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Our knowledge of the interior structure of comets, particularly of the evolution of the outer layers at successive perihelion passages, is almost unconstrained by data and relies instead primarily on [...], Deep Impact collided with comet Tempel 1, excavating a crater controlled by gravity. The comet's outer layer is composed of 1- to 100-micrometer fine particles with negligible strength (1000 kelvins). A large increase in organic material occurred during and after the event, with smaller changes in carbon dioxide relative to water. On approach, the spacecraft observed frequent natural outbursts, a mean radius of 3.0 ± 0.1 kilometers, smooth and rough terrain, scarps, and impact craters. A thermal map indicates a surface in equilibrium with sunlight.

The Deep Impact mission was designed so that much of the mission-critical science would be done from Earth-based telescopes. These facilities would observe the comet before, during, and after impact [...], On 4 July 2005, many observatories around the world and in space observed the collision of Deep Impact with comet 9P/Tempel 1 or its aftermath. This was an unprecedented coordinated observational campaign. These data show that (i) there was new material after impact that was compositionally different from that seen before impact; (ii) the ratio of dust mass to gas mass in the ejecta was much larger than before impact; (iii) the new activity did not last more than a few days, and by 9 July the comet's behavior was indistinguishable from its pre-impact behavior; and (iv) there were interesting transient phenomena that may be correlated with cratering physics.

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

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 (

High-resolution images of the jovian satellite Europa obtained by the Galileo spacecraft present distinct surface features, including domes and spots. The features generally alter and disrupt the pre-existing plains, which are extensive and made up of subparallel ridges and grooves overlapping in successive generations. It appears that the domes, pits and spots exist where mottled plains units occur.

The Galileo mission is a multidisciplinary international effort to explore the jovian system. Its primary objectives are to explore the atmosphere of the giant planet, the large Galilean satellites, and [...], The first images of Jupiter, Io, Europa, and Ganymede from the Galileo spacecraft reveal new information about Jupiter's Great Red Spot (GRS) and the surfaces of the Galilean satellites. Features similar to clusters of thunderstorms were found in the GRS. Nearby wave structures suggest that the GRS may be a shallow atmospheric feature. Changes in surface color and plume distribution indicate differences in resurfacing processes near hot spots on Io. Patchy emissions were seen while Io was in eclipse by Jupiter. The outer margins of prominent linear markings (triple bands) on Europa are diffuse, suggesting that material has been vented from fractures. Numerous small circular craters indicate localized areas of relatively old surface. Pervasive brittle deformation of an ice layer appears to have formed grooves on Ganymede. Dark terrain unexpectedly shows distinctive albedo variations to the limit of resolution.

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.

Using mathematical and statistical means the bulk density of 243 Ida has been calculated with the help of the orbits of its satellite Dactyl. The density calculated shows that Ida is probably stony with a small amount of NiFe. The orbits of Dactyl have been calculated to be elliptical with their periapse distance approaching Ida as their apoapse distance increases beyond a distance equal to the product of the heliocentric distance of the Dactyl-Ida system and the mass of Ida divided by the Mass of the Sun.

The five images of 243 Ida (1), returned after Galileo's 28 August 1993 encounter with the asteroid (2), allow a preliminary characterization of its global properties and its surface geology. [...], The first images of the asteroid 243 Ida from Galileo show an irregular object measuring 56 kilometers by 24 kilometers by 21 kilometers. Its surface is rich in geologic features, including systems of grooves, blocks, chutes, albedo features, crater chains, and a full range of crater morphologies. The largest blocks may be distributed nonuniformly across the surface; lineaments and dark-floored craters also have preferential locations. Ida is interpreted to have a substantial regolith. The high crater density and size-frequency distribution (-3 differential power-law index) indicate a surface in equilibrium with saturated cratering. A minimum model crater age for Ida--and therefore for the Koronis family to which Ida belongs--is estimated at 1 billion years, older than expected.

Imaging objectives at Galileo's second lunar encounter with the moon (EM-2) were to extend multispectral mapping of the western limb and far side (1-5) to the northeastern limb and north [...], Multispectral images obtained during the Galileo probe's second encounter with the moon reveal the compositional nature of the north polar regions and the northeastern limb. Mare deposits in these regions are found to be primarily low to medium titanium lavas and, as on the western limb, show only slight spectral heterogeneity. The northern light plains are found to have the spectral characteristics of highlands materials, show little evidence for the presence of cryptomaria, and were most likely emplaced by impact processes regardless of their age.

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.

On 29 October 1991, the Galileo space-craft flew past 951 Gaspra at a distance of 1600 km and obtained the first close-up views of an asteriod. Because Galileo's high-gain antenna [...], Galileo images of Gaspra reveal it to be an irregularly shaped object (19 by 12 by 11 kilometers) that appears to have been created by a catastrophic collisional disruption of a precursor parent body. The cratering age of the surface is about 200 million years. Subtle albedo and color variations appear to correlate with morphological features: Brighter materials are associated with craters especially along the crests of ridges, have a stronger 1-micrometer absorption, and may represent freshly excavated mafic materials; darker materials exhibiting a significantly weaker 1-micrometer absorption appear concentrated in interridge areas. One explanation of these patterns is that Gaspra is covered with a thin regolith and that some of this material has migrated downslope in some areas.

Observations made by the Galileo spacecraft present evidence that Europa spins more rapidly than the synchronous rate, or that this was the case in the past. This finding is in alignment with the possibility of a global subsurface ocean. If Europa's interior has a sufficient mass distribution asymmetry to tidally lock the moon towards Jupiter, as the gravity results appear to suggest, but the surface shows signs of asynchronous rotation, then the crust has to be mechanically decoupled from the interior. Decoupling could result from a subsurface ocean or a layer of ductile ice.

Dactyl, a satellite associated with the asteroid 243 Ida, was discovered and studied by the spacecraft Galileo. Dactyl, which was formed at the same time as Ida by the breaking of a Koronis body, is ellipsoid and its spectral curvature is greater than that of other members of the Koronis family. This difference is probably due to weathering since the spectral properties of young craters on Ida resemble those of the spectrum of both Ida and Dactyl. The surface of Dactyl contains craters and is smoother than that of Ida because of constant breaking and re-formation with debris from Ida.