In 1978, NASA sent the Pioneer Venus Orbiter to study Venus for more than 10 years. This image shows the extensive cloud cover of the planet. Scientists believe that Venus once contained water and could have been quite similar to Earth a billion years ago. But the most powerful greenhouse gas effect in the solar system has rendered the planet a wasteland of toxicity. Because the atmosphere is mostly carbon dioxide, heat is trapped on the planet's surface. This means that Venus is hotter than Mercury, despite Mercury's closer proximity to the sun.