Spitzer spectral observations of the deep impact ejecta.

Spitzer Space Telescope imaging spectrometer observations of comet 9P/Tempel 1 during the Deep Impact encounter returned detailed, highly structured, 5- to 35-micrometer spectra of the ejecta. Emission signatures due to amorphous and crystalline silicates, amorphous carbon, carbonates, phyllosilicates, polycyclic aromatic hydrocarbons, water gas and ice, and sulfides were found. Good agreement is seen between the ejecta spectra and the material emitted from comet C/1995 O1 (Hale-Bopp) and the circumstellar material around the young stellar object HD100546. The atomic abundance of the observed material is consistent with solar and C1 chondritic abundances, and the dust-to-gas ratio was determined to be greater than or equal to 1.3. The presence of the observed mix of materials requires efficient methods of annealing amorphous silicates and mixing of high- and low-temperature phases over large distances in the early protosolar nebula.

Exposed water ice deposits on the surface of comet 9P/Tempel 1.

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.

Deep Impact: excavating comet Tempel 1.

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 (<65 pascals). Local gravitational field and average nucleus density (600 kilograms per cubic meter) are estimated from ejecta fallback. Initial ejecta were hot (>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.

Deep Impact: observations from a worldwide Earth-based campaign.

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.

Galileo at Io: results from high-resolution imaging.

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 approximately 1 kilometer are rich in volatiles.

Galileo imaging of atmospheric emissions from Io.

The Galileo spacecraft has detected diffuse optical emissions from Io in high-resolution images acquired while the satellite was eclipsed by Jupiter. Three distinct components make up Io's visible emissions. Bright blue glows of more than 300 kilorayleighs emanate from volcanic plumes, probably due to electron impact on molecular sulfur dioxide. Weaker red emissions, possibly due to atomic oxygen, are seen along the limbs, brighter on the pole closest to the plasma torus. A faint green glow appears concentrated on the night side of Io, possibly produced by atomic sodium. Io's disk-averaged emission diminishes with time after entering eclipse, whereas the localized blue glows brighten instead.

High-temperature silicate volcanism on Jupiter's moon Io.

Infrared wavelength observations of Io by the Galileo spacecraft show that at least 12 different vents are erupting lavas that are probably hotter than the highest temperature basaltic eruptions on Earth today. In at least one case, the eruption near Pillan Patera, two independent instruments on Galileo show that the lava temperature must have exceeded 1700 kelvin and may have reached 2000 kelvin. The most likely explanation is that these lavas are ultramafic (magnesium-rich) silicates, and this idea is supported by the tentative identification of magnesium-rich orthopyroxene in lava flows associated with these high-temperature hot spots.

Evidence for a subsurface ocean on Europa.

Ground-based spectroscopy of Jupiter's moon Europa, combined with gravity data, suggests that the satellite has an icy crust roughly 150 km thick and a rocky interior. In addition, images obtained by the Voyager spacecraft revealed that Europa's surface is crossed by numerous intersecting ridges and dark bands (called lineae) and is sparsely cratered, indicating that the terrain is probably significantly younger than that of Ganymede and Callisto. It has been suggested that Europa's thin outer ice shell might be separated from the moon's silicate interior by a liquid water layer, delayed or prevented from freezing by tidal heating; in this model, the lineae could be explained by repetitive tidal deformation of the outer ice shell. However, observational confirmation of a subsurface ocean was largely frustrated by the low resolution (>2 km per pixel) of the Voyager images. Here we present high-resolution (54 m per pixel) Galileo spacecraft images of Europa, in which we find evidence for mobile 'icebergs'. The detailed morphology of the terrain strongly supports the presence of liquid water at shallow depths below the surface, either today or at some time in the past. Moreover, lower-resolution observations of much larger regions suggest that the phenomena reported here are widespread.

Geological evidence for solid-state convection in Europa's ice shell.

The ice-rich surface of the jovian satellite Europa is sparsely cratered, suggesting that this moon might be geologically active today. Moreover, models of the satellite's interior indicate that tidal interactions with Jupiter might produce enough heat to maintain a subsurface liquid water layer. But the mechanisms of interior heat loss and resurfacing are currently unclear, as is the question of whether Europa has (or had at one time) a liquid water ocean. Here we report on the morphology and geological interpretation of distinct surface features-pits, domes and spots-discovered in high-resolution images of Europa obtained by the Galileo spacecraft. The features are interpreted as the surface manifestation of diapirs, relatively warm localized ice masses that have risen buoyantly through the subsurface. We find that the formation of the features can be explained by thermally induced solid-state convection within an ice shell, possibly overlying a liquid water layer. Our results are consistent with the possibility that Europa has a liquid water ocean beneath a surface layer of ice, but further tests and observations are needed to demonstrate this conclusively.

Evidence for non-synchronous rotation of Europa. Galileo Imaging Team.

Non-synchronous rotation of Europa was predicted on theoretical grounds, by considering the orbitally averaged torque exerted by Jupiter on the satellite's tidal bulges. If Europa's orbit were circular, or the satellite were comprised of a frictionless fluid without tidal dissipation, this torque would average to zero. However, Europa has a small forced eccentricity e approximately 0.01 , generated by its dynamical interaction with Io and Ganymede, which should cause the equilibrium spin rate of the satellite to be slightly faster than synchronous. Recent gravity data suggest that there may be a permanent asymmetry in Europa's interior mass distribution which is large enough to offset the tidal torque; hence, if non-synchronous rotation is observed, the surface is probably decoupled from the interior by a subsurface layer of liquid or ductile ice. Non-synchronous rotation was invoked to explain Europa's global system of lineaments and an equatorial region of rifting seen in Voyager images. Here we report an analysis of the orientation and distribution of these surface features, based on initial observations made by the Galileo spacecraft. We find evidence that Europa spins faster than the synchronous rate (or did so in the past), consistent with the possibility of a global subsurface ocean.

Episodic plate separation and fracture infill on the surface of Europa. Galileo Imaging Team.

Images obtained by the Voyager spacecraft revealed dark, wedge-shaped bands on Europa that were interpreted as evidence that surface plates, 50-100 km across, moved and rotated relative to each other. This implied that they may be mechanically decoupled from the interior by a layer of warm ice or liquid water. Here we report similar features seen in higher resolution images (420 metres per pixel) obtained by the Galileo spacecraft that reveal new details of wedge-band formation. In particular, the interior of one dark band shows bilateral symmetry of parallel lineaments and pit complexes which indicates that plate separation occurred in discrete episodes from a central axis. The images also show that this style of tectonic activity involved plates < 10 km across. Although this tectonic style superficially resembles aspects of similar activity on Earth, such as sea-floor spreading and the formation of ice leads in polar seas, there are significant differences in the underlying physical mechanisms: the wedge-shaped bands on Europa most probably formed when lower material (ice or water) rose to fill the fractures that widened in response to regional surface stresses.

Galileo's First Images of Jupiter and the Galilean Satellites

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.

First images of asteroid 243 ida.

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.

Galileo multispectral imaging of the north polar and eastern limb regions of the moon.

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.

Galileo encounter with 951 gaspra: first pictures of an asteroid.

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.

Lunar impact basins and crustal heterogeneity: new Western limb and far side data from galileo.

Multispectral images of the lunar western limb and far side obtained from Galileo reveal the compositional nature of several prominent lunar features and provide new information on lunar evolution. The data reveal that the ejecta from the Orientale impact basin (900 kilometers in diameter) lying outside the Cordillera Mountains was excavated from the crust, not the mantle, and covers pre-Orientale terrain that consisted of both highland materials and relatively large expanses of ancient mare basalts. The inside of the far side South Pole-Aitken basin (>2000 kilometers in diameter) has low albedo, red color, and a relatively high abundance of iron- and magnesium-rich materials. These features suggest that the impact may have penetrated into the deep crust or lunar mantle or that the basin contains ancient mare basalts that were later covered by highlands ejecta.

Images from galileo of the venus cloud deck.

Images of Venus taken at 418 (violet) and 986 [near-infrared (NIR)] nanometers show that the morphology and motions of large-scale features change with depth in the cloud deck. Poleward meridional velocities, seen in both spectral regions, are much reduced in the NIR In the south polar region the markings in the two wavelength bands are strongly anticorrelated. The images follow the changing state of the upper cloud layer downwind of the subsolar point, and the zonal flow field shows a longitudinal periodicity that may be coupled to the formation of large-scale planetary waves. No optical lightning was detected.

P/Halley: the quintessential comet.

Halley's comet is the focus of an international scientific enterprise now under way. Ground-based astronomical observations are already yielding new information about the comet's nucleus and atmosphere. In the coming year, remote and in situ investigations from the ground, Earth orbit, Venus orbit, interplanetary space, and within the comet itself are expected to reveal much more. The climax of the enterprise will be the penetration of an armored European spacecraft into the center of the comet in March 1986.

Extreme ultraviolet observations from voyager 2 encounter with jupiter.

Extreme ultraviolet spectral observations of the Jovian planetary system made during the Voyager 2 encounter have extended our knowledge of many of the phenomena and physical processes discovered by the Voyager 1 ultraviolet spectrometer. In the 4 months between encounters, the radiation from Io's plasma torus has increased in intensity by a factor of about 2. This change was accompanied by a decrease in plasma temperature of about 30 percent. The high-latitude auroral zones have been positively associated with the magnetic projection of the plasma torus onto the planet. Emission in molecular hydrogen bands has been detected from the equatorial regions of Jupiter, indicating planetwide electron precipitation. Hydrogen Lyman alpha from the dark side of the planet has been measured at an intensity of about 1 kilorayleigh. An observation of the occultation of alpha Leonis by Jupiter was carried out successfully and the data are being analyzed in detail.