Some people call Venus our sister planet, but if it is, it's the sister that went very, very bad.
The atmospheric pressure at the surface is a crushing 90 atmospheres. The surface temperature is 470 Celsius (about 900 F). The atmosphere is almost entirely carbon dioxide, and it rains sulphuric acid. To paraphrase Chekov, it's not exactly a garden spot.*
Through a telescope (and by eye for that matter) Venus is beautiful and bright, but featureless. In visible light, the best you can see are very subtle patches on the disk of the planet. The atmosphere is far too thick to see the surface.
But there's still a lot to learn from the planet. The European Space Agency's Venus Express orbiter arrived at the hellish planet in April 2006 and set up shop. It's equipped with an ultraviolet camera, and when viewed in UV Venus is a whole 'nuther place. The chemicals in the atmosphere reflect or absorb UV from the Sun ,creating beautiful global weather patterns reminiscent of Earth's. Here's a recent UV shot:
As you can see, the story is different in UV than in visible. Things is, scientists aren't exactly sure what they're seeing. The bright stripes are due to sulphuric acid droplets in the air (yikes... I mean seriously, yikes). But they're not sure what's causing the darker regions; something is absorbing UV, but it's unknown exactly what it is.
And the weather on Venus is weird, too. The science team was recently amazed to see a bright haze form over the south pole of Venus, then, over the course of several days, grow to cover the southern half of the planet. Then, just as quickly, it receded. What could cause such a thing? No one knows. There are very small amounts of water vapor and sulphur dioxide in Venus's atmosphere, located deeper down (below 70 km in height). If this wells up, the ultraviolet from the Sun can break the molecules apart, which would reform into sulphuric acid, creating the haze. But why would those two molecules suddenly well up to the top of the atmosphere in the first place? Again, no one knows.
The only thing to do is keep looking. Venus Express has been orbiting the planet for nearly two years now, and that allows the long view, so to speak. By examining the data taken over long periods of time, scientists can investigate global properties of the planet and look for trends, connections, cause and effect. Venus has the same mass, size, and density of Earth, but at some point in its past it took a very different path than we did. Studying it carefully will reveal more about the Earth and why things turned out so well for us.
Sure, when you look into the abyss, sometimes it looks back into you. But that can be pretty helpful when you want to learn more about the abyss as well as yourself.
*What, you thought I meant Anton Chekov?