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.