Venus compared with the Earth

Venus is often named as Earth's twin because both worlds share a similar size, surface composition and have an atmosphere with a complex weather system.

The figure on the right compares Venus and Earth spacecraft images. The surface of Venus is shown in orange as radar images while the atmosphere is reproduced on near true colors as it would be seen by the human eye. The upper clouds are brightest in the blue and ultraviolet wavelengths making Venus a white-blue colour planet. Both planets have almost the same size and density and Venus is only a 30% closer to the Sun than Earth. Both share an interesting geological evolution with old volcanoes in Venus and some of them could still be active. One of the biggest misteries of Venus is why its surface is so young on geological time-scales. It is interesting to remark that there is almost no water on Venus' atmosphere.

There are many more differences between both planets.

Whereas Earth rotates in about 24 hours Venus rotates in the contrary sense (retrograde rotation) in 243 days. The orbital period of Venus is 225 days so that a Venus year takes less than a full day. The combination of these two periods results in the Sun appearing from the West and dissapearing over the East with a day-night cycle of 117 days.

The atmosphere of Venus is 90 times more dense than that on Earth and it is made of 96.5% of CO2 and a 3% of nitrogen. This means that both planets have the same amount of Nitrogen on their atmospheres. Surprinsingly the CO2 on Earth is stored on calcite type rocks and if we would convert the CO2 on these rocks into atmospheric CO2 it would amount to the same amount of CO2 that there is on Venus' atmosphere.

Because of the denser atmosphere and the chemical composition Venus experiences an inmense green-house effect that raises the temperature over the surface to more than 470慢.

The figure on the right illustrates the basics of the greenhouse effect on Venus. Long-wave radiation from the Sun is mainly reflected at the upper cloud deck and partially absorbed by the atmosphere but part of it reaches the surface and heats the lowest atmosphere. The hot surface cools down emitting short-wave radiation that is absorbed and re-emitted by the green-house gases of the atmosphere impeding cooling of the planet and originating the high temperatures at the surface. Figure extracted from here.

Clouds are common on Earth but they cover completely Venus' atmosphere. They are made of sulfuric acid droplets at 50-70 km above the surface and at temperatures comparables to Earth's surface temperatures. They are extremely reflective making Venus the most reflecting body in the Solar System.