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.

In this third paper in a series presenting observations by the Cassini Ultraviolet Imaging Spectrometer (UVIS) of the Io plasma torus, we show remarkable, though subtle, spatio-temporal variations in torus properties. The Io torus is found to exhibit significant, near-sinusoidal variations in ion composition as a function of azimuthal position. The azimuthal variation in composition is such that the mixing ratio of S II is strongly correlated with the mixing ratio of S III and the equatorial electron density and strongly anti-correlated with the mixing ratios of both S IV and O II and the equatorial electron temperature. Surprisingly, the azimuthal variation in ion composition is observed to have a period of 10.07 hours-1.5% longer than the System III rotation period of Jupiter, yet 1.3% shorter than the System IV period defined by Brown (1995). Although the amplitude of the azimuthal variation of S III and O II remained in the range of 2-5%, the amplitude of the S II and S IV compositional variation ranged between 5-25% during the UVIS observations. Furthermore, the amplitude of the azimuthal variations of S II and S IV appears to be modulated by its location in System III longitude, such that when the region of maximum S II mixing ratio (minimum S IV mixing ratio) is aligned with a System III longitude of ∼200±15 • , the amplitude is a factor of ∼4 greater than when the variation is anti-aligned. This behavior can explain numerous, often apparently contradictory, observations of variations in the properties of the Io plasma torus with the System III and System IV coordinate systems.

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.

On January 14, 2001, shortly after the Cassini spacecraft's closest approach to Jupiter, the Ultraviolet Imaging Spectrometer (UVIS) made a radial scan through the midnight sector of Io plasma torus. The Io torus has not been previously observed at this local time. The UVIS data consist of 2-D spectrally dispersed images of the Io plasma torus in the wavelength range of 561Å-1912Å. We developed a spectral emissions model that incorporates the latest atomic physics data contained in the CHIANTI database in order to derive the composition of the torus plasma as a function of radial distance. Electron temperatures derived from the UVIS torus spectra are generally less than those observed during the Voyager era. We find the torus ion composition derived from the UVIS spectra to be significantly different from the composition during the Voyager era. Notably, the torus contains substantially less oxygen, with a total oxygen-to-sulfur ion ratio of 0.9. The average ion charge state has increased to 1.7. We detect S V in the Io torus at the 3σ level. S V has a mixing ratio of 0.5%. The spectral emission model used in can approximate the effects of a non-thermal distribution of electrons. The ion composition derived using a kappa distribution of electrons is identical to that derived using a Maxwellian electron distribution; however, the kappa distribution model requires a higher electron column density to match the observed brightness of the spectra. The derived value of the kappa parameter decreases with radial distance and is consistent with the value of κ=2.4 at 8 R J derived by the Ulysses URAP instrument (Meyer-Vernet et al., 1995). The observed radial profile of electron column density is consistent with a flux tube content, NL 2 , that is proportional to r -2 . transport timescale (see review by Thomas et al. 2004, Delamere and Bagenal 2003, Lichtenberg 2001, Schreier et al. 1998). Thus, one aims to relate observations of spatial and temporal variations in torus emissions to the underlying sources, losses and transport processes. Towards this ultimate goal, we present an analysis of observations of the Io torus made by the Cassini spacecraft's Ultraviolet Imaging Spectrograph (UVIS) on January 14, 2001, with emphasis on determining the radial structure. In a companion paper, (Steffl et al.2004), hereafter referred to as paper I, we present examples of the EUV spectra of the torus and its temporal variability as observed during the full 6-month encounter period.Analysis of the temporal structure of the torus is presented in Delamere et al. 2004. II. UVIS DATAUVIS consists of two independent, but coaligned, spectrographs: one optimized for the extreme ultraviolet (EUV), which covers a wavelength range of 561Å to 1181Å and the other optimized for the far ultraviolet (FUV), which covers a wavelength range of 1140Å to 1913Å (McClintock et al. 1993, Esposito et al. 1998, Esposito et al. 2000. Each spectrograph is equipped with a 1024 x 64 pixel imaging microchannel plate detector. UVIS pixels are rectangular, and subte...

A radial scan through the midnight sector of the Io plasma torus was made by the Cassini Ultraviolet Imaging Spectrograph on 14 January 2001, shortly after closest approach to Jupiter. From these data, Steffl et al. (2004a) derived electron temperature, plasma composition (ion mixing ratios), and electron column density as a function of radius from L = 6 to 9 as well as the total luminosity. We have advanced our homogeneous model of torus physical chemistry (Delamere and Bagenal, 2003) to include latitudinal and radial variations in a manner similar to the two‐dimensional model by Schreier et al. (1998). The model variables include: (1) neutral source rate, (2) radial transport coefficient, (3) the hot electron fraction, (4) hot electron temperature, and (5) the neutral O/S ratio. The radial variation of parameters 1–4 are described by simple power laws, making a total of nine parameters. We have explored the sensitivity of the model results to variations in these parameters and compared the best fit with previous Voyager era models (Schreier et al., 1998), Galileo data (Crary et al., 1998), and Cassini observations (Steffl et al., 2004a). We find that radial variations during the Cassini era are consistent with a neutral source rate of 700–1200 kg/s, an integrated transport time from L = 6 to 9 of 100–200 days, and that the core electron temperature is largely determined by a spatially and temporally varying superthermal electron population.