Venus

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Why is Venus so completely different from Earth? Instead of oceans and a pleasant atmosphere, it's bone dry, and has roasting temperatures and choking carbon dioxide air. Yet scientists think that billions of years ago, Earth was similar. Volcanoes churned out carbon dioxide, and no life could live on our planet. On both Earth and Venus, when the planet started to cool, oceans may eventually have formed, either from water issued from the volcanoes or dropping in from a rain of comets.

But being a bit closer to the Sun, water on Venus didn't last long. Carbon dioxide is a greenhouse gas, and traps the heat, so the oceans evaporated. Here on Earth, though, complex molecules in the oceans somehow got together and formed the organisms which we call life. Eventually they started to consume carbon dioxide and give off oxygen, in the process of photosynthesis. Over time, even more carbon dioxide was concentrated into the shells of sea creatures and turned into rock. But hotter Venus had no such luck, and the carbon dioxide still rules.

Another difference is that while on Earth we can find rocks that are billions of years old, on Venus they all seem to be just a few hundred million years old. Just why this might have been is unknown, but even if life did form in its early oceans, it has long since died out when the oceans dried up and any evidence will have disappeared.

Today, the planet is literally a vision of hell, and exploring it will have to be done by heavily engineered robot systems.The air pressure at the surface is similar to that at a depth of 1,000 metres one of Earth's oceans, and the temperature is about 460 degrees C, which is even hotter than on Mercury. At that pressure, even a light breeze on Venus would be enough to knock you over, but some plans use this to propel a surface rover. It would have to go where the wind takes it, but such a system would save on the power requirements.

Several Soviet probes landed there in the 1980s, though much of the information they gleaned has never been released, even today. But one Soviet scientist revealed that because of the dense clouds the lighting at the surface of Venus was equivalent to 'a cloudy winter day in Moscow'.

There's one big difference, though. On Earth a day lasts 24 hours, of course. But Venus has the slowest rotation of any planet - 243 Earth days, which is even longer than its year, so it turns backwards compared with Earth. This means that from noon to noon takes 116 Earth days. Though you couldn't actually see the Sun from the surface of Venus because of the dense clouds, you'd have 58 Earth days of daylight followed by 58 Earth days of night. And Venus doesn't even have a moon, so the night would be pitch black, unless you could see in infrared, in which case there would be a dull glow everywhere. On the whole, I'd prefer a cloudy winter day in Moscow.

Credits: Venus photographed in ultraviolet, from the NASA Pioneer probe - Jet Propulsion Laboratory NASA.