THE EVOLUTION OF VENUS
Because Venus is so close in size, mass, and density to Earth, it is sometimes thought of as Earth's twin. They are also alike in that they comprise fairly similar materials and probably formed at about the same time, roughly 4.5 thousand million years ago.
Their early histories may also have proceeded along similar lines. About 3.8 thousand million years ago, towards the end of heavy bombardment by meteorites, Venus had begun to separate into layers or differentiate. Under the influence of gravity the heavier elements formed a core, while the lighter elements formed a rocky mantle and a crust. As far as we can tell the interior of Venus is like Earth's, with an iron core roughly 3000 kilometres in radius, and a lighter rocky mantle forming the bulk of the planet. The mantle of Venus may even be convecting like Earth's, whereby hot material in the mantle rises before gradually cooling and sinking to be re-heated in a grand cycle taking many millions of years. Before the Magellan mission, one of the biggest questions geologists had was whether Venus had a system of plate tectonics like Earth, with mobile sections of crust or plates jostling against each other.
Venus interior.	Cross-section of Earth showing oceanic and continental crust.
On Earth, the crust is recycled. New crust is formed at mid-ocean ridges, while at the margins of the continents, old ocean crust is drawn downward and re-melted, a process which enables Earth to lose heat. Although there is evidence that convection of the mantle occurs on Venus, there appears to be no plate recycling going on. There are rifts and canyons, but nothing to indicate that they operate in the same way as Earth's mid-ocean ridges or spreading centres.
In bulk Earth and Venus are similar, but they have evolved in very different ways. The high temperatures of Venus' greenhouse atmosphere, as well as the lack of water, are likely to have played a major role in its geological evolution. The high temperatures may mean that the rocks near the surface are too buoyant to sink into the mantle and plate recycling does not take place. Instead Venus loses heat by the movement of hot material to the cooler outer layers.
A rising mass of hot material is known as a "plume", and can create volcanic rises and coronae on the surface. The features that are seen on the surface are a good indication that Venus works vertically rather than horizontally.
As Venus has relatively few craters the surface is thought to be fairly young. Scientists are therefore trying to decide how exactly Venus is renewed or 'resurfaced'. Volcanic eruptions flooded large areas with lava burying the old craters, but what is uncertain is whether these volcanic floods came in patches, here and there, or whether they all happened at the same time, in one catastrophic deluge of molten rock.
Mantle plume.
Geological history
As on the other planets an attempt has been made to provide an outline chronology or timescale of geological evolution. The names of the geological periods are taken from regions which are considered representative of a particular phase in the evolution of Venus. Although the picture emerging of Venus is that of a planet with a complex history, there are just three main geological epochs.
The Fortunian (after Fortuna Tessera) includes surfaces considered to be the oldest which we can see. Pre-Fortunian is used to refer to the early history for which there is no surviving geology. The Guineverian (after Guinevere Planitia), is typified by the formation of extensive volcanic plains, but the period would also have witnessed localised volcanism, corona formation, impact cratering, and surface deformation. More recent geological time is represented by the Aurelian Period (after the surface deposits associated with impact craters), with geological activity ongoing but more intermittently.
Some authorities break the Guineverian period down into a series of sub-units (Sigrun, Lavinia, Rusalka, and Atla). It is an ongoing debate whether the processes thought to typify these units, such as the formation of wrinkle ridges (assigned to the Rusalka group), can used to determine the age of surfaces across the whole planet.