The Pluto system was recently explored by NASA's New Horizons spacecraft, making closest approach on 14 July 2015. Pluto's surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto's atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto's diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto's large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.

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.

Abstract.A series of radio occultation experiments conducted with Mars Global Surveyor in early 1998 has yielded 88 vertical profiles of the neutral atmosphere. The measurements cover latitudes of 29øN to 64øS and local times from 0600 through midnight to 1800 during early summer in the southern hemisphere (Ls -264 ø-308 ø). Retrieved profiles of pressure and temperature versus radius and geopotential extend from the surface to the 10-Pa pressure level. Near-surface uncertainties in temperature and pressure are about i K and 2 Pa, respectively, far smaller than in previous radio occultation measurements at Mars. The profiles resolve the radiativeconvective boundary layer adjacent to the surface and also reveal gravity waves, particularly at northern and equatorial latitudes, which appear to be breaking in some cases. Distinctive meridional gradients of pressure and temperature indicate the presence of a low-altitude westerly jet at latitudes of 15ø-30øS at southern summer solstice. This jet appears in predictions of general circulation models in connection with the strong, seasonal, cross-equatorial Hadley circulation. The pressure gradient at •02 km altitude implies a wind speed of 33 m s -t, stronger than predicted, which may help explain the occurrence of numerous local dust storms within this latitude band in late southern spring. These measurements also characterize the response of the atmosphere to stationary thermal forcing at midsouthern latitudes, where high terrain south of Tharsis and low terrain in Hellas Planitia produce large, zonal temperature variations in the lowest scale height above the surface. Pressure measured at constant geopotential decreases at an average rate of 0.13% per degree Ls, due primarily to condensation of CO2 at the North Pole.

Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto's atmosphere. While the lower atmosphere (at altitudes <200 km) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N 2 ) dominates the atmosphere (at altitudes <1800 km or so), while methane (CH 4 ), acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), and ethane (C 2 H 6 ) are abundant minor species, and likely feed the production of an extensive haze which encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto's atmosphere to space. It is unclear whether the current state of Pluto's atmosphere is representative of its average state-over seasonal or geologic time scales.

Abstract. The Mars Global Surveyor (MGS)Radio