Current Galileo Mission data are giving us the closest views of Jupiter's icy moon, Europa, since Voyager images first revealed the surface 20 years ago. The icy crust is smooth and blocky, with a banded and broken-puzzle appearance. Europa's outer shell, intriguing to geologists and astrobiologists alike, has been cited as evidence supporting a subsurface-ocean hypothesis. Two articles in a recent Galileo Mission special section of the Journal of Geophysical Research (Planets) review the water-ice surface, major geologic units, and the search for current geologic activity on Europa. Ronald Greeley (Arizona State University) and colleagues from universities, NASA, U. S. Geological Survey, and the National Optical Astronomy Observatories provide an extensive compilation of Europa's primary geologic units as a framework for further mapping of the surface. In another analysis of Galileo images, Cynthia Phillips of the University of Arizona (now at the SETI Institute) and colleagues from ASU, Brown, and JPL look for changes on the surface of Europa since Voyager. They also look for evidence of current geologic activity in the form of active plumes. Not finding proof of surface change and plumes, they give estimates of surface age and surface alteration rates on Europa.

Carbon dioxide is present over most of Callisto's surface and is often concentrated in and around morphologically fresh and bright impact craters. The depths of the 4.26‐μm absorption band in reflectance spectra returned by the Near‐Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft imply a CO2 abundance in the top ∼1 mm of the surface that is ≤0.3 wt %. This absorption band is due to CO2 that is contained in the nonice material. Ice on Callisto may also contain CO2, but the ice appears to be large‐grained and therefore is not sufficiently reflective for an absorption band at 4.26‐μm to be visible. The depth of the CO2 absorption band has at most only a very slight photometric dependence. The distribution of CO2 around morphologically fresher impact craters suggests its origin there is related to the impact process, but there is no indication that impactors are the source of the impact‐related CO2, (e.g., known comet chains are not CO2‐rich). These distributions of CO2 also suggest that the enrichment of CO2 in and around impact craters is a transient phenomenon. The increased CO2 abundance around impact craters on Callisto is associated with (1) the dark, less icy material within and nearby the craters, (2) occasionally the icy material inside the craters and their icy rims, (3) their continuous ejecta blankets, and, if existing, (4) their icy ejecta.

[1] Carbon dioxide is present over most of Callisto's surface and is often concentrated in and around morphologically fresh and bright impact craters. The depths of the 4.26-μm absorption band in reflectance spectra returned by the Near-Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft imply a CO2 abundance in the top ∼1 mm of the surface that is ≤0.3 wt %. This absorption band is due to CO2 that is contained in the nonice material. Ice on Callisto may also contain CO2, but the ice appears to be large-grained and therefore is not sufficiently reflective for an absorption band at 4.26-μm to be visible. The depth of the CO2 absorption band has at most only a very slight photometric dependence. The distribution of CO2 around morphologically fresher impact craters suggests its origin there is related to the impact process, but there is no indication that impactors are the source of the impact-related CO2, (e.g., known comet chains are not CO2-rich). These distributions of CO2 also suggest that the enrichment of CO2 in and around impact craters is a transient phenomenon. The increased CO2 abundance around impact craters on Callisto is associated with (1) the dark, less icy material within and nearby the craters, (2) occasionally the icy material inside the craters and their icy rims, (3) their continuous ejecta blankets, and, if existing, (4) their icy ejecta.

Introduction: Tectonic features on Callisto are found (1) preferentially in large multi-ring structures, or (2) in several areas in its dark, cratered plains units outside the large multi-ring structures [1, 2]. In this latter case, tectonism is represented by fractures, scarps, and albedo lineaments. In this paper, we will mainly concentrate on tectonic features outside the large multi-ring structures and investigate (1) their spatial distribution, as seen on high resolution images returned by the Galileo SSI camera, (2) their preferential orientations, (3) their correlation to tectonic features found on lower resolution Voyager images, (4) their time-stratigraphic position, and (5) their correlation to surface degradation processes. Target areas: High-resolution views of Callisto's surface which essentially reveal tectonic features studied in this paper were obtained during Galileo's orbits C9, C10, and C20. In these target areas, stereo data could be used. Red-blue anaglyph images were constructed to aid in the evaluation of these data. Geologic units and crater ages: In orbit C10, an area originally termed smooth plains [3] was targeted at 270 m/pxl and 68 m/pxl (SSI target areas 10CSSMTHPL02 and 01). The area appears bright and smooth in Voyager-1 images. It was interpreted as either a volcanically resurfaced area [3], or as a degraded palimpsest [1]. The higher resolution SSI data showed a rough and knobby surface instead. No evidence is found for volcanic resurfacing in this area, but some more or less concentric scarps indicate an impact origin , most likely a degraded palimpsest. From measurements of the crater distributions, cratering model ages are 4.2±0.05 Gyr in model I [4] and 4.3 Gyr (uncertainty range 4.56 Gyr to 2.1 Gyr) in model II [5], while the surrounding cratered plains have ages of about 4.2 to 4.25±0.05 Gyr in model I, or 4.3 to 4.5 Gyr in model II. Hence the stratigraphic position of the degraded impact structure is close to the base of the system defined by the Asgard impact [6]. In orbit C20, a dark patch interpreted as dark volcanic flow [1] was targeted for high resolution at 430 m/pxl and 108 m/pxl (SSI target areas 20CSDRKFLO02 and 01). Both observations could not reveal features unequivocally indicative of volcan-ism [7]. The dark " flow " is located close to a bright area which appears smooth at Voyager resolution but turned out to be rough and knobby in the SSI frames, comparable to the C10 " smooth plains ". …

The Galileo Orbiter examined several impact features on Europa at considerably better resolution than was possible from Voyager. The new data allow us to describe the morphology and infer the geology of the largest impact features on Europa, which are probes into the crust. We observe two basic types of large impact features: (1) “classic” impact craters that grossly resemble well-preserved lunar craters of similar size but are more topographically subdued (e.g., Pwyll) and (2) very flat circular features that lack the basic topographic structures of impact craters such as raised rims, a central depression, or central peaks, and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them (e.g., Callanish). Our interpretation is that the classic craters (all <30 km diameter) formed entirely within a solid target at least 5 to 10 km thick that exhibited brittle behavior on time scales of the impact events. Some of the classic craters have a more subdued topography than fresh craters of similar size on other icy bodies such as Ganymede and Callisto, probably due to the enhanced viscous relaxation produced by a steeper thermal gradient on Europa. Pedestal ejecta facies on Europa (and Ganymede) may be produced by the relief-flattening movement of plastically deforming but otherwise solid ice that was warm at the time of emplacement. Callanish and Tyre do not appear to be larger and even more viscously relaxed versions of the classic craters; rather they display totally different morphologies such as distinctive textures and a series of large concentric structural rings cutting impact-feature-related materials. Impact simulations suggest that the distinctive morphologies would not be produced by impact into a solid ice target, but may be explained by impact into an ice layer ∼10 to 15 km thick overlying a low-viscosity material such as water. The very wide (near antipodal) separation of Callanish and Tyre imply that ∼10–15 km may have been the global average thickness of the rigid crust of Europa when these impacts occurred. The absence of detectable craters superposed on the interior deposits of Callanish suggests that it is geologically young (<108years). Hence, it seems likely that our preliminary conclusions about the subsurface structure of Europa apply to the current day.

Lofn crater is a 180-km-diameter impact structure in the southern cratered plains of Callisto and is among the youngest features seen on the surface. The Lofn area was imaged by the Galileo spacecraft at regional-scale resolutions (875 m/pixel), which enable the general geology to be investigated. The morphology of Lofn crater suggests that (1) it is a class of impact structure intermediate between complex craters and palimpsests or (2) it formed by the impact of a projectile which fragmented before reaching the surface, resulting in a shallow crater (even for Callisto). The asymmetric pattern of the rim and ejecta deposits suggests that the impactor entered at a low angle from the northwest. The albedo and other characteristics of the ejecta deposits from Lofn also provide insight into the properties of the icy lithosphere and subsurface configuration at the time of impact. The "target" for the Lofn impact is inferred to have included layered materials associated with the Adlinda multiring structure northwest of Lofn and ejecta deposits from the Heimdall crater area to the southeast. The Lofn impact might have penetrated through these materials into a viscous substrate of ductile ice or possibly liquid water. This interpretation is consistent with models of the current interior of Callisto based on geophysical information obtained from the Galileo spacecraft.

Paleotopographic reconstructions based on a synthesis of published geologic information and high‐resolution topography, including topographic profiles, reveal the potential existence of an enormous drainage basin/aquifer system in the eastern part of the Tharsis region during the Noachian Period. Large topographic highs formed the margin of the gigantic drainage basin. Subsequently, lavas, sediments, and volatiles partly infilled the basin, resulting in an enormous and productive regional aquifer. The stacked sequences of water‐bearing strata were then deformed locally and, in places, exposed by magmatic‐driven uplifts, tectonic deformation, and erosion. This basin model provides a potential source of water necessary to carve the large outflow channel systems of the Tharsis and surrounding regions and to contribute to the formation of putative northern‐plains ocean(s) and/or paleolakes.