Solar System information #4: Venus

Venus - the hottest planet

Venus, the second planet out from the Sun, is also the hottest. It is shrouded in thick clouds made up of tiny droplets of sulphuric acid. On the Earth, the heat-trapping effect of our atmosphere (about 1% carbon dioxide) makes the average surface temperature around 46°C warmer than it would be otherwise. The atmosphere on Venus is 90 times denser than the Earth’s, and around 95% carbon dioxide. This means that the greenhouse effect on Venus increases the surface temperature by 400 degrees or more, making the surface hotter than Mercury.

We can never see through the Venusian clouds, but the surface has been mapped by radar (by space probes and by Earth based stations). Radar scans (e.g. by the ESA Venus Express spacecraft) show that 60% of the surface is covered by vast rolling plains, pock marked with craters. Also there are two distinct highland areas which are 4 to 5km higher than the plains which surround them. The highland, known as Ishtar Terra is comparable in size to Australia. The other highland, Aphrodite Terra, is about twice that size and much rougher. Ishtar Terra has the biggest mountain, Maxwell Montes: 11km high (2km higher than Mt Everest). Venus also has a huge canyon 1500km long, 5km deep and up to 400km wide. Hundreds of volcanoes have been found on Venus's surface, and there is evidence that some of them may still be active.

Venus and Earth are similar in size, mass and density; but there are important differences. Earth has water and life. Venus is now much too hot for any form of life as we know it, although billions of years ago, before the greenhouse effect made it so hot, it may have had liquid water oceans and even primitive life, just like the Earth. Carbon dioxide makes up almost 96% of the Venusian atmosphere and nitrogen almost 4%. The planet has a hard rocky core, surrounded by a mantle and a thin surface crust.

Strangely, Venus has a retrograde rotation - it rotates “backwards”. It spins east-west while orbiting west-east. When it is approaching Earth, Venus is seen after sunset and known as the “evening star” but when it is moving away from Earth, it appears just before sunrise and known as the “morning star”. Strangely, Uranus and Pluto also have retrograde rotation. The spin and orbit of Venus combine (spin-orbit coupling) in a way that makes a day on Venus equal to 127 Earth days. Venusian daylight lasts 63.5 days, and night lasts 63.5 days!