IV.

Venus’s Past

Because the size and density of Venus and Earth are so similar, scientists think the two planets originated in the same way. Like Earth, Venus formed about 4.6 billion years ago out of the spinning disk of dust and debris that surrounded the newborn Sun. The materials accreted (clumped together) to form larger and larger objects called planetesimals, resulting in bodies with sizes between those of the Moon and Mars. A number of these bodies with similar or intersecting orbits eventually collided and merged to form both Venus and Earth at difference distances from the Sun.

The earliest history of Venus was probably very similar to that of Earth. However, impacts from leftover Moon or Mars-size bodies in the inner solar system could have given Venus its odd, extremely slow, backward (clockwise) rotation. One or more of these impacts may have created a moon or moons for Venus, much the same way that our Moon is thought to have formed from a giant impact with Earth. Some theories suggest that Venus’s ancient moon or moons eventually crashed back into the planet, reversing its original faster counterclockwise rotation.

Scientists are not certain if Venus had large amounts water like Earth after it formed or if it has always been dry. In the wet scenario, Venus may have had oceans and an atmosphere similar to the early Earth for millions of years. Because it received more sunlight, Venus began to heat up, increasing the water vapor in the atmosphere. Water vapor is an extremely efficient greenhouse gas. The planet eventually suffered a runaway greenhouse effect that raised its surface temperature to a point where the oceans boiled away entirely. Ultraviolet light from the Sun then broke down the water vapor in the atmosphere into hydrogen and oxygen. The light hydrogen atoms escaped into space, carried off by the solar wind, while the oxygen atoms reacted with minerals in the crust. With most of the water lost, carbon dioxide could not combine with water to form carbonate rocks as it had on Earth. The massive amount of carbon dioxide in the atmosphere added to the greenhouse effect begun by the nearly vanished water vapor.

Studies of the impact craters on the surface have led some scientists to propose that the entire surface of the Venus melted and reformed in a planet-wide eruption around 500 million years ago—no older impact craters have been identified. Such a global event may have happened a number of times during the history of Venus. Unlike the constant tectonic plate movement and volcanic activity that gradually reshapes Earth’s crust, Venus may undergo rare but catastrophic tectonic processes that resurface the entire crust all at once.