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NASA Technical Memorandum 86851 

NASA-TM-868S1 

/qgiPf!)008B() 1 

Airborne Astronomy Program 

Medium Altitude Missions Branch 

. Preprint Series 042 

N ? . 

bservations of the J = 10 

Manifold of the Pure 

Rotational Band of 

Phosphine on Saturn 

M.R. Haas, E.F. Erickson, 

O. Goorvitch, 

D.O. McKibbin, 

and D.M. Rank 

January 1986 ..-rIIfIIlIIIIJ----"!JIf 

NI\SI\ 

National Aeronautics and 

Space Administration 

1111111111111 1111 1111111111111111111111111111 

NF00086 

", i, 
I\IASA Technical Memorandum 86851 . Airborne Astronomy Program 

Medium Altitude Missions Branch 

Preprint Series 042 

Observations of the J = 10 Manifold 

of the Pure Rotational Band of 

Phosphine on Saturn 

M. R. Haas, Mycol, Incorporated, Sunnyvale, California 

E. F. Erickson, 

D. Goorvitch, 

D. D. McKibbin, Ames Research Center, Moffett Field, California 

D. M. Rank, University of California, Santa Cruz, California 

January 1986 

NI\S/\ 

National Aeronautics and 

Space Administration 

Ames Research Center 

Moffett Field. California 94035 
OBSERVATIONS OF THE J = 10 MANIFOLD OF THE PURE ROTATIONAL BAND - OF PHOSPHINE ON SATURN 

Manuscr' ipt: 22 

Figures: 5 

Tables: 1 

M. R. Haas 

Mycol, Incorporated 

1067 Mango Avenue 

Sunnyvale, CA 94087 

and 

E. F. Erickson, D. Goorvitch, D. D. McKibbin 

MS 245-6, Space Science Division 

NASA-Ames Research Center 

Moffett Field, CA 94035 

and 

D. M. Rank 

Lick Observatory, University of California 

Santa Cruz, CA 95064 

Received: 
ABSTRACT 

Saturn was observed in the vicinity of the J = 10 manifold of the pure 

rotational band of phosphine on 1984 July 10 and 12 from NASA's Kuiper 

Airborne Observatory with the facility far-infrared cooled grating 

spectrometer. On each night observations of the full disk plus rings were 

made at 4 to 6 discrete wavelengths which selectively sampled the manifold 

and the adjacent continuum. The previously reported detection of this 

manifold is confirmed. After subtraction of the flux due to the rings, the 

data are compared with disk-averaged models of Saturn. It is found that PH 3 

must be strongly depleted above the thermal inversion (-70 mbar). The best 

fitting models consistent with other observational constraints indicate that 

PH 3 is significantly depleted at even deeper atmospheric levels (~500 mbar) , 

implying an eddy diffusion coefficient for Saturn of -104 cm 2 sec-1• 

-3-
I. INTRODUCTION 

The presence of phosphine (PHs) in Saturn's troposphere has been well 

established by observations at near and middle infrared wavelengths (Larson 

~ al., 1980.and references therein; Hanel ~ al., 1981). This was somewhat 

surprising because thermochemical equilibrium calculations indicated that 

for temperatures ~2000 K phosphine would react with H2 0 (Lewis, 1969; 

Barshay and Lewis, 1978). However. its presence may be explainable by some 

combination of modest convection, an upper atmospheric recycling mechanism, 

and low abundances of H2 0 and NHs (Larson ~ al., 1980). The observed PHs 

abundance implies that the P/H ratio is approximately 3 times the solar 

value (Encrenaz and Combes, 1980 and references therein; Courtin ~ al .• 

1984), which may have interesting implications for cosmogony (Gautier and 

Owen, 1983). 

Moreover, the vertical distribution of PHs is important to our 

understanding of its chemistry and the vertical dynamics on Saturn (Prinn 

and Lewis, 1975; Strobel, 1978; Larson ~ al., 1980). From IUE ultraviolet 

spectra Winkelstein et al. (1983) concluded that there is little or no PHs 

above 25 mbar. but from Voyager IRIS spectra Courtin ~ al. (1984) inferred 

a uniform mixing ratio of 3.0 ± 0.5 x 10-6 (5X solar) up to the 3-5 mbar 

level. Given the PHs abundance inferred at shorter wavelengths. the far 

infrared manifolds of the pure rotational band of PHs are predicted to be 

quite strong (Encrenaz and Combes, 1977). This band is expected to reach 

optical depth unity near the thermal inversion (-70 mbar) , thereby probing 

higher levels of the atmosphere than the near and middle infrared 

vibrational transitions (Encrenaz and Combes, 1980). Observations of this 

-4-
band will therefore place new constraints on the PHs distribution in the 

vicinity of the tropopause. 

Haas et al. (1984) previously detected the J = 10 manifold at 103 j.Illl 

1 and Viscuso (1984) may have detected the J = 7 manifold at 141 ~m. In this 

paper we report new observations of the J = 10 manifold with higher 

signal-to-noise and improved calibration. We also describe our ring 

subtraction procedure and present a number of disk-averaged theoretical 

models. The implications of the best fitting models are discussed. 

II. OBSERVATIONS AND DATA ANALYSIS 

Saturn was observed on the nights of 1984 July 10 and 12 with the 

facility far-infrared cooled grating spectrometer (CGS) on the 91 cm 

telescope of the Kuiper Airborne Observatory from an altitude of 12.0 km. 

The FWHM beam size was 40" and the chopper throw was -4'. The CGS was 

operated with six discrete Ge:Ga detectors in the dispersed focal plane with 

a wavelength separation of 0.0314 ~m at 103 j.Illl. The detectors oversampled 

the spectrum in wavelength by a factor of about 1 .8. Additional details 

regarding the CGS design, performance, and typical observing procedures are 

described in Erickson et ale (1984a,bj 1985). 

The resolution of the CGS is high enough that an entire PHs manifold 

(-3 ~m wide) could not be scanned in the available observing time (-15 min). 

Hence observations were made at 4 to 6 discrete wavelengths (grating 

positions) which were chosen to minimize the effects of telluric absorption 

(see Figure 1a) and optimize the wavelength coverage of the important 

spectral features. Four pairs of right-left beam integrations totaling 80 

Ip . .. r1.vate commun1.cat1.on 

-5-
sec were taken at each wavelength. The absolute flux calibration and the 

relative detector response were obtained by observing Mars on the same 

flights. The final spectra are simply the ratio of the signals measured on 

Saturn to those measured on Mars and multiplied by the Mars flux 

corresponding to a brightness temperature of 227 K at all wavelengths as 

predicted by the model of Simpson et ale (1981). At this epoch the diameter 

of the Martian disk was 13.18", the Saturnian disk was 17.39" x 15.71", and 

the inclination of the rings was 18.99° (Vohden and Boksenberg, 1984). 

Following Matthews and Erickson (1977), we calculated the total area of the 

Saturn ring-disk system to be 405.0 square arcsec. The Saturn flux was 

increased by 6% to account for the effects of diffraction in the telescope 

and geometrical losses due to the extended nature of the rings relative to 

the beam. No second order water vapor correction was applied because the 

two planets were observed at a similar elevation and aircraft heading and 

there are no strong telluric features at the wavelengths in question (Figure 

1 a) • 

The brightness temperatures measured for the Saturnian ring-disk system 

are plotted in Figure 1b and listed in Table 1. A decrease in brightness 

temperature of -5 K at the expected position of the J = 10 manifold of PH 3 

is clearly evident in both data sets. No other molecules are expected to 

produce absorption or emission features at these wavelengths; the J = 4 

manifold of NH3 (cf Figure 2) has been carefully avoided. To improve the 

signal-to-noise, the six detectors have been averaged at each grating 

position. Their total bandpass is 0.19 ~ and the uncertainty in the 

wavelength is -0.015~. The quoted error bars are statistical errors only; 

they represent one-standard-deviation-of-the-mean for the six detectors. 

-6-
Systematic uncertainties will increase these errors somewhat, but are 

difficult to estimate. There is a 15% difference in the absolute 

calibration between the two nights' data. However, the good night-to-night 

agreement in the shape of the overall spectrum suggests that the relative 

calibration of the data pOints on a single night is significantly better. 

III. THE RING SUBTRACTION PROCEDURE 

The contrast in the PH, line is reduced by the continuum emission of 

the rings. In order to directly compare our measurements with models of the 

disk of Saturn, we must subtract the ring contribution. To this end, we 

have constructed a model of the ring-disk system for the appropriate epoch. 

Following Haas et ale (1982; henceforth HEMGC) , the total flux from the 

Saturn system is 

(1) 

where Bv is the Planck function, Ot is the total solid angle of the 

ring-disk system, OVD is the solid angle of the visible disk, and IBI is the 

ring tilt angle with respect to the Earth. The average brightness 

temperatures T, To' and TR are those of the system, the disk, and the rings, 

respectively. The i-th ring surface has a normal optical depth 'i' a 

visible solid angle Oi' and a visible solid angle overlapping the disk wi. 

The rings are divided into five separate surfaces as described in HEMGC. 

The optical depths 'i vary as va. TR is computed from a 

thermometric temperature To and an emissivity dependent on the 'its (see 

HEMGC equation 5 and Figure 6). A fit to the data of HEMGC gives a = 1.5 

-7-
and To = 88.7 K for IBI = 21.4°. If the 100 ~ brightness temperature 

variation with IBI has the same form as at 20 ~ (Nolt et al., 1980), then 

To should be decreased by ~2 K. This effect has been ignored since it 

results in a negligible change in the depth of the PH a line. 

This model for the rings was combined with our standard disk model 

(section IV) using equation (1). Since we are primarily concerned with the 

shape of the spectrum and not with its absolute calibration, we then 

renormalized the two nights' data to this model. This renormalization 

removes the difference in absolute calibration between the two data sets, 

forces the relative contributions of the disk and rings to be the same as in 

HEMGC (allowing for the changes in geometry with IBI), and ensures that the 

disk spectrum obtained by subtracting the ring contribution from our data 

will have the same normalization as our standard disk model. The 

renormalization factor is 0.84 for July 10 and 0.98 for July 12. 

The ring contribution was then subtracted from the measured 

(renormalized) spectrum using equation (1) and the ring model described 

above for TH• The inferred disk spectrum for each night is shown in Figure 

2 (circles) along with our standard disk model. The error bars again 

reflect statistical errors only. The measured contrast in the line is just 

over half that of the model. Increasing the contribution of the rings to 

the total system flux will increase the contrast in the deduced disk 

spectrum. For example, if we repeat the subtraction process assuming the 

optical depth of the rings is independent of frequency (a = 0), then TH at 

100 ~ increases from 63 K to 77 K (cf HEMGC's Figure 6) and the contrast in 

the line is increased by several kelvins (Figure 2; triangles). However, 

-8-
there is strong evidence that the brightness temperature of the rings turns 

over beyond 50 ~ (HEMGC; Daniel et al., 1982; Melnick ~al., 1983). 

Therefore, the discrepancy between the measured line profile and our 

standard disk model is probably not a result of uncertainties in the ring 

subtraction procedure. In the following sections we only consider the disk 

spectrum derived assuming that TR decreases as in HEMGC (our case a = 1.5). 

IV. SATURN DISK MODELS 

Disk-averaged models of Saturn were generated using the computer code 

described in HEMGC and Goorvitch et al. (1979). The PHs mixing ratio was 

parameterized as x = xo(P/Po)S for P < Po and as x = Xo for P ~ Po' where P 

is the total pressure and a is the ratio of the dynamical scale height (hp • 

RT/~g) to the scale height for decreasing the PHs mixing ratio hx (cf 

Encrenaz and Combes, 1977; Strobel, 1978; Tokunaga et al., 1980). Our 

standard model uses Po = 100 mbar, a PHs scale height hx = 5 km, 

Xo = 1.2E-06 = 2x0 (twice solar), a helium-to-hydrogen mixing ratio of 

0.13/0.87, an ammonia mixing ratio of 1.5x10-4, and the pressure-temperature 

profile of Tokunaga and Cess (1977), which is very similar to that obtained 

by Voyager (Hanel ~ al., 1983). 

The wavelengths for the pure rotational band of PHs were calculated 

from the formula and constants given by Maki et al., (1973). Because of the 

lack of experimental data, the strengths were calculated using the formulas 

for NHa, which is a spectroscopically similar molecule. We assumed a 

permanent dipole moment of 0.574 Debye (Poynter and Pickett, 1980), adopted 

the pressure broadening data of Pickett et al. (1981), and used a Lorentzian 

-9-
line profile (cf Encrenaz et al., 1971). A ±10% variation in the PH 3 line 

strengths results in a brightness temperature change of less than ±0.3 K. 

As shown in Figure 2, our standard model shows considerably more 

contrast in the PH 3 line than is observed. We have investigated the 

dependence of this contrast on the parameters of the PH 3 distribution (x o, 

Po, and hx)' the [He]/[H] mixing ratio, and the pressure-temperature 

profile. The models are relatively insensitive to the helium-to-hydrogen 

mixing ratio. Varying [He]/[H] from 0.20/0.80 to 0.06/0.94 changes the 

brightness temperature in the continuum by ~0.4 K. Since the core of the 

line is formed near the inversion (- 70 mbar), an inversion temperature of 

-93 K is required to reduce the model contrast to that which is observed. 

Such variations in the pressure-temperature profile are inconsistent with 

past observations, including the Voyager IRIS experiment (Hanel et al., 

1983) and the radio occultation mean (Tyler et al., 1982). 

The brightness temperature in the core of the line roughly corresponds 

to the atmospheric level where the optical depth reaches unity. For a given 

pressure-temperature profile, the most straightforward way to raise the 

brightness temperature in the core is to either increase or decrease the 

opacity such that T = 1 lies above or below the thermal inversion. Figure '3 

shows the effect of decreasing Po' the pressure above which the PH a mixing 

ratio falls exponentially, from 100 to 10 mbar. Such models place 

significant amounts of PH a in the stratosphere. producing a strong emission 

line core. Note that the data points at 102.41 and 102.92 ~ lie near the 

minimum brightness temperature in the manifold, whereas the data point at 

102.78 ~ lies near the maximum of the emission core. These models are 

-10-
inconsistent with the observations, which show a similar brightness 

temperature at all three wavelengths. The strength of the emission core is 

reduced somewhat if we use the pressure-temperature profile deduced from 

Voyager 2 radio occultation measurements (Tyler et al., 1982), because it 

has significantly cooler temperatures above the inversion (cf Orton, 1983). 

However, such models still produce a substantial emission core which is 

inconsistent with the present observations. 

Varying the scale height hx gives results similar to those in Figure 3. 

That is. increasing hx to 10 km puts more PH 3 above the tropopause and 

produces an emission line core similar to the model with Po = 50 mbar. From 

the present observations we therefore conclude that PH 3 must be strongly 

depleted above the inversion. Tokunaga et ale (1980) and Winkelstein et 

ale (1983) arrive at similar conclusions based on observations in the near 

infrared and the ultraviolet, respectively. However, Courtin et ale (1984) 

find evidence in the Voyager IRIS spectra at 10 ~ for a uniform mixing 

ratio of 5X solar to an altitude of 3 to 5 mbar. Prinn et ale (1984) 

suggest that this discrepancy may result from the nonuniqueness of the 

thermal inversion process or to latitudinal or seasonal changes in the PH 3 

vertical profile. Theoretical models predict that phosphine should be 

strongly depleted in the stratosphere because of photolysis by ultraviolet 

radiation (Prinn and Lewis. 1975; Strobel, 1977; 1978). Similar depletions 

have been observed on Jupiter (e.g., Tokunaga et al., 1979; Kunde et al., 

1982) • 

An acceptable fit to the present observations can be obtained if a 

stratospheric "cloud" or haze is included in our standard model with an 

-11-
optical depth of 0.25 at 100~. In such a model the PH 3 manifold is still 

formed above the inversion, but without a significant emission core. 

However, it may be difficult to make particles large enough to absorb at 

100 ~ and still small enough to remain buoyant at pressures ~100 mbar 

(Orton, 1983). Moreover, there is little independent evidence for the 

existence of large stratospheric particles in other spectral bands (Tomasko 

et al. 1984). ---

As discussed above, the brightness temperature in the PH 3 manifold can 

also be raised by forming the line below the inversion. In Figure 4 we show 

the results of varying xo' the uniform mixing ratio assumed for P ~ Po. The 

manifold becomes increasingly narrow as the mixing ratio is decreased. 

However, the brightness temperature in the core remains nearly constant 

until the mixing ratio is sufficiently small (x o ::i 0.05x ) to permit o 

formation of the core (. - 1) below the inversion at higher temperatures. 

Such models are able to fit the observed core of the manifold, but require 

Xo « x0 ' which is inconsistent with the near and middle infrared 

observations which find Xo - 2 - 5x0 in the lower troposphere (Tokunaga et 

al., 1980; Larson et al., 1980; Courtin ~ al., 1984). 

Since the near and middle infrared vibrational transitions probe deeper 

into the troposphere (cf Encrenaz and Combes, 1980), a consistent model can 

be obtained by keeping Xo ~ 2x and increasing Po' Such models maintain a 

o 

large abundance of PH 3 in the lower troposphere, but show substantial 

depletions in the viCinity of the tropopause. Figure 5 shows the effect of 

increasing Po from 100 mbar to 1 bar. The model with Po = 500 mbar provides 

an excellent fit to our measured profile. 

-12-
Moreover, the inferred phosphine distribution is remarkably similar to 

that preferred by Kaye and Strobel (1984), who consider the detailed 

photochemistry of Saturn's atmosphere and derive PH 3 distributions for 

different values of the eddy diffusion coefficient (K). Our best fitting 

distribution (Po = 500 mbar, hx = 5 km, and Xo = 2X0 ) closely corresponds to 

the one they calculate for K = 104 cm2 sec-1 in the upper troposphere. 

Similar values of K are inferred from observations of ortho and para 

hydrogen on the Jovian planets (Massie and Hunten, 1982). 

V. CONCLUSIONS 

The main conclusions of this paper are: 

(1) The detection of the far-infrared J = 10 pure rotational manifold of 

phosphine (PH 3 ) on Saturn has been confirmed, 

(2) The lack of a significant emission core in this manifold implies that 

there is little or no PH 3 above the inversion, and 

(3) The most straightforwar'd explanation for the relatively low brightness 

temperature contrast in the J = 10 manifold is a significant depletion 

of PH 3 in the upper troposphere (Po ~ 500 mbar). This suggests that 

the eddy diffusion coefficient in Saturn's troposphere is -104 cm 2 sec-1• 

Improved sampling of the individual manifolds, as well as observations of 

several additional manifolds, would be useful in further elucidating the 

details of the PH 3 distribution in the vicinity of the tropopause. 

-13-
ACKNOWLEDGEMENTS 

We particularly wish to thank E. Bekstrom for his technical support. 

P. B. Duffy for his work on the flight series and on the detector 

development. and J. P. Simpson for her assistance with the observations. We 

are grateful to P. Viscuso for discussing his observations prior to 

publication and to R. Courtin for a helpful discussion regarding the 

phosphine pressure broadening coefficients. As always. it is a pleasure to 

acknowledge the staff of the Kuiper Airborne Observatory for their 

continuing support. Partial support for this research was provided by the 

NASA Headquarters Office of Astronomy and Astrophysics. 

-14 -
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-16-
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-20-
Table 1 

Brightness Temperatures for Saturn 

Wavelength Brightness 

Date ( jlIII) Temperature (K) 

------------ ---------- --------------- 1984 July 10 97.84 91.2 ± 0.3 

102.78 85.5 ± 0.3 

102.92 86.4 ± 0.3 

104.83 91. 6 ± 0.7 

1984 July 12 97.84 84.9 ± 0.6 

102.41 81. 6 ± 0.6 

102.78 80.3 ± 0.5 

102.92 81.0 ± 0.3 

103.52 83.6 ± 0.3 

104.83 84.4 ± 0.3 

-21-
FIGURE CAPTIONS 

Figure 1. (a) The atmospheric transmission at 12.5 km for 10 

precipitable microns of water (typical) at a resolution of A/~A = 5000. 

(b) The measured brightness temperatures of the ring-disk system of Saturn 

(circles are for July 10; triangles are for July 12). The error bars 

represent statistical errors only. 

Figure 2. The circles represent the data of Figure 1 with the 

contribution of the rings removed assuming their optical depth falls off as 

v1•5 (cf HEMGC). The triangles are the same data with the contribution of 

the rings removed assuming their optical depth is independent of frequency 

(a = 0). The solid curve shows our standard disk-averaged model of Saturn 

at the same resolution. 

Figure 3. The data are from Figure 2 (circles are July 10; triangles 

are July 12) with the ring component removed assuming a 1.5. The curves 

are disk models with different values of Po, the pressure level below which 

the PH 3 mixing ratio is constant rather than exponential. As Po decreases. 

the amount of stratospheric PH 3 increases. The model with Po 100 mbar is 

the standard model described in the text. Recall that the data have been 

normalized to force the two continuum points to fit this model. 

Figure 4. The data are the same as in Figure 3. The models have 

different values of xo' the (uniform) mixing ratio for P S Po = 100 mbar, 

ranging from 0.04 to 4 times solar. 

Figure 5. The data are the same as in Figure 3. The models have 

different values of po. all at or below the tropopause. 

-22-
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<{ 

:s: 
1. Report No. 1 2. Government Accession No. 3. Recipient's Catalog No. 

NASA TM-86851 

4. Title and Subtitle 5. Report Date 

OBSERVATIONS OF THE J = 10 MANIFOLD OF THE PURE January 1986 

ROTATIONAL BAND OF PHOSPHINE ON SATURN 6. Performing Organization Code 

7. Author(s) 8. Performing Organization Report No. 

M. R. Haas*, E. F. Ericksont , D. Goorvitcht, 85434 

D. D. KcKibbint , and D. M. Rank"rt 10. Work Unit No. 

9. Performing Organization Name and Address 

*Mycol, Inc. , 1067 Mango Ave., Sunnyvale, CA 94087 11. Contract or Grant No. tAmes Research Center, Moffett Field, CA 94035 

ttLick Observatory, University of California, Santa 

Cruz CA 95064 13. Type of Report and Period Covered 

12. Sponsoring Agency Name and Address Technical Memorandum 

National Aeronautics and Space Administration 

Washington, 14. Sponsoring Agency Code DC 20546 

352-02-03 

15. Supplementary Notes 

Preprint Series 1142. Supported by NASA grants. 

Point of Contact: L. C. Haughney, Ames Research Center, MS 211-12. Moffett 

Field, CA 94035, (415) 694-5339 or FTS 464-5339 

16. Abstract 

Saturn was observed in the vicinity of the J = 10 manifold of the pure rotational band of phosphine on 1984 July 10 and 12 from NASA's Kuiper Airborne Observatory with the facility far-infrared cooled grating spectrometer. On each night observations of the full disk plus rings were made at 4 to 6 

discrete wavelengths which selectively sampled the manifold and the adjacent continuum. The previously reported detection of this manifold is confirmed. 

After subtraction of the flux due to the rings, the data are compared with 

disk-averaged models of Saturn. It is found that PH 3 must be strongly depleted above the thermal inversion (~70 mbar). The best fitting models consistent ~ith other observational constraints indicate that PH 3 is significantly Idepleted at even deeper atmospheric levels (~500 mbar) , implying an eddy diffusion coefficient for Saturn of ~104 cm2 sec- 1 • 

17. Key Words (Suggested by Author(s)) 18. Distribution Statement 

Saturn Unlimited 

Troposphere 

Planets: phosphine 

Infrared: spectroscopy Subject category - 89 

19. Security Oassif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price" 

Unclassified Unclassified 29 A03 

"For sale by the National Technical Information Service, Springfield, Virginia 22161 
End of Document