The limited relaxation of shapes of impact craters and high correlation between topography and gravity are some of the reasons that support the widely accepted view that the interior of Venus is dry compared to Earth. The fact that the atmospheric abundance of Ar-40 is only similar to 25% of the radiogenic gas produced inside Venus argues that Venus is not thoroughly degassed and its interior has not been dried over time. Therefore Venus must have lost its water very early in its history, before any significant Ar-40 was produced. Current ideas suggest that Venus did not suffer a major impact. Therefore one would not expect it to have been substantially molten. As a result degassing all its water would be difficult and losing all the water without leaving oxygen in the atmosphere would also be very difficult. To overcome the above difficulties in explaining a dry Venus interior, a new hypothesis is proposed that Venus formed by a near head-on collision of two large planetary embryos, as might be expected from favoured oligarchic planetary accretion. Such a collision would be sufficiently large to melt totally and briefly vapourise a significant proportion of both bodies. This would allow much of the released water to react rapidly with iron. Depending upon the reaction hydrogen is either expected to escape to space or enter the core. Oxygen would form FeO, most of which would enter the core, together with other iron reaction products. Most everything else not caught in the hydrodynamic escape driven by any hydrogen stream would be gravitationally retained by the final body. The model can therefore reconcile the Ar-40 data, a virtually oxygen free atmosphere and a dry interior. An appropriate large collision also provides an easy explanation for the retrograde rotation of Venus. The possible implications for inner core and magnetic field formation; and atmosphere evolution including effects on D/H, C, N and inert gases are also discussed. A simple test of this hypothesis is that, in contrast to the current Venus paradigm, little or no hydrated minerals should be found on the surface. (c) 2008 Elsevier B.V. All rights reserved.
