospheric temperature was increased by > 10K above the 6-mbar level. The Infrared Interferometer Spectrometer (IRIS) instrument recorded the thermal emission spectrum from Neptune at 4.3 cm-i spectral resolution during the Voyager 2 encounter in August 1989 (Conrath et al. 1989). BCzard et al. (1991) analyzed the band spectrum of ethane and acetylene (C,H,) centered at 822 cm-’ and 729 cm-’ (Fig. 1) to obtain abundances for ethane and acetylene using a temperature profile updated from Voyager results. An ethane mole fraction of 1.5 (+ 2.5, - 0.5) x 10e6 was retrieved using a distribution uniform with height above the saturation region, In addition, several photochemical model generated ethane and acetylene distribution profiles,were tested. Some of the generated ethane profiles were able to reproduce the observed ethane emission. The chief problem lay in trying to reproduce both the observed acetylene and ethane emission features. The “simultaneous best-fit” models had acetylene mole fractions too high by a factor of 2 and ethane mole fractions too low by the same factor compared to the respective mole fractions which reproduce the molecular bands observed. The ethane mole fraction was also found to be highly sensitive to the value of the eddy diffusion coefficient in the band formation region. A summary of the various C,H, mole fraction determinations is provided in Table I. The infrared heterodyne spectroscopy (IRHS) and Voyager IRIS measurements provide a unique opportunity to directly compare the results from band spectra obtained from spacecraft observations and line spectra obtained
TABLE I Determinations of the Stratospheric Ethane Mole Fraction on Neptune Observation/model Gillett and Rieke (1977) Macy (1980) Orton ef al. (1987, 1990) Romani and Atreya (1989) l Photochemical model Kostiuk ef al. (1990)-Groundbased (using R&A T-profile) 0 Constant profile 0 R&A PC model profile (at 0.27 mbar) (2-5.8 x PC model) Bezard et al. (1991)-Voyager IRIS (using Voyager T-profile, with T-uncertainty) 0 Constant prolile Present work-Groundbased (using Voyager T-profile) 0 Constant profile
Ethane mole fraction 5 x lo-’ -1 x 10-h 6(*/3.2) x 1O-6 52 x 10-e
3.0 (kO.9) x 10-e 3.3 (k1.0) x 10-e
1.5 (+2.5,
-0.5)
x 10-e
1.9(+0.6)
x 1O-6
355
STRATOSPHERIC ETHANE ON NEPTUNE
from the ground just prior to the encounter. The comparison of the two spectra is illustrated in Fig. 1. The whole IRHS spectral bandwidth containing the ethane line is narrower than the thickness of the vertical line on the IRIS spectrum indicating the 840.9763 cm-’ position in the band. Both measurements obtain an effective global average result-the groundbased measurements due to the small angular dimension of Neptune on the sky, and the IRIS result due to the averaging of 2920 spectra needed to obtain an adequate signal-to-noise ratio on the weak C2H6 band emission. These “global averages” were also both weighted towards the southern hemisphere of Neptune, due to the viewing geometry from Earth and the encounter geometry and observational sequencing of the Voyager IRIS. We have reevaluated our heterodyne measurements using the identical thermal profiles and ethane mole fraction distributions used in the Voyager IRIS retrievals. We compare these new results to those of BCzard et al. (1991) to better understand the ethane abundance distribution on Neptune.
Kostiuk,’
lo3
1 40
a ’
60
’
‘;.& 80
The thermal profile used in the initial analysis of the heterodyne data (Kostiuk et al. 1990) and that used by BCzard et al. (1991) in fitting the Voyager IRIS spectrum are shown in Fig. 2. The former profile is just the Appleby profile given in Tokunaga et al. (1983) and was used in the early Romani and Atreya (1989) photochemical model. The profile used by BCzard et al. (1991) is derived from Voyager radio science (RSS) data (Linda1 et al. 1990) and groundbased stellar occultation measurements (Hubbard et al. 1987), both resealed for 19% He abundance. It is interesting to note that this new thermal profile is about 10 K warmer in the 0.02- to O.Cmbar pressure region than that used by Kostiuk et al. (1990) and is consistent with the “warm” profile proposed by them (also given in Fig. 2) as the one providing the best fit of the photochemical model to the heterodyne measurements. The updated BCzard et al. temperature profile for Neptune is used here to fit the heterodyne spectrum with the new photochemically derived C2H6 mole fraction distributions in order to compare the results with Voyager IRIS retrievals. Photochemical
Model
The photochemical model used here, a simple one-dimensional steady-state model, is the same one described in BCzard et al. (1991). The model is an updated and expanded model from the one used in the Kostiuk et al. (1990) analysis. The differences between the two fall into two categories: (1) improvements to the model itself and
100
120
Temperature
INPUT MODELS
Thermal Profile
, , ,
I
I
I
140
i
I 160
I
I 180
, 200
(K)
FIG. 2. The dashed line is the thermal profile used in the analysis of the ground based data by Kostiuk et al. (1990). The dash-dot line is the warm profile proposed by them to reconcile the model-observation disagreement. The solid line is the thermal profile used by Bdzard et al. (1991) in the analysis of the Voyager IRIS data. Horizontal dotted lines mark off the region of sensitivity of the heterodyne and IRIS measurements for all of the tested ethane profiles (extremes of the half maximum values of the contribution functions for either heterodyne or IRIS).
(2) better constraints on the input parameters for the model, based upon further analysis of Voyager data. Model Improvements
The number of chemical reactions was increased and they are listed in BCzard et al. (1991). The rate coefficients were updated. In addition to the solar flux, the local interstellar medium (LISM) Lyman-a flux is now included. Since Neptune is so far from the Sun, the LISM at Lyman-a is an important source of dissociating radiation for methane, and thus affects ethane production. Input Parameters Methane at the lower boundary. The mixing ratio of methane at the lower boundary in the model in the Kostiuk et ul. (1990) paper was 2%. This was based upon a preVoyager analysis of the observed methane emission from Neptune (e.g., Orton et ul. 1990). For the runs presented here it is 3.5 x lo-‘. The lowering of the methane mole fraction is due to the warmer post-encounter-model atmospheres. With this abundance of methane and the new
356
KOSTIUK ET AL.
thermal profile, the groundbased CH, observations can be reproduced (Bezard et al. 1991). Even lower CH, mole fractions are suggested from analysis of Voyager ultraviolet spectrometer (UVS) occultation data (Bishop et al. 1992). This reduction of methane has a minimal effect on the ethane profile as the ethane production is photon limited, not methane limited.
TABLE II
C,H, Mole Fraction Case
Line type
K profile/model
description
A
---
K = 5.0x 10’ cm2 set-’ at the CH4
B
-
K = 5.0x lO’cm* set-’ at the CHI
C
-. .-
K = 2.0 x 10’ cm2 set-’ for pressures greater
homopause, homopause,
Eddy diffuusion. The dependence of the ethane mole fraction on the eddy diffusion coefficient, K, has been discussed in Kostiuk et al. (1990) and BCzard et al. (1991). In the region of the atmosphere probed by the infrared observations ethane is essentially chemically inert. Its source, photochemical production, occurs in the microbar pressure region, while its sink, condensation, takes place at approximately IO-mbar pressure. Transport, via eddy diffusion, connects the source to the sink. Thus, it is the shape and strength of K that determines the ethane mole fraction profile. Prior to the Voyager encounter there was no constraint on the eddy diffusion coefficient in the upper atmosphere. Thus the K profiles in Kostiuk et al. (1990) all had the same height variation (varying as the inverse square root of the atmospheric number density) but different homopause values. Analysis of the He 584-A dayglow line from Voyager UVS observations gave a central value of K at the He homopause of 5 x 10’ cm2 set- 1 with upper and lower error bounds of factors of 3 and 9, respectively (Parkinson et al. 1990). Interpretation of the Voyager UVS solar occultation data yields K values closer to lo7 at the methane homopause (Bishop et al. 1992). The value of K at the homopause has minimal effect on the interpretation of the infrared ethane emission data. As was shown in Bezard et al. (1991), it is Kin the line formation region (0.1-I mbar) that plays a significant role in the predicted mole fraction profile and its retrieval. But, by having constraints on K in the upper atmosphere, and on K in the lower atmosphere by analysis of this data, we can hopefully deduce something about the height variation of K. In Table II we summarize the K profiles and C2H, mole fraction profiles that are used here and in the Bezard et al. (1991) study. In Figs. 3 and 4 are plotted the K profiles and the corresponding ethane mole fraction profiles. The rapid decrease in ethane at the IO-mbar level is due to the onset of condensation. Profiles A-D are from the photochemical model, while E is the “traditional” constant mole fraction above the saturation level profile used in previous studies. Note that the mole fraction presented in E, 1.5 x 10e6, is the IRIS-derived value. Profile A provided a best simultaneous fit to IRIS acetylene and ethane data, while profile B (also shown in Fig. 4 of Bezard et al. (1991)) reproduced the IRIS ethane emission well. Profile C was developed to simulate the change in K due to a breaking wave (Fig. 5 of Bezard et al. (1991)). The
Profiles
D
-.-
E
--
proportional proportional
to N-o.5 to N-o.59
than 4 mbar, rapid increase to 5.0 x 10’ at 0.2 mbar and then constant K = 2.0 x lo3 cm2 set-’ for pressures greater than 4 mbar, discontinuity at 4 mbar, for pressures less than 4 mbar K proportional to N-o.5 and 2.0 x 10’ at the methane homopause C2H6 mixing ratio constant with height above saturation region (constant value is 1.5 x 10v6, from Voyager IRIS observations)
strength of K for pressures greater than 4 mbar comes from the overturning time of the atmosphere deduced by Conrath et al. (1991) (discussed in BCzard et al. (1991)). Note that C shows how K must vary with height to reproduce an E-type profile. Case C also provided a best simultaneous fit to the IRIS spectrum, but note that the ethane profile is quite different from that of case A (Fig. 4). Case D is a hybrid model, taking K in the lower stratosphere to be the same as in C, but then there is a discontinuity in K at 4 mbar and it then follows an A/B-type variation in height. Note that this results in a C,H, profile very similar to B. RESULTS
Reanalysis of the Heterodyne Data Using the results from the improved photochemical model discussed above and improved input parameters based on analysis of Voyager data, such as the thermal profile and Neptune’s rotation period, we reanalyzed our heterodyne spectrum of ethane taken in 1989. The rotation period is an important parameter for very high spectral resolution measurements since rotation of the atmosphere contributes to the broadening of measured emission lines and, thus, affects the value of the ethane mole fractions retrieved. Limaye and Sromovsky (1991) have analyzed cloud motions in im-.ges from Voyager cameras and determined atmospheric rotation as a function of latitude. While the clouds are almost certainly at a lower altitude than the region of ethane line emission, Conrath et al. (1991) have shown that zonal winds decrease slowly with height through the tropopause and lower stratosphere. We have adopted a value of 16.1 hr for the rotation period for our mean viewing latitudes, consistent with the
STRATOSPHERIC
representative distribution for ethane in Neptune’s stratosphere. For Case E, a simple constant with height profile above the saturation region, which is often used to approximate the true ethane distribution and is a standard for comparison between all measurements and retrievals made to date, the fit is also good (multiplicative factor = 1.3). The retrieved mole fraction is 1.9 (20.06) x 10w6, only -27% greater than that retrieved by Bezard et al. (1991) (see Table I).
lo-’
lO-3 2 P E lo-2
357
ETHANE ON NEPTUNE
I--
IRIS Retrievals and Comparison of Results
,I
IO2
,,(,,,,,
102
g1’
d,,,,,, , ,’
103
104
,(,,,,,,
(,,,,,,,,
,,,,,
10s
10s
K (cm2
s-l)
(,,,
IO’
,,,,,,,,,
,
IO8
..,j
109
FIG. 3. The height profiles of the eddy diffusion coefficients used in the photochemical model (see Table II for a description of the K profiles). Horizontal solid lines mark of the region of sensitivity of the heterodyne and IRIS measurements (same meaning as in Fig. 2).
Limaye and Sromovsky (1991) results and the same as the rotation period determined by the Voyager planetary radio astronomy experiment (Warwick et al. 1989). This value iancreases our mole fraction retrieval by -6% as compared to results using 17.7 hr, determined from groundbased observations by Hammel (1989), which was used in the Kostiuk et al. (1990) analysis. As described in Kostiuk et al. (1990), the model-derived ethane mole fraction profiles are adjusted by a multiplicative factor until the calculated emission spectrum best fits the measurements. The heterodyne spectrum and an example of the fit to the data is given in Fig. 1. The best fit (solid line) to the measured ethane heterodyne spectrum (histogram) for the Case B ethane mole-fraction profile is shown. Each step in the histogram corresponds to 100 MHz, except the two extreme right steps, which correspond to 150 and 125 MHz, respectively. Radiance and single-sideband brightness temperature are plotted versus frequency relative to the ethane RR (45) line center position. All spectroscopic and observational input parameters are identical to those described in Kostiuk et al. (1990). For the nonconstant with height photochemically generated mole fraction profiles the emission derived from Case B provided the best match to the data. The profile had to be multiplied by only a factor of 1.2 to best fit the data. Given the thermal profile, the total uncertainty for the retrieval is 30%, indicating that Case B is a good
The band spectrum for Voyager IRIS and a representative synthetic fit to the data (Bezard et al. 1991) are given in Fig. 1. The ethane RR(4,5) line is contained within the vg band of ethane. The lower observed peak radiance, as compared to the heterodyne line measurement, is due to the lower spectral resolution of the IRIS instrument. This spectrum was fit using molecular band parameters consistent with line parameters used in the fits above (i.e., Daunt et al. 1984). The total uncertainty on the retrieval, excluding that due to the thermal profile, was -35%. As in the heterodyne case above, photochemically derived model ethane mole fraction profile B provided the best agreement with the data (multiplicative factor of -1). The results of fitting all the model cases to the IRIS and
lo-5
lo-4
lo-3 ‘;: m 5
lo-2
: 2 : L
10-l
a
IO0
10’
IO2 10-a
lo-’
10-e C,H,
Mixing
10-e Ratio
FIG. 4. The ethane mole fraction profiles used in this analysis. Cases A-D were generated by the photochemical model. The corresponding eddy diffusion coefficient profiles are shown in Fig. 3 and described in Table II. Case E is a constant with height profile above the saturation region at the Voyager IRIS derived value. Horizontal solid lines mark of the region of sensitivity of the heterodyne and IRIS measurements (same meaning as in Fig. 2).
358
KOSTIUK ET AL.
TABLE III Comparison of IRIS and Heterodyne Retrievals Voyager IRIS
Case
Factor”
A
2
Heterodyne
Contribution functionb
Factor’
Contribution functionb
0.02 0.16
0.01 3
0.12
0.75
0.55
0.03 B
1
0.20
0.02 1.2
0.17
0.91
0.64
0.10 C
2
0.75
0.07 2.4
0.36
2.8
1.7
0.03 D
1
0.24
0.02 1.3
0.17
2.0
1.0
0.10 E
1
0.75
0.08 1.3
2.9
0.36 1.7
a Model predicted ethane mole fractions must be multiplied by this factor to agree with observations. b Pressure in millibars where the contribution function peaks and pressures where the contribution function has decreased to half of the peak value.
heterodyne data are given in Table III. The table lists the factors that each model predicted mole fraction must be multiplied by to agree with observations, the pressure region probed, and the peak of the contribution functions for the respective spectral resolution. Cases B, D, and E provide the best fits for both the groundbased and Voyager observations, their multiplicative factors are within the uncertainty of each retrieval. A useful comparison between the two observations requires not only the comparison of the absolute mole fraction profiles retrieved and their agreement with those generated by the photochemical model, but also the comparison of the atmospheric regions probed by the measurements, thus testing the model and hopefully providing useful constraints. If the altitude regions probed by the two measurements are sufficiently different, then the retrieved mole fractions can determine the slope of the ethane distribution and thus help discriminate between candidate profiles used. Knowing the ethane profile would permit us to choose between K varying continuously with height, which produces B-type ethane profiles, or eddy mixing profiles which have discontinuities in them and produce C-type ethane distributions (Figs. 3,4). This can be done by comparing the contribution functions for the
respective measurements and retrievals. The two cases which provided the best model-observational agreement and which represented the two extreme classes of mole fraction profiles are cases B and E. Case B is representative of a nonconstant with height profile derived from the most recent photochemical-condensation model for Neptune. Case E is a simple constant with height profile above the saturation region profile discussed earlier. The contribution functions for the IRIS and heterodyne analyses using these two cases are presented in Figs. 5 and 6. The contribution functions have been calculated for the respective spectral resolution of each measurement, 4.3 cm-’ for IRIS and 0.0033 cm-’ for heterodyne. As expected, the contribution functions are narrower and their peaks and regions probed are at higher altitude (lower pressure) for the heterodyne case than for the IRIS case. For case E the contribution function peaks are separated by about a scale height. This is consistent with higher optical thickness near the line center emission, as measured at high resolution. At 4.3 cm-’ the mean emission is more optically thin, thereby originating deeper in the atmosphere. For a nonconstant mole fraction distribution B, the contribution functions for both measurements are weighted toward the distribution peak. Therefore, the peaks are closer together than for the E profile and are located at comparable pressure levels. Somewhat higher mole fractions are retrieved from the heterodyne results (multiplicative factors 20-30% higher).
1o-3 !
0.0
0.1
0.2 Contribution
0.3
0.4
0.5
0.6
Function
FIG. 5. Comparison of contribution functions for both heterodyne and IRIS retrievals using the Case B ethane profile (see Fig. 4). The horizontal lines mark where the contribution function has dropped to half of the peak value.
STRATOSPHERIC
359
ETHANE ON NEPTUNE
10-3.
1o-3
I
,
I
,
I
Bishop lo-*
(
1
(
I
,
I
,,
I
,
I
:
1
Warm Nominal
)I I t
,
L
1O-2 ‘;: ([I 5 P) 10-l 2 : & loo
10’ 0.0
0.1
0.2 Contribution
0.3
0.4
0.5
0.6
Function
FIG. 6. Comparison of contribution functions for both heterodyne and IRIS retrievals using the Case E ethane profile (see Fig. 4). The horizontal lines mark where the contribution function has dropped to half of the peak value.
It is tempting to interpret the results as information on the altitude distribution of ethane. These results are, however, within the uncertainties of the retrievals. Also, the ranges of pressure probed, as defined by the full width at half maxima of the contribution functions, are wide and show significant overlap (see Table III). These facts do not allow for meaningful conclusions regarding the true altitude distribution of ethane on Neptune.
Dependence on the Thermal Profile We then tested the sensitivity of these results to the assumed temperature profile. Mole fractions were retrieved for two sets of limiting input thermal profiles. In the first case we only varied the temperature in the upper stratosphere (pressures ~0.4 mbar). These thermal structures are shown in Fig. 7 and are taken from Bishop et al. (1992). The Bishop et al. nominal profile is quite close to that of BCzard et al. (1991), also shown in Fig. 7 (and Fig. 2). They are based on the same source data; RSS egress occultation data (Linda1 et al. 1990) and the stellar occultation data of Hubbard et al. (1987), both resealed to a He mole fraction of 0.19. Temperature profiles deduced from other stellar occultation observations (Hubbard et al. 1985, French et al. 1985) fall within the range from the “warm” to “cold” thermal profiles in Fig. 7. In the second retrieval set, the entire thermal profile was shifted + 10 K to create a warm and a cold profile. These were the same limits as in BCzard et al. (1991).
Solid
- BIzard
a ’
8 ’
;\ lo3
E ’ ’ 40 60
60
8 ’
100
120
Temperature
8 ’ 140
c ’
’
160
’ 160
’
4 200
(K)
FIG. 7. Thermal profiles used to test the sensitivity of the retrieved ethane mole fraction to the assumed model atmosphere. Solid line labeled Bezard is the same as in Fig. 2. Curves labeled Bishop, warm, nominal, and cold are from Bishop et al., 1992 and are described in the text. Horizontal dotted lines are the same as in Fig. 2.
We chose to use an ethane mole fraction constant with height above the condensation region (case E profile) to investigate the temperature dependence of the ethane retrieval using both the infrared heterodyne and IRIS data. We have just seen that this type of ethane distribution showed the greatest spread in the peaks of the contribution functions from the heterodyne to IRIS observations, and thus will most likely show any differences between them. The results are summarized in Table IV.
TABLE IV Retrieved Ethane Mole Fractions as a Function of Thermal Profile Thermal profile
IRIS
Heterodyne
A%
Bishop Warm Nominal Cold
1.0 x 10-C 1.3 x 10-G 1.6 x 1O-6
1.3 x 10-e 1.8 x 1O-6 2.4 x 1O-6
30 38 50
BCzard Warm Nominal Cold
1.0 x 10-e 1.5 x 10-6 4.0 x 10-e
1.0 x 10-6 1.9 x 10-e 4.6 x 1O-6
0 27 15
360
KOSTIUK ET AL.
Since the nominal Bishop et al. thermal profile is slightly warmer than the nominal Btzard et al. profile (Fig. 7), using it yielded a slightly smaller ethane mole fraction for both measurements. In both the heterodyne and IRIS cases the retrieved mole fractions increased with decreasing temperature. The heterodyne results, however, showed a greater sensitivity to changes in the upper thermal structure than the IRIS case. The greatest difference between the two retrievals occurred with the “cold” profile, with the heterodyne retrieved abundance increasing to -50% above that of IRIS. This is consistent with the greater sensitivity of the high resolution heterodyne measurements to changes in lower pressure regions as compared to IRIS. For each data set the changes in thermal profile resulted in no more than a ?30% change in the nominal ethane mole fraction, which is comparable to the retrieval uncertainties. This indicates that abundance changes are not a strong function of thermal changes in the upper stratosphere. The results using the Bezard et al. temperature profiles showed a large increase in the ethane mole fraction with decreasing temperature, but with a smaller difference between the heterodyne and IRIS retrievals (A < 26%). This is because in this case, the low altitude (higher pressure) regions where IRIS probes also experience temperature changes and this is reflected in the IRIS retrieval. In this case 10 K thermal changes resulted in significant modification of the retrieved ethane mole fractions (>twofold vs 30% uncertainty). The contribution functions for the IRIS and infrared heterodyne spectra for the warm (solid line) and cold (dashed line) Bishop et al. thermal profiles are shown in Fig. 8. In both measurements the warm contribution functions originate higher in the stratosphere. The pressures where the resultant contribution functions peak and fall to half their value are tabulated in Table V. For the heterodyne retrievals the contribution functions are narrower and show a greater separation between the warm and cold cases. The warm heterodyne contribution function has a narrow peak near 0.13 mbar and is more than one scale height above the cold function peak (at 0.62 mbar). The IRIS warm contribution function shows a small low pressure secondary peak which corresponds to the high altitude temperature increase, but the function is broader than the corresponding heterodyne one. Its mean altitude is at 0.3 mbar, less than one scale height above the cold contribution function peak (0.75 mbar). The heterodyne and IRIS cold cases peak in comparable altitude regions, however, the overall heterodyne contribution function covers lower pressure regions than does the IRIS function. As in the case of abundance profiles discussed above, the contribution functions are greatly overlapping and considering the signal-to-noise ratios on the data and uncertainties in retrieval, we cannot retrieve meaningful
-
10’
-
Cold
~,,,‘,‘,‘.,,‘,,,‘,,,‘,,,~ 0.0 0.1 0.2
0.3
Contribution FIG. thermal the hot cussion
0.4
0.5
0.6
Function
8. Contribution functions for the Bishop er al. cold and warm profiles derived for the IRIS and heterodyne cases. Note that contribution functions peak higher in the stratosphere. See disin the text and Table V.
altitude information from the comparison of the low and high resolution data sets. The heterodyne data can provide narrower and more separated altitude information if sufficient signal-to-noise ratios can be obtained at 25MHz resolution as compared to the present IOO-MHz resolution. CONCLUSIONS
Nearly coincident groundbased and spacecraft measurements of 12-pm ethane emission spectra from Neptune, during the period of Voyager’s Neptune encounter, provided a unique opportunity to determine the ethane abundance in its stratosphere and to test and constrain recent theoretical models for methane photochemistry on Neptune. Constant with height and several photochemical-model-derived mole fraction distributions were tested against the groundbased infrared heterodyne data and compared to similar analyses of the Voyager IRIS data by Bezard et al. (1991). Analyses of the line emission spectrum obtained by heterodyne spectroscopy (X/Ah 105) and the band emission spectrum from Voyager IRIS (A/Ah - 200) retrieved consistent ethane mole fraction distributions for Neptune when identical atmospheric models and spectroscopic parameters were used. For a given ethane mole fraction model, the results obtained from the two measurements agreed to within retrieval uncertainties of -30%. Neptune’s stratospheric tempera-
STRATOSPHERIC
TABLE V IRIS and Heterodyne Contribution Functions for Bishop Thermal Profiles Contribution IRIS
Thermal profile
function0 Heterodyne
0.037 Bishop warm
0.048 0.13
0.3 2.3
I.7
0.20 Bishop cold
0.75
0.14 0.62
2.6
throughout the stratospheric region probed by the instruments (-0.02 to 2 mbar), comparable but much higher sensitivity with temperature was observed in both cases. Contribution functions for warm thermal profiles peaked at higher altitudes as expected, with the heterodyne functions covering lower pressure regions. However, as in the case of ethane altitude distribution tests, contribution functions derived for the various thermal profiles cannot quite provide direct altitude information, due to their width and overlap, as well as to the uncertainties in the rertrievals. Higher quality heterodyne data may be able to separate the altitude regions probed.
1.8
a Pressure in millibars where the contribution function peaks and pressures where the contribution function has decreased to half of the peak value.
ture profile was updated from that used in the Kostiuk et (1990) analyses. The new profile (used by BCzard et al. 1991) is - 10 K warmer in the region of line formation. This is consistent with the conclusion of Kostiuk et al. (1990) that a >lO K warmer stratosphere was needed to best fit the theoretical models to the data. For a nonconstant with height ethane distribution, both data sets were in best agreement with the photochemical model which included an eddy mixing coefficient K = 5.0 x 10’ at the methane homopause, varying as Wo.59 (N = atmospheric number density). This “best model” (Case B) yielded the lowest eddy mixing in the lower stratosphere of Neptune; a result again consistent with predictions by Kostiuk et al. (1990). Both line and band emission spectra were also fit well with the constant with height above the saturation level model, Case E, within the uncertainties of the retrievals (-30%). The higher spectral resolution of the infrared heterodyne measurements probes higher in the stratosphere than that of the Voyager IRIS. For the nonuniform with height ethane distribution, Case B, the pressure region probed is 0.03-0.91 mbar for IRIS, 0.02-0.64 for heterodyne; for the uniform ethane distribution, Case E, 0.10-2.9 mbar for IRIS, 0.08-1.7 mbar for heterodyne. The width of the contribution functions, their degree of overlap, and the uncertainties in the retrievals do not permit the retrieval of direct information on ethane altitude distribution. Therefore, both constant and nonconstant with height profiles remain candidate distributions of stratospheric ethane on Neptune. The ethane retrievals were sensitive to the thermal profile used. Mole fractions retrieved from both heterodyne and IRIS data varied by up to a factor of 4 over the range of reasonable thermal profiles. The heterodyne case was more sensitive to changes at low pressures (co.1 mbar) than was the IRIS case. When thermal changes occurred al.
361
ETHANE ON NEPTUNE
REFERENCES B~ZARD, B., P. ROMANI, B. J. CONRATH, AND W. C. MAGUIRE 1991. Hydrocarbons in Neptune’s stratosphere from Voyager infrared observations. J. Geophys. Res. 96, 18,961-18,975. BISHOP, J., S. K. ATREYA, P. N. ROMANI, B. R. SANDEL, AND F. HERBERT 1992. Voyager 2 UVS solar occultations at Neptune: Constraints on the abundance of methane in the stratosphere. J. Geophys. Res. 97, 11,681-11,694. CONRATH,B., F. M. FLASAR, R. HANEL, V. KUNDE, W. MAGUIRE, J. PEARL, J. PIRRAGILA, R. SAMUELSON,P. GIERASCH, A. WEIR, B. B~ZARD, D. GAUTIER, D. CRUIKSHANK, L. HORN, R. SPRINGER, W. SHAFFER 1989. Infrared observations of the Neptunian system. Science 246, 1454-1459. CONRATH, B. J., F. M. FLASAR, AND P. J. GIERASCH 1991. Thermal structure and dynamics of Neptune’s atmosphere from Voyager measurements. J. Geophys. Res. 96, 18,931-18,939. DAUNT, S. J., A. K. ATAKAN, W. E. BLASS, G. W. HALSEY, D. E. JENNINGS,D. C. REUTER, J. SUSSKIND,AND J. W. BRAULT 1984. The 12 micron band of ethane: High-resolution laboratory analysis with candidate lines for infrared heterodyne searches. Astrophys. J. 280, 921-936; also Atakan, A. K., W. E. Blass, J. W. Brault, S. J. Daunt, G. W. Halsey, D. E. Jennings, D. C. Reuter, and J. Susskind 1983. The 12 Micron Band of Ethane: A Spectral Catalog from 765 cm-‘-900 cm-‘. NASA/GSFC Tech. Memorandum 85108. FRENCH, R. G., P. A. MELROY, R. L. BARON, E. W. DUNHAM, K. J. MEECH, D. J. MINK, J. L. ELLIOT, D. A. ALLEN, M. C. B. ASHLEY, K. C. FREEMAN, E. F. ERICKSON, J. GOGUEN, AND H. HAMMEL 1985. The 1983 June 15 occultation by Neptune. II. The oblateness of Neptune. Astron. .I. 90, 2624-2638. GILLETT, F. C., AND G. H. RIEKE 1977. 5-20 micron observations Uranus and Neptune. Astrophys. J. 218, L141.
of
HAMMEL, H. B. 1989. Neptune cloud structure at visible wavelengths. Science 244, 1165-l 167. HUBBARD,W. B., J. E. FRECKER,J.-A. GEHRELS, T. GEHRELS, D. M. HUNTEN, L. A. LEBOFSKY,B. A. SMITH, D. J. THOLEN, F. VILAS, B. ZELLNER, H. P. AVEY, K. MOTTRAM,T. MURPHY, B. VARNES, B. CARTER, A. NIELSEN, A. A. PAGE, H. H. Fu, H. H. WV, H. D. KENNEDY, M. D. WATERWORTH,AND H. J. REITSEMA1985. Results from observations of the 15 June 1983 occultation by the Neptune system. Astron. J. 90, 655-667. HUBBARD, W. B. P. D. NICHOLSON, E. LELLOUCH, B. SICARDY, A. BRAHIC, F. VILAS, P. BOUCHET, R. A. MCLAREN, R. MILLIS, L. H. WASSERMAN,J. H. ELIAS, K. MATTHEWS, J. D. MCGILL, AND C. PERRIER 1987. Oblateness, radius, and mean stratospheric temperature of Neptune from the 1985 August 20 occultation. Icarus 72, 635-646.
362
KOSTIUK
KOSTIUK, T., F. ESPENAK, P. ROMANI, D. ZIPOY, AND J. GOLDSTEIN 1990. Ethane abundance on Neptune. Icarus 88, 87-96. LIMAYE, S. J., AND L. A. SROMOVSKY1991. Winds ofNeptune: Voyager observations of cloud motions. J. Geophys. Res. 96, l&941-18,960. LINDAL, G. F., J. R. LYONS, D. N. SWEETNAM, V. R. ESHLEMAN, D. P. HINSON, AND G. L. TYLER 1990. The atmosphere of Neptune: Results of radio occultation measurements with Voyager 2 spacecraft. Geophys. Res. Lett. 17, 1733-1736. MACY, W., JR. 1980. Mixing ratios of methane, ethane, and acetylene in Neptune’s stratosphere. Icarus 41, 153. MACY, W., JR., AND W. SINTON 1977. Detection of methane and ethane emission on Neptune but not Uranus. Astrophys. J. 218, L79. ORTON, G. S., D. K. AITKEN, C. SMITH, P. F. ROCHE, J. CALDWELL, AND R. SNYDER 1987. The spectra of Uranus and Neptune at 8-14 and 17-23 pm. Icarus
70, l-12.
ORTON, G. S., K. H. BAINES, J. CALDWELL, P. N. ROMANI, A. T.
ET AL. TOKUNAGA, AND R. A. WEST 1990. Calibration of the 7- to 14-pm brightness spectra of Uranus and Neptune. Icarus 85, 257-265. PARKINSON,C. D., J. C. MCCONNELL, B. R. SANDEL, R. V. YELLE, AND A. L. BROADFOOT1990. He 584 8, dayglow at Neptune. Geophys. Res. Lett. 17, 1709-1712. ROMANI, P. N., AND S. K. ATREYA 1989. Stratospheric aerosols from CH4 photochemistry on Neptune. Geophys. Res. Lett. 16, 941. TOKUNAGA,A. T., G. S. ORTON, AND J. CALDWELL 1983. Newobservational constraints on the temperature inversions of Uranus and Neptune. Icarus 53, 141-146. WARWICK, J. W., D. R. EVANS, G. R. PELTZER, R. G. PELTZER, J. H. ROMIG, C. B. SAWYER, A. C. RIDDLE, A. E. SCHWEITZER, M. D. DESCH, M. L. KAISER, W. M. FARRELL, T. D. CARR, I. DE PATER, D. H. STAELIN, S. GULKIS, R. L. POYNTER, A. BOISCHOT, F. GENOVA, Y. LEBLANC, A. LECACHEUX, B. M. PEDERSEN, P. ZARKA 1989. Voyager planetary radio astronomy at Neptune. Science 246, 1498-1501.
99,
353-362 (1992)
Stratospheric
Ethane on Neptune: Comparison and Voyager IRIS Retrievals
of Groundbased
THEODOR KOSTIUK,’ PAUL ROMANI, AND FRED ESPENAK NASA Goddard Space Flight Center, Greenbelt, Maryland 20771
AND BRUNO BBZARD Observatoire de Paris, Meudon, France
Received March 30, 1992; revised June 26, 1992
Ethane abundances and altitude distributions in Neptune’s stratosphere, retrieved from groundbased single line measurements and from the band spectrum obtained by Voyager IRIS, were compared and used to test current photochemical models. Infrared heterodyne spectroscopic measurements of the CIH, z+,RR (K = 4, J = 5) emission line doublet at 840.9763 cm-’ were taken at the NASA Infrared Telescope Facility in May-June 1989, just prior to the Voyager 2 encounter with Neptune in August of the same year. Analysis of Voyager IRIS spectra has also revealed ethane emission from the broad band centered at 822 cm-’ which includes our single line measurement (Bezard et al., 1991, J. Geophys. Res. 96, l&961-18,975). The infrared heterodyne data were reanalyzed using Voyager updated parameters and thermal profiles and model-determined ethane altitude distributions identical to those used by BCzard et al. Analysis of the line spectrum and band emission yielded consistent mole fraction distributions. Heterodyne-retrieved ethane mole fractions were -30% greater than those from IRIS, but within the experimental uncertainties (30 and 35%, respectively, for a given thermal profile). For a constant with a height ethane distribution profile an ethane mole fraction of 1.9 x 10e6 was obtained for the heterodyne data. Even though the contribution functions for the heterodyne data peak higher in the stratosphere (- 1 scale height), the retrieval uncertainties and widths of the contribution functions do not permit direct retrieval of the altitude distribution of ethane. The new nominal thermal profile and low eddy mixing in the lower stratosphere are consistent with predictions by Kostiuk et al. (1990, Icarus 88, 87-96) using preVoyager atmospheric models. The sensitivity of retrievals to changes in the thermal structure was tested. Both heterodyne and IRIS retrieved ethane mole fractions can vary up to a factor of 4 ’ Visiting astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract with the National Aeronautical and Space Administration. Presented at Neptune/Triton Conference in Tucson, Arizona, during January 6-10, 1992.
over the range of reasonable stratospheric temperature profiles (+ 10 K), with the heterodyne results being more sensitive to changes at low pressures (
INTRODUCTION
Knowledge of the abundances and distributions of hydrocarbon species in the stratosphere of Neptune is important for understanding the photochemical and transport processes, the radiative budget, the thermal structure, and haze formation in the planet’s atmosphere. Photochemical models for methane photolysis on Neptune, which include chemistry, transport, and condensation, predict the abundances and distributions of the product hydrocarbons. Direct measurements of the product species provide constraints on existing photochemical models, allowing for their improvement. Ethane (C,H,) is an important product species that has been previously detected on Neptune in emission near 12.1 pm (Macy and Sinton 1977, Gillett and Rieke 1977, Macy 1980). Volume mixing ratios for constant with height profiles of -10m6 with large uncertainty factors (-5) were derived. Using measurements of the same spectral region at 0.23 cm-’ spectral resolution, Orton et al. (1987, 1990) deduced an ethane mixing ratio of 6 x 10m6 with an uncertainty factor of 3.2 for a mole fraction profile constant above the saturation level. Kostiuk et al. (1990) tested constant and nonconstant C2H6 mole fraction profiles, derived from then-current photochemical-condensation models (Romani and Atreya 1989), using infrared heterodyne measurements of a single ethane doublet emission (RR 4,5 lines in the Z+band) at 840.9763 cm-’ (Fig. 1). The spectrum was obtained at an effective spectral resolution of 0.0033 cm-’ (100 MHz). The observations 353
0019-1035/92 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
KOSTIUK ET AL. N&I’?‘UNE
CZH6;
IRTF
I. 700
.
.
.
I
750
.
.
.
1
.
JUNE
.
800 WAWNUMBER
RR(4,S)
.
1989
.
I
850
.
.
.
..I 900
(cm-‘)
FIG. 1. Ethane line and band spectra on Neptune. The upper ethane line spectrum (histogram) was measured using infrared heterodyne spectroscopy (IRHS). The curve corresponds to the best fit to the data using the same temperature profile as Btzard et al. (1991), and the Case B ethane mole fraction profile (see Fig. 4). The retrieved ethane mole fractions are within 20% of the model input profile, while the total uncertainty, using the given thermal profile, is -30%. The ultimate accuracy of the result is dependent upon the accuracy of the thermal profile used. In the bottom plot a comparison of Voyager IRIS observed (dotted line) and calculated (solid line) spectra in the region of the vs band of C2H2 and of the vs band of CrH, (taken from BCzard et al. 1991) is presented. The observed spectrum (disk-averaged selection) consists of an average of 2920 individual spectra with a field-of-view centered between 10”and 50” S latitude and covering a large fraction of Neptune’s disk. The bar indicates the lu noise level for the selection. The synthetic spectrum was computed for C2H2 and C,H, mole fractions of 6 x lo-* and 1.5 x 10m6respectively, assuming uniform distributions above the condensation levels, Case E. Note that the heterodyne bandwidth is narrower than the thickness of the vertical line indicating the 840.9763 cm-’ position on the IRIS band spectrum.
were made May 29-June 1, 1989 at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. For a nominal Neptune temperature profile (taken from Romani and Atreya 1989), it was determined that in order to fit the observations, 2 to 5.8 times more ethane was required in the 0.02- to 2-mbar pressure region than photochemical models predicted. A mole fraction value of 3 x 10M6near
the 0.27-mbar pressure region was derived. The cumulative uncertainty, excluding that associated with the temperature profile, was 30%. It was concluded that better agreement with models could be obtained if the eddy mixing was reduced in the lower stratosphere and/or the stratospheric temperature was increased by > 10K above the 6-mbar level. The Infrared Interferometer Spectrometer (IRIS) instrument recorded the thermal emission spectrum from Neptune at 4.3 cm-i spectral resolution during the Voyager 2 encounter in August 1989 (Conrath et al. 1989). BCzard et al. (1991) analyzed the band spectrum of ethane and acetylene (C,H,) centered at 822 cm-’ and 729 cm-’ (Fig. 1) to obtain abundances for ethane and acetylene using a temperature profile updated from Voyager results. An ethane mole fraction of 1.5 (+ 2.5, - 0.5) x 10e6 was retrieved using a distribution uniform with height above the saturation region, In addition, several photochemical model generated ethane and acetylene distribution profiles,were tested. Some of the generated ethane profiles were able to reproduce the observed ethane emission. The chief problem lay in trying to reproduce both the observed acetylene and ethane emission features. The “simultaneous best-fit” models had acetylene mole fractions too high by a factor of 2 and ethane mole fractions too low by the same factor compared to the respective mole fractions which reproduce the molecular bands observed. The ethane mole fraction was also found to be highly sensitive to the value of the eddy diffusion coefficient in the band formation region. A summary of the various C,H, mole fraction determinations is provided in Table I. The infrared heterodyne spectroscopy (IRHS) and Voyager IRIS measurements provide a unique opportunity to directly compare the results from band spectra obtained from spacecraft observations and line spectra obtained
TABLE I Determinations of the Stratospheric Ethane Mole Fraction on Neptune Observation/model Gillett and Rieke (1977) Macy (1980) Orton ef al. (1987, 1990) Romani and Atreya (1989) l Photochemical model Kostiuk ef al. (1990)-Groundbased (using R&A T-profile) 0 Constant profile 0 R&A PC model profile (at 0.27 mbar) (2-5.8 x PC model) Bezard et al. (1991)-Voyager IRIS (using Voyager T-profile, with T-uncertainty) 0 Constant prolile Present work-Groundbased (using Voyager T-profile) 0 Constant profile
Ethane mole fraction 5 x lo-’ -1 x 10-h 6(*/3.2) x 1O-6 52 x 10-e
3.0 (kO.9) x 10-e 3.3 (k1.0) x 10-e
1.5 (+2.5,
-0.5)
x 10-e
1.9(+0.6)
x 1O-6
355
STRATOSPHERIC ETHANE ON NEPTUNE
from the ground just prior to the encounter. The comparison of the two spectra is illustrated in Fig. 1. The whole IRHS spectral bandwidth containing the ethane line is narrower than the thickness of the vertical line on the IRIS spectrum indicating the 840.9763 cm-’ position in the band. Both measurements obtain an effective global average result-the groundbased measurements due to the small angular dimension of Neptune on the sky, and the IRIS result due to the averaging of 2920 spectra needed to obtain an adequate signal-to-noise ratio on the weak C2H6 band emission. These “global averages” were also both weighted towards the southern hemisphere of Neptune, due to the viewing geometry from Earth and the encounter geometry and observational sequencing of the Voyager IRIS. We have reevaluated our heterodyne measurements using the identical thermal profiles and ethane mole fraction distributions used in the Voyager IRIS retrievals. We compare these new results to those of BCzard et al. (1991) to better understand the ethane abundance distribution on Neptune.
Kostiuk,’
lo3
1 40
a ’
60
’
‘;.& 80
The thermal profile used in the initial analysis of the heterodyne data (Kostiuk et al. 1990) and that used by BCzard et al. (1991) in fitting the Voyager IRIS spectrum are shown in Fig. 2. The former profile is just the Appleby profile given in Tokunaga et al. (1983) and was used in the early Romani and Atreya (1989) photochemical model. The profile used by BCzard et al. (1991) is derived from Voyager radio science (RSS) data (Linda1 et al. 1990) and groundbased stellar occultation measurements (Hubbard et al. 1987), both resealed for 19% He abundance. It is interesting to note that this new thermal profile is about 10 K warmer in the 0.02- to O.Cmbar pressure region than that used by Kostiuk et al. (1990) and is consistent with the “warm” profile proposed by them (also given in Fig. 2) as the one providing the best fit of the photochemical model to the heterodyne measurements. The updated BCzard et al. temperature profile for Neptune is used here to fit the heterodyne spectrum with the new photochemically derived C2H6 mole fraction distributions in order to compare the results with Voyager IRIS retrievals. Photochemical
Model
The photochemical model used here, a simple one-dimensional steady-state model, is the same one described in BCzard et al. (1991). The model is an updated and expanded model from the one used in the Kostiuk et al. (1990) analysis. The differences between the two fall into two categories: (1) improvements to the model itself and
100
120
Temperature
INPUT MODELS
Thermal Profile
, , ,
I
I
I
140
i
I 160
I
I 180
, 200
(K)
FIG. 2. The dashed line is the thermal profile used in the analysis of the ground based data by Kostiuk et al. (1990). The dash-dot line is the warm profile proposed by them to reconcile the model-observation disagreement. The solid line is the thermal profile used by Bdzard et al. (1991) in the analysis of the Voyager IRIS data. Horizontal dotted lines mark off the region of sensitivity of the heterodyne and IRIS measurements for all of the tested ethane profiles (extremes of the half maximum values of the contribution functions for either heterodyne or IRIS).
(2) better constraints on the input parameters for the model, based upon further analysis of Voyager data. Model Improvements
The number of chemical reactions was increased and they are listed in BCzard et al. (1991). The rate coefficients were updated. In addition to the solar flux, the local interstellar medium (LISM) Lyman-a flux is now included. Since Neptune is so far from the Sun, the LISM at Lyman-a is an important source of dissociating radiation for methane, and thus affects ethane production. Input Parameters Methane at the lower boundary. The mixing ratio of methane at the lower boundary in the model in the Kostiuk et ul. (1990) paper was 2%. This was based upon a preVoyager analysis of the observed methane emission from Neptune (e.g., Orton et ul. 1990). For the runs presented here it is 3.5 x lo-‘. The lowering of the methane mole fraction is due to the warmer post-encounter-model atmospheres. With this abundance of methane and the new
356
KOSTIUK ET AL.
thermal profile, the groundbased CH, observations can be reproduced (Bezard et al. 1991). Even lower CH, mole fractions are suggested from analysis of Voyager ultraviolet spectrometer (UVS) occultation data (Bishop et al. 1992). This reduction of methane has a minimal effect on the ethane profile as the ethane production is photon limited, not methane limited.
TABLE II
C,H, Mole Fraction Case
Line type
K profile/model
description
A
---
K = 5.0x 10’ cm2 set-’ at the CH4
B
-
K = 5.0x lO’cm* set-’ at the CHI
C
-. .-
K = 2.0 x 10’ cm2 set-’ for pressures greater
homopause, homopause,
Eddy diffuusion. The dependence of the ethane mole fraction on the eddy diffusion coefficient, K, has been discussed in Kostiuk et al. (1990) and BCzard et al. (1991). In the region of the atmosphere probed by the infrared observations ethane is essentially chemically inert. Its source, photochemical production, occurs in the microbar pressure region, while its sink, condensation, takes place at approximately IO-mbar pressure. Transport, via eddy diffusion, connects the source to the sink. Thus, it is the shape and strength of K that determines the ethane mole fraction profile. Prior to the Voyager encounter there was no constraint on the eddy diffusion coefficient in the upper atmosphere. Thus the K profiles in Kostiuk et al. (1990) all had the same height variation (varying as the inverse square root of the atmospheric number density) but different homopause values. Analysis of the He 584-A dayglow line from Voyager UVS observations gave a central value of K at the He homopause of 5 x 10’ cm2 set- 1 with upper and lower error bounds of factors of 3 and 9, respectively (Parkinson et al. 1990). Interpretation of the Voyager UVS solar occultation data yields K values closer to lo7 at the methane homopause (Bishop et al. 1992). The value of K at the homopause has minimal effect on the interpretation of the infrared ethane emission data. As was shown in Bezard et al. (1991), it is Kin the line formation region (0.1-I mbar) that plays a significant role in the predicted mole fraction profile and its retrieval. But, by having constraints on K in the upper atmosphere, and on K in the lower atmosphere by analysis of this data, we can hopefully deduce something about the height variation of K. In Table II we summarize the K profiles and C2H, mole fraction profiles that are used here and in the Bezard et al. (1991) study. In Figs. 3 and 4 are plotted the K profiles and the corresponding ethane mole fraction profiles. The rapid decrease in ethane at the IO-mbar level is due to the onset of condensation. Profiles A-D are from the photochemical model, while E is the “traditional” constant mole fraction above the saturation level profile used in previous studies. Note that the mole fraction presented in E, 1.5 x 10e6, is the IRIS-derived value. Profile A provided a best simultaneous fit to IRIS acetylene and ethane data, while profile B (also shown in Fig. 4 of Bezard et al. (1991)) reproduced the IRIS ethane emission well. Profile C was developed to simulate the change in K due to a breaking wave (Fig. 5 of Bezard et al. (1991)). The
Profiles
D
-.-
E
--
proportional proportional
to N-o.5 to N-o.59
than 4 mbar, rapid increase to 5.0 x 10’ at 0.2 mbar and then constant K = 2.0 x lo3 cm2 set-’ for pressures greater than 4 mbar, discontinuity at 4 mbar, for pressures less than 4 mbar K proportional to N-o.5 and 2.0 x 10’ at the methane homopause C2H6 mixing ratio constant with height above saturation region (constant value is 1.5 x 10v6, from Voyager IRIS observations)
strength of K for pressures greater than 4 mbar comes from the overturning time of the atmosphere deduced by Conrath et al. (1991) (discussed in BCzard et al. (1991)). Note that C shows how K must vary with height to reproduce an E-type profile. Case C also provided a best simultaneous fit to the IRIS spectrum, but note that the ethane profile is quite different from that of case A (Fig. 4). Case D is a hybrid model, taking K in the lower stratosphere to be the same as in C, but then there is a discontinuity in K at 4 mbar and it then follows an A/B-type variation in height. Note that this results in a C,H, profile very similar to B. RESULTS
Reanalysis of the Heterodyne Data Using the results from the improved photochemical model discussed above and improved input parameters based on analysis of Voyager data, such as the thermal profile and Neptune’s rotation period, we reanalyzed our heterodyne spectrum of ethane taken in 1989. The rotation period is an important parameter for very high spectral resolution measurements since rotation of the atmosphere contributes to the broadening of measured emission lines and, thus, affects the value of the ethane mole fractions retrieved. Limaye and Sromovsky (1991) have analyzed cloud motions in im-.ges from Voyager cameras and determined atmospheric rotation as a function of latitude. While the clouds are almost certainly at a lower altitude than the region of ethane line emission, Conrath et al. (1991) have shown that zonal winds decrease slowly with height through the tropopause and lower stratosphere. We have adopted a value of 16.1 hr for the rotation period for our mean viewing latitudes, consistent with the
STRATOSPHERIC
representative distribution for ethane in Neptune’s stratosphere. For Case E, a simple constant with height profile above the saturation region, which is often used to approximate the true ethane distribution and is a standard for comparison between all measurements and retrievals made to date, the fit is also good (multiplicative factor = 1.3). The retrieved mole fraction is 1.9 (20.06) x 10w6, only -27% greater than that retrieved by Bezard et al. (1991) (see Table I).
lo-’
lO-3 2 P E lo-2
357
ETHANE ON NEPTUNE
I--
IRIS Retrievals and Comparison of Results
,I
IO2
,,(,,,,,
102
g1’
d,,,,,, , ,’
103
104
,(,,,,,,
(,,,,,,,,
,,,,,
10s
10s
K (cm2
s-l)
(,,,
IO’
,,,,,,,,,
,
IO8
..,j
109
FIG. 3. The height profiles of the eddy diffusion coefficients used in the photochemical model (see Table II for a description of the K profiles). Horizontal solid lines mark of the region of sensitivity of the heterodyne and IRIS measurements (same meaning as in Fig. 2).
Limaye and Sromovsky (1991) results and the same as the rotation period determined by the Voyager planetary radio astronomy experiment (Warwick et al. 1989). This value iancreases our mole fraction retrieval by -6% as compared to results using 17.7 hr, determined from groundbased observations by Hammel (1989), which was used in the Kostiuk et al. (1990) analysis. As described in Kostiuk et al. (1990), the model-derived ethane mole fraction profiles are adjusted by a multiplicative factor until the calculated emission spectrum best fits the measurements. The heterodyne spectrum and an example of the fit to the data is given in Fig. 1. The best fit (solid line) to the measured ethane heterodyne spectrum (histogram) for the Case B ethane mole-fraction profile is shown. Each step in the histogram corresponds to 100 MHz, except the two extreme right steps, which correspond to 150 and 125 MHz, respectively. Radiance and single-sideband brightness temperature are plotted versus frequency relative to the ethane RR (45) line center position. All spectroscopic and observational input parameters are identical to those described in Kostiuk et al. (1990). For the nonconstant with height photochemically generated mole fraction profiles the emission derived from Case B provided the best match to the data. The profile had to be multiplied by only a factor of 1.2 to best fit the data. Given the thermal profile, the total uncertainty for the retrieval is 30%, indicating that Case B is a good
The band spectrum for Voyager IRIS and a representative synthetic fit to the data (Bezard et al. 1991) are given in Fig. 1. The ethane RR(4,5) line is contained within the vg band of ethane. The lower observed peak radiance, as compared to the heterodyne line measurement, is due to the lower spectral resolution of the IRIS instrument. This spectrum was fit using molecular band parameters consistent with line parameters used in the fits above (i.e., Daunt et al. 1984). The total uncertainty on the retrieval, excluding that due to the thermal profile, was -35%. As in the heterodyne case above, photochemically derived model ethane mole fraction profile B provided the best agreement with the data (multiplicative factor of -1). The results of fitting all the model cases to the IRIS and
lo-5
lo-4
lo-3 ‘;: m 5
lo-2
: 2 : L
10-l
a
IO0
10’
IO2 10-a
lo-’
10-e C,H,
Mixing
10-e Ratio
FIG. 4. The ethane mole fraction profiles used in this analysis. Cases A-D were generated by the photochemical model. The corresponding eddy diffusion coefficient profiles are shown in Fig. 3 and described in Table II. Case E is a constant with height profile above the saturation region at the Voyager IRIS derived value. Horizontal solid lines mark of the region of sensitivity of the heterodyne and IRIS measurements (same meaning as in Fig. 2).
358
KOSTIUK ET AL.
TABLE III Comparison of IRIS and Heterodyne Retrievals Voyager IRIS
Case
Factor”
A
2
Heterodyne
Contribution functionb
Factor’
Contribution functionb
0.02 0.16
0.01 3
0.12
0.75
0.55
0.03 B
1
0.20
0.02 1.2
0.17
0.91
0.64
0.10 C
2
0.75
0.07 2.4
0.36
2.8
1.7
0.03 D
1
0.24
0.02 1.3
0.17
2.0
1.0
0.10 E
1
0.75
0.08 1.3
2.9
0.36 1.7
a Model predicted ethane mole fractions must be multiplied by this factor to agree with observations. b Pressure in millibars where the contribution function peaks and pressures where the contribution function has decreased to half of the peak value.
heterodyne data are given in Table III. The table lists the factors that each model predicted mole fraction must be multiplied by to agree with observations, the pressure region probed, and the peak of the contribution functions for the respective spectral resolution. Cases B, D, and E provide the best fits for both the groundbased and Voyager observations, their multiplicative factors are within the uncertainty of each retrieval. A useful comparison between the two observations requires not only the comparison of the absolute mole fraction profiles retrieved and their agreement with those generated by the photochemical model, but also the comparison of the atmospheric regions probed by the measurements, thus testing the model and hopefully providing useful constraints. If the altitude regions probed by the two measurements are sufficiently different, then the retrieved mole fractions can determine the slope of the ethane distribution and thus help discriminate between candidate profiles used. Knowing the ethane profile would permit us to choose between K varying continuously with height, which produces B-type ethane profiles, or eddy mixing profiles which have discontinuities in them and produce C-type ethane distributions (Figs. 3,4). This can be done by comparing the contribution functions for the
respective measurements and retrievals. The two cases which provided the best model-observational agreement and which represented the two extreme classes of mole fraction profiles are cases B and E. Case B is representative of a nonconstant with height profile derived from the most recent photochemical-condensation model for Neptune. Case E is a simple constant with height profile above the saturation region profile discussed earlier. The contribution functions for the IRIS and heterodyne analyses using these two cases are presented in Figs. 5 and 6. The contribution functions have been calculated for the respective spectral resolution of each measurement, 4.3 cm-’ for IRIS and 0.0033 cm-’ for heterodyne. As expected, the contribution functions are narrower and their peaks and regions probed are at higher altitude (lower pressure) for the heterodyne case than for the IRIS case. For case E the contribution function peaks are separated by about a scale height. This is consistent with higher optical thickness near the line center emission, as measured at high resolution. At 4.3 cm-’ the mean emission is more optically thin, thereby originating deeper in the atmosphere. For a nonconstant mole fraction distribution B, the contribution functions for both measurements are weighted toward the distribution peak. Therefore, the peaks are closer together than for the E profile and are located at comparable pressure levels. Somewhat higher mole fractions are retrieved from the heterodyne results (multiplicative factors 20-30% higher).
1o-3 !
0.0
0.1
0.2 Contribution
0.3
0.4
0.5
0.6
Function
FIG. 5. Comparison of contribution functions for both heterodyne and IRIS retrievals using the Case B ethane profile (see Fig. 4). The horizontal lines mark where the contribution function has dropped to half of the peak value.
STRATOSPHERIC
359
ETHANE ON NEPTUNE
10-3.
1o-3
I
,
I
,
I
Bishop lo-*
(
1
(
I
,
I
,,
I
,
I
:
1
Warm Nominal
)I I t
,
L
1O-2 ‘;: ([I 5 P) 10-l 2 : & loo
10’ 0.0
0.1
0.2 Contribution
0.3
0.4
0.5
0.6
Function
FIG. 6. Comparison of contribution functions for both heterodyne and IRIS retrievals using the Case E ethane profile (see Fig. 4). The horizontal lines mark where the contribution function has dropped to half of the peak value.
It is tempting to interpret the results as information on the altitude distribution of ethane. These results are, however, within the uncertainties of the retrievals. Also, the ranges of pressure probed, as defined by the full width at half maxima of the contribution functions, are wide and show significant overlap (see Table III). These facts do not allow for meaningful conclusions regarding the true altitude distribution of ethane on Neptune.
Dependence on the Thermal Profile We then tested the sensitivity of these results to the assumed temperature profile. Mole fractions were retrieved for two sets of limiting input thermal profiles. In the first case we only varied the temperature in the upper stratosphere (pressures ~0.4 mbar). These thermal structures are shown in Fig. 7 and are taken from Bishop et al. (1992). The Bishop et al. nominal profile is quite close to that of BCzard et al. (1991), also shown in Fig. 7 (and Fig. 2). They are based on the same source data; RSS egress occultation data (Linda1 et al. 1990) and the stellar occultation data of Hubbard et al. (1987), both resealed to a He mole fraction of 0.19. Temperature profiles deduced from other stellar occultation observations (Hubbard et al. 1985, French et al. 1985) fall within the range from the “warm” to “cold” thermal profiles in Fig. 7. In the second retrieval set, the entire thermal profile was shifted + 10 K to create a warm and a cold profile. These were the same limits as in BCzard et al. (1991).
Solid
- BIzard
a ’
8 ’
;\ lo3
E ’ ’ 40 60
60
8 ’
100
120
Temperature
8 ’ 140
c ’
’
160
’ 160
’
4 200
(K)
FIG. 7. Thermal profiles used to test the sensitivity of the retrieved ethane mole fraction to the assumed model atmosphere. Solid line labeled Bezard is the same as in Fig. 2. Curves labeled Bishop, warm, nominal, and cold are from Bishop et al., 1992 and are described in the text. Horizontal dotted lines are the same as in Fig. 2.
We chose to use an ethane mole fraction constant with height above the condensation region (case E profile) to investigate the temperature dependence of the ethane retrieval using both the infrared heterodyne and IRIS data. We have just seen that this type of ethane distribution showed the greatest spread in the peaks of the contribution functions from the heterodyne to IRIS observations, and thus will most likely show any differences between them. The results are summarized in Table IV.
TABLE IV Retrieved Ethane Mole Fractions as a Function of Thermal Profile Thermal profile
IRIS
Heterodyne
A%
Bishop Warm Nominal Cold
1.0 x 10-C 1.3 x 10-G 1.6 x 1O-6
1.3 x 10-e 1.8 x 1O-6 2.4 x 1O-6
30 38 50
BCzard Warm Nominal Cold
1.0 x 10-e 1.5 x 10-6 4.0 x 10-e
1.0 x 10-6 1.9 x 10-e 4.6 x 1O-6
0 27 15
360
KOSTIUK ET AL.
Since the nominal Bishop et al. thermal profile is slightly warmer than the nominal Btzard et al. profile (Fig. 7), using it yielded a slightly smaller ethane mole fraction for both measurements. In both the heterodyne and IRIS cases the retrieved mole fractions increased with decreasing temperature. The heterodyne results, however, showed a greater sensitivity to changes in the upper thermal structure than the IRIS case. The greatest difference between the two retrievals occurred with the “cold” profile, with the heterodyne retrieved abundance increasing to -50% above that of IRIS. This is consistent with the greater sensitivity of the high resolution heterodyne measurements to changes in lower pressure regions as compared to IRIS. For each data set the changes in thermal profile resulted in no more than a ?30% change in the nominal ethane mole fraction, which is comparable to the retrieval uncertainties. This indicates that abundance changes are not a strong function of thermal changes in the upper stratosphere. The results using the Bezard et al. temperature profiles showed a large increase in the ethane mole fraction with decreasing temperature, but with a smaller difference between the heterodyne and IRIS retrievals (A < 26%). This is because in this case, the low altitude (higher pressure) regions where IRIS probes also experience temperature changes and this is reflected in the IRIS retrieval. In this case 10 K thermal changes resulted in significant modification of the retrieved ethane mole fractions (>twofold vs 30% uncertainty). The contribution functions for the IRIS and infrared heterodyne spectra for the warm (solid line) and cold (dashed line) Bishop et al. thermal profiles are shown in Fig. 8. In both measurements the warm contribution functions originate higher in the stratosphere. The pressures where the resultant contribution functions peak and fall to half their value are tabulated in Table V. For the heterodyne retrievals the contribution functions are narrower and show a greater separation between the warm and cold cases. The warm heterodyne contribution function has a narrow peak near 0.13 mbar and is more than one scale height above the cold function peak (at 0.62 mbar). The IRIS warm contribution function shows a small low pressure secondary peak which corresponds to the high altitude temperature increase, but the function is broader than the corresponding heterodyne one. Its mean altitude is at 0.3 mbar, less than one scale height above the cold contribution function peak (0.75 mbar). The heterodyne and IRIS cold cases peak in comparable altitude regions, however, the overall heterodyne contribution function covers lower pressure regions than does the IRIS function. As in the case of abundance profiles discussed above, the contribution functions are greatly overlapping and considering the signal-to-noise ratios on the data and uncertainties in retrieval, we cannot retrieve meaningful
-
10’
-
Cold
~,,,‘,‘,‘.,,‘,,,‘,,,‘,,,~ 0.0 0.1 0.2
0.3
Contribution FIG. thermal the hot cussion
0.4
0.5
0.6
Function
8. Contribution functions for the Bishop er al. cold and warm profiles derived for the IRIS and heterodyne cases. Note that contribution functions peak higher in the stratosphere. See disin the text and Table V.
altitude information from the comparison of the low and high resolution data sets. The heterodyne data can provide narrower and more separated altitude information if sufficient signal-to-noise ratios can be obtained at 25MHz resolution as compared to the present IOO-MHz resolution. CONCLUSIONS
Nearly coincident groundbased and spacecraft measurements of 12-pm ethane emission spectra from Neptune, during the period of Voyager’s Neptune encounter, provided a unique opportunity to determine the ethane abundance in its stratosphere and to test and constrain recent theoretical models for methane photochemistry on Neptune. Constant with height and several photochemical-model-derived mole fraction distributions were tested against the groundbased infrared heterodyne data and compared to similar analyses of the Voyager IRIS data by Bezard et al. (1991). Analyses of the line emission spectrum obtained by heterodyne spectroscopy (X/Ah 105) and the band emission spectrum from Voyager IRIS (A/Ah - 200) retrieved consistent ethane mole fraction distributions for Neptune when identical atmospheric models and spectroscopic parameters were used. For a given ethane mole fraction model, the results obtained from the two measurements agreed to within retrieval uncertainties of -30%. Neptune’s stratospheric tempera-
STRATOSPHERIC
TABLE V IRIS and Heterodyne Contribution Functions for Bishop Thermal Profiles Contribution IRIS
Thermal profile
function0 Heterodyne
0.037 Bishop warm
0.048 0.13
0.3 2.3
I.7
0.20 Bishop cold
0.75
0.14 0.62
2.6
throughout the stratospheric region probed by the instruments (-0.02 to 2 mbar), comparable but much higher sensitivity with temperature was observed in both cases. Contribution functions for warm thermal profiles peaked at higher altitudes as expected, with the heterodyne functions covering lower pressure regions. However, as in the case of ethane altitude distribution tests, contribution functions derived for the various thermal profiles cannot quite provide direct altitude information, due to their width and overlap, as well as to the uncertainties in the rertrievals. Higher quality heterodyne data may be able to separate the altitude regions probed.
1.8
a Pressure in millibars where the contribution function peaks and pressures where the contribution function has decreased to half of the peak value.
ture profile was updated from that used in the Kostiuk et (1990) analyses. The new profile (used by BCzard et al. 1991) is - 10 K warmer in the region of line formation. This is consistent with the conclusion of Kostiuk et al. (1990) that a >lO K warmer stratosphere was needed to best fit the theoretical models to the data. For a nonconstant with height ethane distribution, both data sets were in best agreement with the photochemical model which included an eddy mixing coefficient K = 5.0 x 10’ at the methane homopause, varying as Wo.59 (N = atmospheric number density). This “best model” (Case B) yielded the lowest eddy mixing in the lower stratosphere of Neptune; a result again consistent with predictions by Kostiuk et al. (1990). Both line and band emission spectra were also fit well with the constant with height above the saturation level model, Case E, within the uncertainties of the retrievals (-30%). The higher spectral resolution of the infrared heterodyne measurements probes higher in the stratosphere than that of the Voyager IRIS. For the nonuniform with height ethane distribution, Case B, the pressure region probed is 0.03-0.91 mbar for IRIS, 0.02-0.64 for heterodyne; for the uniform ethane distribution, Case E, 0.10-2.9 mbar for IRIS, 0.08-1.7 mbar for heterodyne. The width of the contribution functions, their degree of overlap, and the uncertainties in the retrievals do not permit the retrieval of direct information on ethane altitude distribution. Therefore, both constant and nonconstant with height profiles remain candidate distributions of stratospheric ethane on Neptune. The ethane retrievals were sensitive to the thermal profile used. Mole fractions retrieved from both heterodyne and IRIS data varied by up to a factor of 4 over the range of reasonable thermal profiles. The heterodyne case was more sensitive to changes at low pressures (co.1 mbar) than was the IRIS case. When thermal changes occurred al.
361
ETHANE ON NEPTUNE
REFERENCES B~ZARD, B., P. ROMANI, B. J. CONRATH, AND W. C. MAGUIRE 1991. Hydrocarbons in Neptune’s stratosphere from Voyager infrared observations. J. Geophys. Res. 96, 18,961-18,975. BISHOP, J., S. K. ATREYA, P. N. ROMANI, B. R. SANDEL, AND F. HERBERT 1992. Voyager 2 UVS solar occultations at Neptune: Constraints on the abundance of methane in the stratosphere. J. Geophys. Res. 97, 11,681-11,694. CONRATH,B., F. M. FLASAR, R. HANEL, V. KUNDE, W. MAGUIRE, J. PEARL, J. PIRRAGILA, R. SAMUELSON,P. GIERASCH, A. WEIR, B. B~ZARD, D. GAUTIER, D. CRUIKSHANK, L. HORN, R. SPRINGER, W. SHAFFER 1989. Infrared observations of the Neptunian system. Science 246, 1454-1459. CONRATH, B. J., F. M. FLASAR, AND P. J. GIERASCH 1991. Thermal structure and dynamics of Neptune’s atmosphere from Voyager measurements. J. Geophys. Res. 96, 18,931-18,939. DAUNT, S. J., A. K. ATAKAN, W. E. BLASS, G. W. HALSEY, D. E. JENNINGS,D. C. REUTER, J. SUSSKIND,AND J. W. BRAULT 1984. The 12 micron band of ethane: High-resolution laboratory analysis with candidate lines for infrared heterodyne searches. Astrophys. J. 280, 921-936; also Atakan, A. K., W. E. Blass, J. W. Brault, S. J. Daunt, G. W. Halsey, D. E. Jennings, D. C. Reuter, and J. Susskind 1983. The 12 Micron Band of Ethane: A Spectral Catalog from 765 cm-‘-900 cm-‘. NASA/GSFC Tech. Memorandum 85108. FRENCH, R. G., P. A. MELROY, R. L. BARON, E. W. DUNHAM, K. J. MEECH, D. J. MINK, J. L. ELLIOT, D. A. ALLEN, M. C. B. ASHLEY, K. C. FREEMAN, E. F. ERICKSON, J. GOGUEN, AND H. HAMMEL 1985. The 1983 June 15 occultation by Neptune. II. The oblateness of Neptune. Astron. .I. 90, 2624-2638. GILLETT, F. C., AND G. H. RIEKE 1977. 5-20 micron observations Uranus and Neptune. Astrophys. J. 218, L141.
of
HAMMEL, H. B. 1989. Neptune cloud structure at visible wavelengths. Science 244, 1165-l 167. HUBBARD,W. B., J. E. FRECKER,J.-A. GEHRELS, T. GEHRELS, D. M. HUNTEN, L. A. LEBOFSKY,B. A. SMITH, D. J. THOLEN, F. VILAS, B. ZELLNER, H. P. AVEY, K. MOTTRAM,T. MURPHY, B. VARNES, B. CARTER, A. NIELSEN, A. A. PAGE, H. H. Fu, H. H. WV, H. D. KENNEDY, M. D. WATERWORTH,AND H. J. REITSEMA1985. Results from observations of the 15 June 1983 occultation by the Neptune system. Astron. J. 90, 655-667. HUBBARD, W. B. P. D. NICHOLSON, E. LELLOUCH, B. SICARDY, A. BRAHIC, F. VILAS, P. BOUCHET, R. A. MCLAREN, R. MILLIS, L. H. WASSERMAN,J. H. ELIAS, K. MATTHEWS, J. D. MCGILL, AND C. PERRIER 1987. Oblateness, radius, and mean stratospheric temperature of Neptune from the 1985 August 20 occultation. Icarus 72, 635-646.
362
KOSTIUK
KOSTIUK, T., F. ESPENAK, P. ROMANI, D. ZIPOY, AND J. GOLDSTEIN 1990. Ethane abundance on Neptune. Icarus 88, 87-96. LIMAYE, S. J., AND L. A. SROMOVSKY1991. Winds ofNeptune: Voyager observations of cloud motions. J. Geophys. Res. 96, l&941-18,960. LINDAL, G. F., J. R. LYONS, D. N. SWEETNAM, V. R. ESHLEMAN, D. P. HINSON, AND G. L. TYLER 1990. The atmosphere of Neptune: Results of radio occultation measurements with Voyager 2 spacecraft. Geophys. Res. Lett. 17, 1733-1736. MACY, W., JR. 1980. Mixing ratios of methane, ethane, and acetylene in Neptune’s stratosphere. Icarus 41, 153. MACY, W., JR., AND W. SINTON 1977. Detection of methane and ethane emission on Neptune but not Uranus. Astrophys. J. 218, L79. ORTON, G. S., D. K. AITKEN, C. SMITH, P. F. ROCHE, J. CALDWELL, AND R. SNYDER 1987. The spectra of Uranus and Neptune at 8-14 and 17-23 pm. Icarus
70, l-12.
ORTON, G. S., K. H. BAINES, J. CALDWELL, P. N. ROMANI, A. T.
ET AL. TOKUNAGA, AND R. A. WEST 1990. Calibration of the 7- to 14-pm brightness spectra of Uranus and Neptune. Icarus 85, 257-265. PARKINSON,C. D., J. C. MCCONNELL, B. R. SANDEL, R. V. YELLE, AND A. L. BROADFOOT1990. He 584 8, dayglow at Neptune. Geophys. Res. Lett. 17, 1709-1712. ROMANI, P. N., AND S. K. ATREYA 1989. Stratospheric aerosols from CH4 photochemistry on Neptune. Geophys. Res. Lett. 16, 941. TOKUNAGA,A. T., G. S. ORTON, AND J. CALDWELL 1983. Newobservational constraints on the temperature inversions of Uranus and Neptune. Icarus 53, 141-146. WARWICK, J. W., D. R. EVANS, G. R. PELTZER, R. G. PELTZER, J. H. ROMIG, C. B. SAWYER, A. C. RIDDLE, A. E. SCHWEITZER, M. D. DESCH, M. L. KAISER, W. M. FARRELL, T. D. CARR, I. DE PATER, D. H. STAELIN, S. GULKIS, R. L. POYNTER, A. BOISCHOT, F. GENOVA, Y. LEBLANC, A. LECACHEUX, B. M. PEDERSEN, P. ZARKA 1989. Voyager planetary radio astronomy at Neptune. Science 246, 1498-1501.