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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2004.10.019 Byline: Y.R. Fernandez (a), S.C. Lowry (b), P.R. Weissman (c), B.E.A. Mueller (d), N.H. Samarasinha (d), M.J.S. Belton (e), K.J. Meech (a) Abstract: We present new, near-aphelion, time series of photometry of Comet 2P/Encke in Cousins-R band. With these light curves we find that the dominant, synodic rotational periodicity is either P.sub.0=11.079[+ or -]0.009h or 2P.sub.0=22.158[+ or -]0.012h. This is in contrast to data from the 1980s published by others that are consistent with 15.08- and 22.6-h periods. Those periods do not satisfy our phased light curves, and also the 1980s data are not easily reconciled with our periods. This could be due to P/Encke having non-principal axis rotation or due to a drift in the rotation period caused by outgassing torques. We observed the comet at five epochs: July, August, September, and October 2001, and September 2002, and the comet was at times intrinsically brighter than expected for a bare nucleus, due to an apparent contribution from an unresolved coma. Three-quarters of the data were obtained in the second and fifth epochs, and we analyzed these two time series using both the phase-dispersion minimization and 'WindowCLEAN' techniques. At both epochs and with both techniques strong periodicities were found near frequencies f.sub.0=2.16d.sup.-1 and f.sub.1=4.35d.sup.-1. By then using visual inspection of the phased light curves to corroborate these frequencies, and by using the data from the other three epochs to properly align light curve features, we were able to derive P.sub.0 and 2P.sub.0 as the only solutions that satisfy all our observations. The periodicity due to f.sub.1 is clearly seen in our data, but we cannot tell from our data alone whether it is a manifestation of the nucleus's shape, non-principal axis rotation, or both. Author Affiliation: (a) Institute for Astronomy, University of Hawai'i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA (b) Queen's University Belfast, Department of Physics, Belfast, BT7 1NN, United Kingdom (c) Jet Propulsion Laboratory, MS 183-601, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USA (d) National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson, AZ 85719, USA (e) Belton Space Exploration Initiatives, 430 S. Randolph Way, Tucson, AZ 85716, USA

We present new, near-aphelion, time series of photometry of Comet 2P/Encke in Cousins-R band. With these light curves we find that the dominant, synodic rotational periodicity is either [P.sub.0] = 11.079 [+ or -] 0.009 h or 2[P.sub.0] = 22.158 [+ or -] 0.012 h. This is in contrast to data from the 1980s published by others that are consistent with 15.08- and 22.6-h periods. Those periods do not satisfy our phased light curves, and also the 1980s data are not easily reconciled with our periods. This could be due to P/Encke having non-principal axis rotation or due to a drift in the rotation period caused by outgassing torques. We observed the comet at five epochs: July, August, September, and October 2001, and September 2002, and the comet was at times intrinsically brighter than expected for a bare nucleus, due to an apparent contribution from an unresolved coma. Three-quarters of the data were obtained in the second and fifth epochs, and we analyzed these two time series using both the phase-dispersion minimization and 'WindowCLEAN' techniques. At both epochs and with both techniques strong periodicities were found near frequencies [f.sub.0] = 2.16 [d.sup.-1] and [f.sub.1] = 4.35 [d.sup.-1]. By then using visual inspection of the phased light curves to corroborate these frequencies, and by using the data from the other three epochs to properly align light curve features, we were able to derive [P.sub.0] and 2[P.sub.0] as the only solutions that satisfy all our observations. The periodicity due to [f.sub.1] is clearly seen in our data, but we cannot tell from our data alone whether it is a manifestation of the nucleus's shape, non-principal axis rotation, or both. Keywords: Comets; Encke

On UT 2000 August 21 we obtained simultaneous visible and mid-infrared observations of Comet 9P/Tempel 1, the target of the upcoming NASA Discovery Program mission Deep Impact. The comet was still quite active while 2.55 AU from the Sun (post-perihelion). Two independent analyses of our data, one parameterizing the coma morphology and the other modeling infrared spectrophotometry, show that the nucleus's cross section at the time the data were taken corresponds to an effective radius of 3.0 [+ or -] 0.2 km. Based on visible-wavelength photometry of the comet taken during this observing run and others in the summer of 2000, all of which show the rotational modulation of the nucleus's brightness, we find that the infrared data were obtained near the maximum of the light curve. If we assume that the nucleus's light curve had a peak-to-valley range of 0.6 [+ or -] 0.2 mag, then the mean effective radius is 2.6 [+ or -] 0.2 km. Visible-wavelength photometry of the nucleus, including data published by other groups, lets us constrain the nucleus's R-band geometric albedo: 0.072 [+ or -] 0.016. The nucleus's flux contributed about 85% of the light in the mid-infrared images. Keywords: Comets, Tempel 1; Infrared observations

Images from the Galileo mission to Jupiter have been combined with earlier Voyager images to produce improved geodetic control networks for the Galilean satellites. The improved geometric stability and lack of scene-dependent distortions of the Galileo images compensate for their relatively small number. Triaxial solutions are derived for the principal axes of the satellites by assuming an equilibrium constrained by the gravity potential coefficients. The control network is used in the preparation of planimetric maps by relating the images to the latitude/longitude grid. Key Words: Io; Europa; Ganymede; Callisto; satellites of Jupiter.

The images of the four inner small jovian satellites obtained by the Galileo Solid State Imaging (SSI) experiment have much more detailed shape, color, and photometric information than were provided previously by Voyager images. The satellites are in synchronous rotation and show no binary or bifurcated shapes. Thebe and Amalthea have densities of large craters approximately at 'empirical equilibrium' levels. The leading sides of Metis, Amalthea, and Thebe are all 25-35% brighter than their trailing sides; the global-average, clear-filter ([Lambda] = 0.64 [[micro]meter]) geometric albedos of these three satellites are 0.063, 0.091, and 0.049, respectively. A definite color gradient is observed, with the satellites closer to Jupiter being redder: the mean violet/green ratio (0.42/0.56 [[micro]meter]) decreases from Thebe to Metis. This ratio also is lower for the trailing sides of Thebe and Amalthea than for their leading sides. Bright spots on Amalthea and Thebe are small ( Key Words: satellites; Jupiter; cratering; photometry.

The vertical structure of aerosols on Jupiter is inferred from data obtained by the NASA Galileo Solid State Imaging system during the first six orbits of the spacecraft. Images at 889 nm (a strong methane band), 727 nm (a weaker methane band), and 756 nm (continuum) taken at a variety of lighting and viewing angles are used. The images are displayed and described in the companion paper by Vasavada et al. (1998, Icarus 135, 265-275). Conservative scattering cloud particles with laterally uniform single scattering properties are assumed in the analysis and are shown to be consistent with the data at these wavelengths. Particles are bright, and the darkest locations on Jupiter correspond to the smallest optical thickness of aerosols. Optical depths and vertical positions of aerosol layers vary from place to place and are the retrieved quantities in the analysis. Only mid and low latitudes are sampled in this data set. A stratospheric haze with an optical depth of roughly a tenth and an upper tropospheric haze with an optical depth of 2 to 6 exist over all regions. Both are consistent with previous conclusions based on data of lower spatial resolution (e.g., West et al. 1986, Icarus 65, 161-217). The new data show that these layers contain little lateral structure on scales smaller than the planetary jets. On scales of the jets and ovals, the top and bottom of the upper tropospheric haze vary in elevation. The concentration of particles (optical depth per pressure interval) varies less than does the total optical depth. Near the base of the upper tropospheric haze is a third cloud component, usually at pressure p = 750 [+ or -] 200 mb, which is less than a scale height in geometric thickness. Its optical depth varies from zero to about 20 on regional scales and often varies by 50% on scales of a few tens of kilometers. Optical depth variations in this cloud are the principal cause of the features in Jupiter's atmosphere seen at red and longer wavelengths. It is probably composed of ammonia. The expected N[H.sub.4]SH cloud has not been identified in this work, perhaps because it exists only at locations where it is concealed beneath higher clouds. Our retrievals also cannot rule out a pervasive deep haze without small-scale structure. Finally, in one region northwest of the Great Red Spot, a deeper cloud is identified. Parts of it lie at a pressure greater than four bars. It is associated with a rapidly changing storm system with optical depth of several tens (or more) and a range of cloud heights between p > 4 bars to p [approximately] 400 mb. It is probably composed of water. Key Words: atmospheres, composition, structure; clouds; Jupiter atmosphere; radiative transfer; spectrophotometry.

Galileo CCD images of the limb of Io provide improved data for determining the shape of this synchronously rotating satellite. The best ellipsoidal fit is within 0.3 km of the best equilibrium fit of 1829.7, 1819.2, 1815.8 km. The shape is consistent with substantial mass concentration in a core and with gravity measurements from tracking of the Galileo spacecraft. The surface of Io is largely plains and isolated peaks, with little long-wavelength topography over 1 km in amplitude.

During Galileo's G7 orbit, the Solid State Imaging (SSI) camera acquired pictures of the spacecraft shadow point on Europa's surface as well as a comparison set of images showing the same geographic region at phase angle [Alpha] = 5 [degrees]. Coverage, obtained at three spectral bandpasses (VLT, 0.41 [[micro]meter], GRN, 0.56 [[micro]meter]; and 1MC, 0.99 [[micro]meter]) at a spatial resolution of 404 m/pixel, shows a 162 x 220-km region of Europa's surface located at 30 [degrees] N, 162 [degrees] W. We have used these images to measure the near-opposition spectrophotometric behavior of four primary europan terrain materials: IR-bright icy material, IR-dark icy material, dark lineament material, and dark spot material. The high spatial resolution of the G7 images reveal low-albedo materials in dark spots that are among the darkest features (17% albedo at 0.56 [[micro]meter] and 5 [degrees] phase) yet found on icy Galilean satellites. While material of comparable albedo is found on Ganymede and Callisto, low-albedo europan materials are much redder. All europan surface materials exhibit an opposition effect; however, the strength of the effect, as measured by the total increase in reflectance as phase angle decreases from [Alpha] = 5 [degrees] to [Alpha] = 0 [degrees], varies among terrains. The opposition effects of IR-bright icy and IR-dark icy materials which dominate Europa's surface are about 1.5 times larger than predicted from pre-Galileo studies. Low-albedo materials in dark spots exhibit unusually intense opposition effects (up to four times larger than bright icy europan terrains), consistent with the presence of a strong shadow-hiding opposition surge. The strengths of the opposition surges among average europan terrains systematically vary with terrain albedo and can be explained in terms of the simultaneous contributions of shadow-hiding and coherent-backscatter to the total opposition effect. Coherent backscatter introduces a narrow angular contribution ( Key Words: Europa; photometry; regolith; spectrophotometry; ices; albedo; spectra; geological processes; Galileo; Voyager.

Four distinct classes of lineaments can be described on the basis of Galileo's improved spectral and spatial coverage of Europa: (1) incipient cracks are narrow (

Galileo's flyby of 243 Ida in August 1993 led to the discovery of a small satellite, Dactyl, some 85 km from the asteroid's center. From Earth at mean opposition, the satellite is a V = +20.3 mag object (some 6.7 magnitudes fainter than Ida). Forty-seven images of the satellite at 18 different observing times were played back, including one multicolor sequence in which the satellite is resolved adequately to distinguish surface markings ([approximately]105 m/pxl) and three higher resolution single-color views (89, 39, and 24 m/pxl). The satellite, mean radius = 0.7 km, is an elongated, but not angular body with principal diameters of 1.6 x 1.4 x 1.2 km. In the highest resolution view, the longest axis points approximately in the direction of Ida, and its shortest axis is perpendicular to the orbital plane. The spin period is slow (> 8 hr?) and may be synchronous. The satellite shows no conspicuous sharp edges and is much less irregular in shape than Ida. Limb profiles are remarkably smooth over distances of 200-300 m. The geometric albedos of the two objects are similar (0.20 vs 0.21), as are the 0.4-1.0-[[micro]meter] colors. Like Ida, Dactyl is an S-asteroid, but has a slightly deeper 1-[[micro]meter] band than Ida (by 5-8%). While no identical regions (in color) are seen on Ida, the color difference is consistent with color variations reported within the Koronis family and may be due to a slightly higher pyroxene/olivine ratio on the satellite. More than a dozen craters ranging from [less than or approximately]90 to 280 m diameter are visible in the best image (39 m/pxl at 47 [degrees] phase). The largest contains an off-centered, positive relief feature some 75 m across. The image includes an intriguing crater chain, but no grooves, ridges, or sharp edges are evident. In terms of limb roughness, Dactyl is much smoother than Ida, but comparable to the two satellites of Mars, Phobos and Deimos. While the satellite's origin is uncertain, a likely scenario would have the satellite date from the breakup of the Koronis family. It is interesting that crater densities on the satellite are similar to those on Ida itself.

Ida's shape is more irregular than that of any solar system object previously encountered by spacecraft. Approximately 95% of the surface was imaged by the Galileo spacecraft at better than 1 km/pixel and 30% at better than 40 m/pixel. Ida's volume is 16,100 [+ or -] 1900 [km.sup.3] (mean radius of 15.7 [+ or -] 0.6 km); the surface area is 3900 [+ or -] 300 [km.sup.2]. The maximum and minimum dimensions are 55.8 and 14.8 km. Viewed from the poles Ida has a somewhat crescent shape. Smaller scale overlapping depressions along much of the 110 [degrees] and 260 [degrees] E longitudes give the appearance of a 'waist.' A sharp ridge about 40 km in length spans nearly 150 [degrees] of longitude. The distinctive topography on either side of the 'waist' suggests a difference of mechanical properties along the length of the asteroid. These differences might arise from structures inherited in place from a larger precursor object, or might indicate a two component object. The spin pole aligns with the model maximum principal moment of inertia axis to within 2 [degrees]; this relationship rules out extreme density asymmetries inside Ida, but does not limit modest changes in porosity or composition across the asteroid. The irregular shape, low mean density (2.6 [+ or -] 0.5 gm/[cm.sup.3]), and rapid spin (P = 4.63 hr) of Ida mean that surface accelerations vary greatly (0.3 to 1.1 cm/[sec.sup.2]); gravity is lowest at the greatest and minimum radii due to the effects of rotation and the amount of mass interior to surface location. Maximum slopes are about 50 [degrees], although very little of the surface area has slopes in excess of 35 [degrees].

The surface of 243 Ida is dominated by the effects of impacts. No complex crater morphologies are observed. A complete range of crater degradation states is present, which also reveals optical maturation of the surface (darkening and reddening of materials with increasing exposure age). Regions of bright material associated with the freshest craters might be ballistically emplaced deposits or the result of seismic disturbance of loosely-bound surface materials. Diameter/depth ratios for fresh craters on Ida are [approximately]1:6.5, similar to Gaspra results, but greater than the 1:5 ratios common on other rocky bodies. Contributing causes include rim degradation by whole-body 'ringing,' relatively thin ejecta blankets around crater rims, or an extended strength gradient in near-surface materials due to low gravitational self-packing. Grooves probably represent expressions in surface debris of reactivated fractures in the deeper interior. Isolated positive relief features as large as 150 m are probably ejecta blocks related to large impacts. Evidence for the presence of debris on the surface includes resolved ejecta blocks, mass-wasting scars, contrasts in color and albedo of fresh crater materials, and albedo streaks oriented down local slopes. Color data indicate relatively uniform calcium abundance in pyroxenes and constant pyroxene/olivine ration. A large, relatively blue unit across the northern polar area is probably related to regolith processes involving ejecta from Azzurra rather than representing internal compositional heterogeneity. A small number of bluer, brighter craters are randomly distributed across the surface, unlike on Gaspra where these features are concentrated along ridges. This implies that debris on Ida is less mobile and/or consistently thicker than on Gaspra. Estimates of the average depth of mobile materials derived from chute depths (20-60 m), grooves ([greater than or equal to]30 m), and shallowing of the largest degraded craters (20-50 m minimum, [approximately]100 m maximum) suggest a thickness of potentially mobile materials of [approximately]50 m, and a typical thickness for the debris layer of 50-100 m.

The images of asteroid 951 Gaspra, taken by the Galileo space craft during October 1991, yield the shape model of the asteroid. The shape model reveals that Gaspra is an irregular astronomical body with an average radius of 6.1 kilometers. The rotation axis is aligned with the asteroid's moment of inertia as its mass distribution is uniform. Several flat regions, large and shallow concave crater-like areas characterize the surface of the asteroid.

The grooves on the surface of asteroid 951 Gaspra are fractures modified by the inward drainage of regoliths with depths ranging from 30 to 200 meters. The regoliths are noncohesive substances of various sizes. The presence of grooves indicate that Gaspra was subjected to violent collisions with other astronomical objects.

The jovian ring system was observed during four orbits of Galileo's nominal mission, when 25 clear-filter images of the rings were taken at spatial resolutions of 23 to 134 km/pixel; the ring appeared fortuitously in an additional 11 images. The tenuous jovian ring system (normal optical depths < [10.sup.-5]) has three components: the halo, main ring, and gossamer ring. The innermost component, a toroidal halo, extends radially from approximately 92,000 to about 122,500 km (near the 3: 2 Lorentz resonance) and has a full-width, half-maximum thickness of 12,500 km; its brightness decreases with height off the equatorial plane and decreases as the planet is approached. The main ring reaches from the halo's outer boundary across 6440 to 128,940 km, just interior to Adrastea's orbit (128,980 km); at its outer edge, the main ring takes nearly 1000 km to develop its full brightness. The ring's brightness noticeably decreases around 127,850 km in the vicinity of Metis' semimajor axis (127,980 km). The precise location and nature of the main ring's outer periphery may shift slightly from image to image; the same is true for the 'notch' near Metis. The main ring has a faint, vertically extended component that thickens as the halo region is approached. Brightness variations of [+ or -]10% are visible in the central main ring and may be due to vertical corrugations, density clumps, or 'spokes' in the ring. Unexplained differences between the near- and far-arm brightnesses are visible. We have discovered that the gossamer ring, lying exterior to the main ring, has two primary components, each of which is fairly uniform: one originates just interior to Amalthea's orbit (181,366 km) while the other is situated radially interior to Thebe's orbit (221,888 km). Very faint material continues past Thebe, blending into the background at 250,000 km. The gossamer rings have thicknesses that are comparable to the maximum elevations of these satellites off Jupiter's equatorial plane; from Galileo's nearly equatorial view, the gossamer rings present rectangular end-profiles with greater intensities along their top and bottom surfaces. The rings seem to be derived from the satellites. Key Words: planetary rings; Jupiter, rings; dust.