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During the Galileo Europa Mission (GEM), impact features on Europa were observed with improved resolution and coverage was compared with Voyager or the Galileo nominal mission. We surveyed all primary impact features [is greater than] 4 km in diameter seen on Europa (through orbit E19). The transition from simple to complex crater morphology occurs at a diameter of about 5 km. We calculated the transient crater dimensions and excavation depths of all craters surveyed. The largest impact feature (Tyre) probably had a transient crater depth between 5 and 10 km and transported material to the surface from a depth of not greater than ~4 km. Craters [is less than] 30 km in diameter, such as Manannan and Pwyll, formed within targets whose immediate subcrater materials exhibited nonfluid behavior on time scales of the impact event, and that are capable, especially in the case of Pwyll, of supporting significant local topographic loads such as a central peak. These craters are nevertheless quite shallow, with very subdued floors, and we suspect that Manannan and Pwyll's small depth-to-diameter ratios are due to the isostatic adjustment of large-scale topography, facilitated by warm, plastically deformable ice at depth. Morphological similarities between Callanish and Tyre strongly imply that conclusions reached regarding Callanish in J. Moore et al. (1998, Icarus 135, 127-145) also apply to Tyre, which was that Callanish is the consequence of impact into target materials that are mechanically very weak at depth. New evidence that Callanish's circumferential rings formed before the proximal ejecta became immobile implies a low-viscosity substrate at the time of impact. We also report additional evidence that a component of the proximal ejecta of Callanish was emplaced as a fluid. Our observations of Pwyll secondaries support the conclusions stated in Alpert and Melosh (1999) that impacts on icy bodies eject smaller fragments and that fragment size decreases more gradually as velocity increases than observed for impacts on silicate bodies at equivalent ejection velocities. Examination of Pwyll's secondary craters reveals azimuthal variations, with ejecta fragment sizes being larger near the center of a ray than off the ray. Our initial analysis of the characteristic size distribution of Pwyll's secondary craters shows that they form a differential slope slightly shallower than -4. Similar steep slopes for small craters on Ganymede imply that small craters there are mostly formed by secondary impact, and the jovian system may thus be deficient in small impacts relative to the environment of the terrestrial planets. [C] 2001 Academic Press Key Words: Europa; cratering; satellites of Jupiter.

Using Galileo clear-filter images (effective wavelength [approximately equals] 0.64 [micro]m), we have created the first albedo maps of the small inner jovian satellites Thebe, Amalthea, and Metis. These maps clearly show that the leading sides of all three satellites are significantly brighter than their corresponding trailing sides, confirming and extending a result first reported by P. C. Thomas et al. (1998, Icarus 135, 360-371). In particular, on all three moons the leading side is brighter than the trailing side by 25 to 30%. The fact that the direction and size of this albedo asymmetry is identical from satellite to satellite suggests that one common physical mechanism is governing the global albedo patterns of all three moons. The most plausible such mechanism is the impact of macroscopic meteoroids that originated outside the jovian system. These impacts, which eject the dust that forms Jupiter's ring system (M. E. Ockert-Bell et al., 1999, Icarus 138, 188-213; J. A. Burns et al., 1999, Science 284, 1146-1150), are probably also responsible for brightening the leading sides of these small satellites. [C] 2000 Academic Press Key Words: photometry; satellites of Jupiter; Galileo spacecraft.

We report on the first successful, spatially resolved spectroscopic observations of Io's [SO.sub.2] atmosphere. Observations made with the Hubble Space Telescope Faint Object Spectrograph on 1 August 1996 using the 0.26' aperture have provided detections of [SO.sub.2] gas in absorption in three locations on the Io disk: the Pele volcano, the Ra volcano, and T3 (a control region at latitude 45 [degrees] S, longitude 300 [degrees] W). The column densities of [SO.sub.2] at these three locations have been determined by best-fit models of the geometric albedo and vary by a factor of five, with [MATHEMATICAL EXPESSION NOT REPRODUCIBLE IN ASCII], and 7 x [10.sup.15] [cm.sup.-2] for Pele, Ra, and T3, respectively. Thus, [SO.sub.2] gas is found to be present, and collisionally 'thick' (N [is greater than or equal to] 6 x [10.sup.14] [cm.sup.-2]), in all three locations. The factor of five difference in column densities among the three targets provides the first direct evidence that the Io atmosphere is spatially inhomogeneous. Models of the [SO.sub.2] gas band absorption at different temperatures give best-fit models with temperatures of T = 280 (Pele), 150 (Ra), and 200 K (T3). Addition of SO to the models in the amounts [N.sub.so] ~ 2.5 x [10.sup.15] (Pele), 5 x [10.sup.14] (Ra), and 1.5 x [10.sup.15] [cm.sup.-2] (T3) provides improved ([X.sup.2]) fits to the data for all three locations and gives reasonably good agreement with the previous detection of SO in the Io atmosphere at an abundance ~0.1 times that of [SO.sub.2]. We set an upper limit of 2 x [10.sup.14] [cm.sup.-2] on the abundance of [CS.sub.2]. Observations with the HST WFPC2 obtained on 24 July 1996, 7 days earlier than our FOS spectra, showed an active plume over the Pele volcano. If Pele was still active on 1 August, our results imply that the regions of highest [SO.sub.2] gas density on Io may be associated with active volcanic plumes and not sublimation from the visibly bright [SO.sub.2] frost patches common on the surface of the satellite. As a result of the positive detection of atomic sulfur emission from the [SI] 1900 [Angstron] multiplet from two of our three targets (~3.6 kR, Pele; ~1.6 kR, T3; [is less than] 1.5 kR, Ra) our spectra also provide the first concurrent measurements of S, SO, and [SO.sub.2] gases in the atmosphere and give a ratio of S/ [SO.sub.2] abundance of ~0.003-0.007. The [SO.sub.2] distribution we observe falls off much more slowly with latitude than the best available sublimation atmosphere models, but matches well the latitudinal distribution of the more realistic sublimation-driven atmosphere models that include hydrodynamic flow and photochemistry. Key Words: atmospheres, structure; abundances, atmospheres; Io; satellites of Jupiter.

We develop a basic thermal model for Comet 1P/Halley aimed at explaining the secular behavior and the diurnal amplitude of the observed water production. Our model incorporates time varying insolation of five active regions as determined for 1P/Halley by the nonprincipal axis spin model of M. J. S. Belton et al. (1991, Icarus 93, 183-193). The thermal structure internal to each of the rive active regions is governed by the insolation pattern, cooling by reradiation and sublimation, and the nonlinear heat conduction. The model results are compared with the water production rates calculated from the observed OH production in 1P/Halley (D. G. Schleicher et al., 1998, Icarus 132, 397-419). The models are sensitive to the thermal inertia of cometary ice; an absolute upper limit to the thermal inertia of 0.016 cal [cm.sup.-2] [s.sup.-1/2] [K.sup.-1] is derived based on the model fits to the secular behavior of observational data. This derived thermal inertia of cometary ice is at least a factor of three smaller than that for solid crystalline water ice and therefore consistent with a porous structure for the cometary ice. This observationally based estimate of the thermal inertia of cometary ice is also consistent with the approximate thermal inertia of 0.003 cal [cm.sup.2] [s.sup.-1] [K.sup.-1] suggested by P. R. Weissman (1987, Astron. Astrophys. 187, 873-878) based on the 'turn-on' of water production in Comet 1P/Halley. The models are insensitive to the initial temperature of the nucleus, assumed to be 60 K. We find that the secular behavior of preperihelion model water production, unlike that during the postperihelion, is strongly dependent on the thermal inertia of cometary ice. This underlines the importance of obtaining well sampled accurate preperihelion observations of water (or a daughter species of it such as OH) throughout the entire period when water is the most dominant outgassing species. We obtain approximately [10.sup.12] [cm.sup.2] for the total active area on the nucleus, consistent with that based on spacecraft imagery and other groundbased data.