Tectonics on Earth's "Twin" Planet

Venus had often been called Earth’s twin planet.  While it has many similarities to Earth- size, density, composition, heat production- it turns out to be a very different world.  There are surface temperatures of 740 K (870° F, hot enough to melt lead) and an atmosphere of almost entirely carbon dioxide (a runaway greenhouse effect).   Venus is a dry planet; there is no substantial amount of water on the surface.  The atmosphere, however, has signatures of water leading to the conclusion that Venus was once a wet planet.  It seems Venus has become Earth’s evil twin. 

Fig. 1: A computer generated surface view of the volcano Maat Mons (source: NASA)

Another important difference is its absence of plate tectonics.  Earth appears to be the only planet with plate tectonics, which is the theory that Earth’s lithosphere (the crust and upper mantle) is made of individual mobile plates that can collide (to form mountains), spread apart (to create oceans and basins), or slide past one another (as in the case of the San Andrea fault).  Although Venus currently lacks plate tectonics, it is impossible to know if it existed before 500 million years ago.  It was around this time that Venus experienced a global wide volcanic event that resurfaced the planet.  Instead of plate tectonics, Venus is currently in a stagnant lid convection regime, which is a fancy way of saying that the lithosphere isn’t broken up into discrete pieces and tectonics is instead driven by the mantle convecting underneath the immobile lithosphere (Hot mantle rises and cools, then descends. The same principal as boiling water on the stove- see Fig. 2).

Fig 2: A simulation showing convection in a one-plate planet such as Venus. The light blue arrows indicate motion; warm mantle rises and cold mantle descends. (credit: Walter Kiefer and Louise Kellogg, lpi.usra.edu)

On Earth, plate tectonics require a global network of shear zones (areas of low strength and concentrated deformation, such as faults) throughout the entire lithosphere. The localization processes on Earth are dominated by water content and mineral that are layered like sheets. Venus' lithosphere is dry and lacks layered minerals, and yet the rift zones of Venus are remarkably Earth-like. The are of Beta Regio has often been compared to the East African Rift system due to their similar geologic characteristics (Fig. 3). Beta Regio has a rift called Devana Chasma, which is part of a triple junction system similar to the one seen in East Africa. For Devana Chasm to be as narrow as it is, we expect some form of localization to be active to concentrate the deformation into a localized region. It is likely that the presence of melt will have a significant impact on localization.

Fig.3:  Comparison of the East African Rift and Beta Regio; both exhibit "triple-junction" rift behavior. (credit: (top) Rathbun et al. Formation of Beta Regio, Venus. (bottom) Tesfaye, et al.  Early continental breakup boundary and migration of the Afar triple junction, Ethiopia)

It is thought that the addition of the appropriate localization mechanism(s) and including the fact that tectonics are not driven by the same forces on Venus as they are on Earth, that an agreeable model for rifts on Venus can be developed.  By understanding the tectonic regime of Venus we are better able to constrain its evolutionary history and interior processes.   Venus can also represent an appropriate analogue for early Earth, prior to the initiation of plate tectonics, which will allow us to better understand how it developed here.