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The last two successful flybys of Io by Galileo in 2001 (orbits I31,I32) allowed the Near Infrared Mapping Spectrometer to enrich its collection of IR spectral image cubes of the satellite. These data cover hemispheric portions of Io, several volcanic centers as well as their surroundings with a spatial resolution ranging from 2 to 93 km [pixel.sup.-1]. They map thermal emission from the hot-spots and the distribution of solid S[O.sub.2] in the 1.0-4.7 [micro]m spectral range. We obtain maps of S[O.sub.2] abundance and granularity from the NIMS data using the method of Doute et al. (2002, Icarus 158, 460-482). The maps are correlated to distinguish four different physical units that indicate zones of S[O.sub.2] condensation, metamorphism and sublimation. We relate these information with visible images from Galileo's Solid State Imaging System and with detailed mapping of the thermal emission produced by Io's surface. Our principal goal is to understand the mechanisms controlling how lava, pyroclastics and gas are emitted by different types of volcanoes and how these products evolve. The 800 km diameter white ring of fallout created by a violent 'Pillanian' eruption during summer of 2001 is at least partly composed of solid S[O.sub.2] and has enriched preexisting regional deposits. Orange materials have been recently or are currently emplaced 240 km south from the main eruption site, possibly as sulfur flows. A similar event may have taken place in the past at Ababinili Patera (12.5[degrees] N, 142[degrees] W). Carefull study of S[O.sub.2] maps covering the Emakong region also suggests that sulfur forms the bright channel-fed flow emerging from the south eastern side of the caldera. Within the main caldera of Tvashtar Catena completely cooled patches of crust exist. Elsewhere, the caldera is still cooling from previous episodes of flooding. We confirm that Amirani emits constantly large amount of S[O.sub.2] gas by interaction of fresh lava with the volatiles of the underlying plains. Nevertheless S[O.sub.2] frost is not the major component of the bright white ring seen in the SSI images. Over the whole Gish Bar region, S[O.sub.2] frost seems barely stable and is constantly regenerated. The stability increases along gray filamentary structures which could be faults filled with materials having peculiar thermal properties. Northwest of Gish Bar Patera, a localized bright deposit shows an unusual spectral signature potentially indicative of [H.sub.2]O molecules forming ice crystals or being trapped in a nonidentified matrix. The Chaac region may present a thickened old crust reducing the geothermal flux to levels lower than 0.5 W [m.sup.-2] and thus creating a cold trap for S[O.sub.2]. Looking at the abundance and degree of metamorphose of S[O.sub.2], we establish the relative age of different flows and ejecta for the Sobo Fluctus. Finally the assumption that the white patches in visible images indicate S[O.sub.2] rich deposits is once again challenged. In the Camaxtli region we identify a topographically controlled compact white deposit showing only moderate S[O.sub.2] abundance. In contrast, we detect two spots of quite pure S[O.sub.2] ice on the gray flanks of Emakong. Furthermore, the close association of fumarolic S[O.sub.2] and red [S.sub.2] already noted for several volcanic centers is observed at Tupan.

Galileo's Near-Infrared Mapping Spectrometer (NIMS) obtained its final observations of Io during the spacecraft's fly-bys in August (I31) and October 2001 (I32). We present a summary of the observations and results from these last two fly-bys, focusing on the distribution of thermal emission from Io's many volcanic regions that give insights into the eruption styles of individual hot spots. We include a compilation of hot spot data obtained from Galileo, Voyager, and ground-based observations. At least 152 active volcanic centers are now known on Io, 104 of which were discovered or confirmed by Galileo observations, including 23 from the I31 and I32 Io fly-by observations presented here. We modify the classification scheme of Keszthelyi et al. (2001, J. Geophys. Res. 106 (E12) 33 025-33 052) of Io eruption styles to include three primary types: promethean (lava flow fields emplaced as compound pahoehoe flows with small plumes < 200 km high originating from flow fronts), pillanian (violent eruptions generally accompanied by large outbursts, > 200 km high plumes and rapidly-emplaced flow fields), and a new style we call 'lokian' that includes all eruptions confined within paterae with or without associated plume eruptions). Thermal maps of active paterae from NIMS data reveal hot edges that are characteristic of lava lakes. Comparisons with terrestrial analogs show that Io's lava lakes have thermal properties consistent with relatively inactive lava lakes. The majority of activity on Io, based on locations and longevity of hot spots, appears to be of this third type. This finding has implications for how Io is being resurfaced as our results imply that eruptions of lava are predominantly confined within paterae, thus making it unlikely that resurfacing is done primarily by extensive lava flows. Our conclusion is consistent with the findings of Geissler et al. (2004, Icarus, this issue) that plume eruptions and deposits, rather than the eruption of copious amounts of effusive lavas, are responsible for Io's high resurfacing rates. The origin and longevity of islands within ionian lava lakes remains enigmatic. Keywords: Io; Volcanism; Satellites of Jupiter

We report the first spectroscopic detection of discrete ammonia ice clouds in the atmosphere of Jupiter, as discovered utilizing the Galileo Near-Infrared Mapping Spectrometer (NIMS). Spectrally identifiable ammonia clouds (SIACs) cover less than 1% of the globe, as measured in complete global imagery obtained in September 1996 during Galileo's second orbit. More than half of the most spectrally prominent SIACs reside within a small latitudinal band, extending from 2[degrees] to 7[degrees] N latitude, just south of the 5-[micro]m hot spots. The most prominent of these are spatially correlated with nearby 5-[micro]m-bright hot spots lying 1.5[degrees]-3.0[degrees] of latitude to the north: they reside over a small range of relative longitudes on the eastward side of hot spots, about 37% of the longitudinal distance to the next hot spot to the east. This strong correlation between the positions of hot spots and the most prominent equatorial SIACs suggests that they are linked by a common planetary wave. Good agreement is demonstrated between regions of condensation predicted by the Rossby wave model of A. J. Friedson and G. S. Orton (1999, Bull. Am. Astron. Assoc 31, 1155-1156) and the observed longitudinal positions of fresh ammonia clouds relative to 5-[micro]m hot spots. Consistency is also demonstrated between (1) the lifetime of particles as determined by the wave phase speed and cloud width and (2) the sedimentation time for 10-[micro]m radius particles consistent with previously reported ammonia particle size by T. Y. Brooke et al. (1998, Icarus 136, 1-13). A young age ( Key Words: atmospheres, composition; atmospheres, dynamics; ices; Jupiter, atmosphere; spectrophotometry.

We analyze a series of spectral image cubes acquired by the Galileo Near Infrared Mapping Spectrometer (NIMS) over the Prometheus region of Io. We use S[O.sub.2] frost, a volatile compound ubiquitous on the surface, as a tracer to understand various thermodynamic and volcanic processes acting in this equatorial region. Here we develop a new method to derive, from the 12-wavelength NIMS products, the distribution and physical properties of solid S[O.sub.2]. This method is based on the inversion of a bidirectional reflectance model on two observed spectral ratios sensitive to (1) the areal abundance of S[O.sub.2] and (2) its mean grain size. As a result, reliable and consistent maps of S[O.sub.2] abundance and granularity are obtained which can be correlated to distinguish four different physical units. The distribution of these S[O.sub.2] units indicates zones of condensation, metamorphism, and sublimation linked with the thermodynamic and volcanic processes of interest. Our maps depict equatorial plains undisturbed by any kind of vigorous volcanic activity over 35-40% of their surface. Elsewhere, 10-20% of the equatorial plains display abnormally low frost coverage which may imply the recent presence of positive thermal anomalies with temperatures in the range 110-200 K. Hot-spots such as Prometheus, Culann, Surya, and Tupan (to mention the most persistent) emit a great variety of gases, some of which will condense at Io's surface near their source regions. Associated fields of freshly condensed S[O.sub.2] are easily observed, and deposits of more refractory compounds with higher (e.g., [S.sub.8]) or lower (e.g., NaCl) molecular weight must also be present (although their exact nature is unknown). Three different mechanisms of emission are proposed for the volatile compounds and supported by the distribution maps. These are (a) the interaction between flowing lava and preexisting volatile deposits on the surface, (b) direct degassing from the lava, and (c) the eruption of a liquid aquifer from underground. The geometric elongation of the Prometheus S[O.sub.2] deposition ring being related to the development of a 95-km-long lava field is the best illustration of mechanism (a). Details of the progressive emplacement of the S[O.sub.2] ring by the associated plume are examined by the development of a semiempirical model of material deposition based on a ballistic transfer from the sources to the surface. This model shows that lava emission may have been occuring at Prometheus at a fairly constant rate since Voyager. Mechanism (b) may operate at the hot-spot Surya, which presents a noticeable field of fresh S[O.sub.2] frost but no extended lava flow. Finally, we have noted on the northwestern flank of the volcanic edifice Emakong the existence of an extremely deep [v.sub.1] + [v.sub.3] S[O.sub.2] absorption which is indicative of abundant, pure, and perhaps icy S[O.sub.2] deposits. These could be the result of the eruption of an S[O.sub.2] liquid aquifer (mechanism (c)). Key Words: satellites of Jupiter; Io; infrared observations; ices; volcanism.