The planet Venus is called the sister planet of Earth because it resembles the Earth in many ways but also different in many properties. The structure and size of the planet Venus is similar to the planet Earth. The difference lies in the temperature and composition of atmosphere. The temperature of Venus is much larger than planet Earth and sufficient to melt lead. The atmosphere of Venus is so dense that the atmospheric pressure is 90 times the atmospheric pressure on Earth. The equivalent pressure on Earth exist in the oceans at the depth of 1 km.
The atmosphere on Earth has 78% Nitrogen gas, 21% Oxygen

The initial conditions and structure of Venus is believed to be similar to Earth then what is the reason for large amount of Carbon Dioxide in the atmosphere of Venus?

The excessive amount of Carbon Dioxide is caused by many factors which includes mainly “Runaway Greenhouse Effect” and “Volcanic Eruption”. Carbon dioxide is a greenhouse gas which can trap sunlight and increase the temperature of the planet by significant amount.

The temperature and pressure decreases with the height. The water vapour moves upward and reaches a point where it condenses due to lower temperature and pressure at that height and returns back to the surface. This region is called “cold trap”. The cold trap found on the Earth at height of 9-15 km and at Venus, it is present at the height of about 50 km from the surface due to the closure proximity of the planet Venus to the Sun.

On the Earth, the ozone layer is present much above the cold trap region which absorbs the Ultraviolet Radiation (UV) of sun. Thus ozone layer protects the water molecules from UV radiation which can destroy them by breaking into hydrogen and oxygen atoms.

On the Venus, no such ozone layer is present to protect UV radiation from reaching the water molecules. Very deep layers below the cold trap are opaque for UV but when molecules of water rise towards the cold trap region then they encounter the region where UV radiation breaks them into atoms of hydrogen and oxygen before reaching cold trap region. The hydrogen molecules form the hydrogen gas which leaves the atmosphere and goes into space because the low gravity of Venus cannot hold the Hydrogen gas. The oxygen atom combines with the carbon or carbon monoxide to from the carbon monoxide or carbon dioxide.

As the water continuously loses in the above manner, the carbon dioxide cannot be brought back to the surface by water. So the carbon dioxide keeps on increasing and causing more greenhouse effect which increases the temperature. The increased temperature leads to the increased height of cold trap which increases the region of encounter of water molecules with UV radiation. Thus all this results in decreased amount of water and increased amount of carbon dioxide.

The surface of Venus is covered with large number of volcanoes which usually remains active. The volcanic eruption releases many gases like sulphur dioxide and carbon dioxide. The carbon oxide released from volcanic eruption increases the amount already present in atmosphere of Venus.

You may be surprised to know that both Earth and Venus have nearly same amount of carbon content, the difference is that the Earth has large amount of carbon contain present in the form of rocks, oceans and life forms whereas on Venus it is present as carbon dioxide in the atmosphere. The Earth has many processes which brings the carbon dioxide back to the surface and restore that in solid form like carbonates and other rocks, life forms also convert carbon dioxide to the other compounds in photosynthesis. No such processes are present on Venus to restore the carbon dioxide back to the surface.