Ejecta with flow features and discrete termini surround many fresh Martian craters. Several morphologies of the flow-like ejecta are observed; they are found globally and in nearly all terrains. It is suggested that the morphology of flow ejecta craters is related to the amount of subsurface volatiles and/or to atmospheric drag effects. It was attempted to constrain factors which could contribute to the ejecta morphology such as latitude, elevation, and terrain unit, however, they involved only one or two constrained variables or used global data sets based on Mariner 9 information. Viking-based data sets are becoming available and may provide a better base from which an understanding of the factors which governs ejecta morphology may be obtained. Block sizes of Martian flow ejecta may provide clues to the ejecta emplacement process.
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.
Analyses of landforms on Rhea are used to define three physiographic provinces (cratered terrain 1 undifferentiated, cratered terrain 1 lineated, cratered terrain 2) for which stratigraphic relationships are derived and examined in relation to major impact basins and tectonic features. The formation of multiringed basins may have caused, or at least controlled, the locations of major resurfacing and mantling events. The diameters of central peaks relative to impact crater diameters are significantly larger than those within craters on the moon and Mercury. Rim segments of polygonal craters have preferential orientations. Both crater and noncrater lineaments show regional orientations that do not fit current global or regional stress models. Superposition relationships of provinces and associated features imply the following postaccretional sequence for the geological evolution of Rhea's surface. Cratered terrain 1 undifferentiated formed first. Then linear troughs and chains of coalescing pits developed extensively on cratered terrain 1 lineated. A resurfacing event emplaced cratered terrain 2, on which small craters, scarps, and ridges formed. Small grooves on cratered terrain 1 undifferentiated are tentatively assigned a young age, based on their susceptibility to impact erosion. This geologic history is generally consistent with theoretical models for Rhea's evolution.
Thermal inertia characteristics have been determined for the martian crater Curie from high resolution groundtracks of Viking Thermal Infrared Mapper (IRTM) data. Flow features near the southeastern edge of the ejecta indicate that at least part of the Curie ejecta was emplaced in a manner similar to the ejecta of rampart craters. Within the study region there appears to be a general southeastern trend towards lower thermal inertia values. This trend may be related to the proximity of the Arabia region, which is mainly to the south and east of Curie. Curie is in a region where the overall thermal inertias change over relatively short distances radial to Arabia. Therefore, the observed general decrease in thermal inertia may represent increasing regional dust accumulation in the direction of Arabia.
