High-Phase-Angle Observations of Uranus at 2650 Å: Haze Structure and Particle Properties

ICARUS

127, 508–522 (1997) IS975706

ARTICLE NO.

High-Phase-Angle Observations of Uranus at 2650 A˚: Haze Structure and Particle Properties Wayne R. Pryor Laboratory for Atmospheric and Space Physics, University of Colorado, 1234 Innovation Drive, Boulder, Colorado 80303 E-mail: [email protected]

Robert A. West Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109

and Karen E. Simmons Laboratory for Atmospheric and Space Physics, University of Colorado, 1234 Innovation Drive, Boulder, Colorado 80303 Received August 5, 1996; revised December 23, 1996

Spatially resolved photometric observations of Uranus at 2650 A˚ obtained with the Voyager 2 Photopolarimeter Subsystem (PPS) are presented and analyzed with model atmospheres. A vertically homogenous atmospheric model with Rayleigh scattering and absorption fits Uranus well at 2650 A˚ at phase angles from 168 to 1578; no strong forward scattering typical of abundant large particles is observed. A microphysical model developed for Uranus to fit Voyager imaging system data (K. Rages, J. B. Pollack, M. G. Tomasko, and L. R. Doose, 1991, Icarus 89, 359–376) predicts the vertical distribution and properties of stratospheric haze aerosols at 22.58 and 658S. This ˚ PPS data at low latimicrophysical model also fits the 2650-A tudes. Increasing the aerosol sizes in the Rages et al. size distribution by a factor of 1.2 creates an unobserved forward scattering peak and can be excluded. The low-latitude upper stratospheric aerosols on Uranus are smaller (,0.1 mm radius) and/or much less abundant than the 0.2 to 0.25-mm stratospheric aerosol population found by PPS on Neptune; however, to fit the Uranus 2650-A˚ data at all latitudes requires darker and/or more abundant (1.5–3 times more) aerosols near the sunlit pole.  1997 Academic Press

INTRODUCTION

The January 1986 Voyager 2 flyby of Uranus enhanced our knowledge of atmospheric methane and its photochemical products. The interior of Uranus has a methane gas mixing ratio near 1.6% (Baines et al. 1995), but methane condenses near the 1.2-bar pressure level (Lindal et al. 1987). Because of this condensation, and unusually weak vertical mixing, methane and its photochemical products 508 0019-1035/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.

have small stratospheric abundances (Herbert et al. 1987). Photochemical processes transform stratospheric methane (CH4) into higher hydrocarbons such as diacetylene (C4H2), ethane (C2H6), and acetylene (C2H2), that may condense as haze aerosols (Atreya et al. 1991). There is evidence for stratospheric haze in the Voyager Photopolarimeter Subsystem (PPS) data (Lane et al. 1986, Pryor and Hord 1991), in Voyager Imaging Science Subsystem (ISS) data (Smith et al. 1986; Pollack et al. 1987; Rages et al. 1991), and in International Ultraviolet Explorer (IUE) data (Cochran et al. 1990). Voyager 2 PPS Uranus photometry results were presented in three papers. First, Lane et al. (1986) presented two low-phase-angle observations: a high-spatial-resolution latitudinal swatch (PPLATSCAN, see Table I) and a lower-resolution map of the disk (PPNPMAP). Lane et al. ˚ ) absorption and less found Uranus has less UV (2650 A ˚ UV contrast than was seen by PPS at Jupiter at 2400 A (Hord et al. 1979, West et al. 1981) and at Saturn at 2650 ˚ (Lane et al. 1982, West et al. 1983). The Uranus photomeA try data were modeled with conservative Rayleigh scattering by H2 and He gas and nonconservative Rayleigh scattering by aerosols. Modest latitudinal variations in aerosol distribution were indicated. A second paper (West et al. 1987) analyzed PPS observations of a stellar occultation of Uranus at 68.98N (dark side) that were sensitive to the region near 1 mbar. The occultation data were consistent either with no aerosols in this region or with very thin aerosol layers (with an extinction coefficient kext # p1024 km21) above 1 mbar. A third paper (Pryor and Hord 1991) discussed auroral haze formation on the outer planets, and

509

VOYAGER PPS URANUS OBSERVATIONS

found no evidence for this process in the PPS Uranus data. This paper presents a more thorough study of the PPS ˚ observations, including, for the first time, Uranus 2650-A high-phase-angle observations sensitive to particle size.

instrument response rl is averaged over the PPS UV bandpass:

O p fF r p5 O fF r . l

l

INSTRUMENTATION

The Voyager 2 PPS has been described in Lillie et al. (1977). The instrument measures both light intensity and linear polarization. An f/1.4, Dahl-Kirkham-type Cassegrain telescope with a 6-in. primary mirror collects light, which then passes through a four-position aperture wheel, an eight-position analyzer (polarizer) wheel, and an eightposition filter wheel. The filter wheel holds thin-film interference filters that pass a selected band of wavelengths. Transmitted light is detected by an EMR 510-E-06 photomultiplier tube with a trialkali (S-20) photocathode. The data discussed in this work were obtained with the smallest PPS aperture, which has a nominal ahA8 field of view. When this field of view is convolved with the pointspread function of the optics, the resulting instrument response can be fit as a cone of full width at half-maximum of 0.1288. Two filters were used for the observations described here. One of these, PPS UV filter No. 2 (Lillie et al. 1977), probes stratospheric and upper tropospheric altitudes. When the spectral response of the instrument is convolved with the solar flux (Mount and Rottman 1983), the effective ˚ . The wavelength of this UV filter is found to be 2650 A ˚ other filter, with an effective wavelength of 7500 A, simultaneously samples the top several bars of the Uranus atmo˚ full width at half-maxisphere. Both filters have a 300-A ˚ results. mum bandpass. This paper focuses on the 2650-A The ‘‘lookup table’’ observing sequence used at Uranus involved cycling between two orthogonal polarization ana˚ ) and lyzers while also cycling between infrared (7500 A ˚ UV (2650 A) filters. The filter and analyzer wheels moved every 24 sec, with the filter transition offset from the analyzer transition by 12 sec for Uranus. This PPS cycle repeated every 48 sec, which is also the minimum time between images for Voyager ISS. PPS count rates were read out every 0.6 sec, after 0.4 sec of integration and 0.2 sec of dead time. Intensity and linear polarization values at Uranus were obtained from the count rates in the same way as at Neptune (Pryor et al. 1992). CALIBRATION

The PPS UV calibration for Uranus was described by Pryor and Hord (1991). The PPS data were scaled to match model brightnesses in Uranus models with geometric albedos, p, within 20% of p 5 0.53, the result found from IUE spectra (Wagener et al. 1986) when the product of geometric albedo at wavelength l, pl, solar flux fFl, and

l

l l

l l

The PPS data are calibrated with geometric albedos instead of a stellar calibration because stars do not fill the optics in the same way as a spatially resolved source such as Uranus. Stellar calibrations were used to find the off-axis response of the PPS instrument (used to derive the fieldof-view correction factor described below) and to monitor its photometric stability. The observing geometry was obtained from the original SEDR (Supplemental Experiment Data Record) products and from image-navigated ‘‘C-Smithed’’ products (Wang et al., 1988). The C-Smithed product was used for Uranus observations at low and high phase angles where the Voyager ISS image quality was excellent. Near closest approach, the image-derived geometry was less reliable because the available images generally lacked a well-defined limb. Consequently, for times near closest approach the original SEDR geometry was used. A third estimate of the PPS viewing geometry at Uranus was provided by the Voyager imaging team (J. Pollack and K. Rages, private communication, 1987). They also limbfit a series of ISS pictures and applied the boresight offset of the PPS field of view to derive PPS pointing information. This provided a check on the C-Smithing results. The C-Smithing results and the Pollack and Rages results typically agreed on the PPS pointing direction to within 0.058 for the low- and high-phase-angle observations where the C-Smithing corrected geometry was adopted. The PPS field of view sampled a range of different viewing geometries, particularly near the limb. To account for this, we assumed that over the PPS field of view the planetary intensity, I/F (defined as the specific intensity, I, multiplied by f and divided by the solar flux, fF ), varied as e0 , the cosine of the solar zenith angle. This approximation, which is valid at low phase angles in a deep Rayleigh scattering atmosphere, was used to calculate a 21 3 21 grid of predicted intensities on Uranus. The grid was made up of ‘‘pixels’’ 0.028 on a side. Then, predicted intensities were convolved with the PPS field-of-view response function to find field-of-view correction factors, FOV. Measured I/F values were multiplied by the correction factors, yielding I/F of the field-of-view center. Next, model I/F values for the geometry of the field-of-view center were compared with the corrected I/F values. At the highest phase angles seen, we evaluated a full radiative transfer model for each point in the 21 3 21 grid, rather than using the cosine approximation just described.

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TABLE I PPS Uranus 2650-A˚ Data Used in Models

DATA SELECTION

A subset of the best available low-latitude data with good phase angle coverage was selected for systematic study (Table I). The tabulated I/F value is usually the fieldof-view corrected value, which relies on accurate knowl-

edge of the viewing geometry. The two highest phase angle observations (PPVPHOT5, PPVPHOT6, Table I) involved drifts across a crescent comparable in size to the PPS field of view. The reported values in these two cases are the largest I/F derived directly from the count rate. A synthetic image of the crescent was computed from the full 21 3 21

VOYAGER PPS URANUS OBSERVATIONS

511

TABLE I—Continued

grid of points. The brightest point in the model (after convolution with the field-of-view response) could be directly compared with the brightest point in the data. This approach eliminates any systematic errors from timing uncertainties during the limb drifts.

An additional set of points from the PPLATSCAN observation at 208–228 phase was used to evaluate the latitude dependence of the reflected brightness I/F. In this observation the PPS field of view slowly slewed over the pole from the bright limb to the term-

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PRYOR, WEST, AND SIMMONS

TABLE I—Continued

inator, sampling high latitudes once on each side of the pole. This coverage of high latitudes at two emission angles constrains the acceptable vertical structure models.

DATA ANALYSIS

˚ is domiThe observed UV signal on Uranus at 2650 A nated by Rayleigh scattering of sunlight by atmospheric

VOYAGER PPS URANUS OBSERVATIONS

513

TABLE I—Continued

H2 and He. The atmospheric region sampled can be found from the cross sections for Rayleigh scattering in H2 and He (Ford and Browne 1973, Chan and Dalgarno 1965). Let P0 be the vertical optical depth one pressure for pure ˚ . Since the number density Rayleigh scattering at 2650 A of the Uranus atmosphere is 84.8% H2 and 15.2% He above

the methane cloud (Conrath et al. 1987) and the gravitational acceleration g 5 869 cm sec22 at the equator and g 5 919 cm sec22 at the pole (Lindal et al. 1987), P0 is 178 mbar at the equator and 189 mbar at the pole. For comparison, the tropopause occurs near 100 mbar on Uranus (Lindal et al. 1987). Thus, PPS UV data sample regions

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TABLE I—Continued

1

Spacecraft Event Time in 1986. Distance from the spacecraft to the point being observed. 3 Planetocentric latitude. 4 Phase angle. 5 Cosine of the emission angle. 6 Cosine of the solar incidence angle. 7 Azimuthal scattering angle. 8 The last two points without a field-of-view correction, FOV, have not been corrected. For these two, the I/F values represent the average I/F over the field of view. For the others, I/F refers to the center of the field of view. 2

above and below the tropopause, depending on the slant path of each observation. The observed pressure is estimated to be Pobs 5 2g0 P0 /(1/e 1 1/e0), where e is the cosine of the emission angle, e0 is the cosine of the solar zenith angle, and g0 is the effective singlescattering albedo for a volume element containing both gas and aerosol. A photon scattered from this level traverses the same atmospheric path length as a photon traveling vertically from outside the atmosphere to optical depth one and then back out of the atmosphere. Several radiative transfer models were used to study the PPS UV data. All of them handled multiple scattering with a doubling and adding code (Hansen and Travis 1974) which retains linear polarization information in the phase matrix for scattering. First, vertically homogeneous models using only one free parameter, g0 , the effective single-scattering albedo, were studied. Vibrational Raman scattering in H2 reduces g0 in ˚ if the Rayleigh–Raman clear gas from 1.0 to 0.987 at 2650 A scattering approximation of Pollack et al. (1986) is used. In this approximation vibrational Raman scattering, an incoherent scattering process, occurs both as a sink for

˚ to longer wavelengths photons scattering from near 2650 A ˚ ˚ near 2975 A and as a source of photons near 2650 A ˚ scattered from shorter wavelengths near 2385 A. The derived effective single-scattering albedo is less than one because the solar flux at the source wavelengths near 2385 ˚ contains fewer photons than at the sink wavelengths near A ˚ . We weight the effective single-scattering albedo 2650 A at each wavelength with the relative contribution of that wavelength to the observed PPS UV counts to find an effective single-scattering albedo for the filter of 0.990, which corresponds to a geometric albedo of p 5 0.63. The Pollack approximation does not specifically include rotational Raman scattering in H2 , which involves smaller wavelength shifts. Since the Sun is reasonably gray at low ˚ , rotational dispersion on these smaller scales near 2650 A Raman scattering does not have a large effect on the broadband effective single-scattering albedo; however, we partially include rotational Raman scattering, in the sense that the Rayleigh scattering cross section used is the sum of the Rayleigh scattering and pure rotational Raman scattering cross sections, while the v 5 0 to v 5 1 vibrational Raman scattering cross section used is the sum of the pure vibrational and vibrational–rotational cross sections. In our models, a Rayleigh scattering phase matrix and g0 were used to describe each ‘‘typical’’ scattering event.

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VOYAGER PPS URANUS OBSERVATIONS

˚ with vertically and latitudinally homogeneous Rayleigh scattering and effective FIG. 1. (a) A model (thick solid line) for Uranus at 2650 A single-scattering albedo g0 5 0.985 is compared with the latitude behavior of I/F for the PPLATSCAN data (1 symbols) at phase angles 208–228. This observation went from bright limb to pole to terminator, covering some latitudes twice. A second model (thin solid line) with more absorption (g0 5 0.980) better fits the polar region. A third, brighter model without aerosols (g0 5 0.990) is also shown with a thin line. A banded structure centered on the pole is visible in the data. The data were scaled to the g0 5 0.985 model. This scaling was used to produce I/F in Table I. (b) The ˚ points in the latitude range 108–308S. The ratio of g0 5 0.985 model is compared with the phase angle behavior of I/F for selected PPS 2650-A model brightness to data brightness is shown (1) compared with a solid line of ratio one. The quality-of-fit parameter x 2 is indicated.

Such a model is appropriate for gas that contains absorbing aerosols much smaller than the wavelength of light (2650 ˚ ). As a first approximation, g0 5 0.985 reproduces the A PPLATSCAN Uranus data, which observes each high latitude twice, at two emission angles. This value of g0 5 0.985 corresponds to p 5 0.60, some 13% higher than the IUE result of p 5 0.53 6 0.11, but within the error bars. Lower values of g0 increase the model limb brightening more than is observed. The value of g0 5 0.985 is very similar to the Rayleigh–Raman scattering value of g0 5 0.990. Figure 1a shows models with g0 5 0.990, 0.985, and 0.980 compared with the PPLATSCAN data. When the data at low latitudes are scaled to match the 0.985 model, the polar region is better fit by the 0.980 single-scattering albedo. This corresponds to increasing the polar aerosol absorption per unit volume, with either more abundant or darker (larger imaginary refractive index) aerosols. Figure 1b shows the fit of the g0 5 0.985 model to low-latitude data as a function of phase angle. The fact that a simple homogeneous Rayleigh–Raman model produces a good fit to the ˚ PPS data set indicates that the stratospheric haze 2650-A is vertically diffuse and composed of particles in or near the Rayleigh size regime. Analysis of the IUE Uranus ˚ also requires a vertically spectrum from 2088 to 3350 A diffuse absorbing haze above 100 mbar to create the ob˚ , with an optical depth of served absorption below 2400 A ˚ ˚ IUE 0.05–0.1 at 2650 A (Cochran et al. 1990). The 2650-A data by themselves do not require absorbing stratospheric

haze, primarily because of the large geometric albedo uncertainties. We also tested Uranus stratospheric aerosol profiles taken from Rages et al. (1991) (model R91) on the PPS data. The R91 profiles are derived from a combination of theoretical models and fits to the Voyager imaging team data. The models make stratospheric aerosol by condensing photochemically produced C2H2 , C4H2 , C2H6 , and C6H2 . In the R91 model C2H2 ice condensing between 2.4 and 4.0 mbar dominates the haze production. More recent photochemical models favor production of C4H2 ice, which condenses at pressures greater than 0.15 mbar (Atreya et al. 1991). The new models have a similar total production rate, but the vertical aerosol distribution is not given. Therefore, we compare the PPS data with model R91. In the R91 model, stratospheric aerosols grow and settle with time, with altitude characteristics given in Table IX of Rages et al. (1991). The ices are assigned a density of 0.7 g cm23. A log-normal aerosol size distribution is used of the form n(r) 5

1 1 exp[2(ln r 2 ln rg)2 /(2s 2g)], (2f)1/2s g r

where

E 5E

ln rg 5

s 2g

y

0 y

0

n(r) ln rdr (ln r 2 ln rg)2 n(r) dr.

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FIG. 2. Stratospheric aerosol profiles used in model R91 [Rages et al. (1991) Uranus aerosol model]. (a) Aerosol number density as a function of pressure. (b) Aerosol mean radius as a function of pressure. The upper stratospheric aerosols are much smaller than the 0.265-em PPS wavelength, which implies an almost Rayleigh scattering phase function. (c) Cumulative vertical optical depth for Rayleigh scattering by gas, tray , and for aerosol ˚ (solid lines), and the ISS UV filter at 3500 A ˚ (dashed lines). extinction, taer , as a function of pressure for two wavelengths: the PPS UV wavelength 2650 A

Figure 2 shows the R91 aerosol number density, aerosol mean radius, and cumulative optical depth as a function of pressure. In model R91 the imaginary part of the aerosol refractive index, ni , as a function of wavelength is ni (l) 5 exp(a 1 bl), where the two fitting parameters have values a 5 22.1 6 1.1 and b 5 26.5 6 2.2 em21. This relationship ˚ of ni 5 0.022, with leads to an initial estimate at 2650 A a range from 0.0037 to 0.12. We used a Mie scattering code (Hansen and Travis 1974) to produce a model R91 aerosol phase matrix that was then combined with an H2 and He gas Rayleigh scattering phase matrix (with a Rayleigh– Raman effective single-scattering albedo of 0.990) to provide a mean phase matrix for the atmosphere. This mean phase matrix was used with our doubling and adding multiple scattering code to find predicted I/F and geometric albedo values.

˚ is sensitive to stratospheric and The PPS filter at 2650 A upper tropospheric hazes, but is insensitive to the methane cloud from 1.2 to 1.3 bar (at vertical optical depth 6.5–7). Our nominal R91 model contains stratospheric and upper tropospheric haze down to 1.38 bar (Fig. 2), but has no methane cloud or deep cloud. This model has a geometric albedo p 5 0.606, higher than the Wagener et al. (1986) IUE value of p 5 0.53. We varied the ni of the model and found a minimum value for the geometric albedo of p 5 0.56 when ni 5 1.0. These variations on the standard R91 model all have geometric albedos within the 20% uncertainty of the IUE measurement (Fig. 3). Addition of a methane cloud to the models has almost no effect. For example, adding a methane cloud between 1.2 and 1.3 bar of the type in model R91 appropriate for 2658 latitude, along with a stratospheric haze index ni 5 0.022, yields

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˚ as a function FIG. 3. Derived geometric albedo of Uranus at 2650 A of the imaginary refractive index, ni , in the nominal R91 model. The ˚ ) is linear fit to the ISS-derived ni of R91 (extrapolated by us to 2650 A shown as a point at ni 5 0.022 with error bars. The IUE-derived geometric albedo of Wagener et al. (1986) is shown as a horizontal line with error bars. All tested variations on ni are within the error bars on the IUE measurement, but values of ni larger than 0.022 are favored.

p 5 0.605, near the p 5 0.606 value found for a model with no methane cloud. The methane cloud and deeper layers contribute ,1% of the observed low-phase-angle intensity, making the PPS UV data useful for constraining the stratospheric and upper tropospheric haze properties without confusion from deeper layers. The R91 model with ni 5 0.022 generally matches the phase angle dependence of the low-latitude PPS UV data (Fig. 4a). The PPS 1538 and 1578 phase angle data are sensitive to large forward-scattering aerosols at pressures lower than p40 mbar. The R91 model contains small aerosols (,0.1-em mean radius) at these pressures. Larger aerosols are present at greater pressures in the model, but the PPS UV data are insensitive to their presence. The R91 model has an effective single-scattering albedo for each layer in the range 0.983–0.989, similar to the value of 0.985 used in the Rayleigh-only model. We examined the PPS UV sensitivity to low-latitude aerosol size and number density. The R91 nominal model adequately described the phase angle behavior of the data (Fig. 4a); however, uniformly increasing the aerosol sizes in the R91 model by 20% creates a p10% increase in the calculated forward-scattering peak that is not seen in the data (Fig. 4b). This is a reasonable upper limit to how large the aerosols can be, if their vertical distribution follows the R91 model. Decreasing the aerosol sizes by 20% below the nominal R91 model slightly improves the fit to the phase angle data (Fig. 4c). Increasing the aerosol number density by a factor of 2 also creates an unacceptably large forwardscattering peak (Fig. 4d).

If the nominal R91 model derived for 222.58 and 2658 latitude with imaginary refractive index ni 5 0.022 at 2650 ˚ is forced to fit the PPS data by scaling the calibration, A it does not simultaneously fit all latitudes. The polar region data can be fit by darkening the aerosol distribution in the Rages et al. model so that ni 5 0.2 (Fig. 5). Alternatively, if ni 5 0.022 at all latitudes, the polar region can be fit by increasing the aerosol abundance by a factor of 3 (Fig. 6). If much darker aerosols (ni 5 1.0) are used, the polar data can be fit by scaling the aerosol abundance by 1.5 (Fig. 7). Use of larger imaginary refractive indices in the ultraviolet than in the visible may be reasonable. The haze materials are initially white/colorless in the visible; but C4H2 absorbs ˚ and C6H2 absorbs below 3000 A ˚ (Atreya below 2600 A et al. 1991). Ultraviolet-induced polymerization in the hydrocarbon ices can darken them after they form (Pollack et al. 1987). The polar region is about 6% darker than expected from extrapolating the R91 low-latitude model to polar latitudes. The dark polar area covers only 708–908S, which is a small fraction of the projected area of the disk, so the effect on the derived geometric albedo is only p1%, much less than the 20% uncertainty in the IUE geometric albedo. DISCUSSION

Our modeling efforts constrain the phase integral, q (defined as the spherical albedo/geometric albedo), for ˚ . The nominal Rages et al. stratospheric Uranus at 2650 A haze model applicable to most latitudes has a phase integral value of q 5 1.254. The simpler Rayleigh scattering model with an effective single-scattering albedo of 0.985 has a phase integral of q 5 1.251. Examination of the error budget suggests a phase integral for Uranus of q 5 1.25 6 0.05. This value is slightly smaller than the value of q 5 1.26 6 0.05 derived for Neptune by Pryor et al. (1992), reflecting the presence of more forward-scattering stratospheric aerosols on Neptune. Unfortunately, since the polar region of Uranus faced the Sun in 1986, high-phaseangle observations of the polar aerosols suitable for strongly constraining their scattering properties were not possible. ˚ ), violet (4130 A ˚ , 4310 Contrast-enhanced blue (4770 A ˚ ˚ A), and ultraviolet (3510 A) images of Uranus from Voyager 2 also contain a dark polar region centered on the rotation pole with a radius of about 108 in latitude (Smith et al. 1986). Smith et al. ‘‘interpret the relative darkness of the polar region in violet light as an indication of a large abundance of violet-absorbing haze particles.’’ Because the H2 Rayleigh scattering cross section increases at shorter ˚ does not probe wavelengths, the PPS UV filter at 2650 A as deeply into the atmosphere as the ISS UV and violet filters. Thus, our detection of enhanced polar absorption ˚ indicates that at least some of the polar absorpat 2650 A

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˚ with ni 5 0.022 for the phase angle data FIG. 4. (a) Ratio of model brightness to data brightness (1 symbol) for the R91 model for 2650 A for selected points in the latitude range 0–308S. A solid line of ratio one is shown. The quality-of-fit parameter x 2 is indicated. (b) Ratio of model brightness to data brightness for the R91 model if the aerosol sizes are increased by a factor of 1.2. (c) Same ratio for the case of aerosol sizes decreased by a factor of 0.8. (d) Same ratio for the case of the aerosol densities increased by a factor of 2.0.

tion seen by ISS is taking place at altitudes in the upper troposphere and stratosphere, well above the methane cloud near 1.2 bar. The combined PPS and ISS data show that a realistic stratospheric haze model requires increased haze extinction near the pole. We also compared PPS PPLATSCAN observations with simultaneous Voyager UVS LATSCAN observations (Fig. 4 in Yelle et al. 1989). The UVS LATSCAN data have a low signal-to-noise ratio. Yelle et al. found the polar region to be p30% (2.5s) brighter than neighboring latitudes in ˚ and p10% (1.0s) dimthe wavelength range 1338–1523 A ˚. mer than neighboring latitudes in the range 1541–1650 A ˚ They interpret the polar brightening at 1338–1523 A as due to reduced hydrocarbon absorption, and do not comment

on the (possibly statistically insignificant) polar darkening at longer wavelengths. Yelle et al. (1989) argue that aerosols do not affect the UVS data, based on opacities reported in Pollack et al. (1987); however, these opacities were derived from high-phase-angle equatorial data and do not necessarily apply to the Uranus polar region. Because Rayleigh– Raman scattering optical depth 1 for a clear H2 atmosphere ˚ is near 10 mbar (Fig. 1, Yelle et al. 1989), the at 1600 A longer-wavelength UVS data sample the stratospheric region where aerosol condensation occurs. It is suggestive that the 10% polar darkening seen in the UVS 1541–1650 ˚ data is similar in magnitude to that seen in the PPS data A ˚ and in the ISS data at 3430 A ˚ . UV imaging from at 2650 A Hubble Space Telescope could clarify this issue.

VOYAGER PPS URANUS OBSERVATIONS

519

˚ to the latitudinal behavior of the brightness I/F data at 208–228 phase (PPLATSCAN), showing that FIG. 5. Fit of the R91 model for 2650 A if the low-latitude data are fit by ni 5 0.022, ni 5 0.2 is required to fit the polar region. Models are given with a solid line, data with plus symbols.

˚ latitudinal data with the R91 model is to use ni 5 0.022 at all latitudes and to FIG. 6. An alternative way of fitting the PPLATSCAN 2650-A increase the polar aerosol abundance by a factor of 3. Models are given with a solid line, data with plus symbols.

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˚ brightness data is to use very dark aerosols in the R91 model (ni 5 1.0). In this case the FIG. 7. A third way to fit the PPLATSCAN 2650-A required polar aerosol abundance increase is a factor of 1.5. Models are given with a solid line, data with plus symbols.

Why does Uranus have a small polar region of enhanced aerosols? McMillan (1992) examined the coupling of stratospheric circulation and stratospheric hydrocarbon photochemistry on Uranus, and found the time scale for photochemical destruction of methane to be 4 3 107 sec, much shorter than the meridional transport time of 2.7 3 1010 sec. This implies that the polar hydrocarbon population may be quite different from the low-latitude population. Uranus’ pole faced the Sun during the Voyager encounters, leading to a polar photochemical haze production rate of 2.2 3 10215 g cm22 sec21, 16 times larger than the equatorial value of 1.4 3 10216 g cm22 sec21 (Atreya et al. 1991). The dark polar region is so small that dynamics, and not just the solar zenith angle, must concentrate the aerosols. Schulz (1992) found the haze abundances to be extremely dependent on the assumed temperature profile. For example, at 53 K a 1 K shift in temperature changes the saturation vapor pressure of ethane by a factor of 2.44. Models based on Voyager IRIS (Infrared Interferometer Spectrometer and Radiometer Subsystem) tropospheric temperature measurements indicate that Uranus has a meridional circulation with upwelling into the stratosphere at low southern latitudes and polar subsidence, with polar stratospheric temperatures larger by p1 K than at other latitudes (McMillan 1992). It seems that convergence of the mean meridional circulation and subsidence at the pole concentrates the aerosols into a small polar regions, despite

the larger temperature associated with subsidence that promotes aerosol evaporation. Neptune has larger (p0.2-em radius) aerosols (Pryor et al. 1992) than Uranus (,0.1 em) at stratospheric pres˚ high-phase-angle data. sures probed by the PPS 2650-A These high-phase-angle data sensed pressures less than p15 mbar on Neptune and less than p40 mbar on Uranus. Similar Neptune stratospheric particle sizes (0.13 6 0.02 em mean radius at 28 mbar) have been found from ISS data (Moses et al. 1995). The relevant stratospheric chemistry leading to haze formation is dominated by CH4 and its photolysis products on both Uranus and Neptune, but the stronger vertical mixing on Neptune leads to important differences. We now review some of the reasons for the differences. Because of its larger eddy mixing rate, Neptune has a larger stratospheric methane mixing ratio of 1024 to 1023. On Neptune, optical depth one for methane photolysis by Lyman-a occurs at pressures of 1 ebar or less. Since condensation of hydrocarbon hazes occurs near Neptune’s 10-mbar level, vertical transport between these two pressures is needed for haze formation (Bishop et al. 1995). The low pressure in the photolysis region increases the fraction of CH4 photolysis events that form higher hydrocarbons, leading to a relatively large theoretically calculated haze production rate dominated by C2H6 ice. Photochemical calculations by Moses et al. (1995) found a haze

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production rate of 6.1–6.6 3 10215 g cm22 sec21 for aboveaverage solar activity conditions, while Romani et al. (1993) found 1 3 10214 g cm22 sec21 for solar maximum conditions. Somewhat smaller Neptune haze production rates were inferred from Voyager data [1 2 2 3 10215 g cm22 sec21 in Pryor et al. (1992), 1 3 10215 g cm22 sec21 in Moses et al. (1995)]. The larger hydrocarbon abundances and supersaturations on Neptune encourage diffusive growth, and may be related to the larger particle sizes seen; however, diffusive growth from 0.1 to 0.2 em takes 120 years, three times longer than the fallout time (Romani et al. 1993). An alternative growth process, coagulation of existing particles into larger particles, is ineffective on Uranus and Neptune because of the low stratospheric particle number densities and small mean particle radii (Baines et al. 1995). An additional possible source of stratospheric haze is present on Neptune: the stratospheric methane abundance apparently exceeds the abundance predicted for the cold trap at the tropopause, suggesting that locations exist where methane haze particles from the toposphere can be injected into the stratosphere (Bishop et al. 1995). Stratospheric injection of methane haze and condensation of methane in the lower stratosphere are both possibilities on Neptune that are not included in the haze production calculations of Romani et al. (1993) and Moses et al. (1995). On Uranus, the weak eddy mixing reduces the stratospheric methane mixing ratio to p1–3 3 1027 above 3 mbar. This places optical depth one for methane photolysis by Lyman-a at greater pressures near the 0.05-mbar level. The resulting photochemistry favors production and subsequent condensation of C2H4 and larger unsaturated hydrocarbons such as C4H2 and C6H2 that dominate the stratospheric aerosol production rates (Summers and Strobel 1989, Atreya et al. 1991, Rages et al. 1991). Models in Pollack et al. (1987) initially had ethane ice (C2H6) as the main source of stratospheric aerosol on Uranus, but the observed ISS extinction profiles led them and subsequent authors toward models that favored C2H2 and C4H2 ices, which condense at lower pressures. The CH4 photolysis level on Uranus is near the condensation level for C4H2 (0.1 mbar), so little vertical transport is required to initiate C4H2 haze formation (Pollack et al. 1987). Under steady-state conditions, the aerosol mass production rate must balance mass loss due to sedimentation and eddy diffusion. The estimated stratospheric eddy diffusion rate near 2 mbar on Neptune is K p 1–3 3 103 cm2 sec21 (Bishop et al. 1995), and on Uranus it is K p 2 3 102 cm2 sec21 (Yelle et al. 1987). These values imply that vertical eddy transport of aerosols is much slower than sedimentation and can be neglected in evaluating aerosol lifetimes (Schulz 1992). The sedimentation rate, wp, for spheres (in units of pressure change per unit time) is wp 5

2fm rp rp bg 2 kT 9

!

(Baines and Smith 1990), where m is the mean mass of a gas molecule, rp is the particle density, rp is the particle radius, k is Boltzmann’s constant, b 5 1.63 for large Knudsen numbers, Kn, typical of stratospheric aerosols, (Kn 5 molecular mean free path/rp), g is the local gravity, and T is temperature. The mean surface gravity of Neptune (1.14 3 Earth’s) is larger than on Uranus (0.87 3 Earth’s) and increases the sedimentation rate. The same radius particle of a given density changes its pressure level (1.14/0.87)2 5 1.72 times faster on Neptune than on Uranus. Using the sedimentation rate equation for the 0.2-em-radius stratospheric aerosols Pryor et al. (1992) found on Neptune leads to a fallout rate at 6 mbar of 4 mbar/year. For p0.02-emradius particles characteristic of the Uranus stratosphere (Rages et al. 1991, this work), the fallout rate is 0.2 mbar/ year. The shorter aerosol lifetime on Neptune increases the albedo response of the aerosol population to solar cycle variations in the UV-darkening flux (Baines and Smith 1990). The total haze mass on Neptune is probably larger than on Uranus, because the p20 times longer sedimentation lifetime of aerosols on Uranus does not fully offset the 10–100 times larger theoretical haze production rate on Neptune; however, the relative abundance of the haze particles on Uranus and Neptune is not well constrained by observations. This is because most of the stratospheric particles in the R91 Uranus model (which is generally consistent with both PPS and ISS data) are in the Rayleigh scattering size regime for the PPS and ISS wavelengths and scatter much like H2 gas. The R91 Uranus model has a total stratospheric column density of 2.2 3 108 cm22 of small particles in the size range 0.01–0.13 em radius that carry relatively little total mass. Neptune has a smaller column density of stratospheric haze particles (2.5 3 107 to 6.2 3 107 cm22) that have larger mean radius (p0.2 em) and greater total mass (Pryor et al. 1992). The stratospheric haze mass on Uranus in the successful R91 model is 0.1 eg cm22, compared with a stratospheric haze mass of 0.58– 1.4 eg cm22 derived for Neptune (Pryor et al. 1992). Thus, models of the haze data suggest that a larger total stratospheric haze mass exists on Neptune than on Uranus, as expected from theory. ACKNOWLEDGMENTS Chuck Acton, Mike Wang, and Neil Toy deserve special thanks for reconstructing the Voyager PPS pointing geometry. Kathy Rages and Jim Pollack provided an independent check on this geometry. We acknowledge helpful discussions with Kevin Baines, Charles Hord, Julie Moses, Jim Pollack, Kathy Rages, and Paul Romani. This work was supported by the Neptune Data Analysis Program.

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