IELTS Academic Reading Practice 45

Earth has ample water in its oceans but very little carbon dioxide in its incredibly thin atmosphere. By contrast, Venus is very dry and its thick environment is generally filled with carbon dioxide. The unique atmospheres of both Venus and Earth had been derived at least in part from gases spewed forth, or outgassed, by using volcanoes. The gases that emanate from present-day volcanoes on Earth, such as Mount Saint Helens, are predominantly water vapor, carbon dioxide, and sulfur dioxide. These gases ought to consequently have been important parts of the true atmospheres of each Venus and Earth. Much of the water on both planets is also believed to have come from influences from comets.

In fact, water probably once dominated the Venusian atmosphere. Venus and Earth are similar in dimension and mass, so Venusian volcanoes may also have outgassed as much water vapor as on Earth, and each planet would have had about the identical quantity of comets strike their surfaces. Studies of how stars evolve advocate that the early Sun used to be solely about 70 percent as luminous as it is now, so the temperature in Venus’ early environment needs to have been quite a bit lower. Thus water vapor would have been capable to liquefy and form oceans on Venus. But if water vapor and carbon dioxide were once so frequent in the atmospheres of each Earth and Venus, what became of Earth’s carbon dioxide? And what happened to the water on Venus?

The answer to the first question is that carbon dioxide is still found in abundance on Earth, but now, instead of being in the form of atmospheric carbon dioxide, it is either dissolved in the oceans or chemically bound into carbonate rocks, such as the limestone and marble that formed in the oceans. If Earth grew to become as hot as Venus, a good deal of its carbon dioxide would be boiled out of the oceans and baked out of the crust. Our planet would soon strengthen a thick, oppressive carbon dioxide atmosphere a lot like that of Venus.

To reply the query about Venus’ lack of water, we ought to return to the early records of the planet. Just as on present-day Earth, the oceans of Venus constrained the amount of atmospheric carbon dioxide by means of dissolving it in the oceans and binding it in carbonate rocks. But being nearer to the Sun than Earth is, much of the liquid water on Venus would have vaporized to create a thick cowl of water vapor clouds. Since water vapor is a greenhouse gas, this humid atmosphere—perhaps denser than Earth’s present-day atmosphere, however, significantly less dense than the surroundings that envelop Venus today—would have successfully trapped heat from the Sun. At first, this would have had little effect on the oceans of Venus. Although the temperature would have climbed above 100° C, the boiling point of water at sea level on Earth, the added atmospheric pressure from water vapor would have saved the water in Venus' oceans in a liquid state.

This hot and humid kingdom of affairs might also have endured for a number of hundred million years. But as the Sun’s strength output slowly elevated over time, the temperature on Venus’ surface would eventually have risen above 374°C. Above this temperature, no matter the atmospheric pressure, Venus’ oceans would have begun to evaporate, and the introduced water vapor in the surroundings would have extended the greenhouse effect. This would have made the temperature even greater and prompted the oceans to evaporate faster, producing extra water vapor. That, in turn, would have similarly intensified the greenhouse impact and made the temperature climb higher still.

Once Venus’ oceans disappeared, so did the mechanism for disposing of carbon dioxide from the atmosphere. With no oceans to dissolve it, outgassed carbon dioxide started to accumulate in the atmosphere, intensifying the greenhouse effect even more. Temperatures ultimately became excessive ample to "bake out" any carbon dioxide that used to be trapped in carbonate rocks. This liberated carbon dioxide fashioned the thick atmosphere of present-day Venus. Over time, the rising temperatures would have leveled off, solar ultraviolet radiation having broken down atmospheric water vapor molecules into hydrogen and oxygen. With all the water vapor gone, the greenhouse effect would no longer have accelerated.