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International audience; Re-calibrated near-infrared spectroscopy of the resolved nucleus of Comet 9P/Tempel 1 acquired by the Deep Impact spacecraft has been analyzed by utilizing the post-Stardust-NExT nucleus shape model and spin pole solution, as well as a novel thermophysical model that explicitly accounts for small-scale surface roughness and thermal inertia. We find that the thermal inertia varies measurably across the surface, and that thermal emission from certain regions only can be reproduced satisfactory if surface roughness is accounted for. Particularly, a scarped/pitted terrain that experienced morning sunrise during the flyby is measurably rough (Hapke mean slope angle similar to 45 degrees) and has a thermal inertia of at most 50J m(-2) K-1 s(-1/2), but probably much lower. However, thick layered terrain and thin layered terrain experiencing local noon during the flyby have a substantially larger thermal inertia, reaching 150J m(-2) K-1 s(-1/2) if the surface is as rough as the scarped/pitted terrain, but 200J m(-2) K-1 s(-1/2) if the terrain is considered locally flat. Furthermore, the reddening of the nucleus near-infrared 1.5-2.2 gm spectrum varies between morphological units, being reddest for thick layered terrain (median value 3.4% k angstrom(-1)) and most neutral for the smooth terrain known to contain surface water ice (median value 3.1% k angstrom(-1)). Thus, Comet 9P/Tempel 1 is heterogeneous in terms of both thermophysical and optical properties, due to formation conditions and/or post-formation processing. (C) 2013 Elsevier Inc. All rights reserved.

We derive the spin state of the nucleus of Comet 103P/Hartley 2, its orientation in space, and its short-term temporal evolution from a mixture of observations taken from the DIXI (Deep Impact Extended Investigation) spacecraft and radar observations. The nucleus is found to spin in an excited long-axis mode (LAM) with its rotational angular momentum per unit mass, M, and rotational energy per unit mass, E, slowly decreasing while the degree of excitation in the spin increases through perihelion passage. M is directed toward (RA, Dec; J2000) = 8+/-+/- 4 deg., 54 +/- 1 deg. (obliquity = 48 +/- 1 deg.). This direction is likely changing, but the change is probably

International audience; On February 14, 2011 Stardust-NExT (SN) flew by Comet Tempel 1, the target of the Deep Impact (DI) mission in 2005, obtaining dust measurements and high-resolution images of areas surrounding the 2005 impact site, and extending image coverage to almost two thirds of the nucleus surface. The nucleus has an average radius of 2.83 +/- 0.1 km and a uniform geometric albedo of about 6% at visible wavelengths. Local elevation differences on the nucleus reach up to 830 m. At the time of encounter the spin rate was 213 degrees per day (period = 40.6 h) and the comet was producing some 130 kg of dust per second. Some 30% of the nucleus is covered by smooth flow-like deposits and related materials, restricted to gravitational lows. This distribution is consistent with the view that the smooth areas represent material erupted from the subsurface and date from a time after the nucleus achieved its current shape. It is possible that some of these eruptions occurred after 1609 when the comet's perihelion distance decreased from 3.5 AU to the current 1.5 AU. Much of the surface displays evidence of layering: some related to the smooth flows and some possibly dating back to the accretion of the nucleus. Pitted terrain covers approximately half the nucleus surface. The pits range up to 850 m in diameter. Due to their large number, they are unlikely to be impact scars: rather they probably result from volatile outbursts and sublimational erosion. The DI impact site shows a subdued depression some 50 m in diameter implying surface properties similar to those of dry, loose snow. It is possible that the 50-m depression is all that remains of an initially larger crater. In the region of overlapping DI and SN coverage most of the surface remained unchanged between 2005 and 2011 in albedo, photometric properties and morphology. Significant changes took place only along the edges of a prominent smooth flow estimated to be 10-15 m thick, the margins of which receded in places by up to 50 m. Coma and jet activity were lower in 2011 than in 2005. Most of the jets observed during the SN flyby can be traced back to an apparently eroding terraced scarp. The dust instruments detected bursts of impacts consistent with a process by which larger aggregates of material emitted from the nucleus subsequently fragment into smaller particles within the coma. (c) 2012 Elsevier Inc. All rights reserved.

Observations from the second encounter of Comet 9P/Tempel 1 by the Stardust-NExT spacecraft provide an improved shape model and rotational pole for the nucleus (Thomas, P.C. et al. [2012]. Icarus 222, 453–466) that allows us to greatly improve our knowledge of its rotational evolution beyond that outlined earlier in Belton et al. (Belton, M.J.S. et al. [2011]. Icarus 213, 345–368). Model light curves are shown to fit observations at both perihelia with a single pole direction indicating that polar precession during a single perihelion passage is small. We show that the rotational phasing associated with observations taken far from perihelion in the previous work was incorrectly assessed by approximately half a cycle leading us to a significant reassessment of the evolution of the non-gravitational torques acting on the nucleus. We present an updated spin rate profile (torque model) for the 2005 perihelion passage and show that retardation of the spin rate well before perihelion is no longer a required feature. With the exception of the spin rate before the 2000 perihelion passage, the evolution of rotational rates through the three most recent perihelion passages is largely unaffected as is the prediction of the rotational phase of the comet’s nucleus at the Stardust-NExT near-perihelion encounter. We find a spin rate of 209.4 ± 0.01°/d likely applies in the quiescent period before the 2000 perihelion, a 0.2% change, and that the rotational period shortened by 12.3 ± 0.2 min during the 2000 perihelion passage. We present an analysis of Stardust-NExT time-series photometry that yields a spin rate near 213.3 ± 0.8°/d at the time of encounter. An application of the 2005 torque model suggests that, while roughly similar, the torques were probably weaker during the 2011 perihelion passage.

NASA’s Stardust–NExT mission used the Stardust spacecraft to deliver a scientific payload, including a panchromatic visible camera designated NAVCAM, to a close flyby of Comet 9P/Tempel 1 in February 2011. Proper interpretation of the NAVCAM images depends on accurate calibration of the camera performance. While the NAVCAM had been calibrated during the primary Stardust mission to Comet 81P/Wild 2 in 2004, that calibration was incomplete and somewhat lacking in fidelity. Substantial improvements in the NAVCAM calibration were achieved during Stardust–NExT in the areas of geometric correction, spatial resolution, and radiometric calibration (in particular, zero-exposure signal levels, shutter time offsets, absolute radiometric response, noise, and scattered light characterization). These improvements will allow upgrades to the calibration of images returned from the Stardust primary mission as well as high-quality calibration of the Stardust–NExT images. The upgraded calibration results have been incorporated into the Stardust–NExT image processing pipeline via new routines and updated constants and files in preparation for archiving calibrated images in the NASA Planetary Data System.

The photometric properties of the nucleus of Comet 9P/Tempel 1 as modeled from the Stardust-NExT images agree with those reported by Li et al. (Li, J.-Y. et al. [2007a]. Icarus 187, 41–55; Li, J.-Y., A’Hearn, M.F., McFadden, L.A., Belton, M.J.S. [2007b]. Icarus 188, 195–211) from Deep Impact images. No significant changes are detectable by comparing the two image-sets taken one comet year apart. The overall photometric variations on the ∼70% of the surface of Tempel 1 observed by Deep Impact and Stardust-NExT are small, with albedo variations of ±10% full-width-at-half-maximum and non-detectable variations in phase function and surface roughness. Some bright surface albedo features visible in the outbound images have an albedo about 25% higher than that of surrounding area. No bright albedo features similar to those ice patches reported by Sunshine et al. (Sunshine, J.M., et al. [2006]. Science 311, 1453–1455) are seen on the outbound side, which was not imaged by DI. The similar global photometric properties among cometary nuclei may indicate that these properties are dominated by cometary activity that results in constant resurfacing on comets. Tiny amounts of ice concentration on their surface can significantly change the local photometric properties.

We have studied the photometric properties of the nucleus of a hyperactive comet, 103P/Hartley 2, at visible wavelengths using the DIXI flyby images with both disk-integrated and disk-resolved analyses. The disk-integrated phase function of the nucleus has a linear slope of 0.046 ± 0.002 mag/deg and an absolute magnitude of 18.4 ± 0.1 at V -band. The nucleus displays an overall linear, featureless spectrum between 400 nm and 850 nm. The linear spectral slope is 7.6 ± 3.6% per 100 nm, corresponding to broadband solar-illuminated color indices B – V of 0.75 ± 0.05 and V – R of 0.43 ± 0.04. Disk-resolved photometric analysis with a Hapke model returns a best-fit single-scattering albedo of 0.036 ± 0.006, an asymmetry factor of the Henyey–Greenstein single-particle phase function of −0.46 ± 0.06, and a photometric roughness of 15 ± 10°. The model yields a geometric albedo of 0.045 ± 0.009 and a Bond albedo of 0.012 ± 0.002. The overall photometric variations of the nucleus are small, with an equivalent albedo variation of 15% FWHM, and a color variation of 12% FWHM. Some areas near the terminator visible in the inbound images show an albedo of more than twice the global average value, and a much bluer color than the average nucleus. The overall photometric properties and variations of the nucleus of Hartley 2 are similar to those of the nuclei of Comets Wild 2 and Tempel 1 as studied from previous spacecraft flyby missions at similar resolutions.

A flaw in the pre-launch calibration system resulted in an inability to accurately focus the Deep Impact’s High Resolution Instrument (HRI). This defocus resulted in a significant loss of resolution. The nature of the blurring function allows us to use image restoration techniques to retrieve much of the lost resolution. These techniques can produce artifacts in the image such as noise amplification and unwanted oscillations (at the level of 10% of the peak value in the restored point source) in the restored signal. Much useful information, including photometry of point sources, can be extracted from the restored HRI images of Comet Hartley 2, and other images from the Deep Impact spacecraft. In this paper, we present techniques for evaluating the restored images both qualitatively and quantitatively. Monte Carlo techniques are suggested to assess the accuracy of point source photometry.

We consider the origin of ∼380 quasi-circular depressions (pits) seen to be distributed in a broad band across the surface of 9P/Tempel 1 and show that possibly ∼96% may be due to outburst activity. Of the rest, 4 kg and find that the resulting pit should have a diameter in the range 10–30 m. Published locations of mini-outbursts are revised to account for changes in the nucleus shape, rotation rate, and rotation pole that have resulted from observations made during the Stardust-NExT mission. Both of these locations are found to fall in, or on the edge of, the band of pits that encircles the nucleus. We have identified features in high-resolution images near one of these locations as the possible places of origin of the mini-outbursts. These features show close packing of multiple pits in the appropriate diameter range. We consider the distribution of pit diameters and show that the largest pits follow a power–law with exponent −2.24 ± 0.09. Using the June 14, 2005, mini-outburst and the Deep Impact crater to provide a calibration, we establish empirical relationships between pit diameter, D , the total outburst energy, E , and the visual magnitude change, Δ m abs , which is the visual amplitude of the outburst referenced to a standard initial brightness. We find Log 10 D ∼ 0.107(±0.004)Δ m abs + 1.3(±0.4) and Log 10 E ∼ 0.32(±0.01)Δ m abs + 10.1(±1.2) where the uncertainties represent the range of values for the coefficient rather than formal error. We apply these approximate relationships to the mega -outburst on 17P/Holmes and predict that it left a pit-like scar on the surface with a diameter in the range 160–1300 m, that the total energy released was in the range 7 × 10 12 –3 × 10 15 J, and that between 6 × 10 7 and 1.3 × 10 11 kg of material was ejected from the surface. While these predictions are crude they encompass, particularly near the upper end of the range, the results on kinetic energy release and mass loss found by Reach et al. (Reach, W.T., Vaubaillon, J., Lisse, C.M., Holloway, M., Rho, J. [2010]. Icarus 208, 276–292) based on IR observations of 17P.

The evolution of the spin rate of Comet 9P/Tempel 1 through two perihelion passages (in 2000 and 2005) is determined from 1922 Earth-based observations taken over a period of 13 year as part of a World-Wide observing campaign and from 2888 observations taken over a period of 50 days from the Deep Impact spacecraft. We determine the following sidereal spin rates (periods): 209.023 +/- 0.025 degrees/dy (41.335 +/- 0.005 h) prior to the 2000 perihelion passage, 210.448 +/- 0.016 degrees/dy (41.055 +/- 0.003 h) for the interval between the 2000 and 2005 perihelion passages, 211.856 +/- 0.030 degrees/dy (40.783 +/- 0.006 h) from Deep Impact photometry just prior to the 2005 perihelion passage, and 211.625 +/- 0.012 degrees/dy (40.827 +/- 0.002 h) in the interval 2006-2010 following the 2005 perihelion passage. The period decreased by 16.8 +/- 0.3 min during the 2000 passage and by 13.7 +/- 0.2 min during the 2005 passage suggesting a secular decrease in the net torque. The change in spin rate is asymmetric with respect to perihelion with the maximum net torque being applied on approach to perihelion. The Deep Impact data alone show that the spin rate was increasing at a rate of 0.024 +/- 0.003 degrees/dy/dy at JD2453530.60510 (i.e., 25.134 dy before impact), which provides independent confirmation of the change seen in the Earth-based observations. The rotational phase of the nucleus at times before and after each perihelion and at the Deep Impact encounter is estimated based on the Thomas et al. (Thomas et al. [2007]. Icarus 187, 4-15) pole and longitude system. The possibility of a 180 error in the rotational phase is assessed and found to be significant. Analytical and physical modeling of the behavior of the spin rate through of each perihelion is presented and used as a basis to predict the rotational state of the nucleus at the time of the nominal (i.e., prior to February 2010) Stardust-NExT encounter on 2011 February 14 at 20:42. We find that a net torque in the range of 0.3-2.5 x 10(7) kg m(2) s(-2) acts on the nucleus during perihelion passage. The spin rate initially slows down on approach to perihelion and then passes through a minimum. It then accelerates rapidly as it passes through perihelion eventually reaching a maximum post-perihelion. It then decreases to a stable value as the nucleus moves away from the Sun. We find that the pole direction is unlikely to precess by more than similar to 1 degrees per perihelion passage. The trend of the period with time and the fact that the modeled peak torque occurs before perihelion are in agreement with published accounts of trends in water production rate and suggests that widespread H(2)O out-gassing from the surface is largely responsible for the observed spin-up. (C) 2011 Elsevier Inc. All rights reserved.

The Deep Impact flyby spacecraft includes a 1.05 to 4.8 μm infrared (IR) spectrometer. Although ice was not observed on the surface in the impact region, strong absorptions near 3 μm due to water ice are detected in IR measurements of the ejecta from the impact event. Absorptions from water ice occur throughout the IR dataset beginning three seconds after impact through the end of observations, ∼45 min after impact. Spatially and temporally resolved IR spectra of the ejecta are analyzed in conjunction with laboratory impact experiments. The results imply an internal stratigraphy for Tempel 1 consisting of devolatilized materials transitioning to unaltered components at a depth of approximately one meter. At greater depths, which are thermally isolated from the surface, water ice is present. Up to depths of 10 to 20 m, the maximum depths excavated by the impact, these pristine materials consist of very fine grained ( ∼ 1 ± 1 μm ) water ice particles, which are free from refractory impurities.

The results of recognition of cosmic ray (CR) signatures on a single image were analyzed for several codes written by several authors. For most visual images made during low solar activity at exposure time t>4 s, the number of clusters of bright pixels on an image per second per sq. cm of CCD was about 2-4, both for dark and normal sky images. At high solar activity, it sometimes exceeded 10. The ratio of the number of CR signatures consisting of 'n' pixels obtained at high solar activity to that at low solar activity was greater for greater 'n'. The number of clusters detected as CR signatures on a single infrared image is by at least a factor of several greater than the actual number of CR signatures; the number of clusters based on analysis of two successive dark frames is in agreement with an expected number of CR signatures. Some glitches of false CR signatures include bright pixels presented on different infrared images. Our interactive code allows one to delete long CR signatures and prevents removal of false CR signatures near the edge of a comet., Paper submitted to the journal "Advances in Space Research" (Proceedings of COSPAR-2006)

Galileo's Solid State Imager (SSI) observed Tvashtar Catena four times between November 1999 and October 2001, providing a unique look at a distinctive high latitude volcanic complex on Io. The first observation (orbit I25, November 1999) resolved, for the first time, an active extraterrestrial fissure eruption; the brightness temperature was at least 1300 K. The second observation (orbit I27, February 2000) showed a large ( ∼ 500 km 2 ) region with many, small, hot, regions of active lava. The third observation was taken in conjunction with Cassini imaging in December 2000 and showed a Pele-like, annular plume deposit. The Cassini images revealed an ∼ 400 km high Pele-type plume above Tvashtar Catena. The final Galileo SSI observation of Tvashtar (orbit I32, October 2001), revealed that obvious (to SSI) activity had ceased, although data from Galileo's Near Infrared Mapping Spectrometer (NIMS) indicated that there was still significant thermal emission from the Tvashtar region. In this paper, we primarily analyze the style of eruption during orbit I27 (February 2000). Comparison with a lava flow cooling model indicates that the behavior of the Tvashtar eruption during I27 does not match that of simple advancing lava flows. Instead, it may be an active lava lake or a complex set of lava flows with episodic, overlapping eruptions. The highest reliable color temperature is ∼ 1300 K . Although higher temperatures cannot be ruled out, they do not need to be invoked to fit the observed data. The total power output from the active lavas in February 2000 was at least 10 11 W .

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.

In 1998, Comet 9P/Tempel 1 was chosen as the target of the Deep Impact mission (A’Hearn, M. F., Belton, M. J. S., and Delamere, A., Space Sci. Rev., 2005) even though very little was known about its physical properties. Efforts were immediately begun to improve this situation by the Deep Impact Science Team leading to the founding of a worldwide observing campaign (Meech et al., Space Sci. Rev., 2005a). This campaign has already produced a great deal of information on the global properties of the comet’s nucleus (summarized in Table I) that is vital to the planning and the assessment of the chances of success at the impact and encounter. Since the mission was begun the successful encounters of the Deep Space 1 spacecraft at Comet 19P/Borrelly and the Stardust spacecraft at Comet 81P/Wild 2 have occurred yielding new information on the state of the nuclei of these two comets. This information, together with earlier results on the nucleus of comet 1P/Halley from the European Space Agency’s Giotto, the Soviet Vega mission, and various ground-based observational and theoretical studies, is used as a basis for conjectures on the morphological, geological, mechanical, and compositional properties of the surface and subsurface that Deep Impact may find at 9P/Tempel 1. We adopt the following working values (circa December 2004) for the nucleus parameters of prime importance to Deep Impact as follows: mean effective radius = 3.25 ± 0.2 km, shape — irregular triaxial ellipsoid with a/b = 3.2 ± 0.4 and overall dimensions of ∼14.4 × 4.4 × 4.4 km, principal axis rotation with period = 41.85 ± 0.1 hr, pole directions (RA, Dec, J2000) = 46 ± 10, 73 ± 10 deg (Pole 1) or 287 ± 14, 16.5 ± 10 deg (Pole 2) (the two poles are photometrically, but not geometrically, equivalent), Kron-Cousins (V–R) color = 0.56 ± 0.02, V-band geometric albedo = 0.04 ± 0.01, Rband geometric albedo = 0.05 ± 0.01, R-band H(1, 1, 0) = 14.441 ± 0.067, and mass ∼7 × 1013 kg assuming a bulk density of 500 kgm−3. As these are working values, i.e., based on preliminary analyses, it is expected that adjustments to their values may be made before encounter as improved estimates become available through further analysis of the large database being made available by the Deep Impact observing campaign. Given the parameters listed above the impact will occur in an environment where the local gravity is estimated at 0.027–0.04 cm s−2 and the escape velocity between 1.4 and 2ms−1. For both of the rotation poles found here, the Deep Impact spacecraft on approach to encounter will find the rotation axis close to the plane of the sky (aspect angles 82.2 and 69.7 deg. for pole 1 and 2, respectively). However, until the rotation period estimate is substantially improved, it will remain uncertain whether the impactor will collide with the broadside or the ends of the nucleus.

We present the observations of Io acquired by the Solid State Imaging (SSI) experiment during the Galileo Millennium Mission (GMM) and the strategy we used to plan the exploration of Io. Despite Galileo's tight restrictions on data volume and downlink capability and several spacecraft and camera anomalies due to the intense radiation close to Jupiter, there were many successful SSI observations during GMM. Four giant, high-latitude plumes, including the largest plume ever observed on Io, were documented over a period of eight months; only faint evidence of such plumes had been seen since the Voyager 2 encounter, despite monitoring by Galileo during the previous five years. Moreover, the source of one of the plumes was Tvashtar Catena, demonstrating that a single site can exhibit remarkably diverse eruption styles—from a curtain of lava fountains, to extensive surface flows, and finally a ∼400 km high plume—over a relatively short period of time (∼13 months between orbits I25 and G29). Despite this substantial activity, no evidence of any truly new volcanic center was seen during the six years of Galileo observations. The recent observations also revealed details of mass wasting processes acting on Io. Slumping and landsliding dominate and occur in close proximity to each other, demonstrating spatial variation in material properties over distances of several kilometers. However, despite the ubiquitous evidence for mass wasting, the rate of volcanic resurfacing seems to dominate; the floors of paterae in proximity to mountains are generally free of debris. Finally, the highest resolution observations obtained during Galileo's final encounters with Io provided further evidence for a wide diversity of surface processes at work on Io.

Venus Exploration of Volcanoes and Atmosphere (VEVA) is a potential Discovery mission. It will provide a multidisciplinary investigation of the atmosphere and surface of Venus by returning the first-ever aerial photography of the surface and definitive, in situ determination of its atmospheric composition in the lower scale height. VEVA involves delivering an entry vehicle to Venus housing a payload consisting of an instrumented atmospheric sonde and a balloon–gondola system. After entry, the atmospheric sonde is released to fall to the surface, and the balloon package inflates, which arrests its descent at a 60-km float height. The balloon carries an instrumented gondola with battery power for 7 days as it circles Venus. The gondola carries four small imaging sondes for release and descent to the surface at different times during the 7-day mission. Data from the sondes are radioed to the gondola for relay to Earth.

The Solid-State Imaging (SSI) instrument provided the first high- and medium-resolution views of Io as the Galileo spacecraft closed in on the volcanic body in late 1999 and early 2000. While each volcanic center has many unique features, the majority can be placed into one of two broad categories. The "Promethean" eruptions, typified by the volcanic center Prometheus, are characterized by long-lived steady eruptions producing a compound flow field emplaced in an insulating manner over a period of years of decades. In contrast, "Pillanian" eruptions are characterized by large pyroclastic deposits and short-lived but high effusion rate eruptions from fissures feeding open-channel or open-sheet flows. Both types of eruptions commonly have ~100-km-tall, bright, SO2-rich plumes forming near the flow fronts and smaller deposits of red material that mark the vent for the silicate lavas.

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 >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

During late 1999/early 2000, the solid state imaging experiment on the Galileo spacecraft returned more than 100 high-resolution (5 to 500 meters per pixel) images of volcanically active Io. We observed an active lava lake, an active curtain of lava, active lava flows, calderas, mountains, plateaus, and plains. Several of the sulfur dioxide–rich plumes are erupting from distal flows, rather than from the source of silicate lava (caldera or fissure, often with red pyroclastic deposits). Most of the active flows in equatorial regions are being emplaced slowly beneath insulated crust, but rapidly emplaced channelized flows are also found at all latitudes. There is no evidence for high-viscosity lava, but some bright flows may consist of sulfur rather than mafic silicates. The mountains, plateaus, and calderas are strongly influenced by tectonics and gravitational collapse. Sapping channels and scarps suggest that many portions of the upper ∼1 kilometer are rich in volatiles.

We present a brief synopsis of the nature of SSI data that was taken during the nominal Galileo mission. Significant results are briefly described. These include evidence for geologic activity on Europa that supports the hypothesis of liquid water under a thin ice shell; a demonstration of the ubiquitous presence of a thick crust and high temperature silicate volcanism on Io; a demonstration that, together with results from other experiments, the molecular composition in Jupiter's atmosphere depends on local meteorology; the resolution of lightning flashes and their association with cloud type; the identification of cloud structures at or below the 4-bar level that are presumably composed of water; the location and morphology of visible aurorae in Jupiter's Northern hemisphere; improved geodetic control nets for the Galilean satellites; measurement of the sizes and shapes of Metis, Adrastea and Amalthea; new features of the main and gossamer rings; discovery of visible Io-glow and surface hotspots especially at the sub- and anti-jove regions on Io; evidence for solid state convection in Europa's ice shell and its probable non-synchronous rotation; the discovery of visible Europa-glow; evidence that tectonism has dominated the resurfacing of bright grooved terrain on Ganymede; the discovery of a thick blanket of dark material on Callisto's surface and that mass-wasting and sublimation are significant degradation processes on this satellite.

It has been proposed that Jupiter's satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H2O layer about 100 km thick but cannot determine the current physical state of this layer (liquid or solid). Here we summarize the geological evidence regarding an extant subsurface ocean, concentrating on Galileo imaging data. We describe and assess nine pertinent lines of geological evidence: impact morphologies, lenticulae, cryovolcanic features, pull-apart bands, chaos, ridges, surface frosts, topography, and global tectonics. An internal ocean would be a simple and comprehensive explanation for a broad range of observations; however, we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm, soft ice with only localized or partial melting. Two different models of impact flux imply very different surface ages for Europa; the model favored here indicates an average age of ∼50 Myr. Searches for evidence of current geological activity on Europa, such as plumes or surface changes, have yielded negative results to date. The current existence of a global subsurface ocean, while attractive in explaining the observations, remains inconclusive. Future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within Europa today.

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

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 α = 5°. Coverage, obtained at three spectral bandpasses (VLT, 0.41 μm, GRN, 0.56 μm; and 1MC, 0.99 μm) at a spatial resolution of 404 m/pixel, shows a 162 × 220-km region of Europa's surface located at 30°N, 162°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 μm and 5° 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 α = 5° to α = 0°, 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 (

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 (λ = 0.64 μm) 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 μm) 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 (

On November 9, 1996 and again on April 2, 1997, the Galileo spacecraft's Solid State Imaging (SSI) camera targeted the northern auroral region of Jupiter. These observations represent (i) the first spatially resolved images of the jovian auroral oval either at visible wavelengths or on the nightside of the planet, (ii) the first image at visible wavelengths of an auroral footprint of the Io Flux Tube (IFT), (iii) the first unambiguous detection at visible wavelengths of auroral emission on the jovian limb, and (iv) the first images of the aurora with spatial resolution below 100 km per pixel (46 and 35 km, respectively). Relative to many prior expectations, the visible aurora is (i) lower in altitude, (ii) associated with magnetic field lines that cross the equator closer to the planet, and (iii) more variable in time and space. The 1996 images used a clear (broadband) filter, while the 1997 images used both the clear filter and five narrower filters over wavelengths ranging from violet to 968 nm. The filtered images imply that the visible auroral emission contains atomic hydrogen lines, although there is also a continuum component. We were able to position the aurora in three-dimensional space and found the limb emission to be ∼240 km above the surface of a standard (P≈ 1 bar) reference ellipsoid. Our most accurate analysis of the equatormost part of the oval placed it at 54.5° planetocentric latitude and 168° west longitude. Combined with the latest magnetic field models, our results imply that the particles that cause the aurora originate in Jupiter's equatorial plane ∼13 R_J from the center of the planet. The oval was brighter and wider in the 1996 images than in the 1997 images. The broadband radiance of a typical place on the oval as seen directly overhead varied from ∼80 kR in 1997 to ∼300 kR in 1996. Our estimates of the full width of the oval varied from under 500 km to over 8000 km, partly depending on the signal-to-noise ratio of the image. The radiated power per unit length along the oval ranged from ∼60 to ∼700 W/m, with the associated radiated power from the entire oval varying from ∼109 to ∼9 × 10^(10) W. Appreciable auroral emission also occurred both north and south of the main oval. One image contains the northern footprint of the IFT, which appears as a central ellipse with a tail of emission that lies downstream with respect to the plasma flow past Io. The central ellipse is ∼1200 km downstream by ∼500 km cross stream. The IFT is comparable in brightness to the nearby auroral oval (∼250 kR) and has a total radiated power of ∼3 × 10^8 W.

Images of Europa from the Galileo spacecraft show a surface with a complex history involving tectonic deformation, impact cratering, and possible emplacement of ice-rich materials and perhaps liquids on the surface. Differences in impact crater distributions suggest that some areas have been resurfaced more recently than others; Europa could experience current cryovolcanic and tectonic activity. Global-scale patterns of tectonic features suggest deformation resulting from non-synchronous rotation of Europa around Jupiter. Some regions of the lithosphere have been fractured, with icy plates separated and rotated into new positions. The dimensions of these plates suggest that the depth to liquid or mobile ice was only a few kilometers at the time of disruption. Some surfaces have also been upwarped, possibly by diapirs, cryomagmatic intrusions, or convective upwelling. In some places, this deformation has led to the development of chaotic terrain in which surface material has collapsed and/or been eroded.

During the first six orbits of the Galileo spacecraft's prime mission, the Solid State Imaging (SSI) system acquired multispectral image mosaics of Jupiter's Great Red Spot, an equatorial belt/zone boundary, a “5-μm hot spot” similar to the Galileo Probe entry site, and two of the classic White Ovals. We present mosaics of each region, approximating their appearance at visible wavelengths and showing cloud height and opacity variations. The local wind field is derived by tracking cloud motions between multiple observations of each region with time separations of roughly 1 and 10 hr. Vertical cloud structure is derived in a companion paper by Banfieldet al. (Icarus135, 230–250). Galileo's brief, high-resolution observations complement Earth-based and Voyager studies and offer local meteorological context for the Galileo Probe results. Our results show that the dynamics of the zonal jets and large vortices have changed little since Voyager, with a few exceptions. We detect a cyclonic current within the center of the predominantly anticyclonic Great Red Spot. The zonal velocity difference between 0° S and 6° S has increased by 20 m sec^(−1). We measure a strong northeast flow approaching the hot spot. This flow indicates either massive horizontal convergence or the presence of a large anticyclonic vortex southeast of the hot spot. The current compact arrangement of two White Ovals and a cyclonic structure greatly perturbs the zonal jets in that region.

High-resolution Galileo imaging has provided important insight into the origin and evolution of grooved terrain on Ganymede. The Uruk Sulcus target site was the first imaged at high resolution, and considerations of resolution, viewing geometry, low image compression, and complementary stereo imaging make this region extremely informative. Contrast variations in these low-incidence angle images are extreme and give the visual impression of topographic shading. However, photometric analysis shows that the scene must owe its character to albedo variations. A close correlation of albedo variations to topography is demonstrated by limited stereo coverage, allowing extrapolation of the observed brightness and topographic relationships to the rest of the imaged area. Distinct geological units are apparent across the region, and ridges and grooves are ubiquitous within these units. The stratigraphically lowest and most heavily cratered units (“lineated grooved terrain”) generally show morphologies indicative of horst-and-graben-style normal faulting. The stratigraphically highest groove lanes (“parallel ridged terrain”) exhibit ridges of roughly triangular cross section, suggesting that tilt-block-style normal faulting has shaped them. These extensional-tectonic models are supported by crosscutting relationships at the margins of groove lanes. Thus, a change in tectonic style with time is suggested in the Uruk Sulcus region, varying from horst and graben faulting for the oldest grooved terrain units to tilt block normal faulting for the latest units. The morphologies and geometries of some stratigraphically high units indicate that a strike-slip component of deformation has played an important role in shaping this region of grooved terrain. The most recent tectonic episode is interpreted as right-lateral transtension, with its tectonic pattern of two contemporaneous structural orientations superimposed on older units of grooved terrain. There is little direct evidence for cryovolcanic resurfacing in the Uruk Sulcus region; instead tectonism appears to be the dominant geological process that has shaped the terrain. A broad wavelength of deformation is indicated, corresponding to the Voyager-observed topography, and may be the result of ductile necking of the lithosphere, while a finer scale of deformation probably reflects faulting of the brittle near surface. The results here form a basis against which other Galileo grooved terrain observations can be compared.

Active volcanism on Io has been monitored during the nominal Galileo satellite tour from mid 1996 through late 1997. The Solid State Imaging (SSI) experiment was able to observe many manifestations of this active volcanism, including (1) changes in the color and albedo of the surface, (2) active airborne plumes, and (3) glowing vents seen in eclipse. About 30 large-scale (tens of kilometers) surface changes are obvious from comparison of the SSI images to those acquired by Voyager in 1979. These include new pyroclastic deposits of several colors, bright and dark flows, and caldera-floor materials. There have also been significant surface changes on Io during the Galileo mission itself, such as a new 400-km-diameter dark pyroclastic deposit around Pillan Patera. While these surface changes are impressive, the number of large-scale changes observed in the four months between the Voyager 1 and Voyager 2 flybys in 1979 suggested that over 17 years the cumulative changes would have been much more impressive. There are two reasons why this was not actually the case. First, it appears that the most widespread plume deposits are ephemeral and seem to disappear within a few years. Second, it appears that a large fraction of the volcanic activity is confined to repeated resurfacing of dark calderas and flow fields that cover only a few percent of Io's surface. The plume monitoring has revealed 10 active plumes, comparable to the 9 plumes observed by Voyager. One of these plumes was visible only in the first orbit and three became active in the later orbits. Only the Prometheus plume has been consistently active and easy to detect. Observations of the Pele plume have been particularly intriguing since it was detected only once by SSI, despite repeated attempts, but has been detected several times by the Hubble Space Telescope at 255 nm. Pele's plume is much taller (460 km) than during Voyager 1 (300 km) and much fainter at visible wavelengths. Prometheus-type plumes (50–150 km high, long-lived, associated with high-temperature hot spots) may result from silicate lava flows or shallow intrusions interacting with near-surface SO2. A major and surprising result is that ∼30 of Io's volcanic vents glow in the dark at the short wavelengths of SSI. These are probably due to thermal emission from surfaces hotter than 700 K (with most hotter than 1000 K), well above the temperature of pure sulfur volcanism. Active silicate volcanism appears ubiquitous. There are also widespread diffuse glows seen in eclipse, related to the interaction of energetic particles with the atmosphere. These diffuse glows are closely associated with the most active volcanic vents, supporting suggestions that Io's atmopshere is dominated by volcanic outgassing. Globally, volcanic centers are rather evenly distributed. However, 14 of the 15 active plumes seen by Voyager and/or Galileo are within 30° of the equator, and there are concentrations of glows seen in eclipse at both the sub- and antijovian points. These patterns might be related to asthenospheric tidal heating or tidal stresses. Io will continue to be observed during the Galileo Europa Mission, which will climax with two close flybys of Io in late 1999.

Galileo's Solid State Imaging camera recorded six images at wavelengths from 0.41 to 0.99 μm of Europa's trailing hemisphere (∼1.6 km/pixel resolution) during the G1 orbit (1st orbit—target Ganymede) of the nominal mission. We have photometrically corrected these data and extracted spectra representing Europa's diverse geologic terrains. The goals of the analysis of these spectra are (1) to determine whether Europa's geologic units differ spectrally from one another, (2) to determine the number of color components necessary to explain Europa's dark material spectral behavior, and (3) to examine how europan dark materials may change in color with time. Our data indicate that europan dark spots, lineaments and triple band side materials represent a single “dark” and reddish endmember component. We see no evidence for more than one dark endmember; however the dark endmember seen in these G1 data is probably not a “pure” exposure of europan dark material. Mottled terrains, brighter lineaments, and aging triple bands can be modeled by a mixture of bright plains materials and the dark component. Exposures of europan dark materials may thus be brightening with time, eventually blending into the surrounding bright plains.

We report the direct detection of solid water ice deposits exposed on the surface of comet 9P/Tempel 1, as observed by the Deep Impact mission. Three anomalously colored areas are shown to include water ice on the basis of their near-infrared spectra, which include diagnostic water ice absorptions at wavelengths of 1.5 and 2.0 micrometers. These absorptions are well modeled as a mixture of nearby non-ice regions and 3 to 6% water ice particles 10 to 50 micrometers in diameter. These particle sizes are larger than those ejected during the impact experiment, which suggests that the surface deposits are loose aggregates. The total area of exposed water ice is substantially less than that required to support the observed ambient outgassing from the comet, which likely has additional source regions below the surface.

High-temperature hot spots on Io have been imaged at ∼50 km spatial resolution by Galileo's CCD imaging system (SSI). Images were acquired during eclipses (Io in Jupiter's shadow) via the SSI clear filter (∼0.4–1.0 µm), detecting emissions from both small intense hot spots and diffuse extended glows associated with Io‧s atmosphere and plumes. A total of 13 hot spots have been detected over ∼70% of Io–s surface. Each hot spot falls precisely on a low-albedo feature corresponding to a caldera floor and/or lava flow. The hot-spot temperatures must exceed ∼700 K for detection by SSI. Observations at wavelengths longer than those available to SSI require that most of these hot spots actually have significantly higher temperatures (∼1000 K or higher) and cover small areas. The high-temperature hot spots probably mark the locations of active silicate volcanism, supporting suggestions that the eruption and near-surface movement of silicate magma drives the heat flow and volcanic activity of Io.

International audience; NASA's EPOXI mission used the Deep Impact (DI) Flyby spacecraft to deliver a payload of three scientific instruments, two visible cameras and an IR spectrometer, to a close flyby of Comet 103P/Hartley 2 in November 2010. Interpretation of the scientific measurements made using these instruments depends on accurate calibration of the instruments' performance. Updates to the instrument calibrations achieved during the Deep Impact primary mission and results of continued monitoring of their performance during EPOXI are reported here. The instruments' performance has remained remarkably stable over the nearly 7 years of flight. Significant improvements in the understanding and calibration of the IR spectrometer response non-linearity, time-varying background level, flat field, wavelength map, and absolute spectral response have been achieved. Techniques for reducing some semi-coherent horizontal noise stripes in the visible cameras' readouts were developed, and some adjustments have been made to their absolute radiometric conversion constants. The data processing pipeline has been updated to incorporate the improvements in the instrument calibrations. (C) 2013 Elsevier Inc. All rights reserved.

International audience; The nucleus of comet Tempel 1 has been investigated at close range during two spacecraft missions separated by one comet orbit of the Sun, 51/2 years. The combined imaging covers similar to 70% of the surface of this object which has a mean radius of 2.83 +/- 0.1 km. The surface can be divided into two terrain types: rough, pitted terrain and smoother regions of varying local topography. The rough surface has round depressions from resolution limits (similar to 10 m/pixel) up to similar to 1 km across, spanning forms from crisp steep-walled pits, to subtle albedo rings, to topographic rings, with all ranges of morphologic gradation. Three gravitationally low regions of the comet have smoother terrain, parts of which appear to be deposits from minimally modified flows, with other parts likely to be heavily eroded portions of multiple layer piles. Changes observed between the two missions are primarily due to backwasting of scarps bounding one of these probable flow deposits. This style of erosion is also suggested by remnant mesa forms in other areas of smoother terrain. The two distinct terrains suggest either an evolutionary change in processes, topographically-controlled processes, or a continuing interaction of erosion and deposition. (c) 2012 Elsevier Inc. All rights reserved.

International audience; The Deep Impact spacecraft flew by Comet 103P/Hartley 2 on November 4th, 2010 (EPOXI mission) and Comet 9P/Tempel 1 on July 4th, 2005 (Deep Impact mission). During the two flybys, spatially resolved infrared (1.05-4.8 mu m) spectra of the surface of the nucleus were acquired by the HRI-IR instrument. The analysis of these two data sets, obtained by the same instrument, offers a unique opportunity to understand, compare and contrast the surface thermal properties of these two comets. For this paper, we use spectral cubes with a spatial resolution of 30 m/pixel to 40 m/pixel for Hartley 2 and 160 m/pixel for Tempel 1. We focus our analysis on the color, temperature, thermal inertia and roughness of the nucleus. The two comets have the same color, moderately red, with an average slope of 3.0 +/- 0.9% per k angstrom to 3.5 +/- 1.1% per k angstrom. There are very small variations of the color across the surface, except for regions with water ice that are neutral to blue, and two dark spots with redder (4.5 +/- 1.4% per k angstrom) materials on Hartley 2. The nucleus thermal emission at all resolved spatial scales differs from that of a gray body with an infrared emissivity of 0.9-1.0, the discrepancy being more important for larger incidence angles. Moreover, the color temperature of Comets Hartley 2 and Tempel 1 is relatively homogeneous across the surface and does not vary strongly with incidence angle. These two effects mainly result from surface roughness and associated projected shadows. From the temperature rise on the morning terminator, we derive a thermal inertia lower than 250 W/K/m(2)/s(1/2) for Hartley 2 and lower than 45 W/K/m(2)/s(1/2) for Tempel 1 (3 sigma upper limits). For Hartley 2 and Tempel 1, the temperature of the regions with exposed water ice is more than 100 K above the sublimation temperature of water ice (similar to 200 K). This observation indicates that the thermal emission is dominated by dust, and that water ice is not intimately mixed with dust at the scale of observation, with water ice patches at the meter or sub-meter scale. (c) 2012 Elsevier Inc. All rights reserved.

International audience; Data from the Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) mission show Comet 103P/Hartley 2 is a bi-lobed, elongated, nearly axially symmetric comet 2.33 km in length. Surface features are primarily small mounds \textless40 m across, irregularly-shaped smooth areas on the two lobes, and a smooth but variegated region forming a “waist” between the two lobes. Assuming parts of the comet body approach the shape of an equipotential surface, the mean density of Hartley 2 is modeled to be 200-400 kg m(-3). Such a mean density suggests mass loss per orbit of \textgreater1%. The shape may be the evolutionary product of insolation, sublimation, and temporary deposition of materials controlled by the object's complex rotation. (C) 2012 Elsevier Inc. All rights reserved.

Galileo SSI color data between 0.4 and 1.0 μm demonstrate that both Ida and Dactyl are S-type asteroids with similar, but distinct spectra. Small but definite color variations are also observed on Ida itself and involve both the blue part of the spectrum and the depth of the 1-μm pyroxene–olivine band. Ida's surface can be classified into two color terrains: Terrain A has a shallower 1-μm absorption and a steeper visible red slope than does Terrain B. Qualitatively, the color–albedo systematics of these two terrains follow those noted for color units on Gaspra and the variations in 1-μm band depth with weathering described by Gaffeyet al.(Gaffey, M. J., J. F. Bell, R. H. Brown, T. H. Burbine, J. Piatek, K. L. Reed, and D. A. Chaky 1993.Icarus106, 573–602). Terrain A, with its slightly lower albedo, its shallower 1-μm band, and its slightly steeper visible red slope relative to Terrain B could be interpreted as the “more processed,” “more mature,” or the “more weathered” of the two terrains. Consistent with this interpretation is that Terrain A appears to be the ubiquitous background on most of Ida, while Terrain B is correlated with some small craters as well as with possible ejecta from the 10-km Azzurra impact structure. Because of these trends, it is less likely that differences between Terrains A and B are caused by an original compositional inhomogeneity within the body of Ida, although they do fall within the range known to occur within the Koronis family. The spectrum of Dactyl is similar to, but definitely different from, that of Terrain B on Ida. It does not conform to the pattern that obtains between the colors and albedos of Terrains A and B: the satellite's 1-μm band is deeper than that of Terrain B, but its albedo is lower, rather than higher. By itself, the deeper band depth could be interpreted, following Gaffeyet al., to mean that Dactyl is a less weathered version of Terrain B on Ida, but such an interpretation is at odds with Dactyl's redder spectral slope. Thus, the explanation for the color difference between Dactyl and Ida is likely to be different from that which accounts for the differences between the two terrains on Ida. Given that Dactyl and Ida have very similar photometric properties (Helfenstein, P., J. Veverka, P. C. Thomas, D. P. Simonelli, K. Klassen, T. V. Johnson, F. Fanale, J. Granahan, A. S. McEwen, M. J. S. Belton, and C. R. Chapman 1996Icarus120, 48–65), thus ruling out any dramatic texture differences between the two surfaces, the most likely explanation is that the satellite has a slightly different composition (more pyroxene?) than Ida. The spectral difference is within the range reported by Binzelet al.(Binzel, R. P., S. Xu, and S. J. Bus 1993.Icarus106, 608–611.) for members of the Koronis family, and could be caused by compositional inhomogeneities of the Koronis parent body rather than by post-breakup regolith processes.

Galileo imaging observations over phase angles 19.5° to 109.8° are combined with near-opposition Earth-based data to derive the photometric properties of Ida. To first order these properties are uniform over the surface and well modeled at λ = 0.55 μm by Hapke parameters ω0= 0.22,h= 0.020,B0= 1.5,g= −0.33, and θ = 18° with corresponding geometric albedop= 0.21±0.030.01and Bond albedoAB= 0.081±0.0170.008. Ida's photometric properties are more similar to those of “average S-asteroids” (P. Helfenstein and J. Veverka 1989,Asteroids II, Univ. of Arizona Press, Tucson) than are those of 951 Gaspra. Two primary color units are identified on Ida: Terrain A exhibits a spectrum with relatively shallower 1-μm absorption and a relatively steeper red spectral slope than average Ida, while Terrain B has a deeper 1-μm absorption and a less steep red slope. The average photometric properties of Ida and Terrain A are similar while those of Terrain B differ mostly in having a slightly higher value of ω0(0.22 versus 0.21), suggesting that Terrain B consists of slightly brighter, more transparent regolith particles. Galileo observations of Ida's satellite Dactyl over phase angles 19.5° to 47.6° suggest photometric characteristics similar to those of Ida, the major difference being Dactyl's slightly lower albedo (0.20 compared to 0.21).

We provide an overview of the execution, data, and results of the solid state imaging (SSI) experiment at the encounter of the Galileo spacecraft with the asteroid 243 Ida. Ninety-six images of the asteroid, representing 18 time samples during a rotation period (4.633 h), were transmitted to Earth as a result of the UT 1993 August 28.70284 encounter. This provided coverage of ∼95% of the surface and achieved ground resolutions as high as 25 m/pixel. Coverage of most of Ida's surface is available in four colors, with limited regions in five colors, at resolutions up to 105 m/pixel. A natural satellite of Ida, called Dactyl, was discovered in a prograde (with respect to Ida's spin), near-equatorial, orbit moving slowly (∼ 6 m/sec) with a separation of 85 km from Ida.s Ida's shape is highly irregular; by comparison, Dactyl's global topography is quite smooth. The best fit ellipsoid to Ida's shape has principal dimensions 59.8 × 25.4 × 18.6 km, mean radius 15.7 km, and volume 16,100 ± 1900 km3. Dactyl's mean radius is only 0.7 km. Ida's spin axis (right ascension: 348.76° ± 7.5°; declination: 87.10° ± 0.4°; J2000) was found to align with the principal axis of inertia to within the error of measurement. This is consistent with a homogeneous density distribution. Dactyl's rotation rate is unknown, but its long axis was pointed in the direction of Ida at the time of observation, suggesting synchronism of its orbital motion and spin. Constraints on Dactyl's orbit yield 4.2 ± 0.6 × 1019g for Ida's mass and 2.6 ± 0.5 g/cm3for its bulk density. Unless Ida's bulk porosity is exceptionally high, Ida has moderate to low NiFe content. Subtle color variations across the surface of Ida are associated with fresh craters, but, unlike the case for Asteroid 951 Gaspra, are not correlated with topographic features such as ridges. This difference may be a reflection of a deeper and/or more mobile regolith on Ida. Dactyl's spectral reflectance is similar to, but quantitatively distinct from the surface of Ida itself. This difference may reflect compositional differences between Dactyl and Ida, which in turn may have originated in an only partially differentiated Koronis parent body. Results on the origin, collisional history, and geology of Ida and Dactyl are the subject of many of the papers in this special issue. There is general agreement that these asteroids originated in the catastrophic breakup of the Koronis parent body and that the formation of asteroid–satellite systems may be relatively common in such events. The age and collisional history of the pair present a dilemma: using standard interpretations of the cratering record on Ida's surface, an age > 1 byr. is indicated. However, the lifetime of Dactyl against collisional disruption is many times less than this. Novel ideas are presented concerning the collisional history of these two small objects that may resolve this dilemma. These ideas result from analysis of the geological record on the surface of Ida, Dactyl, and, by comparison, Gaspra—all of which are examined in this special issue. The execution of the Galileo flybys of Gaspra, Ida, and Dactyl provide important lessons for future flybys of small bodies. We present our views on the limitations faced by the Galileo imaging experimenters and indicate how future missions can be made more quantitative and productive through the application of innovative electronic control systems and detector technology.

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 (∼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 × 1.4 × 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-μm colors. Like Ida, Dactyl is an S-asteroid, but has a slightly deeper 1-μm 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 ≲90 to 280 m diameter are visible in the best image (39 m/pxl at 47° 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.

DURING its reconnaissance of the asteroid 243 Ida, the Galileo spacecraft returned images of a second object, 1993(243)1 Dactyl1— the first confirmed satellite of an asteroid. Sufficient data were obtained on the motion of Dactyl to determine its orbit as a function of Ida's mass. Here we apply statistical and dynamical arguments to constrain the range of possible orbits, and hence the mass of Ida. Combined with the volume of Ida2, this yields a bulk density of 2.6 ± 0.5 g cm−3. Allowing for the uncertainty in the porosity of Ida, this density range is consistent with a bulk chon-dritic composition, and argues against some (but not all) classes of meteoritic igneous rock types that have been suggested as compositionally representative of S-type asteroids like Ida.

MASTER (Mainbelt Asteroid Exploration/Rendezvous) focuses on the comprehensive global study of important large mainbelt asteroids. A three instument, four-investigation payload can be placed into orbit around 4 vesta within the cost/complexity envelope of the Discovery program using a Delta-II launch vehicle.

International audience; We describe recent results on the CO/CO2/H2O composition of comets together with a survey of older literature (primarily for CO/H2O) and compare these with models of the protoplanetary disk. Even with the currently small sample, there is a wide dispersion in abundance ratios and little if any systematic difference between Jupiter-family comets (JFCs) and long-period and Halley-type comets (LPCs and HTCs). We argue that the cometary observations require reactions on grain surfaces to convert CO to CO2 and also require formation of all types of comets in largely, but not entirely, overlapping regions, probably between the CO and CO2 snow lines. Any difference in the regions of formation is in the opposite direction from the classical picture with the JFCs having formed closer to the Sun than the LPCs. In the classical picture, the LPCs formed in the region of the giant planets and the JFCs formed in the Kuiper Belt. However, these data suggest, consistent with suggestions on dynamical grounds, that the JFCs and LPCs formed in largely overlapping regions where the giant planets are today and with JFCs on average forming slightly closer to the Sun than did the LPCs. Presumably at least the JFCs passed through the scattered disk on their way to their present dynamical family.

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.

On 29 October 1991, the Galileo spacecraft carried out the first ever encounter of a minor planet, flying by Asteroid 951 Gaspra at a distance of 1600 km. We summarize findings derived from the 57 images obtained by Galileo's SSI camera which cover about 80% of the surface of this S-asteroid. The highest resolution achieved was 54 m/pixel. The analysis of the imaging data is described in the series of papers making up this special issue of Icarus . Gaspra is a highly irregular body with principal diameters of 18.2, 10.5, and 8.9 km (average radius = 6.1 ± 0.4 km). Gaspra's irregular shape and the prominence of grooves, linear depressions 100-300 m wide and tens of meters deep, suggest that the asteroid was derived from a larger body by catastrophic collision. Features that appear to reflect structural grain, including ridges, grooves, and flat surfaces, suggest that Gaspra is a single coherent body and not a binary or a "rubble pile." Craters on Gaspra can be divided into two categories: a very subdued, apparently older, population and a population of much crisper (younger?) craters. The latter is characterized by an index (exponent of the power law describing the cumulative distribution of craters as a function of diameter) of -3.1 ± 0.3, definitely steeper than the theoretically derived value of -2.5 expected for collisional equilibrium. Estimates of the cratering age of Gaspra's surface give very young values ranging from 20 to 300 myr. Estimated characteristic times for the collisional disruption of Gaspra-sized bodies at 2.2 AU range from 200 to 1000 myr. Analysis of spectral imaging data (0.40 to 1.10 μm) reveals small but significant color variations over the asteroid's surface. The spectrally most distinct materials on Gaspra are distinguished by a more prominent 1-μm absorption band and tend to be slightly brighter and bluer than average Gaspra. Often such materials are associated with small, fresh-appearing craters along ridges. A strong correlation exists between the infrared/violet color ratio and elevations on Gaspra, a correlation which can be explained in terms of downhill migration of a regolith. Gaspra's photometric properties are lunar-like, but with higher values of ϖ 0 and θ (0.36 and 29°, respectively). The average geometric albedo is 0.23 near 0.56 μm. An important result is the confirmation that pre-Galileo inferences from telescopic observations about Gaspra's size, shape, albedo, and rotation state are accurate.

Galileo images of Gaspra make it possible for the first time to determine a main-belt asteroid's photometric properties accurately by providing surface-resolved coverage over a wide range of incidence and emission angles and by extending the phase angle coverage to phases not observable from Earth. We combine Earth-based telescopic photometry over phase angles 2° ≤ α ≤ 25° with Galileo whole-disk and disk-resolved data at 33° ≤ α ≤ 51° to derive average global photometric properties in terms of Hapke's photometric model. The microscopic texture and particle phase-function behavior of Gaspra's surface are remarkably like those of other airless rocky bodies such as the Moon. The macroscopic surface roughness parameter, θ = 29°, is slightly larger than that reported for typical lunar materials. The particle single scattering albedo, ω0 = 0.36 ± 0.07, is significantly larger than for lunar materials, and the opposition surge amplitude, B0 = 1.63 ± 0.07, is correspondingly smaller. We determine a visual geometric albedo pv = 0.22 ± 0.06 for Gaspra, in close agreement with pv = 0.22 ± 0.03 estimated from Earth-based observations. Gaspra's phase integral is 0.47, and the bolometric Bond albedo is estimated to be 0.12 ± 0.03. An albedo map derived by correcting Galileo images with our average global photometric function reveals subdued albedo contrasts of ±10% or less over Gaspra's northern hemisphere. Several independent classification algorithms confirm the subtle spectral heterogeneity reported earlier (S. Mottola, M. DiMartino, M. Gonano-Beurer, H. Hoffman, and G. Neukum, 1993, Asteroids, Comets, Meteors, pp. 421-424; M. J. S. Belton et al., 1992, Science 257, 1647-1652). Whole-disk colors (0.41 ≤ λ ≤ 0.99 μm) vary systematically with longitude by about ±5%, but color differences as large as 30% occur locally. Colors vary continuously between end-member materials whose areal distribution correlates with regional topography. Infrared: violet (0.99:0.41-μm) color ratios on Gaspra are strongly correlated with local elevation, being largest at lower elevations and smaller at higher elevations. No correlation was detected between elevation and the green:violet (0.56:0.41-μm) color ratio. Bright materials with a strong 1-μm absorption occur primarily in association with craters along ridges, while darker materials with 30% weaker 1-μm signatures occur downslope. The variations of color and albedo cannot be easily explained by grain-size effects alone or by differences in photometric geometry. The trends observed are consistent with those revealed by laboratory studies of the effects of comminution, glass formation, and segregation of metal from silicate components in chondritic meteorites and also in some silicate mixtures. The relative importance of these various processes on Gaspra remains to be determined.