In 1962, Mariner 2 flew by Venus, carrying, among other instruments, magnetometers to measure the strength of the planet's magnetic field. None was detected, and subsequent Soviet and U.S. missions, carrying more sensitive detectors, have confirmed this finding. Venus, with an average density similar to Earth's, probably has a similar overall composition and a partially molten iron-rich core. The lack of any detectable magnetic field on Venus, then, is almost surely the result of the planet's extremely slow rotation and consequent lack of dynamo action. (Sec. 7.4)

Having no magnetosphere, Venus has no protection from the solar wind. Its upper atmosphere is continually bombarded by high-energy particles from the Sun, keeping the topmost layers permanently ionized. However, the great thickness of the atmosphere prevents any of these particles from reaching the surface.

None of the Venera landers carried seismic equipment, so no direct measurements of the planet's interior have been made, and theoretical models of the interior have very little hard data to constrain them. However, to many geologists the surface of Venus resembles the young Earth, at an age of perhaps a billion years. At that time, volcanic activity had already begun, but the crust was still relatively thin and the convective processes in the mantle that drive plate tectonic motion were not yet fully established.

Why has Venus remained in that immature state and not developed plate tectonics like Earth? That question remains to be answered. Some planetary geologists have speculated that the high surface temperature on Venus has inhibited the planet's evolution by slowing the planet's cooling. Possibly the high surface temperature has made the crust too soft for Earth-style plates to develop. Or perhaps the high temperature and soft crust have led to more volcanism, tapping the energy that might otherwise go into convective motion. It may also be that the presence of water plays an important role in lubricating mantle convection and plate motion, so that arid Venus could not evolve along the same path as Earth.