Venus

The scientists guess that Venus is lifeless.

Venus and Earth are commonly regarded as twin planets but not identical twins.

With a significant amount of internal heat, Venus may continue to be geologically active with volcanoes, rifting, and folding.

Space probes suggest that Venus may have started with as much water as Earth, but it was unable to keep its water in liquid form.

Because Venus receives more heat from the Sun, water released from the interior evaporated and rose to the upper atmosphere where the Sun's ultraviolet rays broke the molecules apart.

Much of the freed hydrogen escaped into space, and Venus lost its water.

Without water, Venus became less and less like Earth and kept an atmosphere filled with carbon dioxide.

Without liquid water to remove carbon from the atmosphere, the level of carbon dioxide in the atmosphere of Venus remains high.

Like Venus, Earth is large enough to be geologically active and for its gravitational field to hold an atmosphere.

Unlike Venus, it is just the right distance from the Sun so that temperature ranges allow water to exist as a liquid, a solid, and a gas.

But just because a particular mineral is found on, say Mars or Venus, we really shouldn't assume that whatever caused it to turn up there, must be the same process that formed that mineral here on Earth.

By contrast, Venus is very dry and its thick atmosphere is mostly carbon dioxide.

The original atmospheres of both Venus and Earth were derived at least in part from gases spewed forth, or outgassed, by volcanoes.

These gases should therefore have been important parts of the original atmospheres of both Venus and Earth.

Venus and Earth are similar in size and mass, so Venusian volcanoes may well have outgassed as much water vapor as on Earth, and both planets would have had about the same number of comets strike their surfaces.

Studies of how stars evolve suggest that the early Sun was only about 70 percent as luminous as it is now, so the temperature in Venus’ early atmosphere must have been quite a bit lower.

Thus water vapor would have been able to liquefy and form oceans on Venus.

But if water vapor and carbon dioxide were once so common in the atmospheres of both Earth and Venus, what became of Earth’s carbon dioxide?

If Earth became as hot as Venus, much of its carbon dioxide would be boiled out of the oceans and baked out of the crust.

Our planet would soon develop a thick, oppressive carbon dioxide atmosphere much like that of Venus.

To answer the question about Venus’ lack of water, we must return to the early history of the planet.

Just as on present-day Earth, the oceans of Venus limited the amount of atmospheric carbon dioxide by dissolving it in the oceans and binding it up in carbonate rocks.

But being closer to the Sun than Earth is, enough of the liquid water on Venus would have vaporized to create a thick cover of water vapor clouds.

Since water vapor is a greenhouse gas, this humid atmosphere—perhaps denser than Earth’s present-day atmosphere, but far less dense than the atmosphere that envelops Venus today—would have efficiently trapped heat from the Sun.

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 kept the water in Venus' oceans in the liquid state.

Above this temperature, no matter what the atmospheric pressure, Venus’ oceans would have begun to evaporate, and the added water vapor in the atmosphere would have increased the greenhouse effect.

Once Venus’ oceans disappeared, so did the mechanism for removing carbon dioxide from the atmosphere.

This liberated carbon dioxide formed the thick atmosphere of present-day Venus.

It is no accident that Maxwell Mons on Venus and the Hawaiian shield volcanoes on Earth rise to about the same height (about 10 kilometers) above their respective bases-Earth and Venus have similar surface gravity.

First, the level of sulfur dioxide gas above Venus's clouds shows large and very frequent fluctuations.