262 SOLAR SYSTEM/Venus lithosphere Depending on the rheological properties of the material, horizontal scales of the instabilities, and the time-scale at which the instabilities exist, this process may lead either to transient plate tectonics or to large-scale mantle overturn Both scenarios imply a cyclic nature of the heat loss mechanisms and may lead to a planet-wide resurfacing event on the surface The second variant of the catastrophic resurfacing is based on the secular cooling of the interior of the planet during most of the geological history of Venus According to this scenario, the observable geological Figure 28 A typical nova is characterized by a star like pattern of broad radial fractures and graben originating at its centre Fragment of C1 MIDR.30S279; illumination is from the left history of Venus begins after the transition from vigorous mantle convection under thin lithosphere to stagnant lid convection under thick lithosphere In this model, tessera terrain is the remnant of the previous, thin-lithosphere regime, and the rifted dome-shaped rises topped with large volcanoes are the manifestation of the current regime of mantle convection under thicker lithosphere Models of Geological History on Venus The high quality and global coverage of the Magellan data provide the possibility of detailed geological mapping of the surface based on defining distinct units and structures and establishing their relative ages The results of the mapping efforts have led to two proposed end-member models for the correlation of regionally observed sequences of units and structures In the first model, sequences of distinctive units mapped in different regions appear to have similar repetitive sequences in different places This model has been called a ‘‘directional’’ geological history, implying a specific set of global trends in the evolution of Venus For example, the consistently oldest relative age of tesserae suggests that a tectonic style of tessera formation has changed, with subsequent tectonic styles that led to the formation of other types of terrains The important attribute of this model is its ‘synchronism’, implying that the sequences of events observed in different regions are broadly synchronous globally For instance, regional wrinkle-ridged plains Figure 29 The spatial distribution of coronae (large circles), arachnoids (black dots), and novae (black diamonds) on the surface of Venus The majority of these features, especially the novae, is concentrated in the Beta, Atla, and Themis regiones province and tends to be off of the large plateau shaped tessera highlands, such as Ovda and Thetis regiones The map is in simple cylindrical projection