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The bright southern hemisphere, comprising Uhlanga Regio, is perhaps the most poorly understood geologic province on Triton. The entire bright southern hemisphere has been described as a bright polar 'cap', implying a seasonal origin, or as a permanent geologic terrain distinct from the equatorial terrains. Also, thermal models have predicted seasonal migration of frosts and ices from the presently sun-lit south latitudes to the dark northern latitudes. The distribution of frosts and geologic history of this region must be determined observationally. We reexamine the geology of this terrain with the goal of answering these questions.

Comprehension and analysis of geologic features on any planet is enhanced manyfold by a clear perception between albedo and topography. On many of the icy satellites significant albedo contrasts due to mixtures of dark rocky and bright icy materials can be associated with topographic features. Subtle topographic features can be masked by albedo variation and under high solar illumination albedo and topography can be difficult to separate. To this end we are compiling an atlas of stereo image pairs of the outer solar system based on Voyager imaging for the investigation of various geologic problems and for general use. For the icy satellites, general perceptions of topography are usually gleaned from shape-from-shading information in the images processed by the human brain (i.e. visual inspection). With few exceptions, actual topography has been measured on a spot-by-spot basis using shadow heights or photoclinometry, or along limb profiles (where geographic context may be unavailable). Shadow heights are limited to regions within approximately 10 deg of the terminator and images with resolution better than approximately 1 km/pixel. Photoclinometric scans can be used more widely but are subject to a variety of errors, primarily uncertain assumptions of uniform scene albedo or poorly understood photometry. Stereoscopic analysis, where available, has the potential for greatly expanding topographic perception.

The penultimate meeting in the Mars Surface and Atmosphere Through Time (MSATT) series of workshops was held on the campus of the University of Alaska in Fairbanks, Alaska, 12-13 Aug. 1993. This meeting, entitled 'The Martian Northern Plains: Sedimentological, Periglacial, and Paleoclimatic Evolution,' hosted by the Geophysical Institute at the University of Alaska, was designed to help foster an exchange of ideas among researchers of the Mars science community and the terrestrial glacial and periglacial science community. The technical sessions of the workshop were complemented by field trips to the Alaska Range and to the Fairbanks area and a low-altitude chartered overflight to the Arctic Costal Plain, so that, including these trips, the meeting lasted from 9-14 Aug. 1993. The meeting, field trips, and overflight were organized and partially funded by the Lunar and Planetary Institute and the MSATT Study Group. The major share of logistical support was provided by the Publications and Program Services Department of the Lunar and Planetary Institute. The workshop site was selected to allow easy access to field exposures of active glaciers and glacial and periglacial landforms. In all, 25 scientists attended the workshop, 24 scientists (plus 4 guests and the meeting coordinator) participated in the field trips, and 18 took part in the overflight. This meeting reaffirmed the value of expertly led geologic field trips conducted in association with topical workshops.

Voyager observations reveal impact craters on Ganymede that are characterized by the presence of broad, high albedo, topographic domes situated within a central pit. Fifty-seven craters with central domes were identified in images covering approx. 50% of the surface. Owing to limitations in resolution, and viewing and illumination angles, the features identified are most likely a subset of dome craters. The sample appears to be sufficiently large to infer statistically meaningful trends. Dome craters appear to fall into two distinct populations on plots of the ratio of dome diameter to crater rim diameter, large-dome craters and small-dome craters. The two classes are morphologically distinct from one another. In general, large dome craters show little relief and their constituent landforms appear subdued with respect to fresh craters. The physical attributes of small-dome craters are more sharply defined, a characteristic they share with young impact craters of comparable size observed elsewhere in the solar system. Both types of dome craters exhibit central pits in which the dome is located. As it is difficult to produce domes by impact and/or erosional processes, an endogenic origin for the domes is reasonably inferred. Several hypotheses for their origin are proposed. These hypotheses are briefly reviewed.

Similarities between terrestrial shield volcanoes and a volcano on Io observed in Voyager I imagery of the satellite at 30 deg S, 246 deg W are delineated. A photoclinometry model was used to numerically estimate the slope based on the Minnaert photometric function. The slope values are accurate to within 10 deg on the sun-facing slope and 1 deg on the shadow side. As found with shield volcanoes, the feature has a central edifice, 40-50 km in diameter, and a broad, elliptical base, 77 x 90 km across. The summit of the Io volcano is 2.2-2.8 km above the surrounding plane and contains a caldera about 5 km in diameter. The similarity in shape between basaltic terrestrial shield volcanoes and the Io volcano indicates that the Io feature may also be composed of basalt. The composition could be sulfur if the heat flow was under 0.05 W/sq m, as it might have been in later stages of formation.

Rhea was imaged to a resolution of approximately 1 km/lp by the Voyager spacecraft, providing the most detailed view of any Saturnian satellite. A preliminary study of Rhea divided the northern hemisphere into population 1 cratered terrain (between 20 deg and 120 deg) and population 2 cratered terrain (between 300 deg and 360 deg). Population 1 includes craters that are 40 km and were formed before the termination of population 2 bombardment, which formed craters primarily 40 km. Several geomorphic features on Rhea are classified and interpreted including three physiographic provinces, multiringed basins, craters, megascarps, ridges and scarps, and troughs and coalescing pit chains. A generalized chronology for Rhea is constructed from an analysis of the superposition relationships among the landforms and physiographic provinces.

A volcano was identified whose morphology, especially topography, supports and extends current evidence that shield volcanoes occur on Io. The feature is centered at 30 deg. S, 246 deg w and lies in a region near the terminator. The presence of shield volcanism on Io was inferred from the planimetric configuration of flaws on such features as Ra Patera. A central edifice roughly 40 to 50 km in diameter is situated on intervent plains material, and a broad, slightly elliptical base 75 to 90 km across. The size of this feature is comparable to large shield volcanoes found on Earth. In addition, the topography is similar to some terrestrial shield volcanoes that exhibit a change in slope; the steepening may be explained by shorter and/or thicker flows emplaced during late stage eruptions.

Morphological analyses of landforms on Rhea are used to define three physiographic provinces: cratered terrain 1 undifferentiated; cratered terrain 1 lineated; and cratered terrain 2. The important statigraphic relationships between the different provinces are examined with respect to major impact basins and tectonic features. It is shown that the formation of multiringed basins may have caused, or at least controlled the locations of major resurfacing and mantling events. The diameters of the central peaks relative to the impact crater diameters are found to be significantly larger than those within the craters of the moon or Mercury. Both cratered and noncrater lineaments have regional orientations which do not fit current global or regional stress models. On the basis of the morphological analysis, a chronological order is established for the origin of the three provinces: the cratered terrain 1 province was formed first; and cratered terrain 1 lineated and cratered terrain 2 were formed second, and last, respectively. It is shown that the chronological order is generally consistent with current theoretical models of the evolution of Rhea.

The saturnian satellites were imaged by the Voyager spacecraft at sufficient resolutions to reveal landforms that indicate histories of extensional tectonics for several of these bodies. The relationships among landforms on various satellites imply that extensional tectonism is a consequence of several different energy sources. Case histories of several satellites are discussed to illustrate the interaction of various phenomena associated with extensional tectonism.