VENUS: STILL THE LABORATORY FOR UNDERSTANDING THE ORIGIN AND EVOLUTION OF HABITABLE PLANETS

Venus has long been recognized as important for understanding our own planet because of its similarity to Earth in size and position in the Solar System. This continues to be as true today as it was fifty years ago when the Soviet Venera 1 was launched towards Venus. Some 26 missions have explored Venus with impactors, flybys, atmospheric probes, hard landers, soft landers, balloons, and orbiters. Add to this some remarkable Earth-based radar images.

After decades of intense study of the ancient surfaces of the Moon and Mars, perhaps we should not have been surprised when Venus, based on impact crater density and morphology, as revealed by Magellan, turned out to be more like Earth in the apparent age of its surface. We have learned much about Venus, but spirited debate continues regarding issues such as the age of the surface, whether there is active volcanism and the nature of the physical and chemical interaction of the surface and atmosphere.

Perhaps Venus, rather than being Earth’s “evil twin”, has in fact evolved exactly as should be expected for a planet of its mass and placement in a stellar system. The oxygen-rich atmosphere of Earth is far nastier and more corrosive, and far more unusual than the bit of H₂SO₄ in the cloudy atmosphere of Venus. So which is really the “evil twin?” Yes, Venus has a dense, cloudy atmosphere and a hot surface. Earth and Venus have nearly identical carbon budgets, but Earth has had oceans and evolved life, both capable and necessary for removing CO_2, from the atmosphere and fixing it in rock and organic matter, from as early as we can know. All the gaseous forms of carbon from the interior of Venus remain in the atmosphere.

The last thirty years of exploration of Venus span from the Pioneer Venus Orbiter mission to Venus Express, currently returning data from orbit. We summarize what has been learned about the interrelationships of the processes and evolution of the interior, surface, and atmosphere.