During Voyager 2's occultation by Uranus the radio link from the spacecraft probed the atmosphere of the planet at latitudes ranging from 2 ø to 7 ø south. The measurements, which were conducted at two coherently related wavelengths, namely, 13 cm (S band) and 3.6 cm (X band), did not show any clear signs of microwave absorption. However, the Doppler frequency perturbations observed on the radio link have provided new data on the equatorial radius and atmosphere of the planet. From integral inversion of the Doppler data, profiles in height of the electron number density in the ionosphere and the gas rcfractivity, number density, pressure, temperature, and methane abundance in the troposphere and stratosphere have been determined. The gas data were acquired in the pressure range from about 0.3 mbar to 2.3 bars over an altitude interval of approximately 250 km. At the 2.3-bar level the nominal model has a temperature of 101 K with a la uncertainty of 2 K when the uncertainty in the composition is assumed to be negligible. The corresponding temperature lapse rate is 0.95 + 0.1 K/km. A 2-to 4-km-thick layer with a small refractivity scale height was detected during ingress and egress, which is consistent with the presence of a methane cloud layer centered at the 1.2-bar level. For the nominal model the methane mixing ratio below the base of the cloud is 2.3% by number density. At the tropopause, which was observed near the 100-mbar level, the temperature is 53 + 1 K. A comparison with infrared data acquired with the infrared interferometer spectrometer instrument on board the Voyager spacecraft indicates that the gas in this region consists of 85 + 3% hydrogen with the remainder being mostly helium. Above the tropopause the gas temperature increases with increasing altitude, reaching 114 + 10 K near the 0.5-mbar level. Several warm layers, which may be produced by absorption of solar radiation by hydrocarbon aerosols, were detected in the stratosphere. From the data acquired at ingress and egress the shape and size of the isobaric surfaces of Uranus have been determined. The shape indicates that the gas in the region probed by the link rotates with an average period of about 18 hours, which corresponds to a zonal wind velocity of 110 m/s relative to the magnetic field. This implies that the equatorial atmosphere rotates slower than the interior, in contrast to the situation at Jupiter and Saturn. The 1-bar isobaric surface has an equatorial radius of 25,559 + 4 km. Extrapolating from the equator to the south pole by using available data on the gravity field and the zonal wind velocities gives a polar radius of 24,973 + 20 km. The corresponding oblateness, (Req -Rp)/Req, is 0.02293 + 0.00080. 14,987 14,988 LINDAL ET AL.' THE ATMOSPHERE OF URANUS ECLIPTIC NORTH VOYA(;ER 2 TRAJECTORY of the nightside Uranian stratosphere from Voyager 2 photopolarimeter stellar occultation measurements, J. Geophys. Res., this issue.

• Unexpectedly complex influx of ring material found in Saturn's equatorial upper atmosphere, including organics, water and nanograins. • Ring influx leads to reduction in major ions (H + and H 3 +); heavier molecular ions dominate Saturn's low-altitude equatorial ionosphere. • Major molecular ions at low-altitude still uncertain, but are likely to include H 3 O + and HCO + , and the mean modeled ion mass is 11 Da.

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Coherently related S (2.3 GHz) and X band (8.4 GHz) signals transmitted from Voyager 1 and 2 have been used to probe the Jovian atmosphere during occultations of the spacecraft by Jupiter. The observations have yielded profiles in height of the gas refractivity, molecular number density, pressure, temperature, and microwave absorption in the troposphere and stratosphere of Jupiter at latitudes ranging from 0° to about 70°S. The data cover a pressure range from 1000 to 1 mbar over a height interval of 160 km. At the 1000‐mbar level, the temperature was 165±5 K, and the lapse rate was equal to the adiabatic value of 2.1 K/km, within the resolution of the measurements. The ammonia abundance in this region of the atmosphere was about 0.022±0.008%, in approximate agreement with the value derived from cosmic abundance considerations. The tropopause, which was detected near the 140‐mbar level, had a temperature of 110 K. Above the tropopause, the temperature increased with increasing altitude, reaching 160±20 K in the 10‐ to 1‐mbar region of the stratosphere. Significant horizontal density variations were detected in the stratosphere. This may imply a nonuniform temperature and aerosol distribution across the Jovian disk or high‐ and low‐pressure regions due to local atmospheric dynamics. The zenoid or gravity equipotential surface which best fits the 100‐mbar isobaric surface has an equatorial radius of 71,541±4 km and a polar radius of 66,896±4 km.

The Voyager 2 encounter with the Neptune system included radio science investigations of the masses and densities of Neptune and Triton, the low-order gravitational harmonics of Neptune, the vertical structures of the atmospheres and ionospheres of Neptune and Triton, the composition of the atmosphere of Neptune, and characteristics of ring material. Demanding experimental requirements were met successfully, and study of the large store of collected data has begun. The initial search of the data revealed no detectable effects of ring material with optical depth tau [unknown] 0.01. Preliminary representative results include the following: 1.0243 x 10(26) and 2.141 x 10(22) kilograms for the masses of Neptune and Triton; 1640 and 2054 kilograms per cubic meter for their respective densities; 1355 +/- 7 kilometers, provisionally, for the radius of Triton; and J(2) = 3411 +/- 10(x 10(-6)) and J(4) = -26(+12)(-20)(x10(-6)) for Neptune's gravity field (J>(2) and J(4) are harmonic coefficients of the gravity field). The equatorial and polar radii of Neptune are 24,764 +/- 20 and 24,340 +/- 30 kllometers, respectively, at the 10(5)-pascal (1 bar) pressure level. Neptune's atmosphere was probed to a pressure level of about 5 x 10(5) pascals, and effects of a methane cloud region and probable ammonia absorption below the cloud are evident in the data. Results for the mixing ratios of helium and ammonia are still being investigated; the methane abundance below the clouds is at least 1 percent by volume. Derived temperature-pressure profiles to 1.2 x 10(5) pascals and 78 kelvins (K) show a lapse rate corresponding to "frozen" equilibrium of the para- and ortho-hydrogen states. Neptune's ionosphere exhibits an extended topside at a temperature of 950 +/- 160 K if H(+) is the dominant ion, and narrow ionization layers of the type previously seen at the other three giant planets. Triton has a dense ionosphere with a peak electron concentration of 46 x 10(9) per cubic meter at an altitude of 340 kilometers measured during occultation egress. Its topside plasma temperature is about 80 +/- 16 K if N(2)(+) is the principal ion. The tenuous neutral atmosphere of Triton produced distinct signatures in the occultation data; however, the accuracy of the measurements is limited by uncertainties in the frequency of the spacecraft reference oscillator. Preliminary values for the surface pressure of 1.6 +/- 0.3 pascals and an equivalent isothermal temperature of 48 +/- 5 K are suggested, on the assumption that molecular nitrogen dominates the atmosphere. The radio data may be showing the effects of a thermal inversion near the surface; this and other evidence imply that the Triton atmosphere is controlled by vapor-pressure equilibrium with surface ices, at a temperature of 38 K and a methane mixing ratio of about 10(-4).