Galileo views of the geology of Callisto

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Abstract

Callisto was imaged by the Galileo spacecraft on 10 orbits, half of which involved close flybys, producing images with resolutions as high as 15 m/pixel. The Valhalla, Asgard, and Adlinda multi-ring structures were imaged, along with two features which might also be large impacts, Utgard and Heimdall (both provisionally named). The high albedo interior zones of these structures are relatively rugged and include small knobs. The interior ring zone of Valhalla includes outward facing scarps, as discovered on Voyager images, while the inner zones of the other, smaller multi-ring structures include grabens and inward facing scarps. The outer ring zones consist of grabens which are sinuous and include multiple parallel faults and fractures. High albedo areas associated with the fractures had been suggested previously as resurfacing zones; they are now seen to consist of numerous, closely spaced fractures. Crater size frequency distributions for the units associated with multi-ring structures suggest that all were formed essentially contemporaneously with their respective impact events.

Galileo images show that surface degradation occurs on a variety of scales, including large landslides from the rims of craters and apparent sub-decameter surface modifications, leading to the removal of small carters in some areas. Similar styles of degradation are not observed on Ganymede, suggesting fundamental differences in the properties of the surface materials. Catenae (crater chains) were also observed; some appear to be secondaries in which their primary source crater can be identified, while others are probably the result of the breakup of primary objects, as previously proposed.

Although most of Callisto’s surface consists of ancient cratered plains, some small ‘smooth’ areas had been suggested to result from cryovolcanic resurfacing. Representative bright and dark areas were viewed in high resolution; bright areas were found to be rugged at the 10s of meters scale, show no indications of flow features, and are not considered to be cryovolcanic. Some smooth dark areas appear to mantle and embay the surrounding cratered plains. Although this is consistent with possible cryovolcanic processes, it is not definitive evidence of endogenic processes.

The extension of imaging data, both in coverage and resolution, enable about 90% of the surface to be mapped photogeologically and gives new insight into the nature of surface features and the timing of their development. Cratered plains form the oldest recognizable unit on Callisto, but subtle differences in color and crater frequencies suggest regional differences in its development. The formation of Adlinda, Asgard, Valhalla, Heimdall, and Lofn crater appear to follow in that order. The morphology of associated features is consistent with a differentiated interior for Callisto and the presence of a thin icy lithosphere at the time of the impacts.

Introduction

Callisto is the outermost of the Galilean satellites. It is 4818.6 km in diameter, or slightly smaller than Mercury. Callisto’s density of 1.86 gm/cm3 is the least of the Galilean satellites and suggests that it is composed of a 52:48 to 58:42 mixture of water-ice to rock (Schubert et al., 1986). Callisto has the lowest albedo (0.2) of the Galilean satellites, indicating the presence of abundant non-ice materials on its surface (Dollfus, 1975, Roush et al., 1990). Callisto was first viewed from a spacecraft by Pioneers 10 and 11 in 1974 (Gehrels et al., 1974), which showed a generally dark and mottled surface, but it was not seen in detail until the flybys of the Voyager spacecraft in 1979 (Smith and the Voyager Imaging Team, 1979a, Smith and the Voyager Imaging Team, 1979b). However, the best Voyager images have a resolution of only about 950 m/pixel and suffer from image smear resulting from an offset in the camera pointing sequence.

Despite the low resolution views of Callisto afforded by the Voyager spacecraft, these images enabled the first geological study of the surface, as reported by Smith and the Voyager Imaging Team, 1979a, Smith and the Voyager Imaging Team, 1979b and Passey and Shoemaker (1982). Global geological studies were made by Schenk, 1991, Schenk, 1995) and photogeological mapping was conducted by Bender et al. (1997a) for the areas imaged by Voyager.

As outlined by Carr et al. (1995), the investigations for Callisto using the Galileo (GLL) Solid State Imaging (SSI) system focused on the: (1) morphology and origin of multi-ring structures, including associated scarps, troughs, and ridges; (2) morphology and origin of palimpsests; (3) morphology of impact craters formed in icy-targets; (4) morphology and origin of catenae (crater chains); (5) morphology and origin of candidate endogenic deposits; (6) exploration of areas not imaged or poorly imaged by Voyager; (7) presence and mobility of volatile frosts at high latitudes; and (8) spectral indications of ice/silicate mixtures across the surface.

To achieve the goals outlined above for the SSI experiment, data for Callisto were taken on 10 orbits of Galileo spacecraft and include limited coverage in color (Table 1). Images ranged in resolution from 15 to 17,000 m/pixel and include areas not previously observed (Fig. 1). Each orbit is designated by the first letter of the target-body satellite and the orbit number. For example, G1 corresponds to the first orbit around Jupiter with Ganymede as the prime targeted satellite. Callisto was the targeted body on orbits C3, C9, C10, C20, and C21 during which high-resolution data were obtained. Imaging data were acquired on ‘non-targeted’ encounters as well, but at a lower maximum resolution. The SSI system, as described by Klaasen et al., 1984, Klaasen et al., 1999), has eight filters, centered at 414, 559, 625 (broad-band, clear), 664, 731, 757, 889, and 989 nm. Near infrared mapping spectrometer (NIMS) data were also acquired for parts of Callisto, the initial results of which have been reported by Carlson et al. (1996).

In addition to visual images, NIMS data, and other remote sensing observations, gravity measurements were obtained for Callisto by tracking the Galileo spacecraft (Anderson et al., 1998). Data were also obtained by the magnetometer on three flybys (Khurana et al., 1998, Kivelson et al., 1999). These geophysical data provide an insight into the interior of Callisto which has implications for the evolution of the satellite and its geological surface features. Callisto’s moment of inertia of C/MR2=0.359±0.005 allows Callisto’s interior to be modeled as a three-layer interior involving a central silicate core, an intermediate rock and ice zone, and an outer icy layer 0–500 km thick (Anderson et al., 1998). However, this configuration is model dependent, and a completely differentiated case cannot be separated from an undifferentiated case (Showman and Malhotra, 1999).

Data from the Galileo magnetometer produced some surprising results that suggest the presence of a conducting sub-surface layer. Although several models can be suggested to explain these data, an internal zone of liquid water ∼10 km thick and with a salinity comparable to sea water on Earth appears to be the best case (Kivelson et al., 1999).

This paper reviews the geology of Callisto as revealed by the new Galileo data and assesses the geophysical results and models in terms of the surface features observed in the images.

Section snippets

Global observations

Voyager images revealed that Callisto is a heavily cratered object most of whose surface is considered to date from the early accretion of the Jovian system (Cassen et al., 1980, Passey and Shoemaker, 1982; and others). Topographic relief is generally less than 2 km and is limited to that formed by impact craters. Maps of the major terrains show a relatively homogeneous, low albedo surface, punctuated with large multi-ring impact structures and bright circular spots indicative of bright craters

Multi-ring structures

Multi-ringed structures are recognized on many planets and satellites. As reviewed by McKinnon and Melosh, 1980, Schultz and Merrill, 1981, and Spudis (1993), these structures are inferred to result from impact processes and provide insight into the nature of the planetary ‘target’ at the time of the impact (Melosh, 1989). Schenk (1995) classified various large impact features on Callisto, including multi-ring structures, various palimpsests, and ‘cryptic’ ring structures. Seven multi-ring

Morphology

A variety of impact crater morphologies is seen on Callisto (Fig. 9). The smallest craters are simple bowl-shaped to flat-floored depressions that range in diameter from about 7 km down to ∼50 m, the limit of identification. They exhibit a range of degradation states (Moore et al., 1999), including fresh craters with high-albedo ejecta blankets and rays, circular rims, and walls with little or no evidence of slumping. The floor textures of small craters vary from smooth to pitted or hummocky.

Geological mapping

Among the solid-surface planets and satellites, Callisto is relatively simple in terms of geological mapping at the global scale because there are so few units. Mappable material units include cratered plains, light plains, smooth dark plains, various units associated with multi-ring structures, and other impact crater deposits. Fig. 2 is a generalized photogeological map based on these units and expanded from that of Bender et al. (1997a) using Galileo images to fill areas not observed by

Discussion and conclusions

In this overview, we have addressed most of the principal geology goals identified for the Galileo SSI experiment for Callisto (Carr et al., 1995, and Section 1). The following summarizes the findings and discusses the implications for the surface history of this Jovian satellite.

Acknowledgements

We are grateful to Wing Ip and the European Geophysical Society for the invitation to present these results for Callisto at the 1999 annual meeting and to submit this paper for publication.

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