Blame it on carbon dioxide/CO2 which constitutes 96% of Venusian atmosphere. The 250 kilometers thick gaseous envelope of this oddball planet has enough CO2 to fill about 1,00,00,00,000 billion fire extinguishers. Composed of molecules having one atom of carbon and two atom of oxygen, CO2 is a strong greenhouse gas; whereas nitrogen, the most abundant molecule in the Earth’s atmosphere, is not. However, this does not mean that Venus as a whole is richer in CO2 than the Earth. This gas which might otherwise be in the Earth’s atmosphere has largely been removed by various plants and is locked up in deposits of limestone.
Moving around the Sun at an average distance of 108.2 million kilometers, Venus is truly smoldering greenhouse, where no astronaut would survive the hellish environment. At 470º Celsius the surface temperature of Venus is more than enough to melt the lead and still have 100º left over to make the steel red hot. Even at an altitude of 49 kilometers, where the dense clouds begin, the temperature drops only to 70º Celsius. At about 68 kilometers above Venus, however, the temperature finally drops to nearly 0º Celsius.
It was in 1960s that earthbound radars indicated the piping hot Venusian environment; although many scientists were skeptical at that time. The result, however, were verified by the increasingly sophisticated Venus space probes a decade later. The planet Venus, it turns out, is a greenhouse without a keeper, whose thermostat has broken down long in the past. Sunlight comes in as short wave radiation and heats up the atmosphere as well as the surface, but the heat is then trapped. (See diagram above.) Very little reflected heat, now in the form of long wave infrared, can escape through the dense atmosphere of carbon dioxide.
This process has been called the runaway greenhouse effect which goes on without being regulated by any checks and balances. It is a self-feeding machine which requires no further input of CO2 to keep it up. Presently, our own planet seems to be heading for the similar disaster. Unless the mankind immediately puts an end to carbon dioxide emissions, the Earth will eventually go the way planet Venus went in the past.