c. 4.5 Billion BCE

Venus

It’s fun to ponder the relative importance of “nature versus nurture” in determining the origin of people’s personalities and characteristics. Twins, for example, make great case studies. Well, the same is true for planets, and one of the best examples to consider is Venus, a near-twin of Earth in some ways but profoundly different in others.

Venus is only about 5 percent smaller than Earth and has about the same density—meaning that it is essentially a rocky, terrestrial planet very much like our own. Both planets have atmospheres, and Venus even orbits in the same general neighborhood of the inner solar system as we do, at an average distance of 0.72 astronomical units compared to Earth’s 1.0. But that’s where the similarities end. Venus is barely spinning, taking about 243 Earth days to spin once on its axis—backward! The Venusian atmosphere is much thicker than ours, with 90 times the pressure at its surface. That thick atmosphere sports violent upper-level wind speeds of more than 218 miles (350 kilometers) per hour and is almost entirely carbon dioxide, with only scant traces of the nitrogen dioxide, oxygen, and water found in Earth’s atmosphere. The carbon dioxide molecule is transparent to visible light but is exceedingly good at trapping heat radiation (like a greenhouse), causing the surface of Venus to be very hot—more than 750 kelvins, or about 300 degrees hotter than an oven!

Astronomers are trying to understand how Earth and Venus ended up with such radically different surface conditions. Understanding carbon dioxide may be the key. Earth has as much carbon dioxide as Venus, but it dissolves in our oceans and is trapped in rocky carbonate minerals. Any ocean on early Venus, slightly closer to the Sun, would have since evaporated away, however, leaving no way to remove the carbon dioxide.

Venus is a case study of carbon dioxide gone wild and is a prime example of how studying other planets can help us understand what may be in store for our own world.

In 2009 the European Space Agency’s Venus Express orbiter acquired this false-color composite of infrared heat emitted from the planet’s night side (lower left, red) and sunlight reflected from the planet’s swirling day-side clouds (upper right).