Venusian Novae and arachnoids: Characteristics, differences and the effect of the geological environment

Novae and arachnoids are Venusian structures, both supposedly formed by a volcanic uplifting (Janes et al., Lunar Planet. Sci. XXVII (1996) 605; Head et al., J. Geophys. Res. 97 (E8) (1992) 13,153). Corona-like features and radially fractured domes, which could be considered as novae, have been connected to the coronae or the corona formation (Squyres et al., J. Geophys. Res. 97 (E8) (1992) 13,153; Stofan et al., J. Geophys. Res. 97 (E8) (1992) 13,347). Arachnoids are also thought to be a sub-type of coronae (Price and Suppe, Earth Moon Planets 71 (1995) 99) or corona-like features (Head et al., 1992). Despite the fact that they both belong to the same broad class of corona and corona-like features, these structures seem to have very divergent basic characteristics generally. In addition to morphological differences, the novae are mostly elevated in a distinct way with lava flows and radial fractures while the majority of the arachnoids are structures with depressed interior, radial ridges and they are without lava flows. The distribution map indicates that the novae are located in sparse chains on the deformation belts and the arachnoids are in groups or in clusters on the plains. On the area to the south of Atla Regio, novae and arachnoids seem to be rather densely spaced, but also within this region the novae are on the extension zones and most of the arachnoids are located on the plains or adjoining a ridge belt. Only the few arachnoids which are located in some uncommon location, such as close to an extension zone or within a volcanic area, display some properties that usually are found in novae. This indicates that the geologic environment is a significant factor in the formation process of novae and arachnoids.     

Mapping and analysis of a chain of channeled basins in the eastern rim region of Hellas basin,Mars

The study is a detailed look on one of the several fluvial systems located on the eastern rim region of the Hellas basin on Mars. We analyzed the morphologic and morphometric characteristics of an extensive channel system, which extends for over 650 km from 35.8°S, 106.4°E in Hesperia Planum to Reull Vallis at 39.5°S, 98.1°E, and has a drainage area of 35,000–40,000 km². During its traverse the channel changes its characteristics many times, indicating variations in the surface properties. Based on cross-cutting relations, the fluvial system post-dates the emplacement of the early Hesperian lava plains in Hesperia Planum but predates the Amazonian deposits. We describe the geomorphology and evolution of the system and provide evidence of both surface flow and groundwater sapping processes. A chain of channeled paleolake basins in the central parts of the system (38°S, 102°E) provides a rough estimate for the water volume (250–300 km³) which was required to form the system. The minimum volume of surface materials eroded by the channel system is ～74 km³. Although this study presents the detailed analysis of only one fluvial system, the presence of many similar channel systems along the margin of Hellas suggests that late-stage surface runoff has played a significant role in the degradation of the rim of the basin and also in the transportation of materials towards Hellas floor.